CA1155562A - Realtime radiation exposure monitor and control apparatus - Google Patents
Realtime radiation exposure monitor and control apparatusInfo
- Publication number
- CA1155562A CA1155562A CA000370579A CA370579A CA1155562A CA 1155562 A CA1155562 A CA 1155562A CA 000370579 A CA000370579 A CA 000370579A CA 370579 A CA370579 A CA 370579A CA 1155562 A CA1155562 A CA 1155562A
- Authority
- CA
- Canada
- Prior art keywords
- detector
- layer
- charge
- selenium
- exposure
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
- G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
- G01T1/16—Measuring radiation intensity
- G01T1/24—Measuring radiation intensity with semiconductor detectors
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
- G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
- G01T1/02—Dosimeters
- G01T1/08—Photographic dosimeters
Landscapes
- Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- General Physics & Mathematics (AREA)
- High Energy & Nuclear Physics (AREA)
- Molecular Biology (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Measurement Of Radiation (AREA)
Abstract
ABSTRACT
An apparatus for measuring radiation exposure in realtime by measuring the charge flowing in an external circuit when radiation creates electron hole pairs, and thus allows discharge of a duo-dielectric detector.
The apparatus comprises a radiation source adapted to irradiate a multi-layered detector structure, an external circuit connected to the transparent insulat-ing layer and to a conductive layer of the multi-layered detector structure, and a device for measuring the flow of current through this external circuit when the multi-layered detector structure is irradiated. This exposure measur-ing apparatus is unique in that it measures the voltage of the output electric signal as a function of the surface charge present at the spot where the radi-ation source strikes the photoconductive layer of the detector.
An apparatus for measuring radiation exposure in realtime by measuring the charge flowing in an external circuit when radiation creates electron hole pairs, and thus allows discharge of a duo-dielectric detector.
The apparatus comprises a radiation source adapted to irradiate a multi-layered detector structure, an external circuit connected to the transparent insulat-ing layer and to a conductive layer of the multi-layered detector structure, and a device for measuring the flow of current through this external circuit when the multi-layered detector structure is irradiated. This exposure measur-ing apparatus is unique in that it measures the voltage of the output electric signal as a function of the surface charge present at the spot where the radi-ation source strikes the photoconductive layer of the detector.
Description
~ 15~5~
RrALTItl~ RADIATIO,I EXPOSUREilOi~I,OR
A'ID CO'ITP~OL APPARATUS
TECHNICAL FIELD
The present invention relates generally to apparatus and methods used to obtaln image information from modulation of a unlform flux.
More specifically, the present invention relates to methods and apparatus used to produce a radio-graph by producing a latent image in a photocon-ductor sandwich structure and then optically reading out the latent image.
BACKGROUND ~RT
.
a. Fa~slmile and Vidicons Facsimile systems modulate an electrical signal in response to light that is reflected from a small portion of an image. Facsimile requires production of a real image.
Vidicon tubes store a latent image as a charge distribution pattern in a semiconductor target. This latent imase is scanned by an elec tron beam and the current variation induced by ~he different charges on separate portions of the target comprises a video signal. Vidicons require use of a high vacuum and precise focusing of an electron beam that must be shielded from e~ternal electric and magnetic fields.
b. Conventional Radiographic_Methods Latent images stored in silver halide film or selenium xeroradioyraphic plates must be chemically or powder cloud developed to produce real images. Use of calcium tungstate crystals or a high atomic weight gas for image intensification ,~,.
degrades image quality and still re~lires expo-sures of from 1 to 5 R per typical clinical mammo-gram. Powder cloud development of latent xero-radiographic images requires high differert1al charge denslties on the xeroradiographic plate to attract and hold powder par-ticles prior to fusing.
This high charge differential is generated by x-rays, or ions, impinging on the surface of a charged selenium plate. The higher t~he diffex_ ential charge needed to produce an irnage, khe mor~
X-ray ~xposure is required.
The reader is directed to the following publications for detailed discussion of this prior art.
XERORADIOGRAPHY, J. W. Bo~g, ~ ~ Med.
Bio _, 1973, Vol. 18, No. 1, pp. 3-37; Principals of Rad ographic Exposure_and Processin~, Arthur W. Fuchs, 1958, Chapter 14, "X-Ray Lntensifying Screens", pp. 158-164.
U.S. Patent No. 3,860,817; Reducing Patient X-Ray Dose Durinq Fluoroscopy With an Image System.
U.S. Patent No. 3,828,191, Gas Handling System for Electronradiography Imaging Chamber.
U.S. Patent No. 3,308,23~, Xerographic Facsimile Printer ~laving Light Beam Scanning and Electrical Charging With Transparent Conductive Belt.
Recent research by the present inventors and others indicates that a detailed latent image can be created in a xeroradiographic plate by very low levels (under 10 n~) of X-ray exposure. Ak this exposure level latent image is present, but its associated electrostatic field is not of sufficient intensiky to produce a real image by the powder cloud method of image development. For details of this research see, Grant Application, "Radiomammography With Less than 150 m~ Per Expo~
sure", available from the Department of Experimen-tal Radiology, M.D. Anderson Hospital, Houston, Texas 77025.
c. Semlconductor Art The prior art of reading charge storage patterns out of multilayer se~iconductor sandwich structures lies largely in the area of electro-nics, especially exotic computer memories.
Charge patterns on certain MOS struc-tures can be "read out" using a photon beam. See, Ima~ nq and Stora~e Wlth a Uniform MOS Structure, Applied Physics Letters, Vol. 11, Number 11, pp.
359--361. This technology functions by modifying a depletion layer and then charging the layer to saturation. The few microns thick depletion layer is the only active structure.
Electric fields can also be impressed across two separable photoconductive insulating films in pressure contact that are precharged to the same polarity. See, Increasin~ the Sensitlv-ity of Xer~raphic Photoconductors, IBM Technical Disclosure Bulletin, Vol. 6, No. 10, 1~64, page 60.
IBM has also developed an exotic charge storage beam addressable memory comprising a semiconductor sandwich wherein the semiconductor is totally insulated from both electrodes in the sandwich. See, IBM Technical Disclosure ~ulletin, Vol. 9, No. 5, 1966, pp. S55 556. This device allows data readout by shifting charge population to one side or the other of the insula-ted semicon-ductor.
Finally, some theoretical work has been done on the ~ehavior of light sensitive capaci-~ :~5~$~
tors, see, ~naly.s-is _ I PerEo _ance of_a I g t_S_lsitive Capacitor, Proceedings of the IEEE, ~pril 1965, p. 378.
The present inventlon is an exposure measuring apparatus with a radiatioll source adapted to irradiate a detector structure with radiation.
The detector structure comprises a duo--dielectric sandwich, an external cir-cuit, and a measuring device responsive to the E]ow of current in the external circuit when the detector structure is irradiated by the radiation source.
The duo-dielectric sandwich includes a transparent conducting layer, a transparent insulating layer, a high resistant photoconductor layer, and a conductive layer. The transparent conducting layer overlies the trans-parent insulating layer. The transparent insulating layer is affixed to a high resistant photoconductor layer. The photoconductor layer overlays the conductive layer. Within this duo-dielectric sanclwich, the radiation is capable oF producing electron hole pairs in the photoconductor layer oE thLs detector structure.
The external circuit connects the transparent conducting layer and the conductive layer.
The measuring device measures the flow of current from only a part of the detector structure. This part of the detector structure is electrically segmented from the greater portion of the detector structure. The current flow is measured from several small segmented portlons of the detector.
A control device is responsive to the measuring device so as to stop the irradiation when the total charge reaches a preselected limited.
It is an object of the present invention to provide an X-ray sensing system whose output is an analog or digital video signal that may be selectively displayed on a television monitor, recorded on film, or directly stored or processed in a computer for image enhancement or pattern recognition.
It is another object o~ the present invention to provide a novel ~ 155~2 method and apparatus for converting a charge distribution on a semiconducting surface to a modu:Latecl electric signal.
Another purpose of the present lnvention -is to provide an appara-tus and method th~t combines the edge enhancement eEEect of xeroradiography with a low patient dose level.
Still another purpose oE the present invention is to provide a novel low noise method and apparatus fcr reading out a latent image stored as a pattern of electrical charges by selectively discharging said charges in the presence of a reverse bias electric -field.
It is still another purpose of the present invention to prov:ide an X-ray sensing system that is capable oE directly timing X-ray exposure.
BRIEF DESCRIPTION OF DRAWINGS
~ igure ] is a greatly enlarged cross-sectional view of a portion of one embodimen-t of the multi-layered photon detector apparatus taught by the present invention;
. , .
; 2 ~ IGURE lA is a schematic electrical diagram illustr~ting the electric circuit analog of ~he structure shown in Fiqure l;
FIGURE 2 is a view of the structure ~hown in Fi~ure 1 being flooded wlth photon~i ~ IGURE 2A is a schematic showirlg the electric circuit analoq of Figure 2;
FIGUR~ 3 is a view of the cletector ~tructure after it i~ chaxged, floode~d with pho-ton~ and ~he surface charge has been redistri-buted;
FIGURE 3A i~ the electrical schematic analog of the detector as it is shown in Figure 3;
FI~URE 4 illustrates the detector struc-ture of a preferred embodiment o the present invention as it i5 used in an X-ray system;
FIG~E S is a gxeatly enlar(3ed schematic cross-sectional view of a portion of Figure 4 illustrating the effect of a modulated X-ray flu~
on the detector;
FIGURE 6 is a ~chematic view of ~he detector graphically illustrating ~he change in de~ector ~urface charge caused by X-ray exposure;
FIGURS 7 is a schematic vie~ of the detecto~ apparatu~ taught by ~he presen~ invention being read out by a thin, scanning photon beam, an o~tput wave for~ of the video electric signal i5 illustrated together with a schematic representa-tion of the surface charge of the detector;
FIGURE 8 i a highly enl~rged schematic cross~sectional view of a small portion of the detector structure shown in Figure 7 illustr~ting ~he electron hole production mechanism in the photoconductor layer used by the present inven-tion;
~55~2 FIGURE 9 shows a partially cut-away ~iew of an apparatus adapted to practice the pr~ferred embodiment of the present invention;
FIGUR~ 10 is a simpl.ified diagram show-ing the major parts of the apparatus shown in Figure 9;
FIGURE 11 is a schematic block diagram illustrating a diagnostic X-ray system constructed according to a preferred embodiment of the present invention;
FIGURE 12 shows a cross-sectional view of a clinical apparatus for conducting mass chest radiographic screeniny using the present inven-tion;
FIGURE 13 is a highly enlarged schematic cross-sectional view of an embodiment of the present invention used in the apparatus shown in Figure 12.
- FIGURE 14 is a graph illustrating the theoretical maximum charge obtain~ble from an experimental detector structure per unit area of the detector as a function o the voltage applied across its photoconductive layer. Figure 14 also shows a plot of specific points representing experimental results from tests run on this ex-perimental detector structure for a given mylar thickness and selenium thickness.
FIGURE 15 is a graph illustrating the voltage across the selenium layer of a theoretical detector in kilovolts as a function of the value of C2, in farads, for that detector.
FIGURE 16 is a graph illustrating the charge, in coulombs collected by the experimental detector structure per unit of exposure as a function of total exposure. The values plotted in Figure 16 were obtained using a Bakalite shutter;
l 15~62 ~7 9~
FIGURE 17 illustrates the same informa-tion as Figure 16, but the data was obtained using a PVC shutter;
FIGURE la is a graph showi ng the effi-ciency of the present invention, as expressed in coulombs of charge obtained per roentgen of expo~
sure, plotted against supply voltage for various levels of total exposure. This figure illustrates the increased efficiency of the present invention at low radiation levels;
FIGURE 19 is a graph that plots the total charge collected by an experimental embodi-ment of the present invention as a function of applied voltage across its photoconductive layer.
Fiqure 19 illustrates increase in latitude of the present invention's detector that is observed as applied voltage across the photoconductor is ncreased;
FIGURE 20 is a functional block diagr~1 of an experimental prototype system of the present invention.
FIGURE 21 is a schematic diagram showing a portion of the detector structure used a~ a radiation exposure detector automatically timing the exposure.
FIGURE 22 is an electrical schematic showing the reversed biased mode of detector operation.
FIGURE 23 is an electrical schematic o the preamplifier used in experimental system 3.
FIGURE 24 is a schematic diagram of ~he X-position interface module used in experimental system 3.
FIGURE 25 is the characteristic curvP of the detector structure.
il~5S~
q BEST MODE FOR CARRYING OUT THE INVENTION
In Figure 1, multilayered photon detec tor apparatus 10 has first electrode 12 conduc-tively connected to a first conductive layer 14.
A photoconductive layer 16 is in physical and electrical contact with plate 14 which may be made o al~inum or any other conductor. An oxide layer 11 may act as a blocking contact between layer 16 and plate 14. Layers 14, 16, and 11 may be provided by using a conventional xeroradiogra-phic plate. Transparent insulating layer 18 overlays and may be integrally affixed to selenium photoconductor layer 16. A transparent conductive layex 20 may be integrally affixed to and overlies insulator 18. These lay~rs may be made by vapor deposition or by adhesively bonding the individual components together. Layer ~0 is electrically connected to lead 22.
D SCUSS ION OF EXPERIMEN~AL SYSTEM #l The experimental plates used to test the present invention as early as late-1975 were made from conventional xeroradiographic plate~. These plates are made for mamography by the Xerox Com~
pany. To make the present invention's detector, a plate is first cut to the correct size and ~hen covered with mylar. The conductive coating, which is either Nesa glas~ or a few angstroms of gold, is placed on the mylar.
In the preferred embodiment of the present invention discussed in this section o the specification, the aluminum backing plate 14 is approximately 1/10 of an inch thick and the layer of amorphous selenium 16 is approximately lS0 microns thick. The physical properties of sele-nium are listed in Table I. The transparent insulator 18 is mylar. The transparent conductor /~
~,~
20 may be Nesa glass, a thin fllm of metal deposi-ted directly on the transparent insulator 1~, or a plastic film with a conductive coating, i.e., gold covered mylar. The entire structure 10 may be made by depositing successive layers of selenium, mylar and a thin film of metal onto an aluminum plate. Assembly may be accomplished by vapor deposition, spu-ttering, or any other technique useful to deposi~ even thickness films. This technology is well developed in the art of 5emi-conductor electronics and glass manufacturing.
The thickness of the selenium layer must be selected to maximize quantum efficiency of the detector. This optimum thickness will be a u~c-tion of the photoconductor's characteristi.cs, the potential at which the detector is operated and the energy of the X-ray photons or other particles to which the detector is exposed that act to discharqe it.
Basically, the thicker the selenium, the more it interacts with a given energy of exposing radiation and the more electron-hole pairs a given quantity of radiation produces. ConYersely, as the selenium layer is made thinner ~he electric field acting on these electron-hole pairs becomes stronger (the same potential over less distance~.
Thus a very thin layer of selenium would not interact with the exposing radiation enough to produce electron hole pairs while a very thick layer of selenium would result in the production of lots of electron-hole pairs, bu~ they would all recombine when the exposure stopped.
The optimum thickness of the photocon-ductor layer of the detector will depend on the characteristics of the photoconductor and the energy of the X-ray photons it is designed to ~ .~555~2 sense. In the experimental embodiments of the present invention applicants have found that from one hundred microns to four hundred microns thick-ness of amorphous selenium is optimu~ for the X ray photon energies commonly used in diagnostic radiology.
There are several facts the inventors discovered experimentally that shouldl be men-tioned.
When several experimental prototype detector structures were built under laboratory "clean room" conditions they worked poorly.
Regular aluminum backed selenium-xeroradiography plates covered with gold covered mylar plastic and assem~led with optical cement under less than ideal conditions work very well.
It appears ~hat a very thin al-~inum oxide layer between the selenium and the aluminum acts as a blocking contact, i.e., a diode, to retain the positive charge on the surface of the selenium. Thi~ blocking layer has a blocking potential that must be overcome for current to flow through the contact.
Also, when the experimental detectors built by the inventors were scanned fast, a higher signal strength output was obtained than when they were scanned slowly. Since the light intensity was the same, the faster scan was predicted to result in lower signal strength. Applicants cannot yet explain this phenomena, but, by pulse modulating their light source for between 2 nano-seconds and 10 microseconds on and of, this effect can be used to increase signal strength from the present invention.
Finally, repeated use of the prototype plate resulted in the development of 'lartifacts", 1 L555~2 /~
- 3~
i.e., lines and trash on the image. Repeated use also caused an overall loss in detector sensiti-vity. It was accidentally discovered that heating the prototype plate for a short time with a heat gun used to shrink plastic tubing removed ~lese artifacts and otherwise generally restored the detector's performance.
In Fi~lre 1, Rl represents the resis-tance of the photoconductive layer when it is flooded with light. R2 represents the resistance of the photoconductive layer when it is in the dark. C2 represents the electrical capacity manifested by the charge separation across the transparent insulator 18. One charge pol~rity resides on the transparent conductor 20 and the other on the surface of selenium, which is in immediate contact with insulator 18. Cl repre-sents the electrical capacity of the detector measured between the aluminum conductor 14 and the selenium surface 16. The selenium is thus the dielectric material of Cl.
