GB2082873A - Compton scatter diagnostic apparatus for determining the internal structure of a body - Google Patents

Abstract

In Compton scatter apparatus for determining the distribution of local electron density in a body, a source 1 which provides three photon energies E1, E2, E3, directs a pencil beam 3 through the body 5. Radiation 6, 6', scattered from respective elements P along the beam, passes via a transverse slit 8, 8', and is detected by corresponding detectors 10, 10'. Each detector output is fed to three window circuits and corresponding element scatter signals for each energy S(E1). S(E2), S(E3), are fed to a computer. The improvement comprises initially carrying out similar measurements on a reference phantom resembling the body, storing the corresponding reference scatter signals V(E1), V(E2), V(E3), in a store and causing the computer to generate the quotients S(E1)/V(E1) ...etc., for each photon energy. The local electron density is then computed from these quotients using known data relating to the reference phantom. <IMAGE>

Description

SPECIFICATION Compton scatter diagnostic apparatus for determining the internal structure of a body The invention relates to a diagnostic apparatus for determining the internal structure of a body, comprising radiation source means for generating a primary beam of penetrating radiation of small cross-section which is directed so as to penetrate the body, said source means being arranged to generate radiation having any of at least three distinct radiation photon energies, at least one slit diaphragm which is situated outside the primary beam path and which is provided with a slip-shaped aperture which extends in a direction approximately perpendicular to a plane containing the primary radiation beam, a detector device which extends in a direction transverse to that of the longitudinal dimension of the slit and which comprises individual detectors for the detection of scattered radiation which is produced in the body by the primary beam and which passes through the slit-shaped aperture, and an electrical device for the processing and display of signals derived from the detectors.

An apparatus of this kind is known from German Offenlegungsschrift 27 13 581. However, such an apparatus is only suitable for forming a qualitative representation of, for example, a sectional image of a three-dimensional body unless additional correction steps are performed. For example, if the attenuation of the radiation along the path followed by the primary beam or by the scattered radiation, is also to be taken into account the measurement values obtained by means of the apparatus must be corrected in accordance with the correction methods which are also known from German Offenlegungsschrift 27 13 581, or the corresponding U.K. Patent Application No. 1 1670/78, thus necessitating the use of a digital computer.

For the correction of measurements made on a sectional region of a body using that method, it is assumed, for example, that initially a first line path through,the body section is scanned, the scattered radiation from which reaches the detector device without attenuation by intermediate tissue. The scattered radiation emitted by a first notional elemental cell along this line path will not yet have been attenuated, so that it can be used directly as a measure of the local density in this cell.Primary radiation reaching the second elemental cell along this line will have been attenuated by an amount equal to the energy converted into scattered radiation by the first elemental cell and because this energy is known from the measurement made on the first cell, it can be taken into account by correspondingly increasing the magnitude of the output signal of the detector associated with the second elemental cell by comparison with the value of the output signal of the detector associated with the first cell. Similarly, for a third cell along this line path, the attenuation by the first two cells must be taken into account, and so on.For a first elemental cell of the next line path situated behind the first line path with respect to the detector, the primary beam will also not have been attenuated, but the scattered radiation from this cell will be attenuated by the cells of the first or preceding line path, which are situated between the detector slit and the relevant radiation scattering cell. Because the attenuation of radiation by these cells has already been determined during previous measurements, the measurement value associated with the first cell of the second line can be corrected accordingly.When processing the output signal of the detector which measures the scattered radiation produced by the second elemental cell of the second line path it will therefore be necessary to take into account not only the attenuation of the primary beam by the first cell of this line, but also the attenuation of the scattered radiation by the elemental cells in adjacent line paths located between the scatter point and the detector.

Thus, this correction method can only enable a correct image of internal regions of a body section to be generated if the outer regions of the body section under examination are also irradiated and measured. Furthermore, if this correction method were used, only the scattered radiation generated in the body by the primary beam and propagating substantially in the plane of the body section to be imaged should be measured, because the corresponding attenuation coefficients for the individual pixels of the sectional image will not be disturbed by any regions exhibiting strong absorption (bones, gas inclusions etc). which are situated outside the body section to be imaged.

It is an object of the invention to provide a Compton scatter diagnostic apparatus for the determination of the structure of a body which can enable improved sectional images to be formed in a simple manner without the need for using such a correction method.