Figure lA shows an electrical schematic illustrating an electronic analog of detector sandwich structure 10 shown in Figure 1. Rl, R2, Cl and C2 are shown schematically connected in Figure lA as electrical symbols. Switch 24, which is shown open, is used to represent the photocon-ductive nature of selenium layer 16 of Figure l.
Selenium has a dark resistance of approximately 10l6 ohm centimeter. A portion of this resistance .is caused by a blocking contact formed at the Se-Al interface. When photons strike the photo~
conductor its resistance is lowered because the photons create electron hole pairs that carry current. This lower resistance is illustrated schematically in the electrical analogs contained ~ ~S~2 ~4 in this specification by a closure of switch 24 leaving only residual resistance Rl, which repre-sents the forward resistance of the blocking contact in series with the resistance of the selenium layer when it is flooded with photons.
For a current to flow when the photoconductor is flooded with light the blocking potential of blocking contact 11 must be overcome~
TAE~LE I
Ph~sica1 ProE~ertles of Selenlum (2,3 Atomic Number 3~
Density (g cm 3) 4.25 Dielectric Constant 6 . 6 Resistivity at 20C. (~ cm) lol3_1ol6 Thermal Conductivity at 20C.
( -lK-l) 2 x 10 3 Optical Band Gap ( 2V ) 2 . 3 Photo Response Edge (AE) 4600 K absorption Edge (KeV) 12.7 Mobility of Holes (cm2s lv 1 ) O .14 Mobility of Electrons (cm2s lv 1) 5 x 10 3 Energy for Production of Charge Carriers (eV) 7 ______.__._ _____________ * W - 2.67 x Eg 4 0.86 eV
in which Eg is thP optical band gap 1 ;~S~2 Figure 2 illustrates detector 10 being flooded with photons 26. Simultaneously a nega-tive high voltage, i.e., 2,000 vol-ts, is applied to terminal 22 hence to transparent conductor 20.
Figure 2A, the electrical schematic analog of Figure 2, shows switch 24 closed by iight 26. The 2,000 volt potential, Vs, is impressed across electrodes 12 and 22.
Figure 3 shows the semiconductor detec-tor sandwich 10 after light 26 has been exting-uished and leads 12 and 22 shorted together. The resulting positive surface charge on the amorphous ~elenium layer is illustrated diagrammatically by dotted line 28. This surface charge is uniformly distributed over the entire surface.
Figure 3A is Figure 3's electrical analog. The detector is in darkness, so switch 24 is illustrated as open. Diode 15 and battery 13 represent the blocking contact and potential, respectively, of blocking junction 11. The elec-tric charge originally present on capacitor C2 has redistributed so that a portion of that char~e is present on capacitor Cl, the exact portion being dependen-t upon the ratio of Cl to (Cl + C2).
Figure 4 shows the charged detector illustrated in Figure 3 used ~s an image receptor structure for an X-ray image. Uniform X-ray flux 30 is generated by a convenient source of radia-tion, such as an X-ray tube, not shown. The detector will work with any radiation source capable of generatiny electron-hole pairs in the photoconductor. This uniform photon flux encoun-ters and interacts with object 32, which may be any object o interest. Ob~ect 32 is placed directly over detector 10. For simplicity, object 32 is represented as an oblate spheroid of uniform density.
/~
Figure 5 shows a greatly enlarged sche-matic view of a portion of detector structure 10 of Figure 4. Modulated X-ray flux 34 from the radia~ion that has passed through object 32, creates electron-hole pairs 36 in selenium layer 16.
Figure 6 shows detector 10 of Figure 4 after the X-ray exposure has been completed. The detector is now storing a latent image. The modulated surface charge compris1ng the image is schematically illustrated by a humped dotted line 38. This dotted line represents the change in potential created by the electron-hole pairs generated by the X-ray exposure.
Figure 7 show~ detector 10 having a latent image stored as a modula-ted surface charge 38. A thin beam of light 40 is shown scanning the surface of photoconductive layer 16 in a regular raster pattern. In the experimental system number 1 embodiment of the present invention, this thin scanning light beam is produced by a ~-Cd lasPr.
It should be understood ~hat the photon beam need not be coherent. It may be of any frequency capable of creating electron-hole pairs in photoconductive layex 16 o detector 10.
Electrode 41 is connected to electrical ground and electrode 43 carries a video electric signal whose wave form is a function of modulated surface charge 38 in detector 10 scanned by light be~m 40.
Arrow 42 indicates the direction of movement of scanning light beam 40. Output wave form 44 indicates the voltage variation of the output video signal o~tained by said beam's scan-ning the latent image.
l l~S~)fi2 1~
Figure 8 is a schematic, highly enlarged cross-sectional view of a portion of the detector structure being scanned by light beam 40 of Figure 7. Light beam 40 penetrates the transparent conductor and transparent insulator to generate electron-hole pairs 36 in that portion of selenium layer 16 irradiated by the beam.
Functionally, detector 10 operates by placing a uniform surface charge on selenium layer 16 and then selectively discharging part of this surface charge by X-ray exposure to form a latent image.
As shown in Figure 2, when light photons 100d detector 10, selenium photoconductor layer 16 becomes cond~ctive. If, as shown, a -2,000 volt potential is applied between the aluminum backplate ~at ground) and the transparent conduc-tor 20 (at -2,000 volts potential~, then the detector will charge as a capacitor~ Figure 2A
illustrates how this charge is applied to the system. Light flood 26 increases conductivity in the selenium layer 16. The applied voltage Vs is -~,000 volts. This causes a charge to build up across capacitor C2, which represents the capacity between the surface o the selenium in contact with the transparent insulator and the transparent conductor of detector 10. Capacitor C2 thus charges to potential Vs of 2,000 volts.
After capacitor C2 has charged to poten-tial Vsl the light flood is removed and detector 10 is stored in darkness. In the absence of photons, amorphous selenium layer 15 becomes nonconductive and its dark resistivity in syner-gistic co~bination with blocking contact 11 pre-vents the surface charge on the selenium rom leaking off. This condition is illustrated by ~, Figures 3 and 3A. After detector 10 is in dark-ness, terminals 22 and 12 are shorted toge-ther.
This causes the electric charge on capacitor C2 to redistribute between capacitor C2 and Cl, each of which is then charged to a potPntial of one-half Vs, i.e., 1,~00 volts if Cl-C2. Capacitor Cl represents capacitance between the surface of the photoconductive layer 16 and the aluminum backing 14. This 1,000 volt surface charge is clistributed uniformly over the selenium layer's surface. This surface charge is maintained across the selenium layer by the high darX resistivity of the selenium in conju~ction with the now reverse biased block-ing junction at the Al Se interace. The effec-tive blocking potential of this junction has been experimentally d~termined to be about 150 to 300 volts in the experimental systems discussed below.
Resistance Rl, representing resistance of the selenium layer when it is exposed to light, is relatively low compared to its dark resistance R2.
Each photon, striking the pho-toconductor depending on its energy, generates a specific number of electro~-hole pairs within photocon-ductive layer 16 (see Table II~. Each electron-hole pair discharges a portion of the surface charge at the spot it is generated.
Figure 4 illustrates how this effect is used to impress a Iatent image on the detector.
Figure 4 is a cross-section schematic view of the photon detector structure taught by the present invention being used as the image receptor in an X-ray system.
A uniform flux af X-ray photons 30 is modulated, i.e., partially absorbed, by its pass-age through an object being X-rayed 32. The modulated X-ray flux then strikes detector 10.
~ ~5~2 ,~i Detector 10 has a l,000 volt surface charge, as was illustrated by charge 28 in Figure 3, impressed on selenium layer 16. As the X-ray pho-tons strike the selenium layer, they generate electron hole pairs. For monochromatic X-rays the number of electron hole pairs generated is a direct function of the number of X-ray photons striking the seleniurn detector layer.
Some of the X-ray photons in uniform flux 30 are absorbed by object 3~. Thus the modulated flux striking photoconductive layer 16 contains information about the internal structure of the object being x-rayed. This infor~ation is contained in the number of photons striking each portion of the detector.
Figure 5 illustrates how this modula-ted X-ray flux creates a modulated surface charge on the surface of the detector. Modulated X-ray flux 34 generates electron-hole pairs 36 in selenium layer 16 of detec-tor 10. The number of el~ctron hole pairs generated at each point on the surface of the detector i5 a function of the number of X-ray photons impinging on the photoconductive layer Each electron hole pair 36 discharges a portion of the surface charge impressed on the photoconductive layer at the point where it is generated. The greatest nu~ber of electron hole pairs will be generated where the X-ray photons from uniform flux 30 strike the detector without any absorption from the object being x-rayed. A
lesser number of X-ray photons will strike the photoconductive layer under the objec-t being X-rayed. The radio-opacity of the object being X-rayed is thus reproduced in the surface charge of the detector structure after X-ray exposure.
Figure 6 schematically illustrates the detector structure1s modulated surface charge after the X-ray exposure described in connection with Figures 4 and 5.
The surface charge is lower where no object absorbed X-ray photons. The object being X-rayed, which is assumed to be uniformly opague to X-rays, absor~s a portion of the uniform X-ray flux. This results in the generation of fewer electron 'nole pairs under the object and a higher surface charge on the detector under the object being X-rayed. This lS represented by modulated surface charge dotted line 38.
Figure 7 illustrates schematically the readout method used to extract an electric video signal from ~he modulated surface charge which forms the latent image on a detector structure taught by the present invention.
- The detector structure is kept in dark-ness and a thin beam of light, preferably gene-rated by a helium-cadmium laser, is scanned in a r~gular raster pattern across the surface of photoconductive layer l6.
As the scanning light beam 40 moves in direction illustrated by arrow 42, it produces a small moving spot on the surface of photoconduc~
tive layer 16. The size of this spot determines the resolution of the final image. It is there-fore desirable that this spot size be kept small.
As laser beam 40 scans the surface of photoconductive layer 16, it generates electron-hole pairs as is illustrated in detail by Figure 8. These electron-hole pairs are mobile within the photoconductive layer and discharge a portion of the surface charge. In the preferred embodi-ment of the present invention the electrons move 1 ~S~j62 2l toward the positively charged selenium surface of the ~etector sandwich ~tructure 10. ~he voltage drop produced across the resistor connected be-tween ground connection 41, which is attached to transparent conductor 20 and video CUtpUt elec-trode 43, which is attached to alumlnum conductive backplate 14, will be a function of the intensity of the surface charge at the point where the laser beam generates the electron-hole pair.
Scanning laser beam 4Q thus produces a modulated electric signal 44 at output electrode 43. The voltage of this output electric si~nal will be a function of the surface charge present at the spot where the laser beam strlkes the photoconductive layer of the detector. The cur-rent present in the output circuit is a function of the frequency and intensity of the scanning light beam and is a further function of the speed with which the light beam scans the surface of the photoconductor. Video signal 44 can be electro-nically processèd to produce an imaqe that repro duces the latent ima~e found ln the surface charge of the photoconductive layer of the detector.
Depending on the intensity of ~he sur-face charge, the beam scanning speed, and the frequency and intensity of the light beam used, the surface charge may be repetitively scanned several timPs hy the beam of light.
Table II lists the values for the thick-ness o the selenium layer, the X-ray photon energy, the fraction of this energy absorbed by the selenium layer, quantum conversion between X-ray photons and electron-hole pairs, dark resis-tance of the selenium and the total receptor area of detector 10 used in calculating the operating parameters of a sample detector constructed ac~
, r~ j 2 -~3-cording to the preferred embod1ment of the present invention.
TABLE II
Assumed Values Thickness of Selenium Layer lOQ microns Energy Radiation ~1 keV
Abcorbed Fraction of photons 85%
Quantum Conversion 3000 electron,~
photon Surface Voltage `1000 V
Photon Flux/Roentgen 5 + 10 cm Dark Resistivity lol6 Q ~m Total Receptor Area 560 cm Table III lists the calculated capacity of the detector sandwich structure, the charge density present in this structure when the surface potential is 1,000 volts, the number of elementary charges present per square centimeter in the structure and the dark resistance and dark current of the detector at a thousand volts charge.
1~5$~2 ` I` 2, TABLE III
Calculated Values Capacitance 5 8 x 1o~llF/cm2 Charge Density at 1000 Volts 5.8 x 10 Coul/cm2 Number of Elem~tary Charge 3 6 ~ 1olle1/Cm2 Dark Resistance 1014 Q /cm2 Dark Current at 1000 Volts 10 11A/Cm2 Using the values from these tables it is possible to calculate operatirlg parameters for a typical detector system constructed according to the preferred embodiment of the present invention.
If detector 10 has an area of 560 square centimeters and is exposed to uniform flux of 21 keV X-rays resulting in an exposure of Np X-ray photons per unit area, then the total number of electron hole pairs generated will be 3,000 times ~p. Information theory requires that three times as many electron hole pairs be generated as are present at the statistical noise level of the system for the latent image to be detectable.
There are approximately 5 times 109 photons per square centimeter per roentgen of X-ray exposure at 21 Kev. Thus the minimum de~
tectable latent radiation exposure that will produce a detectable latent image in the selenium sandwich structure taught by the preferred embodi-ment of -the present invention (for a 560 square centimeter detector) is approximately 23 micro roentgens per exposure. This may be thought of as i 155r~62 the theoretical lower limit of exposure to gene-rate a 10 line pair per millimeter image with the present invention.
Turning now to a practical example, if the X-ray exposure of a patient scan is 100 mR and at the thinnest section of the X-rayed object 50%
of this radiation is absorbed by the sample, then exposure at the detector at the brightest portion of the image amounts to 50 mR while the lowest theoretical detectable dose, as was discussed above, amounts to 23~R. This theoretically yields a latent image represented by modulation of the surface charge present on the detector at a ratio between the brightest and the darkest areas of the image of approximately 2,000.
The example discussed above shows that the detector structure taught by the present invention is theoretically capable of recording an image using a selenium photoconductive layer with a resolution of 10 line pairs per millimeter and a brightness range of over 2,000 after a total X-ray exposure of only 100 mR. An example of how this latent image can be read out of the detector follows.
Figure 9 shows a partially cut-away view of an apparatus capable of reading out the image whose generation was discussed above.
Light-tight housing 46 contains a light source 48 aligned so as to project a light beam 50 onto the front of a scanning means 52 which may be a rotating multi-sided mirror. Mirror S2 is mounted on an axis 54 which is operably attached to a scanning motor 56. The mirror axis and scan-ning motor are affixed to a platform 58. This platform is mounted on an axis 60 which is ortho-gonal to axis 54. Axis 60 is mounted at one end ~ ~555~
to first upright support 62 and its other end to second upright support 64. The end of axis 60 is swingably mounted in second upright 64 and is operably attached to stepping mo-tor 66.
The entire scanning means comprising the mirror and associated positioning components is available from Texas Medical Instruments, Inc., 12108 Radium, San Antonio, Texas 78216.
Mirror 52 and its associated scanning mechanism is located at one end of housing 46. A
multilayered photon detector apparatus 10 is removably inserted through a light tight opening 68 into a holder at the other end of housing 46 such that deposited transparent con~uctive layer 20 faces toward mirror 52.
Light source 48 is preferably a Helium-Cac~nium laser such as a Liconix model #402 laser with an optical modulator. This laser produces a beam of intense light at appro~imately 4,400 angstroins.
Scanning motor 56 rotates multi-sided mirror 52 on axis 54 to cause light beam 50 to scan horizontally across the surface of detect~r 10. This causes spot 70 to intersect the selenium photoconductive layer as was dlscussed in connec-tion with Figures 7 and 8 above. Each time spot 70 moves from the left to the right side of detec-tor 10, stepping motor 66 moves platform 58 throuqh sufficient arc to deflect spot 70 vertically 1/20th of a millime-ter. This stepping-scanning function may be controlled mechanically or electri.
cally. By convention, spot 70 begins scanning the surface of detector 10 at its upper left hand corner. After raster scanning the entire surface of the detec-tor, the scanning mechanism may ~e ~ ~S;~2 .;. I ,,~.
programmed either to turn off or to rescan the plate.
Figure 10 is a simplified schematic view of the scanning readout mechanism shown in Figure 9. Light beam 50 from laser 48 reflects off of the polygonal surface of multi~sided mirror 52 as that mirror rotates Oll axis 54. This causes light beam 50 to generate flying spot 70 which moves across the surface of detector structure 10. As was discussed in connection with Figures 7 and 8l above, this causes a video signal to be generated across the detector structure.
Theoretically, photons from light source 48 have a wave length of approximately 4,400 angstroms. The quantum yield of these photons and selenium approaches unity when field strength nears 105 volts per centimeter. In a hundred and fifty micron selenium plate, such as is used in the preferred embodiment of the present invention, this corresponds to a thousand volts surface potential.
It is possible to pulse the laser to decrease the sca~ning time required to obtain an image from the detector structure. In the present example, the laser beam may be pulsed for a 100 nanoseconds period with 3 to 4 microsecond spacing between pulse~. This illuminates each image element sufficiently to read out a latent image stored in the detector. The practi~al advantage of such a regime is to permit much shorter samp-ling times. It may also result in higher signal strengths.
Figure 11 is a block diagram of a radio graphic X-ray system taught by the preferred e~bodiment of the present invention. Laser 7~
produces a small intense beam of light 74 which is ; 2 expanded by optical objective 76 and lenses 78 and 80 into a reasonably wide parallel beam 82.
Fo~using mirror 84 reflects beam 82 on the reflec-tive surface 86 of scanning mechanism 88. Scan ning mechanism 88 is substantlally the same as the scanning mechanism discussed in connection with Figure 9 above. This scanning mechanism may be any apparatus capable of moving the laser beam over the detector and film. For example it may be a se-t of computer controlled mirrors or hologra-p~liC optics.
Beam 82 projects a small flying spot 90 onto the surface of detector structure 10. Detec-tor structure 10 was discussed in detail in con-nection with Figures 1 through 8, above.
Detector structur-e 10 is electrically connected by lines 92 and 94 to cooled ampliier 96. Amplifier 96 may be a low noise amplifier.
Amplifier 96 is connected by line 98 to lines 100 and 102 which electrically connect the output of cooled ampliier 96 to sisnal processor 104 and second signal processor 106, respectively. Signa]
processor 104 is connected by line 108 to beam modulator 110. Laser 112 produces an intense beam of coherent light 114 which is modulated by beam modulator 110 and spread by o~jective 116 in lenses 118 and 120 to coherent modulated beam 122.
Modulated beam 122 is focused by focusing mirror 124 onto a second mirror surface 126 of optical scanner 88. This surace may be another surface of the same scanner or may be a separate optical scanning system.
Mechanical movement of scanning system 88 causes beam 122 to form flying spot 128 which scans the surface of recording medium 130. This process allows the very low intensity latent image ~2~5 on detector structure 10 to be electrically ampli fied by cooled amplifier 96 and signal processor 104 and then to be rewritten as an intensified image on a photographic or xeroradiographlc plate 130.
The electrical output of cooled ampli-fier 96 also is fed to signal processor 106 by lines 98 and 102. The output of signal processor 106 is connected by line 132 to a digital computer 134. Computer 134 is used to digitally store and manipulate the information content imparted to it by the electrical signal produced by cooled ampli-fier 96 by way of signal processor 106. The images may be stored on magnetic tape or disc files and can be manipulated within the computer by algorithms or image edge enhancement or pat-tern recognition or automated diagnosis.
The output of signal processor 106 also goes by line 136 to mass storage system 1~8.
Mass storage system 138 contains a high resolution display tube 140 which produces an analog image 142 which is focused by focusing optics 144 onto film plane 146 of a mass film storage system 148. This mass film storage sys-tem may be a 35 or 70 mm cassette system.
- The type of radiographic image proces-sing and storage system described in connection with Figure 11 is especially useful for inter-facing mass radiographic data acquisition equip-ment with the central computing facilities of a large hospital complex. Digital storage of the radiographic images allow the data to be accessed by a remote radiologist with speed and precision.
Computer-based pattern recognition algorithms allow inexpensive gross screening of large pa-tient populations for radiographic anomalies. The 1 1 ~ 2 ., ~Y
system' 5 ability to rewrite informakion obtained at very low radiation dosages onto conventional xeroradiographic plates or films permits the system to protect the patient while interfacing with presently existing radiographic data storage for~ats.
As will be discussed further below, the radiographic data comprising the latent image on detector structure 10 may be read out in real time to produce a video image which may be studied by the radiologist.
Figure 12 shows a mass screening system constructed according to the preferred embodiment of the present invention. The embodiment shown in Figure 12 could be used for real time read out of chest films in a mass screening program. X-ray source 150 produces a uniform flux of X-rays 152.
Uniform flux 152 passes through and is modulated by patient 154. The modulated X ray flux impinges onto multi-layered photon detector apparatus 156.
Figure 13 shows a highly enlarged cross-sectional view of a portion of detector sandwich structure 156 used in Figure 12.
Detector structure 156 is like detector structure 10 described in Figures 1 through 8, above, éxcept that it is positioned so the modu-lated X-ray flux impinges on the selenium photo-conductive layer after it passes through the aluminum backplate. Additionally, a thin insula-ting layer 158 has been add~d to the outer surface of the aluminum support structure to electrically insulate the aluminum layer from patient 154.
Detector sandwich structure lS6 is mounted within light-tight housing 160. Housing 160 also contains a means for scanning the detec-tor structure with a thin beam of light, i.e., laser scanning system 162 which projects light beam 164 in a raster pattern onto the back side, i.e., the conductive transparent coating side, of detector 156. Base 166 of housing 160 con-tains control electronics 168 and signal electronics 170. The entire housing and base may be mounted on legs 172 to raise detector structure 156 to chest height of patient 154.
Control means 16~ comprises electronics necessary to control the scanning pattern of laser scanner 162 so as to cause light beam 16~ to scan the photoconductive side of detector 156 in a regular xaster pattern. The control electronics also controls X-ray source 150 and synchronizes exposure and readout operations.
Signal acquisition means 170 comprises electric amplifiers connected to the video output of detector 156. Signal electronics 170 amplifies the detector output and provides a video output capable oE operating video dlsplay means 174, which may be a high resoIution video display monitor.
Operationally, the mass screening system shown ln Figure 12 is a small portable unit.
Patient 154 steps up to the unit and presses his chest against thin lnsulating layer 158 over the aluminum back plate of detector structure 156.
X-ray source 150 is then turned on by the control means and irradiates the patient with under 10Q mE~
of X-ray radiation. As is shown in Figure 13, this modulated X-ray flux penetrates thin insula-ting layer 158 and the aluminum backplate of detector 156. The X-ray flux then generates electron hole pairs in the selenium detector structure as was discussed in connection with Figures 2 through 6, above. The effect of X-rays ~5S~2 ,~/
being absorbed in the selenium with conseguent neu-tralization of surface changes is a displace-ment current that flows as C2 redistributes its charge over C1. When this current is integrated over time it will reach a preset value. Control circuitry will then terminate the X--ray exposure.
Use of such circuitry insures images oE equivalent quality regardless of the thickness of the patien-t.
The above device or operation is called "automatic photo-timing" or "automatic exposure control".
Referring again to Figure 12, as soon as ~he exposure is completed, control electronics 168 causes laser scanner 162 to sca.n light beam 164 across the surface of the selenium photoconductive layer in a regular raster pattern as was described in connection with Figures 7 through 10, above.
This causes detector 156 to produce a video signal containing image information. This video signal is fed to sigrlal electronics 170 where it is amplified to provide a video output to TV monitor 174.
Such a system may be used to make a clinical radiograph of any body part, includin~
breast, chest, head, etc.
The detector itself, being a replacement for film in a radiographic system, can be used with intensifying screens. One embodiment of the detector structure would use a phosphor, prefer-ably a high Z rare earth phosphor, in place of insulating layer 18 in Figure 1. The X~rays striking the phosphor would generate light which would create ion~hole pairs in photoconductive layer 16.
5 ~ 2 ,;
DER I VAT I ON O F THE CHARACTER I ST I C CURVE OF
THE DETECTOR
This section refers to the derivation of the curve shown in Figure 25.
When the experimental system is exposed to light, a total charge of QL coulombs will flow in the external circuit where QL = 2VsC22 coul/pixel (1) Cl+C2 Exposure to X~radiation prior to such a light exposure would simply reduce QL proportional to ~e X-ray exposure dose.
One may compute the charges placed in motion due to absorption of X-ra~ photons by first deflning the photogeneration efficiency after Fender d E ev x Volts) ~ dn (ion palr ~ (2) where ~ is the energy deposited within the sele-nium in ev due to absorbed photons, N is the number of ions which neutralize the charge on the surface of the absorber, and E is ~he average electric field across the absorber. Also we note, n = QxA and dn = A dQ (3) e e where Qx is the surface charge neutralized in coulombs per square centimeter, A the area of the absorber in square centimeters and e the electron-ic charge in coulombs/ion-pair. The energy absorbed may be calculated as 1~55~2 ~3 ~ = fkXA and d~ =f~AdX (4) where f is the fraction of X-ray phQtons absorbed in the selenium, k is the energy fluence in air per roentgen exposure and X is the exposure in roentgens. Also, since the electric field is that which exists wi-thin the selenlum, = Q 1 (5) Cldl where Q*} is the instantaneous charge existing across the selenium, ~1 the capacitance of the selenium layer, and dl the thickness of the sele-nium. Substituting (3)(4) and (S) into (2) W2 have tl = eQ*l dX
Cldl dQx (6) From (1) we may express Q*l as Q*l = 2VsClC2 _ Q~Cl Cl +C2 Cl~C2 substituting in (6), we have the differential equation, dQx ~ fke Qx ~ 2kfeC~Vs = O (7 dX d~(Cl~C2 ~(Cl~C2) d the solution of which i5 Qx = 2C2VS (1 - exp ~ fkeX ~ ) (8) ~rldl(Cl C2 ) J
t3 1~ 2 if QR QX (C2 ~ (9) Cl+C2 then the total charges placed in motion when illuminating a pixel which has absorbed X roent-gens of X-ray is, Qsig = Q~ - OR (10) from (1) and (10), Qsig = 2VsC2 - 2VsC2 (1 - exp~ rk 3) ~
Qsig = [2V C~ exp~ - ~X)] (12) s 2 Cl+C2 where ~ - fke rldl ( Cl~C~ ) Equation (12) is of the form described by Boag in which he states that the radiographic discharge of electrostatic plates is exponential.
Figure 19 is a plot of QR versus Vs for various doses of X-radia~ion. From such data we may solve equation ~8) for ~, the photogeneration efficiency. From Figure 19 we also note the emperical data predicts a loss of charge which may be due to recombination; however, it may also be indicativ~ of deep hole traps within the selenium that establish a residual potential below which the system cannot be discharged.
; 2 _ 3~_ DESCRIPTION AND DISCUSSION OF METHODS
OF CHARG I NG THE DETECTOR
Chargin~ Method #l To review briefly, the semiconductor sandwichlike struc-ture, illustrated by F1gures 1 and lA above, appears as a pair of capacitors in series. C2 is the capacitor ormed in the experi-mental system by Nesa conductive glass and the surface of the selenium proximate the Nesa. R
mylar insulating layer acts as a dielectric. Cl is the capacitor formed by the surface of the selenium and the aluminum substrate, the selenium photoconductor acting as the dielectric. Because photoconductors are not dielectrics when they are illuminated, Cl will exist only when it is in the dark. This ignores such things as deep hole traps, dark currents, etc., but they will be discussed in turn.
Referring to Figure 1, when a volta~e is applied between terminals 22 and 12, a current will flow charging capacitors Cl and C2 to volt-ages Vl and V2 respectively, where:
Vs ~ ~1 + ~2 (1) In which case:
V2 = VS C
. _ Cl ~ C2 (2) Where Vs is the supply voltage.
andVl = V
(3) I the selenium layer is illuminated while there is a potential between terminals 22 3 155~
.~
and 12, capacitor Cl becomes a conductor and allows additional current to flow to C2. This additional charge Q1 is the charge initially residing on Cl. That is, since Ql = ClVl ~4~
substituting (3) in-to (4), ClC2 Ql = Vs Cl+C2 ~5) The initial voltage across C2 is deter-mined as in (2). The increase in voltage across C2 due to Cl becoming conductive is:
Ql V~C~
V~ = Cl Cl~C~ (6) thus the total voltage across C2 is now (2) plus (6) or Cl ~sc~
V2 = Vs Cl~C2 Cl+C2 = Vs ~7) Because of (1), Vl is now found to be equal to zero. The charge across C2 is now the total Q or QT = VSC2 (8) If the illumination is removed, this voltage distribution will persist until the poten--tial is removed and terminals 22 and 12 are con-nected together. C2 will now partially discharge into Cl resulting in an equal voltage appearing ~ 1$5~2 'i 'I
across Cl and C2 since they are now essentially in parallel. This voltage will be V parallel = 52 total C total = V C = V = --V
s 2 1 2 Cl+C2 Note that Vl and V2 are ec~al, however, of opposite polarity.
It will also be noted that no supply voltage Vs is in the circuit during this redistri-bution of charge. The above operation has resul-ted in placing a charge on the surface of the selenium much as is accomplished in the Xeroradio-graphic procedure. In the present invention, however, the charge has been applied to a selenium surface that is physically inaccessible. :
The system is now ready for exposure to X-rays. An integrating ~mmeter or a coulomb meter -., is placed in the circuit between Cl and~R2 such ` ',r that the current, or net charge movement due to radiation exposure may be measured. This flow of current is proportional to exposure and thus can .-~e used to control the exposure. This is advan-tageous because it allows the exposure time to be controlled by the amount of radiation actually reaching the detector.
Alternatively, the surface of capacitor C2, which is a thin conductive film, may be etched to form a small island as shown in Figure 21. In Fiyure 21 coulomb meter 2101 may be connected between island electrode 2103 and the aluminum substrate 2105. During X-radiation exposure meter 2101 will accumulate a charge which may be used to ;IS~2 3~
,, , photo-time the exposure. This is advantageous because regardless of the thickness of the patient, the X-ray machine will stay turned on until the correct amount of charge is collected hy the detector, thus insuring a properly exposed image.
Several of these isolated islands may be etched in the plate and used as exposure meters.
When the detector is exposed to radia-tion, electron-hole pairs are created wi~hin the selenium which neutralize the charge across Cl.
~s these charges are neutralized, the charge on C2 redistributes. This redistribution maintains the two capacitors at the same potential. Thus the total charge flowing through the external circuit, i.e., through load resistor R2, is the initial charge placed on C2 alone. From (9) we can obtain the ~oltage across C2, i.e., the total charge on C~ prior to X ray exposure:
Q~ = C2Vs C2 cl+c2 ( 10 ) Equation (10) is the total charge that will move if the system is not exposed to radia-tion. Should X-ray exposure occur to create a latent image, a portion, Qx' of the charges on C
will be neutralized. The charge remaining on C2 will he:
Qsig (QT QX) C2 cl+c2 ( 11 ) Where QT = c2v5 as in (8). If QX = , then Equa-tion (11) becomes (10).
1 153~62 ~Y
This charge can be accumulated by a coulomb meter and would be a functlon of the number of electron-hole pairs generated by the X~rays absorbed in the selenium. Whe~ the detec-tor is scanned by the laser beam, each pixel, i.e.
the area illuminated by the laser b~eam at rest, is sequentially discharged. Functionally, Cl is made conductive where the light shines. When Cl be-comes conductive, the charge on C2 can flow through the external circuit. The current which flows through the external circuit is a function of the latent image stored in the photoconductor.
As the laser beam scans across Cl, the IR drop appearing across resistor R2 is a video signal.
Char~inq Method #2 Consider the same sandwich-like detector structure in a circuit configuration analogous to the one shown in Figure 22.
Structurally, three-position switch 2201 is connected to one side of the plate structure 2203 illustrated as capacitor Cl and C2 in series.
One side of coulomb meter 2205 is connected to the other side of plate structure 2203. The other ~ide of coulomb meter 2205 is connected to one side of resistor 2207. The other side of resistor 2207 is connected to the positive side of battery 2209; directly to terminal 2211 and to the nega-tive side of battery 2213. The positive side of battery 2213 is connected to terminal 1 of three--position switch 2201. Terminal 2211 is connected to terminal 2 of switch 2201. The negative side of battery 2209 i5 connected to terminal 3 of three-position switch 2201.
Three position switch 2201 is first placed in the position 1. The voltages across C
4~
and C2 are the same as shown in Figure 1 and lA.
If the selenium is now illuminated, Cl becomes conductive as before and V2 = Vs as in (7). If the illumination is now removed this voltage dis-tribution again persists. Moving switch 2201 to position 2 results in the charge on C2 being distributed over both Cl and C2 as before with V
= -V2 as given in (9).
If switch 2201 is moved to position 3, a voltage will exist across C2 which will be that ratio of Vs due to capacitance Cl and C2 less the voltage due to trapped charges (9), i.e., V = V C - V C
RrALTItl~ RADIATIO,I EXPOSUREilOi~I,OR
A'ID CO'ITP~OL APPARATUS
TECHNICAL FIELD
The present invention relates generally to apparatus and methods used to obtaln image information from modulation of a unlform flux.
More specifically, the present invention relates to methods and apparatus used to produce a radio-graph by producing a latent image in a photocon-ductor sandwich structure and then optically reading out the latent image.
BACKGROUND ~RT
.
a. Fa~slmile and Vidicons Facsimile systems modulate an electrical signal in response to light that is reflected from a small portion of an image. Facsimile requires production of a real image.