According to the invention there is provided a diagnostic apparatus for determining the internal structure of a body, comprising radiation source means for generating a primary beam of penetrating radiation of small cross section which is directed so as to penetrate the body, said source means being arranged to generate radiation having any of at least three distinct radiation photon energies, at least one slit diaphragm which is situated outside the primary radiation beam path and which is provided with a slit-shaped aperture which extends in a direction approximately perpendicular to a plane containing the primary radiation beam, a detector device which extends in a direction transverse to that of the longitudinal dimension of the slit and which comprises individual detectors for the detection of scattered radiation which is produced in the body by the primary beam and which passes through the slit-shaped aperture, and an electrical device for the processing and display of signals derived from the detectors characterised in that each detector supplies an output signal which is representative of the photon energy of the incident radiation and is connected to an electrical circuit for forming respective measured scatter signals relating to the corresponding individual photon energies of the primary radiation, the apparatus further comprising a memory for the storage of corresponding reference scatter signals which have been measured and recorded in respect of a predetermined reference body in a similar manner by means of a similar diagnostic apparatus, said memory being connected to the processing device which is arranged to compare the measured scatter signals with the stored reference scatter signals for each individual radiation photon energy, and to determine the internal structure of the body from the measured scatter signals and the stored reference scatter signals thus compared, As has already been stated, the measured scatter signals (S) and the reference scatter signal (V) (scattered radiation intensities) measured at a given primary radiation photon energy from an elemental region (cell) of the body under examination and of the reference body respectively, when irradiated by the primary beam, will be dependent on the electron density in that body region, on the attenuation of the primary radiation by the body before reaching that body region, and on the attenuation of the scattered radiation by the body before reaching the detector. These three variables are unknown quantities. When the measured scatter signals are compared with the reference scatter signals previously recorded for a known reference body, for example, by forming the quotient of the measured scatter signal and the reference scatter signal and then forming the logarithm ( 1 n(S/V)) thereof, the three unknown variables can be determined therefrom if the procedure is performed for each body point (i.e. notional elemental cell) using respectively at least three distinct primary radiation photon energies.

The corresponding variables for the reference body are known. The comparison of a measured scatter signal with a reference scatter signal, however, should always be performed for the same primary photon energy. This enables a simple determination to be made of, for example, the electron density distribution in the body region irradiated by the primary beam and hence the generation of, for example, a sectional image of the body if this procedure is executed for a large number of, for example parallel beam paths situated in the plane of the body section.

Such sectional images will not need to be corrected for the attenuation of the primary radiation or of the scattered radiation. Notably the generation of a high-quality sectional image of parts which are situated within a body section will thus also be possible without the need to measure the entire crosssection of the body. It will be apparent that sectional images of a body, which are not situated in a plane can also be formed.

It is also advantageous that, for example, variables such as the sensitivity of individual detectors, the energy dependency of multiple scattering in the body etc. no longer significantly adversely affect the quality of the resulting image of the body section or body structure. During the comparison of the measured scatter signals and the reference scatter signals, these variables will tend to cancel one another if the body and the reference body have both been measured using the same scatter diagnostic apparatus in the same manner, i.e. if they have been correspondingly irradiated with primary radiation of the same photon energy. The body and the reference body should resemble one another as much as possible.

In a preferred embodiment in accordance with the invention, the radiation source consists of an Xray source or of at least three substances respectively emitting gamma rays of different photon energy.

It is thus achieved that radiation of at least three distinct photon energies can be provided in a simple manner.

In a further embodiment in accordance with the invention, the detector device comprises a plurality of individual detectors arranged respectively in corresponding rows which extend across the detector, each row being parallel to the direction of the longitudinal dimension of the slit-shaped aperture.

The radiation scattered by a given elemental cell is measured independently by each detector in a detector row and the respective measurements are then used together for the determination of the internal body structure, for example, the electron density of the body substance in the corresponding body region from which the radiation was scattered. Subsequently, the electron densities thus obtained are averaged. For example, strongly absorbing body structures between the irradiated body region (scattering cell) and the detectors can then be located and taken into account, for example, by suitably weighting the electron densities associated with the corresponding detectors during the process of averaging. The reconstruction accuracy can be improved by such a detector arrangement.The image quality is additionally improved because a larger number of the scattered photons emitted by the scatter centre are intercepted and measured. Because the strip-shaped detector described in DE 27 13 581 does not have local resolution along the line direction, it is not suitable for the reconstruction of the image of a body section in which highly absorbing body structures (bones, air, inclusions, etc.) are situated between the irradiated body region and the detector device because these structures cannot be taken into account, with the result that the electron density in the irradiated body region is not properly reproduced.

Embodiments in accordance with the invention will now be described by way of example, with reference to the accompanying drawings, of which~ Figure 1 is a sectional view of a diagnostic apparatus in accordance with the invention, Figure 2 is a block diagram illustrating the processing of the detector output signals from one of the detectors, Figure 3 is a perspective view of part of the diagnostic apparatus, and Figure 4 shows a radiation source arrangement comprising three individual radiation sources of different respective photon energies.

Figure 1 is a sectional view of a diagnostic apparatus in accordance with the invention. It comprises, for example, an X-ray source 1 whose polychromatic radiation is limited by means of a diaphragm 2 in order to form a primary beam 3 of small cross-section which irradiates a body 5 positioned on a table 4. The primary beam 3 follows a primary beam path defined by this beam. The scattered radiation 6, 6' produced in the region of the body 5 irradiated by the primary beam 3, reaches a respective detector device 9, 9' via a corresponding slit diaphragm 7, 7' respectively arranged to either side of the primary beam 3, and whose slit-shaped apertures 8, 8' the width of which is preferably adjustable, extend perpendicularly to a plane containing the primary beam 3.The detector devices 9, 9' consist of individual detectors 10, 10' which are arranged side by side along a straight line which extends parallel to the primary beam 3. The detectors 10, 10' may be, for example, strip-shaped and be arranged so that their principal dimension extends parallel to the slit-shaped apertures 8, 8'. For scanning different regions of the body, the body 5 and the diagnostic apparatus are arranged to be displaceable relative to one another.

Each detector of the detector devices 9, 9' supplies detector output signals l(E) which are dependent on the photon energy E of the scattered radiation incident thereon. The photon energy E of the scattered radiation is determined for a given photon energy Eof the primary radiation and for a given scatter angle 6 at which the detected scattered radiation is scattered with respect to the primary beam 3, inaccordance with the generally known Compton equation. The position of the slit diaphragms 7, 7' defines the angles 6 at which the scattered radiation is measured for any point (elemental region) along the primary beam 3. Apart from the foregoing the energy of the scattered radiation, therefore, is determined only by the photon energy of the primary radiation.

Figure 2 shows a block diagram for processing the detector output signals. Each detector, for example, the detector 10, is connected to a corresponding electronic circuit 1 1 which selectively registers scattered photons occurring at only three different energies, that is to say l(E1), l(E2) and I(E3), from the scattered photons of different energy which arrive at the detector 10 and which are generated by a polychromatic X-ray beam. Scatter signals S(E1), S(E2), S(E3) are then formed from the detector output signals, each associated with a corresponding one of the scattered radiation energies El, E2, E3.To achieve this, the electronic circuit 1 1 may comprise, for example, three circuits 12 which define energy windows and which generate an output signal only if the input signal (detector output signal) is within a given range which corresponds to a predetermined photon energy range of the scattered radiation. The respective output signals of each circuit 12 associated with a given photon energy range, are then counted during a predetermined period in order to provide corresponding scatter signals S(E 1), S(E2), S(E3); this operation can also be performed in the electronic circuit 1 1.

Scatter signal recorded by means of a detector can be represented as follows:

This relationship (1 ) is equally applicable to each of the photon energies El, E2, E3. As has already been stated S(E1 ) represents the intensity of the scattered radiation with a photon energy El, N(E1) is the intensity of the primary radiation outside the body 5 with a photon energy El, d~(t1 )/do) is the differential effective cross section for scattering the primary radiation, vk is the electron density in the body 5 at the body point P under consideration (see Figure 1), the fourth term represents the attenuation of the primary beam 3 between the radiation source 1 and the relevant body point P (path 11), and the fifth term represents the attenuation of the scattered radiation between the body point P and the detector measuring the scattered radiation (path 12).