Vidicon tubes store a latent image as a charge distribution pattern in a semiconductor target. This latent imase is scanned by an elec tron beam and the current variation induced by ~he different charges on separate portions of the target comprises a video signal. Vidicons require use of a high vacuum and precise focusing of an electron beam that must be shielded from e~ternal electric and magnetic fields.
b. Conventional Radiographic_Methods Latent images stored in silver halide film or selenium xeroradioyraphic plates must be chemically or powder cloud developed to produce real images. Use of calcium tungstate crystals or a high atomic weight gas for image intensification ,~,.
degrades image quality and still re~lires expo-sures of from 1 to 5 R per typical clinical mammo-gram. Powder cloud development of latent xero-radiographic images requires high differert1al charge denslties on the xeroradiographic plate to attract and hold powder par-ticles prior to fusing.
This high charge differential is generated by x-rays, or ions, impinging on the surface of a charged selenium plate. The higher t~he diffex_ ential charge needed to produce an irnage, khe mor~
X-ray ~xposure is required.
The reader is directed to the following publications for detailed discussion of this prior art.
XERORADIOGRAPHY, J. W. Bo~g, ~ ~ Med.
Bio _, 1973, Vol. 18, No. 1, pp. 3-37; Principals of Rad ographic Exposure_and Processin~, Arthur W. Fuchs, 1958, Chapter 14, "X-Ray Lntensifying Screens", pp. 158-164.
U.S. Patent No. 3,860,817; Reducing Patient X-Ray Dose Durinq Fluoroscopy With an Image System.
U.S. Patent No. 3,828,191, Gas Handling System for Electronradiography Imaging Chamber.
U.S. Patent No. 3,308,23~, Xerographic Facsimile Printer ~laving Light Beam Scanning and Electrical Charging With Transparent Conductive Belt.
Recent research by the present inventors and others indicates that a detailed latent image can be created in a xeroradiographic plate by very low levels (under 10 n~) of X-ray exposure. Ak this exposure level latent image is present, but its associated electrostatic field is not of sufficient intensiky to produce a real image by the powder cloud method of image development. For details of this research see, Grant Application, "Radiomammography With Less than 150 m~ Per Expo~
sure", available from the Department of Experimen-tal Radiology, M.D. Anderson Hospital, Houston, Texas 77025.
c. Semlconductor Art The prior art of reading charge storage patterns out of multilayer se~iconductor sandwich structures lies largely in the area of electro-nics, especially exotic computer memories.
Charge patterns on certain MOS struc-tures can be "read out" using a photon beam. See, Ima~ nq and Stora~e Wlth a Uniform MOS Structure, Applied Physics Letters, Vol. 11, Number 11, pp.
359--361. This technology functions by modifying a depletion layer and then charging the layer to saturation. The few microns thick depletion layer is the only active structure.
Electric fields can also be impressed across two separable photoconductive insulating films in pressure contact that are precharged to the same polarity. See, Increasin~ the Sensitlv-ity of Xer~raphic Photoconductors, IBM Technical Disclosure Bulletin, Vol. 6, No. 10, 1~64, page 60.
IBM has also developed an exotic charge storage beam addressable memory comprising a semiconductor sandwich wherein the semiconductor is totally insulated from both electrodes in the sandwich. See, IBM Technical Disclosure ~ulletin, Vol. 9, No. 5, 1966, pp. S55 556. This device allows data readout by shifting charge population to one side or the other of the insula-ted semicon-ductor.
Finally, some theoretical work has been done on the ~ehavior of light sensitive capaci-~ :~5~$~
tors, see, ~naly.s-is _ I PerEo _ance of_a I g t_S_lsitive Capacitor, Proceedings of the IEEE, ~pril 1965, p. 378.
The present inventlon is an exposure measuring apparatus with a radiatioll source adapted to irradiate a detector structure with radiation.
The detector structure comprises a duo--dielectric sandwich, an external cir-cuit, and a measuring device responsive to the E]ow of current in the external circuit when the detector structure is irradiated by the radiation source.
The duo-dielectric sandwich includes a transparent conducting layer, a transparent insulating layer, a high resistant photoconductor layer, and a conductive layer. The transparent conducting layer overlies the trans-parent insulating layer. The transparent insulating layer is affixed to a high resistant photoconductor layer. The photoconductor layer overlays the conductive layer. Within this duo-dielectric sanclwich, the radiation is capable oF producing electron hole pairs in the photoconductor layer oE thLs detector structure.
The external circuit connects the transparent conducting layer and the conductive layer.
The measuring device measures the flow of current from only a part of the detector structure. This part of the detector structure is electrically segmented from the greater portion of the detector structure. The current flow is measured from several small segmented portlons of the detector.
A control device is responsive to the measuring device so as to stop the irradiation when the total charge reaches a preselected limited.
It is an object of the present invention to provide an X-ray sensing system whose output is an analog or digital video signal that may be selectively displayed on a television monitor, recorded on film, or directly stored or processed in a computer for image enhancement or pattern recognition.
It is another object o~ the present invention to provide a novel ~ 155~2 method and apparatus for converting a charge distribution on a semiconducting surface to a modu:Latecl electric signal.
Another purpose of the present lnvention -is to provide an appara-tus and method th~t combines the edge enhancement eEEect of xeroradiography with a low patient dose level.
Still another purpose oE the present invention is to provide a novel low noise method and apparatus fcr reading out a latent image stored as a pattern of electrical charges by selectively discharging said charges in the presence of a reverse bias electric -field.
It is still another purpose of the present invention to prov:ide an X-ray sensing system that is capable oE directly timing X-ray exposure.
BRIEF DESCRIPTION OF DRAWINGS
~ igure ] is a greatly enlarged cross-sectional view of a portion of one embodimen-t of the multi-layered photon detector apparatus taught by the present invention;
. , .
; 2 ~ IGURE lA is a schematic electrical diagram illustr~ting the electric circuit analog of ~he structure shown in Fiqure l;
FIGURE 2 is a view of the structure ~hown in Fi~ure 1 being flooded wlth photon~i ~ IGURE 2A is a schematic showirlg the electric circuit analoq of Figure 2;
FIGUR~ 3 is a view of the cletector ~tructure after it i~ chaxged, floode~d with pho-ton~ and ~he surface charge has been redistri-buted;
FIGURE 3A i~ the electrical schematic analog of the detector as it is shown in Figure 3;
FI~URE 4 illustrates the detector struc-ture of a preferred embodiment o the present invention as it i5 used in an X-ray system;
FIG~E S is a gxeatly enlar(3ed schematic cross-sectional view of a portion of Figure 4 illustrating the effect of a modulated X-ray flu~
on the detector;
FIGURE 6 is a ~chematic view of ~he detector graphically illustrating ~he change in de~ector ~urface charge caused by X-ray exposure;
FIGURS 7 is a schematic vie~ of the detecto~ apparatu~ taught by ~he presen~ invention being read out by a thin, scanning photon beam, an o~tput wave for~ of the video electric signal i5 illustrated together with a schematic representa-tion of the surface charge of the detector;
FIGURE 8 i a highly enl~rged schematic cross~sectional view of a small portion of the detector structure shown in Figure 7 illustr~ting ~he electron hole production mechanism in the photoconductor layer used by the present inven-tion;
~55~2 FIGURE 9 shows a partially cut-away ~iew of an apparatus adapted to practice the pr~ferred embodiment of the present invention;
FIGUR~ 10 is a simpl.ified diagram show-ing the major parts of the apparatus shown in Figure 9;
FIGURE 11 is a schematic block diagram illustrating a diagnostic X-ray system constructed according to a preferred embodiment of the present invention;
FIGURE 12 shows a cross-sectional view of a clinical apparatus for conducting mass chest radiographic screeniny using the present inven-tion;
FIGURE 13 is a highly enlarged schematic cross-sectional view of an embodiment of the present invention used in the apparatus shown in Figure 12.
- FIGURE 14 is a graph illustrating the theoretical maximum charge obtain~ble from an experimental detector structure per unit area of the detector as a function o the voltage applied across its photoconductive layer. Figure 14 also shows a plot of specific points representing experimental results from tests run on this ex-perimental detector structure for a given mylar thickness and selenium thickness.
FIGURE 15 is a graph illustrating the voltage across the selenium layer of a theoretical detector in kilovolts as a function of the value of C2, in farads, for that detector.
FIGURE 16 is a graph illustrating the charge, in coulombs collected by the experimental detector structure per unit of exposure as a function of total exposure. The values plotted in Figure 16 were obtained using a Bakalite shutter;
l 15~62 ~7 9~
FIGURE 17 illustrates the same informa-tion as Figure 16, but the data was obtained using a PVC shutter;
FIGURE la is a graph showi ng the effi-ciency of the present invention, as expressed in coulombs of charge obtained per roentgen of expo~
sure, plotted against supply voltage for various levels of total exposure. This figure illustrates the increased efficiency of the present invention at low radiation levels;
FIGURE 19 is a graph that plots the total charge collected by an experimental embodi-ment of the present invention as a function of applied voltage across its photoconductive layer.
Fiqure 19 illustrates increase in latitude of the present invention's detector that is observed as applied voltage across the photoconductor is ncreased;
FIGURE 20 is a functional block diagr~1 of an experimental prototype system of the present invention.
FIGURE 21 is a schematic diagram showing a portion of the detector structure used a~ a radiation exposure detector automatically timing the exposure.
FIGURE 22 is an electrical schematic showing the reversed biased mode of detector operation.
FIGURE 23 is an electrical schematic o the preamplifier used in experimental system 3.
FIGURE 24 is a schematic diagram of ~he X-position interface module used in experimental system 3.
FIGURE 25 is the characteristic curvP of the detector structure.
il~5S~
q BEST MODE FOR CARRYING OUT THE INVENTION
In Figure 1, multilayered photon detec tor apparatus 10 has first electrode 12 conduc-tively connected to a first conductive layer 14.
A photoconductive layer 16 is in physical and electrical contact with plate 14 which may be made o al~inum or any other conductor. An oxide layer 11 may act as a blocking contact between layer 16 and plate 14. Layers 14, 16, and 11 may be provided by using a conventional xeroradiogra-phic plate. Transparent insulating layer 18 overlays and may be integrally affixed to selenium photoconductor layer 16. A transparent conductive layex 20 may be integrally affixed to and overlies insulator 18. These lay~rs may be made by vapor deposition or by adhesively bonding the individual components together. Layer ~0 is electrically connected to lead 22.
D SCUSS ION OF EXPERIMEN~AL SYSTEM #l The experimental plates used to test the present invention as early as late-1975 were made from conventional xeroradiographic plate~. These plates are made for mamography by the Xerox Com~
pany. To make the present invention's detector, a plate is first cut to the correct size and ~hen covered with mylar. The conductive coating, which is either Nesa glas~ or a few angstroms of gold, is placed on the mylar.
In the preferred embodiment of the present invention discussed in this section o the specification, the aluminum backing plate 14 is approximately 1/10 of an inch thick and the layer of amorphous selenium 16 is approximately lS0 microns thick. The physical properties of sele-nium are listed in Table I. The transparent insulator 18 is mylar. The transparent conductor /~
~,~
20 may be Nesa glass, a thin fllm of metal deposi-ted directly on the transparent insulator 1~, or a plastic film with a conductive coating, i.e., gold covered mylar. The entire structure 10 may be made by depositing successive layers of selenium, mylar and a thin film of metal onto an aluminum plate. Assembly may be accomplished by vapor deposition, spu-ttering, or any other technique useful to deposi~ even thickness films. This technology is well developed in the art of 5emi-conductor electronics and glass manufacturing.
The thickness of the selenium layer must be selected to maximize quantum efficiency of the detector. This optimum thickness will be a u~c-tion of the photoconductor's characteristi.cs, the potential at which the detector is operated and the energy of the X-ray photons or other particles to which the detector is exposed that act to discharqe it.
Basically, the thicker the selenium, the more it interacts with a given energy of exposing radiation and the more electron-hole pairs a given quantity of radiation produces. ConYersely, as the selenium layer is made thinner ~he electric field acting on these electron-hole pairs becomes stronger (the same potential over less distance~.
Thus a very thin layer of selenium would not interact with the exposing radiation enough to produce electron hole pairs while a very thick layer of selenium would result in the production of lots of electron-hole pairs, bu~ they would all recombine when the exposure stopped.
The optimum thickness of the photocon-ductor layer of the detector will depend on the characteristics of the photoconductor and the energy of the X-ray photons it is designed to ~ .~555~2 sense. In the experimental embodiments of the present invention applicants have found that from one hundred microns to four hundred microns thick-ness of amorphous selenium is optimu~ for the X ray photon energies commonly used in diagnostic radiology.
There are several facts the inventors discovered experimentally that shouldl be men-tioned.
When several experimental prototype detector structures were built under laboratory "clean room" conditions they worked poorly.
Regular aluminum backed selenium-xeroradiography plates covered with gold covered mylar plastic and assem~led with optical cement under less than ideal conditions work very well.
It appears ~hat a very thin al-~inum oxide layer between the selenium and the aluminum acts as a blocking contact, i.e., a diode, to retain the positive charge on the surface of the selenium. Thi~ blocking layer has a blocking potential that must be overcome for current to flow through the contact.
Also, when the experimental detectors built by the inventors were scanned fast, a higher signal strength output was obtained than when they were scanned slowly. Since the light intensity was the same, the faster scan was predicted to result in lower signal strength. Applicants cannot yet explain this phenomena, but, by pulse modulating their light source for between 2 nano-seconds and 10 microseconds on and of, this effect can be used to increase signal strength from the present invention.
Finally, repeated use of the prototype plate resulted in the development of 'lartifacts", 1 L555~2 /~
- 3~
i.e., lines and trash on the image. Repeated use also caused an overall loss in detector sensiti-vity. It was accidentally discovered that heating the prototype plate for a short time with a heat gun used to shrink plastic tubing removed ~lese artifacts and otherwise generally restored the detector's performance.
In Fi~lre 1, Rl represents the resis-tance of the photoconductive layer when it is flooded with light. R2 represents the resistance of the photoconductive layer when it is in the dark. C2 represents the electrical capacity manifested by the charge separation across the transparent insulator 18. One charge pol~rity resides on the transparent conductor 20 and the other on the surface of selenium, which is in immediate contact with insulator 18. Cl repre-sents the electrical capacity of the detector measured between the aluminum conductor 14 and the selenium surface 16. The selenium is thus the dielectric material of Cl.
Figure lA shows an electrical schematic illustrating an electronic analog of detector sandwich structure 10 shown in Figure 1. Rl, R2, Cl and C2 are shown schematically connected in Figure lA as electrical symbols. Switch 24, which is shown open, is used to represent the photocon-ductive nature of selenium layer 16 of Figure l.
Selenium has a dark resistance of approximately 10l6 ohm centimeter. A portion of this resistance .is caused by a blocking contact formed at the Se-Al interface. When photons strike the photo~
conductor its resistance is lowered because the photons create electron hole pairs that carry current. This lower resistance is illustrated schematically in the electrical analogs contained ~ ~S~2 ~4 in this specification by a closure of switch 24 leaving only residual resistance Rl, which repre-sents the forward resistance of the blocking contact in series with the resistance of the selenium layer when it is flooded with photons.
For a current to flow when the photoconductor is flooded with light the blocking potential of blocking contact 11 must be overcome~
TAE~LE I
Ph~sica1 ProE~ertles of Selenlum (2,3 Atomic Number 3~
Density (g cm 3) 4.25 Dielectric Constant 6 . 6 Resistivity at 20C. (~ cm) lol3_1ol6 Thermal Conductivity at 20C.
( -lK-l) 2 x 10 3 Optical Band Gap ( 2V ) 2 . 3 Photo Response Edge (AE) 4600 K absorption Edge (KeV) 12.7 Mobility of Holes (cm2s lv 1 ) O .14 Mobility of Electrons (cm2s lv 1) 5 x 10 3 Energy for Production of Charge Carriers (eV) 7 ______.__._ _____________ * W - 2.67 x Eg 4 0.86 eV
in which Eg is thP optical band gap 1 ;~S~2 Figure 2 illustrates detector 10 being flooded with photons 26. Simultaneously a nega-tive high voltage, i.e., 2,000 vol-ts, is applied to terminal 22 hence to transparent conductor 20.
Figure 2A, the electrical schematic analog of Figure 2, shows switch 24 closed by iight 26. The 2,000 volt potential, Vs, is impressed across electrodes 12 and 22.
Figure 3 shows the semiconductor detec-tor sandwich 10 after light 26 has been exting-uished and leads 12 and 22 shorted together. The resulting positive surface charge on the amorphous ~elenium layer is illustrated diagrammatically by dotted line 28. This surface charge is uniformly distributed over the entire surface.
Figure 3A is Figure 3's electrical analog. The detector is in darkness, so switch 24 is illustrated as open. Diode 15 and battery 13 represent the blocking contact and potential, respectively, of blocking junction 11. The elec-tric charge originally present on capacitor C2 has redistributed so that a portion of that char~e is present on capacitor Cl, the exact portion being dependen-t upon the ratio of Cl to (Cl + C2).