In the case of biological material, for energies in excess of 100 KeV scattering forms the major component of the attenuation coefficient,', (see formula 1), so that it can be expressed as follows: ,u E, 11 = b (E). q7(1) (2) Therein, b(E) is the overall Klein-Nishina effective scatter cross-section and (I) is the locationdependent electron density.

The measured scatter signals (SE 1), S(E2), S(E3) are applied to an electronic computer 13 to which the electronic circuit 11 is electrically connected and which forms part of an electronic processing device 14. Also connected to the electronic computer 13 is an electronic memory 15 which stores reference scatter signals V(E1), V(E2), V(E3) which correspond to the measured scatter signals S(E1), S(E2), S(E3) and which have been recorded under the same circumstances as the latter signals, that is to say with the same primary radiation photon energies E1, E2, E3, but using a known reference body which corresponds to the body to be examined and by means of the same diagnostic apparatus.

The reference body (not shown) may be, for example, a water phantom.

A comparison between the measured scatter signals and the reference scatter signals is executed in the computer 13 so that for each photon energy E1, E2, E3, the corresponding quotient e.g.

S(El )/V(El), of a scatter signal and a reference scatter signal, is formed and the logarithm thereof is provided. Thus, a set of three equations with three unknowns is obtained. The equation for scattered radiation having a photon energy El is obtained, after substituting from (2) into (1), as follows:

Similar equations can be formed for E2 and E3. The index k denotes the body 5 under examination, whilst the index v denotes the reference body.

From the set of equations (3) (i.e. corresponding equations for El, E2, E3) the electron density vk can thus be determined for each body point P (elemental cell) irradiated by the primary beam 3. The electron densities 97v (Il) and Yv(12) of the reference body on the path 11 of the primary beam and the path 12 of the scattered radiation respectively are known, as are the Klein-Nishina effective scatter cross-sections b(E1), S(E1 ) (similarly for the photon energies E2, E3). As has already been stated, the photon energy of the scattered radiation El can be calculated from the photon energy E1 of the primary radiation by means of the Compton equation.It will be apparent that photons can also be selectively registered in respect of more than three energy levels. For this the electronic circuit 1 1 would only require additional circuits 12 (or additional channels of a multiple channel) to be provided. The set of equations thus obtained would subsequently be suitably minimized.

Each detector 10, 10' is thus connected to a corresponding electronic circuit 1 only one of which is shown in Figure 2, all said circuits 1 1 being connected to the same computer 13. The memory 15 also contains the reference scatter signals for all corresponding points on the primary beam 3 passing through the reference body. The electron densities vk determined by the computer 13, or the variables derived therefrom, can be displayed on a monitor 16 or can be stored in a bulk memory 17 (for example on magnetic tape or memory disk).

Figure 3 is a perspective view of a Compton scatter diagnostic apparatus in accordance with the invention. The slit diaphragm 7 has an elongate slit-shaped aperture 8, arranged so that a scattered radiation beam 6 having a very large angular spread a which starts from the body point P, is provided by scattering from the primary radiation beam 3. The scattered radiation beam 6 reaches a detector row which consists of individual detectors 1 Oa, b etc. which are situated in a row which extends parallel to the slit-shaped aperture 8 and which extends perpendicularly to a plane containing the primary beam 3.

In an extreme case, the entire detector device 9 and the slit diaphragm 7 can alternatively completely surround the primary beam, for example, in a cylindrical manner, so that the primary beam 3 extends along the cylinder axis. Each individual detector of the detector device 9 (which latter can be shaped as a cylinder or as a two-dimensional detector matrix) is connected to a corresponding electronic circuit 1 1 (not shown) via the connections a-d etc. Respective local values of a variable, for example the electron density vk which characterise the internal structure of the body 5, can be derived from each corresponding detector of the detector device. The electron density values derived from the detectors in a a given detector row will relate to the same corresponding body point P.An improved value for the electron density at the body point P can be determined from these density values, for example by weighted averaging.

It will be apparent that the individual detectors 1 Oa, b etc. of a detector row can alternatively be replaced by a single, rod-shaped detector which has a local resolution ability along the direction of the row, so that the scattered radiation intensity can be measured for different row sections (see Figure 2).