Figure 4 shows the charged detector illustrated in Figure 3 used ~s an image receptor structure for an X-ray image. Uniform X-ray flux 30 is generated by a convenient source of radia-tion, such as an X-ray tube, not shown. The detector will work with any radiation source capable of generatiny electron-hole pairs in the photoconductor. This uniform photon flux encoun-ters and interacts with object 32, which may be any object o interest. Ob~ect 32 is placed directly over detector 10. For simplicity, object 32 is represented as an oblate spheroid of uniform density.
/~
Figure 5 shows a greatly enlarged sche-matic view of a portion of detector structure 10 of Figure 4. Modulated X-ray flux 34 from the radia~ion that has passed through object 32, creates electron-hole pairs 36 in selenium layer 16.
Figure 6 shows detector 10 of Figure 4 after the X-ray exposure has been completed. The detector is now storing a latent image. The modulated surface charge compris1ng the image is schematically illustrated by a humped dotted line 38. This dotted line represents the change in potential created by the electron-hole pairs generated by the X-ray exposure.
Figure 7 show~ detector 10 having a latent image stored as a modula-ted surface charge 38. A thin beam of light 40 is shown scanning the surface of photoconductive layer 16 in a regular raster pattern. In the experimental system number 1 embodiment of the present invention, this thin scanning light beam is produced by a ~-Cd lasPr.
It should be understood ~hat the photon beam need not be coherent. It may be of any frequency capable of creating electron-hole pairs in photoconductive layex 16 o detector 10.
Electrode 41 is connected to electrical ground and electrode 43 carries a video electric signal whose wave form is a function of modulated surface charge 38 in detector 10 scanned by light be~m 40.
Arrow 42 indicates the direction of movement of scanning light beam 40. Output wave form 44 indicates the voltage variation of the output video signal o~tained by said beam's scan-ning the latent image.
l l~S~)fi2 1~
Figure 8 is a schematic, highly enlarged cross-sectional view of a portion of the detector structure being scanned by light beam 40 of Figure 7. Light beam 40 penetrates the transparent conductor and transparent insulator to generate electron-hole pairs 36 in that portion of selenium layer 16 irradiated by the beam.
Functionally, detector 10 operates by placing a uniform surface charge on selenium layer 16 and then selectively discharging part of this surface charge by X-ray exposure to form a latent image.
As shown in Figure 2, when light photons 100d detector 10, selenium photoconductor layer 16 becomes cond~ctive. If, as shown, a -2,000 volt potential is applied between the aluminum backplate ~at ground) and the transparent conduc-tor 20 (at -2,000 volts potential~, then the detector will charge as a capacitor~ Figure 2A
illustrates how this charge is applied to the system. Light flood 26 increases conductivity in the selenium layer 16. The applied voltage Vs is -~,000 volts. This causes a charge to build up across capacitor C2, which represents the capacity between the surface o the selenium in contact with the transparent insulator and the transparent conductor of detector 10. Capacitor C2 thus charges to potential Vs of 2,000 volts.
After capacitor C2 has charged to poten-tial Vsl the light flood is removed and detector 10 is stored in darkness. In the absence of photons, amorphous selenium layer 15 becomes nonconductive and its dark resistivity in syner-gistic co~bination with blocking contact 11 pre-vents the surface charge on the selenium rom leaking off. This condition is illustrated by ~, Figures 3 and 3A. After detector 10 is in dark-ness, terminals 22 and 12 are shorted toge-ther.
This causes the electric charge on capacitor C2 to redistribute between capacitor C2 and Cl, each of which is then charged to a potPntial of one-half Vs, i.e., 1,~00 volts if Cl-C2. Capacitor Cl represents capacitance between the surface of the photoconductive layer 16 and the aluminum backing 14. This 1,000 volt surface charge is clistributed uniformly over the selenium layer's surface. This surface charge is maintained across the selenium layer by the high darX resistivity of the selenium in conju~ction with the now reverse biased block-ing junction at the Al Se interace. The effec-tive blocking potential of this junction has been experimentally d~termined to be about 150 to 300 volts in the experimental systems discussed below.
Resistance Rl, representing resistance of the selenium layer when it is exposed to light, is relatively low compared to its dark resistance R2.
Each photon, striking the pho-toconductor depending on its energy, generates a specific number of electro~-hole pairs within photocon-ductive layer 16 (see Table II~. Each electron-hole pair discharges a portion of the surface charge at the spot it is generated.
Figure 4 illustrates how this effect is used to impress a Iatent image on the detector.
Figure 4 is a cross-section schematic view of the photon detector structure taught by the present invention being used as the image receptor in an X-ray system.
A uniform flux af X-ray photons 30 is modulated, i.e., partially absorbed, by its pass-age through an object being X-rayed 32. The modulated X-ray flux then strikes detector 10.
~ ~5~2 ,~i Detector 10 has a l,000 volt surface charge, as was illustrated by charge 28 in Figure 3, impressed on selenium layer 16. As the X-ray pho-tons strike the selenium layer, they generate electron hole pairs. For monochromatic X-rays the number of electron hole pairs generated is a direct function of the number of X-ray photons striking the seleniurn detector layer.
Some of the X-ray photons in uniform flux 30 are absorbed by object 3~. Thus the modulated flux striking photoconductive layer 16 contains information about the internal structure of the object being x-rayed. This infor~ation is contained in the number of photons striking each portion of the detector.
Figure 5 illustrates how this modula-ted X-ray flux creates a modulated surface charge on the surface of the detector. Modulated X-ray flux 34 generates electron-hole pairs 36 in selenium layer 16 of detec-tor 10. The number of el~ctron hole pairs generated at each point on the surface of the detector i5 a function of the number of X-ray photons impinging on the photoconductive layer Each electron hole pair 36 discharges a portion of the surface charge impressed on the photoconductive layer at the point where it is generated. The greatest nu~ber of electron hole pairs will be generated where the X-ray photons from uniform flux 30 strike the detector without any absorption from the object being x-rayed. A
lesser number of X-ray photons will strike the photoconductive layer under the objec-t being X-rayed. The radio-opacity of the object being X-rayed is thus reproduced in the surface charge of the detector structure after X-ray exposure.
Figure 6 schematically illustrates the detector structure1s modulated surface charge after the X-ray exposure described in connection with Figures 4 and 5.
The surface charge is lower where no object absorbed X-ray photons. The object being X-rayed, which is assumed to be uniformly opague to X-rays, absor~s a portion of the uniform X-ray flux. This results in the generation of fewer electron 'nole pairs under the object and a higher surface charge on the detector under the object being X-rayed. This lS represented by modulated surface charge dotted line 38.
Figure 7 illustrates schematically the readout method used to extract an electric video signal from ~he modulated surface charge which forms the latent image on a detector structure taught by the present invention.
- The detector structure is kept in dark-ness and a thin beam of light, preferably gene-rated by a helium-cadmium laser, is scanned in a r~gular raster pattern across the surface of photoconductive layer l6.
As the scanning light beam 40 moves in direction illustrated by arrow 42, it produces a small moving spot on the surface of photoconduc~
tive layer 16. The size of this spot determines the resolution of the final image. It is there-fore desirable that this spot size be kept small.
As laser beam 40 scans the surface of photoconductive layer 16, it generates electron-hole pairs as is illustrated in detail by Figure 8. These electron-hole pairs are mobile within the photoconductive layer and discharge a portion of the surface charge. In the preferred embodi-ment of the present invention the electrons move 1 ~S~j62 2l toward the positively charged selenium surface of the ~etector sandwich ~tructure 10. ~he voltage drop produced across the resistor connected be-tween ground connection 41, which is attached to transparent conductor 20 and video CUtpUt elec-trode 43, which is attached to alumlnum conductive backplate 14, will be a function of the intensity of the surface charge at the point where the laser beam generates the electron-hole pair.
Scanning laser beam 4Q thus produces a modulated electric signal 44 at output electrode 43. The voltage of this output electric si~nal will be a function of the surface charge present at the spot where the laser beam strlkes the photoconductive layer of the detector. The cur-rent present in the output circuit is a function of the frequency and intensity of the scanning light beam and is a further function of the speed with which the light beam scans the surface of the photoconductor. Video signal 44 can be electro-nically processèd to produce an imaqe that repro duces the latent ima~e found ln the surface charge of the photoconductive layer of the detector.
Depending on the intensity of ~he sur-face charge, the beam scanning speed, and the frequency and intensity of the light beam used, the surface charge may be repetitively scanned several timPs hy the beam of light.
Table II lists the values for the thick-ness o the selenium layer, the X-ray photon energy, the fraction of this energy absorbed by the selenium layer, quantum conversion between X-ray photons and electron-hole pairs, dark resis-tance of the selenium and the total receptor area of detector 10 used in calculating the operating parameters of a sample detector constructed ac~
, r~ j 2 -~3-cording to the preferred embod1ment of the present invention.
TABLE II
Assumed Values Thickness of Selenium Layer lOQ microns Energy Radiation ~1 keV
Abcorbed Fraction of photons 85%
Quantum Conversion 3000 electron,~
photon Surface Voltage `1000 V
Photon Flux/Roentgen 5 + 10 cm Dark Resistivity lol6 Q ~m Total Receptor Area 560 cm Table III lists the calculated capacity of the detector sandwich structure, the charge density present in this structure when the surface potential is 1,000 volts, the number of elementary charges present per square centimeter in the structure and the dark resistance and dark current of the detector at a thousand volts charge.
1~5$~2 ` I` 2, TABLE III
Calculated Values Capacitance 5 8 x 1o~llF/cm2 Charge Density at 1000 Volts 5.8 x 10 Coul/cm2 Number of Elem~tary Charge 3 6 ~ 1olle1/Cm2 Dark Resistance 1014 Q /cm2 Dark Current at 1000 Volts 10 11A/Cm2 Using the values from these tables it is possible to calculate operatirlg parameters for a typical detector system constructed according to the preferred embodiment of the present invention.
If detector 10 has an area of 560 square centimeters and is exposed to uniform flux of 21 keV X-rays resulting in an exposure of Np X-ray photons per unit area, then the total number of electron hole pairs generated will be 3,000 times ~p. Information theory requires that three times as many electron hole pairs be generated as are present at the statistical noise level of the system for the latent image to be detectable.
There are approximately 5 times 109 photons per square centimeter per roentgen of X-ray exposure at 21 Kev. Thus the minimum de~
tectable latent radiation exposure that will produce a detectable latent image in the selenium sandwich structure taught by the preferred embodi-ment of -the present invention (for a 560 square centimeter detector) is approximately 23 micro roentgens per exposure. This may be thought of as i 155r~62 the theoretical lower limit of exposure to gene-rate a 10 line pair per millimeter image with the present invention.
Turning now to a practical example, if the X-ray exposure of a patient scan is 100 mR and at the thinnest section of the X-rayed object 50%
of this radiation is absorbed by the sample, then exposure at the detector at the brightest portion of the image amounts to 50 mR while the lowest theoretical detectable dose, as was discussed above, amounts to 23~R. This theoretically yields a latent image represented by modulation of the surface charge present on the detector at a ratio between the brightest and the darkest areas of the image of approximately 2,000.
The example discussed above shows that the detector structure taught by the present invention is theoretically capable of recording an image using a selenium photoconductive layer with a resolution of 10 line pairs per millimeter and a brightness range of over 2,000 after a total X-ray exposure of only 100 mR. An example of how this latent image can be read out of the detector follows.
Figure 9 shows a partially cut-away view of an apparatus capable of reading out the image whose generation was discussed above.
Light-tight housing 46 contains a light source 48 aligned so as to project a light beam 50 onto the front of a scanning means 52 which may be a rotating multi-sided mirror. Mirror S2 is mounted on an axis 54 which is operably attached to a scanning motor 56. The mirror axis and scan-ning motor are affixed to a platform 58. This platform is mounted on an axis 60 which is ortho-gonal to axis 54. Axis 60 is mounted at one end ~ ~555~
to first upright support 62 and its other end to second upright support 64. The end of axis 60 is swingably mounted in second upright 64 and is operably attached to stepping mo-tor 66.
The entire scanning means comprising the mirror and associated positioning components is available from Texas Medical Instruments, Inc., 12108 Radium, San Antonio, Texas 78216.
Mirror 52 and its associated scanning mechanism is located at one end of housing 46. A
multilayered photon detector apparatus 10 is removably inserted through a light tight opening 68 into a holder at the other end of housing 46 such that deposited transparent con~uctive layer 20 faces toward mirror 52.
Light source 48 is preferably a Helium-Cac~nium laser such as a Liconix model #402 laser with an optical modulator. This laser produces a beam of intense light at appro~imately 4,400 angstroins.
Scanning motor 56 rotates multi-sided mirror 52 on axis 54 to cause light beam 50 to scan horizontally across the surface of detect~r 10. This causes spot 70 to intersect the selenium photoconductive layer as was dlscussed in connec-tion with Figures 7 and 8 above. Each time spot 70 moves from the left to the right side of detec-tor 10, stepping motor 66 moves platform 58 throuqh sufficient arc to deflect spot 70 vertically 1/20th of a millime-ter. This stepping-scanning function may be controlled mechanically or electri.
cally. By convention, spot 70 begins scanning the surface of detector 10 at its upper left hand corner. After raster scanning the entire surface of the detec-tor, the scanning mechanism may ~e ~ ~S;~2 .;. I ,,~.
programmed either to turn off or to rescan the plate.
Figure 10 is a simplified schematic view of the scanning readout mechanism shown in Figure 9. Light beam 50 from laser 48 reflects off of the polygonal surface of multi~sided mirror 52 as that mirror rotates Oll axis 54. This causes light beam 50 to generate flying spot 70 which moves across the surface of detector structure 10. As was discussed in connection with Figures 7 and 8l above, this causes a video signal to be generated across the detector structure.
Theoretically, photons from light source 48 have a wave length of approximately 4,400 angstroms. The quantum yield of these photons and selenium approaches unity when field strength nears 105 volts per centimeter. In a hundred and fifty micron selenium plate, such as is used in the preferred embodiment of the present invention, this corresponds to a thousand volts surface potential.
It is possible to pulse the laser to decrease the sca~ning time required to obtain an image from the detector structure. In the present example, the laser beam may be pulsed for a 100 nanoseconds period with 3 to 4 microsecond spacing between pulse~. This illuminates each image element sufficiently to read out a latent image stored in the detector. The practi~al advantage of such a regime is to permit much shorter samp-ling times. It may also result in higher signal strengths.
Figure 11 is a block diagram of a radio graphic X-ray system taught by the preferred e~bodiment of the present invention. Laser 7~
produces a small intense beam of light 74 which is ; 2 expanded by optical objective 76 and lenses 78 and 80 into a reasonably wide parallel beam 82.
Fo~using mirror 84 reflects beam 82 on the reflec-tive surface 86 of scanning mechanism 88. Scan ning mechanism 88 is substantlally the same as the scanning mechanism discussed in connection with Figure 9 above. This scanning mechanism may be any apparatus capable of moving the laser beam over the detector and film. For example it may be a se-t of computer controlled mirrors or hologra-p~liC optics.
Beam 82 projects a small flying spot 90 onto the surface of detector structure 10. Detec-tor structure 10 was discussed in detail in con-nection with Figures 1 through 8, above.
Detector structur-e 10 is electrically connected by lines 92 and 94 to cooled ampliier 96. Amplifier 96 may be a low noise amplifier.
Amplifier 96 is connected by line 98 to lines 100 and 102 which electrically connect the output of cooled ampliier 96 to sisnal processor 104 and second signal processor 106, respectively. Signa]
processor 104 is connected by line 108 to beam modulator 110. Laser 112 produces an intense beam of coherent light 114 which is modulated by beam modulator 110 and spread by o~jective 116 in lenses 118 and 120 to coherent modulated beam 122.
Modulated beam 122 is focused by focusing mirror 124 onto a second mirror surface 126 of optical scanner 88. This surace may be another surface of the same scanner or may be a separate optical scanning system.
Mechanical movement of scanning system 88 causes beam 122 to form flying spot 128 which scans the surface of recording medium 130. This process allows the very low intensity latent image ~2~5 on detector structure 10 to be electrically ampli fied by cooled amplifier 96 and signal processor 104 and then to be rewritten as an intensified image on a photographic or xeroradiographlc plate 130.
The electrical output of cooled ampli-fier 96 also is fed to signal processor 106 by lines 98 and 102. The output of signal processor 106 is connected by line 132 to a digital computer 134. Computer 134 is used to digitally store and manipulate the information content imparted to it by the electrical signal produced by cooled ampli-fier 96 by way of signal processor 106. The images may be stored on magnetic tape or disc files and can be manipulated within the computer by algorithms or image edge enhancement or pat-tern recognition or automated diagnosis.
The output of signal processor 106 also goes by line 136 to mass storage system 1~8.
Mass storage system 138 contains a high resolution display tube 140 which produces an analog image 142 which is focused by focusing optics 144 onto film plane 146 of a mass film storage system 148. This mass film storage sys-tem may be a 35 or 70 mm cassette system.
- The type of radiographic image proces-sing and storage system described in connection with Figure 11 is especially useful for inter-facing mass radiographic data acquisition equip-ment with the central computing facilities of a large hospital complex. Digital storage of the radiographic images allow the data to be accessed by a remote radiologist with speed and precision.
Computer-based pattern recognition algorithms allow inexpensive gross screening of large pa-tient populations for radiographic anomalies. The 1 1 ~ 2 ., ~Y
system' 5 ability to rewrite informakion obtained at very low radiation dosages onto conventional xeroradiographic plates or films permits the system to protect the patient while interfacing with presently existing radiographic data storage for~ats.
As will be discussed further below, the radiographic data comprising the latent image on detector structure 10 may be read out in real time to produce a video image which may be studied by the radiologist.
Figure 12 shows a mass screening system constructed according to the preferred embodiment of the present invention. The embodiment shown in Figure 12 could be used for real time read out of chest films in a mass screening program. X-ray source 150 produces a uniform flux of X-rays 152.
Uniform flux 152 passes through and is modulated by patient 154. The modulated X ray flux impinges onto multi-layered photon detector apparatus 156.
Figure 13 shows a highly enlarged cross-sectional view of a portion of detector sandwich structure 156 used in Figure 12.
Detector structure 156 is like detector structure 10 described in Figures 1 through 8, above, éxcept that it is positioned so the modu-lated X-ray flux impinges on the selenium photo-conductive layer after it passes through the aluminum backplate. Additionally, a thin insula-ting layer 158 has been add~d to the outer surface of the aluminum support structure to electrically insulate the aluminum layer from patient 154.
Detector sandwich structure lS6 is mounted within light-tight housing 160. Housing 160 also contains a means for scanning the detec-tor structure with a thin beam of light, i.e., laser scanning system 162 which projects light beam 164 in a raster pattern onto the back side, i.e., the conductive transparent coating side, of detector 156. Base 166 of housing 160 con-tains control electronics 168 and signal electronics 170. The entire housing and base may be mounted on legs 172 to raise detector structure 156 to chest height of patient 154.
Control means 16~ comprises electronics necessary to control the scanning pattern of laser scanner 162 so as to cause light beam 16~ to scan the photoconductive side of detector 156 in a regular xaster pattern. The control electronics also controls X-ray source 150 and synchronizes exposure and readout operations.
Signal acquisition means 170 comprises electric amplifiers connected to the video output of detector 156. Signal electronics 170 amplifies the detector output and provides a video output capable oE operating video dlsplay means 174, which may be a high resoIution video display monitor.
Operationally, the mass screening system shown ln Figure 12 is a small portable unit.
Patient 154 steps up to the unit and presses his chest against thin lnsulating layer 158 over the aluminum back plate of detector structure 156.
X-ray source 150 is then turned on by the control means and irradiates the patient with under 10Q mE~
of X-ray radiation. As is shown in Figure 13, this modulated X-ray flux penetrates thin insula-ting layer 158 and the aluminum backplate of detector 156. The X-ray flux then generates electron hole pairs in the selenium detector structure as was discussed in connection with Figures 2 through 6, above. The effect of X-rays ~5S~2 ,~/
being absorbed in the selenium with conseguent neu-tralization of surface changes is a displace-ment current that flows as C2 redistributes its charge over C1. When this current is integrated over time it will reach a preset value. Control circuitry will then terminate the X--ray exposure.
Use of such circuitry insures images oE equivalent quality regardless of the thickness of the patien-t.
The above device or operation is called "automatic photo-timing" or "automatic exposure control".
Referring again to Figure 12, as soon as ~he exposure is completed, control electronics 168 causes laser scanner 162 to sca.n light beam 164 across the surface of the selenium photoconductive layer in a regular raster pattern as was described in connection with Figures 7 through 10, above.
This causes detector 156 to produce a video signal containing image information. This video signal is fed to sigrlal electronics 170 where it is amplified to provide a video output to TV monitor 174.
Such a system may be used to make a clinical radiograph of any body part, includin~
breast, chest, head, etc.
The detector itself, being a replacement for film in a radiographic system, can be used with intensifying screens. One embodiment of the detector structure would use a phosphor, prefer-ably a high Z rare earth phosphor, in place of insulating layer 18 in Figure 1. The X~rays striking the phosphor would generate light which would create ion~hole pairs in photoconductive layer 16.
5 ~ 2 ,;
DER I VAT I ON O F THE CHARACTER I ST I C CURVE OF
THE DETECTOR
This section refers to the derivation of the curve shown in Figure 25.
When the experimental system is exposed to light, a total charge of QL coulombs will flow in the external circuit where QL = 2VsC22 coul/pixel (1) Cl+C2 Exposure to X~radiation prior to such a light exposure would simply reduce QL proportional to ~e X-ray exposure dose.
One may compute the charges placed in motion due to absorption of X-ra~ photons by first deflning the photogeneration efficiency after Fender d E ev x Volts) ~ dn (ion palr ~ (2) where ~ is the energy deposited within the sele-nium in ev due to absorbed photons, N is the number of ions which neutralize the charge on the surface of the absorber, and E is ~he average electric field across the absorber. Also we note, n = QxA and dn = A dQ (3) e e where Qx is the surface charge neutralized in coulombs per square centimeter, A the area of the absorber in square centimeters and e the electron-ic charge in coulombs/ion-pair. The energy absorbed may be calculated as 1~55~2 ~3 ~ = fkXA and d~ =f~AdX (4) where f is the fraction of X-ray phQtons absorbed in the selenium, k is the energy fluence in air per roentgen exposure and X is the exposure in roentgens. Also, since the electric field is that which exists wi-thin the selenlum, = Q 1 (5) Cldl where Q*} is the instantaneous charge existing across the selenium, ~1 the capacitance of the selenium layer, and dl the thickness of the sele-nium. Substituting (3)(4) and (S) into (2) W2 have tl = eQ*l dX
Cldl dQx (6) From (1) we may express Q*l as Q*l = 2VsClC2 _ Q~Cl Cl +C2 Cl~C2 substituting in (6), we have the differential equation, dQx ~ fke Qx ~ 2kfeC~Vs = O (7 dX d~(Cl~C2 ~(Cl~C2) d the solution of which i5 Qx = 2C2VS (1 - exp ~ fkeX ~ ) (8) ~rldl(Cl C2 ) J
t3 1~ 2 if QR QX (C2 ~ (9) Cl+C2 then the total charges placed in motion when illuminating a pixel which has absorbed X roent-gens of X-ray is, Qsig = Q~ - OR (10) from (1) and (10), Qsig = 2VsC2 - 2VsC2 (1 - exp~ rk 3) ~
Qsig = [2V C~ exp~ - ~X)] (12) s 2 Cl+C2 where ~ - fke rldl ( Cl~C~ ) Equation (12) is of the form described by Boag in which he states that the radiographic discharge of electrostatic plates is exponential.
Figure 19 is a plot of QR versus Vs for various doses of X-radia~ion. From such data we may solve equation ~8) for ~, the photogeneration efficiency. From Figure 19 we also note the emperical data predicts a loss of charge which may be due to recombination; however, it may also be indicativ~ of deep hole traps within the selenium that establish a residual potential below which the system cannot be discharged.
; 2 _ 3~_ DESCRIPTION AND DISCUSSION OF METHODS
OF CHARG I NG THE DETECTOR
Chargin~ Method #l To review briefly, the semiconductor sandwichlike struc-ture, illustrated by F1gures 1 and lA above, appears as a pair of capacitors in series. C2 is the capacitor ormed in the experi-mental system by Nesa conductive glass and the surface of the selenium proximate the Nesa. R
mylar insulating layer acts as a dielectric. Cl is the capacitor formed by the surface of the selenium and the aluminum substrate, the selenium photoconductor acting as the dielectric. Because photoconductors are not dielectrics when they are illuminated, Cl will exist only when it is in the dark. This ignores such things as deep hole traps, dark currents, etc., but they will be discussed in turn.
Referring to Figure 1, when a volta~e is applied between terminals 22 and 12, a current will flow charging capacitors Cl and C2 to volt-ages Vl and V2 respectively, where:
Vs ~ ~1 + ~2 (1) In which case:
V2 = VS C
. _ Cl ~ C2 (2) Where Vs is the supply voltage.
andVl = V
(3) I the selenium layer is illuminated while there is a potential between terminals 22 3 155~
.~
and 12, capacitor Cl becomes a conductor and allows additional current to flow to C2. This additional charge Q1 is the charge initially residing on Cl. That is, since Ql = ClVl ~4~
substituting (3) in-to (4), ClC2 Ql = Vs Cl+C2 ~5) The initial voltage across C2 is deter-mined as in (2). The increase in voltage across C2 due to Cl becoming conductive is:
Ql V~C~
V~ = Cl Cl~C~ (6) thus the total voltage across C2 is now (2) plus (6) or Cl ~sc~
V2 = Vs Cl~C2 Cl+C2 = Vs ~7) Because of (1), Vl is now found to be equal to zero. The charge across C2 is now the total Q or QT = VSC2 (8) If the illumination is removed, this voltage distribution will persist until the poten--tial is removed and terminals 22 and 12 are con-nected together. C2 will now partially discharge into Cl resulting in an equal voltage appearing ~ 1$5~2 'i 'I
across Cl and C2 since they are now essentially in parallel. This voltage will be V parallel = 52 total C total = V C = V = --V
s 2 1 2 Cl+C2 Note that Vl and V2 are ec~al, however, of opposite polarity.
It will also be noted that no supply voltage Vs is in the circuit during this redistri-bution of charge. The above operation has resul-ted in placing a charge on the surface of the selenium much as is accomplished in the Xeroradio-graphic procedure. In the present invention, however, the charge has been applied to a selenium surface that is physically inaccessible. :
The system is now ready for exposure to X-rays. An integrating ~mmeter or a coulomb meter -., is placed in the circuit between Cl and~R2 such ` ',r that the current, or net charge movement due to radiation exposure may be measured. This flow of current is proportional to exposure and thus can .-~e used to control the exposure. This is advan-tageous because it allows the exposure time to be controlled by the amount of radiation actually reaching the detector.
Alternatively, the surface of capacitor C2, which is a thin conductive film, may be etched to form a small island as shown in Figure 21. In Fiyure 21 coulomb meter 2101 may be connected between island electrode 2103 and the aluminum substrate 2105. During X-radiation exposure meter 2101 will accumulate a charge which may be used to ;IS~2 3~
,, , photo-time the exposure. This is advantageous because regardless of the thickness of the patient, the X-ray machine will stay turned on until the correct amount of charge is collected hy the detector, thus insuring a properly exposed image.
Several of these isolated islands may be etched in the plate and used as exposure meters.
When the detector is exposed to radia-tion, electron-hole pairs are created wi~hin the selenium which neutralize the charge across Cl.
~s these charges are neutralized, the charge on C2 redistributes. This redistribution maintains the two capacitors at the same potential. Thus the total charge flowing through the external circuit, i.e., through load resistor R2, is the initial charge placed on C2 alone. From (9) we can obtain the ~oltage across C2, i.e., the total charge on C~ prior to X ray exposure:
Q~ = C2Vs C2 cl+c2 ( 10 ) Equation (10) is the total charge that will move if the system is not exposed to radia-tion. Should X-ray exposure occur to create a latent image, a portion, Qx' of the charges on C
will be neutralized. The charge remaining on C2 will he:
Qsig (QT QX) C2 cl+c2 ( 11 ) Where QT = c2v5 as in (8). If QX = , then Equa-tion (11) becomes (10).
1 153~62 ~Y
This charge can be accumulated by a coulomb meter and would be a functlon of the number of electron-hole pairs generated by the X~rays absorbed in the selenium. Whe~ the detec-tor is scanned by the laser beam, each pixel, i.e.
the area illuminated by the laser b~eam at rest, is sequentially discharged. Functionally, Cl is made conductive where the light shines. When Cl be-comes conductive, the charge on C2 can flow through the external circuit. The current which flows through the external circuit is a function of the latent image stored in the photoconductor.
As the laser beam scans across Cl, the IR drop appearing across resistor R2 is a video signal.
Char~inq Method #2 Consider the same sandwich-like detector structure in a circuit configuration analogous to the one shown in Figure 22.
Structurally, three-position switch 2201 is connected to one side of the plate structure 2203 illustrated as capacitor Cl and C2 in series.
One side of coulomb meter 2205 is connected to the other side of plate structure 2203. The other ~ide of coulomb meter 2205 is connected to one side of resistor 2207. The other side of resistor 2207 is connected to the positive side of battery 2209; directly to terminal 2211 and to the nega-tive side of battery 2213. The positive side of battery 2213 is connected to terminal 1 of three--position switch 2201. Terminal 2211 is connected to terminal 2 of switch 2201. The negative side of battery 2209 i5 connected to terminal 3 of three-position switch 2201.
Three position switch 2201 is first placed in the position 1. The voltages across C
4~
and C2 are the same as shown in Figure 1 and lA.
If the selenium is now illuminated, Cl becomes conductive as before and V2 = Vs as in (7). If the illumination is now removed this voltage dis-tribution again persists. Moving switch 2201 to position 2 results in the charge on C2 being distributed over both Cl and C2 as before with V
= -V2 as given in (9).
If switch 2201 is moved to position 3, a voltage will exist across C2 which will be that ratio of Vs due to capacitance Cl and C2 less the voltage due to trapped charges (9), i.e., V = V C - V C
2 S 1 5 2 Cl+C2 Cl+C2 ( 12 ) or V2 =`Vs (~
Cl~C2 2C2 = Vs Cl+C2 f 2C2 ~
= VS ~ Cl C2 ) 2VsC2 (13) V2 = V ~ Cl+C2 ~ ~ 5~2 ~/
~,~
since Vl ~ V2 = Vs we find Vl = 2Vs C2 (14) Cl+C~
that is, the voltage across the selenium is now twice that obtained in the other charging method #1.
Now the total charge on C2 is from (13j 2VsC2~
Q2 = C2 ~s ~ Cl+C2 J
2VsC2C2 (15) C2Vs Cl+C2 Under these conditions if the ratio of C2~Cl+C2)is near unity, ~hen V2 will be opposite in polarity to Vl as well as at a lesser poten-tial.
If we assume C2 = 10 Cl, the ratio C2 ~ 1+C2)= 10/11, also assuming Vs = lOOV, from Vl = 2 x lOOV ~ 10/11 = 181.8 volts and V2 ~ -81.8 volts Thus the Q2 as shown in Equation (15) represents a char~e opposite to that on Cl and that of the voltage supply Vs.
Upon exposure to radiation and/or subse-~uent read-out by the laser, the total Qsig moving through the external circuit will be that residing 6 ~
on C2 initially, plus that required to charge C2 to a voltage of Vs in the opposite polarity. Thus Qsig Q2 QT
2VSC2C2 + C2~S
= -c2vs ~ Cl+C.~
orQsig = 2vs C2 C2 Cl+C2 ~16) If the detector has been exposed to X-rays then, /
Qsig ( 2~T Qx ~ C2 ( 17 ) J~ Cl+C2 here, as before, QX are those charges neutralized by the electron-hole created in the seleniu~ by absorbed X-ray photons.
In summary, use of charge method #2 results in doubling the voltage across the sele-nium without doubling the voltage applied across mylar capacitor C2. This allows capacitor C2 to have a thinner dielectric and thus a higher capaci-tance. Increasing the value of C2 relative to the value of Cl brings the fraction C~ ~ l+C2)nearer to unity, which urther increases the detector's signal output. A further advantage of doubling the voltage across the selenium is that the in-creased electric field strength decreases electron-hole pair recombination in the selenium and in~
creases the efficiency of ion-pair collection.
This increases the efficiency of the detector.
~5~
~,~
--4~
Increasing C2, or rather making Cl as small as possible, has two beneficlal results:
A. Reducing the capacity of Cl is accomplished by increasing the thickness of the selenium layer in the detector, which increases the detector's X-ray ~bsorption efficiency.
B. Reducing Cl while using the highest possible value of C2 reduces the total capaci-tance of the detector, since Cl and C2 are in series. This lower total capacitance in-creases the scan speed that ~e invention can achieve. This is important because the priox art only teaches the segmenting of a plate into small sections to avoid this large capacitance.
CHARGING METHQD #3 It is also possible to use a detector wherein the selenium layer has high effectiv~e dark resistivity, but not a bIocking contact, which is not charged prior to x-ray exposure. That is, a detector whose photoconductive layer has no sur-face charge. In ~his method, an electric poten~
tial is placed across the previously uncharged detector only during exposure to the x-rays. This procedure will result in a modulated surface charge conforming to the radio-opacity of the object being x-rayed being placed on the photocon-ductive layer. The modulated surface charge may be read with a scanning laser beam as described above.
EXPERIMENTAL SY_TEM #2 The Experimental Apparatus To evaluate the operational character~
istics of the detector and readout means of the present invention, a model detector as in Figure 1 1 ~ 5,~ S~ 2 ft~7f was built and mounted on an optical bench which contained a He-Cd laser, collimating lens and field stop. The C2 was formed of approximately 5 mil mylar plastic coated with a few angstroms of gold, i.e., to where the surface resistance of the mylar is equal to 20 ohms per square (layer 20, Figure 1). The mylar was bonded to the selenium layer with optical cement. The laser was pulsed via a pulse generator. Charge method #2 was used as described above. The detector was loaded with various values of resistances in accordance with the applied voltage. An amplifier with a gain of 1000 was used to condition the det~ctor's output signal prior to display on a storage oscilloscope.