For this purpose, use can be made of, for example, rod-shaped scintillators provided with respective photomultipliers which are arranged at either end of the rod, the output signals of said photomultipliers being processed in accordance with the Anger camera principle. The longitudinal direction of the rod should be parallel to the slit-shaped aperture 8 and hence perpendicular to a plane containing the primary beam 3.

A further radiation source 1 ' for the emission of primary radiation with at least three different radiation photon energies is shown in Figure 4. The radiation source 1' comprises three radiation sources 1 8a-c which emit gamma rays, for example respective sources comprising 137Cs (0.66MeV), 203Hg (0.28 MeV) and 57Co(0.l2 MeV). The three individual radiation sources 1 8a-c are situated, for example, inside a rotatable disk 19 which has a corresponding radial duct 20 to the radiation exit for each individual radiation source. The disk 19 rotates about a shaft 21 at the correct angular velocity, so that the separate radiation sources 1 8a-c are successively positioned in front of an exit opening 22 of a housing 23 which shields the radiation. The primary radiation beam 24 which passes through the exit aperture 22, is collimated by means of a diaphragm 25. In the present embodiment the electronic circuit 1 1 will comprise three circuits 12 which form corresponding energy windows, for example, pulse amplitude analyzers, which are adapted to the photon energies of the individual radiation sources 1 8ac.

It will be apparent that the individual radiation sources 1 8a-c can be arranged or displaced with respect to the radiation exit aperture in an alternative manner, for example, linearly. For a further form of radiation source use can alternatively be made of a mixture of said three substances emitting gamma rays, said mixture being arranged in the rotating disk 19 at the location of one of the radiation sources 1 8a-c.

Claims (9)

1. A diagnostic apparatus for determining the internal structure of a body, comprising radiation source means for generating a primary beam of penetrating radiation of small cross-section which is directed so as to penetrate the body, said source means being arranged to generate radiation having any of at least three distinct radiation photon energies, at least one slit diaphragm which is situated outside the primary beam path and which is provided with a slip-shaped aperture which extends in a direction approximately perpendicular to a plane containing the primary radiation beam, a detector device which extends in a direction transverse to that of the longitudinal dimension of the slit and which comprises individual detectors for the detection of scattered radiation which is produced in the body by the primary beam and which passes through the slit-shaped aperture, and an electrical device for the processing and display of signals derived from the detectors, characterized in that each detector supplies an output signal which is representative of the photon energy of the incident radiation and is connected to an electrical circuit for forming respective measured scatter signals relating to the corresponding individual photon energies of the primary radiation, the apparatus further comprising a memory for the storage of corresponding reference scatter signals which have been measured and recorded in respect of a predetermined reference body in a similar manner by means of a similar diagnostic apparatus, said me'mory being connected to the processing device which is arranged to compare the measured scatter signals with the stored reference scatter signals for each individual radiation photon energy, and to determine the internal structure of the body from the measured scatter signals and the stored reference scatter signals thus compared.

2. An apparatus as claimed in Claim 1, characterised in that the radiation source is an X-ray source.

3. An apparatus as claimed in Claim 1, characterised in that the radiation source comprises at least three substances which respectively emit gamma rays of a different photon energy.

4. An apparatus as claimed in Claim 3, characterised in that three substances emitting gamma rays are individually accommodated and can be successively positioned in front of an aperture forming part of a collimator device which is arranged to limit the primary beam.

5. An apparatus as claimed in Claim 3 or 4, characterised in that the substances emitting gamma rays are 137Cs (0.66 MeV), 203Hg(0.28 MeV) and 57Co(0.l 2 MeV).

6. An apparatus as claimed in Claim 1, characterised in that the detector device comprises a plurality of individual detectors arranged respectively in corresponding rows extending across the detector, each row being parallel to the direction of the longitudinal dimension of the slit-shaped aperture.

7. An apparatus as claimed in Claim 1, characterised in that the detectors comprise rod-shaped scintillators with respective photomultipliers which are arranged at the corresponding extremities of each rod, the longitudinal direction of the rod extending parallel to the direction of the longitudinal dimension of the slit-shaped aperture.

8. An apparatus as claimed in any one of Claims 1 to 7, characterised in that the slit diaphragm and the detector device cylindrically surround the primary beam, the primary beam following the cylinder axis.

9. A diagnostic apparatus for determining the internal structure of a body by Compton scatter measurements, substantially as herein described with reference to the accompanying drawings.