A schema-tic of the amplifier is given in Figure 23. From photographs taken of the signals on the oscilloscope's screen the total area under the discharge curve was determined via planimeter to estimate the charge set in motion by the laser.
This procedure was repeated after exposure to measure exposure to X-ray radiation to determine the number of coulombs of charge discharyed by the X-ray exposure.
Experimental Results Experiments were designed to determine if detector and readout means of the present invention performed as predicted. Using Equation (17), the total possible signa~, in coulombs per square centimeter, was calculated assuming Qx = ~
Cl -- 3.69 x 10 11 and C~ = 2.93 x 10 11 farads/
cm . Figure 14 is a graph of the number of cou lombs theoretically predicted by such a model as a function of supply voltage applied across the detector. Also shown on Figur~ 14 are several experimentally measured values of charges col-lected from experimental system #2l which was ~3~
S
constructed and tested in the experimental radio-logy laboratory of M.D. Anderson Hospital in Houston/ Texas.
In experimental system #2 the selenium thickness was measured as 150 microns. Five mil mylar was employed as the second dielectrlc. All charges calculated in Figure 14 assumed an active area or pixel size of .3 cm2 and represent total discharge of ~he pixel.
As can be seen from Figure 14, the correlation of calculated and measured charge collected is very good if one assumes a loss o
Cl~C2 2C2 = Vs Cl+C2 f 2C2 ~
= VS ~ Cl C2 ) 2VsC2 (13) V2 = V ~ Cl+C2 ~ ~ 5~2 ~/
~,~
since Vl ~ V2 = Vs we find Vl = 2Vs C2 (14) Cl+C~
that is, the voltage across the selenium is now twice that obtained in the other charging method #1.
Now the total charge on C2 is from (13j 2VsC2~
Q2 = C2 ~s ~ Cl+C2 J
2VsC2C2 (15) C2Vs Cl+C2 Under these conditions if the ratio of C2~Cl+C2)is near unity, ~hen V2 will be opposite in polarity to Vl as well as at a lesser poten-tial.
If we assume C2 = 10 Cl, the ratio C2 ~ 1+C2)= 10/11, also assuming Vs = lOOV, from Vl = 2 x lOOV ~ 10/11 = 181.8 volts and V2 ~ -81.8 volts Thus the Q2 as shown in Equation (15) represents a char~e opposite to that on Cl and that of the voltage supply Vs.
Upon exposure to radiation and/or subse-~uent read-out by the laser, the total Qsig moving through the external circuit will be that residing 6 ~
on C2 initially, plus that required to charge C2 to a voltage of Vs in the opposite polarity. Thus Qsig Q2 QT
2VSC2C2 + C2~S
= -c2vs ~ Cl+C.~
orQsig = 2vs C2 C2 Cl+C2 ~16) If the detector has been exposed to X-rays then, /
Qsig ( 2~T Qx ~ C2 ( 17 ) J~ Cl+C2 here, as before, QX are those charges neutralized by the electron-hole created in the seleniu~ by absorbed X-ray photons.
In summary, use of charge method #2 results in doubling the voltage across the sele-nium without doubling the voltage applied across mylar capacitor C2. This allows capacitor C2 to have a thinner dielectric and thus a higher capaci-tance. Increasing the value of C2 relative to the value of Cl brings the fraction C~ ~ l+C2)nearer to unity, which urther increases the detector's signal output. A further advantage of doubling the voltage across the selenium is that the in-creased electric field strength decreases electron-hole pair recombination in the selenium and in~
creases the efficiency of ion-pair collection.
This increases the efficiency of the detector.
~5~
~,~
--4~
Increasing C2, or rather making Cl as small as possible, has two beneficlal results:
A. Reducing the capacity of Cl is accomplished by increasing the thickness of the selenium layer in the detector, which increases the detector's X-ray ~bsorption efficiency.
B. Reducing Cl while using the highest possible value of C2 reduces the total capaci-tance of the detector, since Cl and C2 are in series. This lower total capacitance in-creases the scan speed that ~e invention can achieve. This is important because the priox art only teaches the segmenting of a plate into small sections to avoid this large capacitance.
CHARGING METHQD #3 It is also possible to use a detector wherein the selenium layer has high effectiv~e dark resistivity, but not a bIocking contact, which is not charged prior to x-ray exposure. That is, a detector whose photoconductive layer has no sur-face charge. In ~his method, an electric poten~
tial is placed across the previously uncharged detector only during exposure to the x-rays. This procedure will result in a modulated surface charge conforming to the radio-opacity of the object being x-rayed being placed on the photocon-ductive layer. The modulated surface charge may be read with a scanning laser beam as described above.
EXPERIMENTAL SY_TEM #2 The Experimental Apparatus To evaluate the operational character~
istics of the detector and readout means of the present invention, a model detector as in Figure 1 1 ~ 5,~ S~ 2 ft~7f was built and mounted on an optical bench which contained a He-Cd laser, collimating lens and field stop. The C2 was formed of approximately 5 mil mylar plastic coated with a few angstroms of gold, i.e., to where the surface resistance of the mylar is equal to 20 ohms per square (layer 20, Figure 1). The mylar was bonded to the selenium layer with optical cement. The laser was pulsed via a pulse generator. Charge method #2 was used as described above. The detector was loaded with various values of resistances in accordance with the applied voltage. An amplifier with a gain of 1000 was used to condition the det~ctor's output signal prior to display on a storage oscilloscope.
A schema-tic of the amplifier is given in Figure 23. From photographs taken of the signals on the oscilloscope's screen the total area under the discharge curve was determined via planimeter to estimate the charge set in motion by the laser.
This procedure was repeated after exposure to measure exposure to X-ray radiation to determine the number of coulombs of charge discharyed by the X-ray exposure.
Experimental Results Experiments were designed to determine if detector and readout means of the present invention performed as predicted. Using Equation (17), the total possible signa~, in coulombs per square centimeter, was calculated assuming Qx = ~
Cl -- 3.69 x 10 11 and C~ = 2.93 x 10 11 farads/
cm . Figure 14 is a graph of the number of cou lombs theoretically predicted by such a model as a function of supply voltage applied across the detector. Also shown on Figur~ 14 are several experimentally measured values of charges col-lected from experimental system #2l which was ~3~
S
constructed and tested in the experimental radio-logy laboratory of M.D. Anderson Hospital in Houston/ Texas.
In experimental system #2 the selenium thickness was measured as 150 microns. Five mil mylar was employed as the second dielectrlc. All charges calculated in Figure 14 assumed an active area or pixel size of .3 cm2 and represent total discharge of ~he pixel.
As can be seen from Figure 14, the correlation of calculated and measured charge collected is very good if one assumes a loss o
3.93 x 10 9 coulombs. This departure from the theoretically obtained value may be due to incom plete discharge caused by the existence of a blockiny contact at the selenium/aluminum inter-face. The fact that the emperical experimental data does not extrapolate to zero may also imply that as the plate nears total discharge, fewer charges are collected because more recombination takes place at lower field stxength.
Figure 15 is a graph illustrating the ~lect~ic field strength across the selenium layer of the detector as a function of mylar thickness, i.e. C2, for a given selenium thickness and vari- -ous supply voltages applied by charge method ~2.
This graph is useful in predicting the d~lamic range of the system in coulombs for various combi-nations of C2 and Vs. In experimental system #2, Cl is fixed at 3.69 x 10 11 farads/cm2 by the 150 micron thickness of selenium. Referring again to Figure 15, if C2 is 8xlO 11 farads/cm2 and Vs at 2000V, we find the voltage applied to the selenium will be 2500V, using charge method #2, and the dynamic range of the detector will be lxlO 7 coul/cm2. This means that for each square centi-1 ;lS;~62 meter of plate illuminated, a -total of lxlO 7 coulombs will flow In the external circuit. X-ra~
exposure to produce a latent image would decrease this total.
The detector's sensitivity to X-ray exposure is of great importance. That is, how many coulombs o charge will be set in motion and collected per absorption of an X-ray photon.
Figure 16 is a graph showing the coulombs of charge collected per roentgen exposure as a func-tion of total exposure (measured in MAS (Milli-amperes X seconds) applied to an X-ray tube).
The X-ray system employed with experi-mental system #2 was a Siemens Mammomat designed for operation with a xeroradiographic system. The X ray unit was operated at 44 kVp and had a half-value-layer ~HVL) of approximately 1.5 mm. of aluminum. The receptor was housed in a cassette made of 1/16th inch PVC and had a Bakelite shut-ter.
Figure 16 shows that the sensitivity (Q/roent.) of the present invention is higher for higher applied voltages and that the invention is unexpectedly sensitive to low exposure levels.
The present invention's efficiency of collection, expressed in coulombs per roentgen, is much higher at low X-ray exposure levels than at high exposure levels.
Figure 17 describes the same data as Figure 16; however, the X-ray detector cassette is equipped with a 1/16" thick PVC shutter. The high attenuation of PVC to the X~rays ~mployed in these experiments was not appreciated at the time this data was collected.
Figure 18 illustrates the efficiency of the present invention in coulombs/roentgen, as a t 1;7j~562 function of supply voltage. Again the system is clearly more efficient at lower radiation levels.
Table IV simply presents the same data in tabular form.
., 1 1 5556~
f ~
Figure 15 is a graph illustrating the ~lect~ic field strength across the selenium layer of the detector as a function of mylar thickness, i.e. C2, for a given selenium thickness and vari- -ous supply voltages applied by charge method ~2.
This graph is useful in predicting the d~lamic range of the system in coulombs for various combi-nations of C2 and Vs. In experimental system #2, Cl is fixed at 3.69 x 10 11 farads/cm2 by the 150 micron thickness of selenium. Referring again to Figure 15, if C2 is 8xlO 11 farads/cm2 and Vs at 2000V, we find the voltage applied to the selenium will be 2500V, using charge method #2, and the dynamic range of the detector will be lxlO 7 coul/cm2. This means that for each square centi-1 ;lS;~62 meter of plate illuminated, a -total of lxlO 7 coulombs will flow In the external circuit. X-ra~
exposure to produce a latent image would decrease this total.
The detector's sensitivity to X-ray exposure is of great importance. That is, how many coulombs o charge will be set in motion and collected per absorption of an X-ray photon.
Figure 16 is a graph showing the coulombs of charge collected per roentgen exposure as a func-tion of total exposure (measured in MAS (Milli-amperes X seconds) applied to an X-ray tube).
The X-ray system employed with experi-mental system #2 was a Siemens Mammomat designed for operation with a xeroradiographic system. The X ray unit was operated at 44 kVp and had a half-value-layer ~HVL) of approximately 1.5 mm. of aluminum. The receptor was housed in a cassette made of 1/16th inch PVC and had a Bakelite shut-ter.
Figure 16 shows that the sensitivity (Q/roent.) of the present invention is higher for higher applied voltages and that the invention is unexpectedly sensitive to low exposure levels.
The present invention's efficiency of collection, expressed in coulombs per roentgen, is much higher at low X-ray exposure levels than at high exposure levels.
Figure 17 describes the same data as Figure 16; however, the X-ray detector cassette is equipped with a 1/16" thick PVC shutter. The high attenuation of PVC to the X~rays ~mployed in these experiments was not appreciated at the time this data was collected.
Figure 18 illustrates the efficiency of the present invention in coulombs/roentgen, as a t 1;7j~562 function of supply voltage. Again the system is clearly more efficient at lower radiation levels.
Table IV simply presents the same data in tabular form.
., 1 1 5556~
f ~
4~ -~D t~J l'f ~ N r_ ~ a~ o V ~D a~ N O
N t~) ~ m ID
~0 I o a~ o I ~
~ a~
.~; .- I o o ~ I a~
~ ~ 1 0 ~ ~ ~ U~
O v N
t'~ C::l ~) -) O ~') N r~
o O 'D C ~n o u~
~ vc ~ :) ~D ~) ~ L''> N ~) C~.
C ~æ I N 0 ~ o 0 V
., 0 o a~ I
F~ ~J) ~ U~ N
O `f O a5 c~
C`J
O O O N a~, I ~ ~ 1~) .-- N
E o ~, +, +
o~ . 0 C~J
a~ , r~ ~ a~ ~1 L~ o o o o o o :-C o o o o o o i-- ~D N Lt'l ~ d' 1--~- N N N
U~O
N t~) ~ m ID
~0 I o a~ o I ~
~ a~
.~; .- I o o ~ I a~
~ ~ 1 0 ~ ~ ~ U~
O v N
t'~ C::l ~) -) O ~') N r~
o O 'D C ~n o u~
~ vc ~ :) ~D ~) ~ L''> N ~) C~.
C ~æ I N 0 ~ o 0 V
., 0 o a~ I
F~ ~J) ~ U~ N
O `f O a5 c~
C`J
O O O N a~, I ~ ~ 1~) .-- N
E o ~, +, +
o~ . 0 C~J
a~ , r~ ~ a~ ~1 L~ o o o o o o :-C o o o o o o i-- ~D N Lt'l ~ d' 1--~- N N N
U~O
5 r~ ~ 2 ~q When a Bakelite shutter is used at 320 MAS and 2700V supply voltage, 93.2% of all avail-able charges transfer. This represents near total discharge. Further, in view of the variance shown in Table IV in total charges available, the above example does indeed represent total discharges within experimental error.
Figure 19 illustrates total charge collected as a function of voltage applied across the detector. The curves diverge for the various exposures as -the applied voltage is increased.
This can be interpreted as meaning that the lati-tude of the detector, i.e. its ability to diffe-rentiate exposures of various levels, increases as applied voltage increases. Optimum latitude can be determined as a trade of between the dynamic range of the system and the signal to noise (S/N) ratio of the detector OUtp~lt signal.
Estimates of the values o Cl and C~
have been made based on calculations of capaci-tance assuming selenium thickness of 150 microns and a dielectric constant of 6.3; mylar thickness of 4.3 mils and dielectric constant of 3.5. The experimental results allow certain conclusions to be drawn about the present invention:
(1) Use of charge method ~2 has allowed use of increased voltage across the selenium while maintaining a lower voltage across the mylar.
(2) For a given thickness of selenium, the dynamic range of the system may be varied by varying the mylar thickness and applied voltage.
(3) The system's dynamic range must necessarily be greater than the range in charges neutralized by radiation absorption, ~ ~5~5~2 i.e. the x-rays cannot be allowed to comple-tely discharge any part of the detector.
This is necessary so as to maintain a field across the selenium even in areas of greatest radiation exposure. Doing so will maintain a good collection efficiency.
It is empirically es-timated that any one pixel should not be discharged beyond 50% of its initial charge. This, however, is only an esti-mate and has yet to be derived theoretically or by optimization procedures.
~4) Using the data collected, an ex-ample of the operational charac-teristics of the system can be computed as follows.
Assuming 44 kVp, 320 ~AS ~-ray source and a 6 cm. lucite phantom, the maximum exposure to the receptor is 370 mR. When operating the invention at 2700V applied, 6.0xlO 9 coulombs are placed in motion due to irradiating an area of .3 cm2. ~iven a load resistance of 1000 Q and readout time of 4xlO 6 sec. (cor-responds to 125,000 hz band width), an ampli-fier gain of 1000, and a pixel size of 50 micron, the output signal is:
E - 6xlO coul/cm x 2.5 x 10 cm x 1000 ~ 1000 out 4 1o~6 sec = 4.8 x 10 2 E t = 48mv System noise may be evaluated as ampli-fier noise plus Nyquist noise of the input resis-tance ~ .15~2 ~/
E = \/4kTR,~F
n , ~ c = ~ 4x1.37xlO ~x300xlOOOx1.5xlO' ~ 1.57~v Using an amplifier with nois~ figure of .8 nano volt/~ ~ and 1.5x105 hz, En = .8xlO 9 (1.5x105)1/2 - .8xlO 9x3.87x102 . 309,uv Hence, the amplifier noise is much less than the Nyquist noise of the input resistor.
Therefore, sl~nal to noise ratio is:
48.
S/N = 1 57 = 30.5:1 this dynamic range will produce an image with approximately 9-10 shades of gray. Here a shade of gray is defined as increase in signal. That is, Shades o. Gray = ~ g-~log 2 EXPERIMENTAL SYSTEM #3 Figure 20 is a block diagram that illu-strates the major components of the third experi-mental system built at the experimental radiology department of M.D. Anderson Hospital in Houstonr Texas. This experimental system was built to test the entire integrated system of the present inven-tion and to provide a breadboard system for ini-tial clinical evaluation. This system is fully computerized.
As is shown in Figure 20, microcomputer 2007, which is a Southwest Technical Products 6800, controls laser blanker 2015; 16-bit d/a converter 2011; x-position interface module 2017;
1 ;il5~5~2 . .~
12-bit d/a converter 2009; and teletype 2005.
Teletype 2005 is a model 33ASR. Sixteen bit d/a converter 2011 is an Analog Devices 1136 and 12-bit d/a converter 2009 is an Analog ~evices 1132. Laser blanking unit and laser 201S are a Liconix model 4110 helium/cadmium laser.
Laser 2015 is in a light~tight housing, not shown, such as the one functionally illu-strated in Fi~lre 9, above.
The photon beam from laser 2015 scans detector 2019. The output of detector 2019 is connected to the input of preamp 2021. The output of preamp 2021 is cor~ected to scan rate converter 2003; a/d converter 2023; and write laser modula-tor and laser 2025.
The output of a/d converter 2023 is an input to microcomputer 2007. Teletype 2005 is in two way communication with microcomputer 2007.
It would be appreciated by looking at Figure 20 that laser 2015 is a read laser adapted to scan detector 2019 and laser 2025 is a write laser adapted to write information out on a film or xerographic plate. This type of system is generally described in ConneGtion with Flgure 11, above.
Both the read and write laser beams are controlled by the microcomputer. The output of 12-bit d/a converter 2009 drives read laser x-posi tion scanner, controller and mirror transducer 2013. It also drives write laser x-position scanner, controller and mirror transducer 2027.
Similarly, 16-bit d/a converter 2011 drives both read laser y-position scanner, controller and mirror transducer 2029 and write laser y-position scanner, controller and mirror transducer 2031.
It will easily be appreciated that the beams from 11S5~2 ~ 3 -5~-bo-th the read laser and the write laser scan in sychronization, although both of them need no-t be turned on at the same time or modulated by the same signal.
Functionally, the microprocesser accepts instructions from the teletype. These instruc-tions operate on a computer program. A copy of this program's listing is included with this application for the Examiner's convenience. It is the inventors' request that it be inserted in the record so it will be available to the public as part of the File Wrapper of this application.
When the microprocessor addresses the 16-bit d/a converter and increments the nul~er stored in it, the y-position mirrors of both lasers are deflected to a certain angle. In experimental system $~3 this angle is sufficient to move the spot that is illuminated on detector 2019 S0 microns. Thus the y-positioning of the laser beam on the detector occurs in 50 micron incre-ments. The program included with this application allows specification of line sizing on ~he y-axis to S0, 100, 200, or 400 micxons. This translates to a resolution in the image produced by the system of 10, 5, 2.5 and 1.25 line pairs per millimeter.
The 12-bit d/a converter is not directly addressed by the computer. The computer addresses a special interface that controls the 12-bit d/a converter. This interface allows the computer to specify the starting and ending positions of the laser beam on the detector surface and along the x-axis. A schematic of x-position interface module 2017 is included as Figure 24. The compu-ter then issues a start comman~ to the interface which enables a switch selectable clock to start 5 ~ 2 incrementing the d/a to a specific start address.
The clock continues to increment the d/a until the stop address is encountered, then the clock is disabled ~y the interface. This x-position inter-face accomplishes two objectives. First, the data readout range can be varied. Secondly, difEerent slze detector plates can be read out without overscanning them. This is especially valuable on an experimental system.
Computer 2007 also controls laser blank-ing, which just means that it turns the laser on and off. The computer blanks the laser after each x-axis scan is completed. The laser is then moved back across the detector and turned back on for the next scan line.
To read out a plate using experimental system #3:
(1~ The user specifies plate size and x-axis line spacing. The computer program will ask for these values.
~ 2) The x-axis interface is given the appropriate start and stop addresses by the computer.
~ 3), The x-axis line spacing and total number of required steps is determined by the computer.
~ 4) The computer blanks the CRT and puts the scan rate converter into the write mode.
~ 5~ The laser is turned on by the computer.
Figure 19 illustrates total charge collected as a function of voltage applied across the detector. The curves diverge for the various exposures as -the applied voltage is increased.
This can be interpreted as meaning that the lati-tude of the detector, i.e. its ability to diffe-rentiate exposures of various levels, increases as applied voltage increases. Optimum latitude can be determined as a trade of between the dynamic range of the system and the signal to noise (S/N) ratio of the detector OUtp~lt signal.
Estimates of the values o Cl and C~
have been made based on calculations of capaci-tance assuming selenium thickness of 150 microns and a dielectric constant of 6.3; mylar thickness of 4.3 mils and dielectric constant of 3.5. The experimental results allow certain conclusions to be drawn about the present invention:
(1) Use of charge method ~2 has allowed use of increased voltage across the selenium while maintaining a lower voltage across the mylar.
(2) For a given thickness of selenium, the dynamic range of the system may be varied by varying the mylar thickness and applied voltage.
(3) The system's dynamic range must necessarily be greater than the range in charges neutralized by radiation absorption, ~ ~5~5~2 i.e. the x-rays cannot be allowed to comple-tely discharge any part of the detector.
This is necessary so as to maintain a field across the selenium even in areas of greatest radiation exposure. Doing so will maintain a good collection efficiency.
It is empirically es-timated that any one pixel should not be discharged beyond 50% of its initial charge. This, however, is only an esti-mate and has yet to be derived theoretically or by optimization procedures.
~4) Using the data collected, an ex-ample of the operational charac-teristics of the system can be computed as follows.
Assuming 44 kVp, 320 ~AS ~-ray source and a 6 cm. lucite phantom, the maximum exposure to the receptor is 370 mR. When operating the invention at 2700V applied, 6.0xlO 9 coulombs are placed in motion due to irradiating an area of .3 cm2. ~iven a load resistance of 1000 Q and readout time of 4xlO 6 sec. (cor-responds to 125,000 hz band width), an ampli-fier gain of 1000, and a pixel size of 50 micron, the output signal is:
E - 6xlO coul/cm x 2.5 x 10 cm x 1000 ~ 1000 out 4 1o~6 sec = 4.8 x 10 2 E t = 48mv System noise may be evaluated as ampli-fier noise plus Nyquist noise of the input resis-tance ~ .15~2 ~/
E = \/4kTR,~F
n , ~ c = ~ 4x1.37xlO ~x300xlOOOx1.5xlO' ~ 1.57~v Using an amplifier with nois~ figure of .8 nano volt/~ ~ and 1.5x105 hz, En = .8xlO 9 (1.5x105)1/2 - .8xlO 9x3.87x102 . 309,uv Hence, the amplifier noise is much less than the Nyquist noise of the input resistor.
Therefore, sl~nal to noise ratio is:
48.
S/N = 1 57 = 30.5:1 this dynamic range will produce an image with approximately 9-10 shades of gray. Here a shade of gray is defined as increase in signal. That is, Shades o. Gray = ~ g-~log 2 EXPERIMENTAL SYSTEM #3 Figure 20 is a block diagram that illu-strates the major components of the third experi-mental system built at the experimental radiology department of M.D. Anderson Hospital in Houstonr Texas. This experimental system was built to test the entire integrated system of the present inven-tion and to provide a breadboard system for ini-tial clinical evaluation. This system is fully computerized.
As is shown in Figure 20, microcomputer 2007, which is a Southwest Technical Products 6800, controls laser blanker 2015; 16-bit d/a converter 2011; x-position interface module 2017;
1 ;il5~5~2 . .~
12-bit d/a converter 2009; and teletype 2005.
Teletype 2005 is a model 33ASR. Sixteen bit d/a converter 2011 is an Analog Devices 1136 and 12-bit d/a converter 2009 is an Analog ~evices 1132. Laser blanking unit and laser 201S are a Liconix model 4110 helium/cadmium laser.
Laser 2015 is in a light~tight housing, not shown, such as the one functionally illu-strated in Fi~lre 9, above.
The photon beam from laser 2015 scans detector 2019. The output of detector 2019 is connected to the input of preamp 2021. The output of preamp 2021 is cor~ected to scan rate converter 2003; a/d converter 2023; and write laser modula-tor and laser 2025.
The output of a/d converter 2023 is an input to microcomputer 2007. Teletype 2005 is in two way communication with microcomputer 2007.
It would be appreciated by looking at Figure 20 that laser 2015 is a read laser adapted to scan detector 2019 and laser 2025 is a write laser adapted to write information out on a film or xerographic plate. This type of system is generally described in ConneGtion with Flgure 11, above.
Both the read and write laser beams are controlled by the microcomputer. The output of 12-bit d/a converter 2009 drives read laser x-posi tion scanner, controller and mirror transducer 2013. It also drives write laser x-position scanner, controller and mirror transducer 2027.
Similarly, 16-bit d/a converter 2011 drives both read laser y-position scanner, controller and mirror transducer 2029 and write laser y-position scanner, controller and mirror transducer 2031.
It will easily be appreciated that the beams from 11S5~2 ~ 3 -5~-bo-th the read laser and the write laser scan in sychronization, although both of them need no-t be turned on at the same time or modulated by the same signal.
Functionally, the microprocesser accepts instructions from the teletype. These instruc-tions operate on a computer program. A copy of this program's listing is included with this application for the Examiner's convenience. It is the inventors' request that it be inserted in the record so it will be available to the public as part of the File Wrapper of this application.
When the microprocessor addresses the 16-bit d/a converter and increments the nul~er stored in it, the y-position mirrors of both lasers are deflected to a certain angle. In experimental system $~3 this angle is sufficient to move the spot that is illuminated on detector 2019 S0 microns. Thus the y-positioning of the laser beam on the detector occurs in 50 micron incre-ments. The program included with this application allows specification of line sizing on ~he y-axis to S0, 100, 200, or 400 micxons. This translates to a resolution in the image produced by the system of 10, 5, 2.5 and 1.25 line pairs per millimeter.
The 12-bit d/a converter is not directly addressed by the computer. The computer addresses a special interface that controls the 12-bit d/a converter. This interface allows the computer to specify the starting and ending positions of the laser beam on the detector surface and along the x-axis. A schematic of x-position interface module 2017 is included as Figure 24. The compu-ter then issues a start comman~ to the interface which enables a switch selectable clock to start 5 ~ 2 incrementing the d/a to a specific start address.
The clock continues to increment the d/a until the stop address is encountered, then the clock is disabled ~y the interface. This x-position inter-face accomplishes two objectives. First, the data readout range can be varied. Secondly, difEerent slze detector plates can be read out without overscanning them. This is especially valuable on an experimental system.
Computer 2007 also controls laser blank-ing, which just means that it turns the laser on and off. The computer blanks the laser after each x-axis scan is completed. The laser is then moved back across the detector and turned back on for the next scan line.
To read out a plate using experimental system #3:
(1~ The user specifies plate size and x-axis line spacing. The computer program will ask for these values.
~ 2) The x-axis interface is given the appropriate start and stop addresses by the computer.
~ 3), The x-axis line spacing and total number of required steps is determined by the computer.
~ 4) The computer blanks the CRT and puts the scan rate converter into the write mode.
~ 5~ The laser is turned on by the computer.
(6) The computer instructs the inter-face to turn on the clock.
(7) The x-axis mirror sweeps the laser across the plate.
Jl Jl5~5~i2 ,~
Jl Jl5~5~i2 ,~
(8) The computer t~arns off the laser.
(9) The computer increments the y~axis d/a the requ~ired number of steps which moves the laser beam through the specified line spacing.
(10) The computer then repeats steps 5 9 until the entire plate has been scanned.
(11~ The computer puts the scan rate converter in the read mode and the image is displayed.
The read laser x-position scan control-ler and y-position SGan controller both feed into scan rate converter 20Q3. The scan rate converter allows a relatively slow video image, such as that formed by experimental system #3, to be interfaced with a television display. Scan rate converter 2003 outputs a normal video signal to CRT display 2001, where the image may be viewed, moved about on the screen, or expanded to examine detail.
The example and sugges-ted systems illu-strated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use their invention. Nothing in this specifica-tion should be considered as limiting the scope of the present invention. Many changes could be made by those skilled in the art to produce eguivalent systems without departing from the invention. For example, it is possible to use a photoconductor other than selenium as part of the detector sand-wich. Holographic optics could be used in the scanning system in place of the mechanical systems used in the example. It may be desirable to alter the relative thicknesses of the various layers making up the sandwich structure or to provide for different frequency of light beams to read out the 115~S~2 .radiographic image or to sense different x-ray potentials. The present invention should only be limited by the following claims and their equiva lents.
(11~ The computer puts the scan rate converter in the read mode and the image is displayed.
The read laser x-position scan control-ler and y-position SGan controller both feed into scan rate converter 20Q3. The scan rate converter allows a relatively slow video image, such as that formed by experimental system #3, to be interfaced with a television display. Scan rate converter 2003 outputs a normal video signal to CRT display 2001, where the image may be viewed, moved about on the screen, or expanded to examine detail.
The example and sugges-ted systems illu-strated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use their invention. Nothing in this specifica-tion should be considered as limiting the scope of the present invention. Many changes could be made by those skilled in the art to produce eguivalent systems without departing from the invention. For example, it is possible to use a photoconductor other than selenium as part of the detector sand-wich. Holographic optics could be used in the scanning system in place of the mechanical systems used in the example. It may be desirable to alter the relative thicknesses of the various layers making up the sandwich structure or to provide for different frequency of light beams to read out the 115~S~2 .radiographic image or to sense different x-ray potentials. The present invention should only be limited by the following claims and their equiva lents.
Claims (4)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An exposure measuring apparatus comprising: a radiation source adapted to irradiate a detector structure with radiation, said detector structure comprising: a duo-dielectric sandwich including a transparent conducting layer overlying a transparent insulating layer which is affixed to a high resistant photoconductor layer which overlays a conductive layer, said radiation capable of producing electron hole pairs in said photoconductor layer of said detector structure; an external circuit connecting said trans-parent conducting layer and said conductive layer; and measuring means res-ponsive to the flow of current in said external circuit when said detector structure is irradiated by said radiation source.
2. An apparatus as in Claim 1 wherein the flow of current is measured from only a part of said detector structure, said small part being electrically segmented from the greater portion of the detector.
3. An apparatus as in Claim 2 wherein current flow is measured from several small segmented portions of the detector.
4. An apparatus as in Claim 1, 2, or 3 including control means re-sponsive to said measuring means for stopping said irradiation when the total charge reaches a preselected limit.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
CA000370579A CA1155562A (en) | 1981-02-11 | 1981-02-11 | Realtime radiation exposure monitor and control apparatus |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
CA000370579A CA1155562A (en) | 1981-02-11 | 1981-02-11 | Realtime radiation exposure monitor and control apparatus |
Publications (1)
Publication Number | Publication Date |
---|---|
CA1155562A true CA1155562A (en) | 1983-10-18 |
Family
ID=4119150
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
CA000370579A Expired CA1155562A (en) | 1981-02-11 | 1981-02-11 | Realtime radiation exposure monitor and control apparatus |
Country Status (1)
Country | Link |
---|---|
CA (1) | CA1155562A (en) |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0318807A1 (en) * | 1987-12-01 | 1989-06-07 | Noranda Inc. | Systeme for measuring the charge distribution on a photoreceptor surface |
-
1981
- 1981-02-11 CA CA000370579A patent/CA1155562A/en not_active Expired
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0318807A1 (en) * | 1987-12-01 | 1989-06-07 | Noranda Inc. | Systeme for measuring the charge distribution on a photoreceptor surface |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
US4521808A (en) | Electrostatic imaging apparatus | |
US4268750A (en) | Realtime radiation exposure monitor and control apparatus | |
US4539591A (en) | Method of impressing and reading out a surface charge on a multi-layered detector structure | |
US4554453A (en) | Apparatus for recording X-ray images | |
JPS6252981B2 (en) | ||
US4763002A (en) | Photon detector | |
EP0602475A2 (en) | Method and apparatus for acquiring an X-ray image using a solid state device | |
US5925890A (en) | Apparatus for recording and reading out a pattern of penetrating electromagnetic radiation | |
JPH077831B2 (en) | Solid-state radiation sensor array panel | |
US5510626A (en) | System and method for conditioning a radiation detector | |
US5166524A (en) | Element, device and associated method for capturing a latent radiographic image | |
WO1994004943A1 (en) | Method and apparatus for acquiring an electrical signal representing a radiographic image | |
US5127038A (en) | Method for capturing and displaying a latent radiographic image | |
EP0748115A1 (en) | Method for recording and reading out a pattern of penetrating electromagnetic radiation | |
CA1155562A (en) | Realtime radiation exposure monitor and control apparatus | |
CA1162332A (en) | Method of impressing and reading out a surface charge on a multilayered detector structure | |
AU536855B2 (en) | Realtime radiation exposure monitor and control apparatus | |
CA1156772A (en) | Photon detector | |
CA1159507A (en) | Electrostatic imaging | |
US6794655B2 (en) | Radiation image detecting system | |
JPH1033519A (en) | Megavolt image forming method utilizing coupling of photoreceptor and high-energy photon converter and amplifier | |
JPS595773A (en) | Image detector | |
Zermeno et al. | Laser readout of electrostatic images | |
JPS59226569A (en) | X-ray picture reading device | |
JPS6111685A (en) | Radiation image reading device |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
MKEX | Expiry |