WO2007089962A1

WO2007089962A1 - Spect detector eliminating the depth-of-interaction problem

Info

Publication number: WO2007089962A1
Application number: PCT/US2007/060365
Authority: WO
Inventors: Carsten Degenhardt; Herfried Wieczorek
Original assignee: Koninklijke Philips Electronics, N.V.; U.S. Philips Corporation
Priority date: 2006-01-30
Filing date: 2007-01-11
Publication date: 2007-08-09

Abstract

At least one radiation detection head (16) is disposed adjacent a field of view (20) to detect radiation from the field of view (20). The radiation detection head (16) includes a collimator (60), including an opening (62), through which radiation rays emanating from the field of view (20) pass, and a detection system (44) forming an arcuate surface (68) focused on the collimator opening (62) for detecting radiation so that radiation rays received through the collimator opening (62) strike the arcuate surface substantially perpendicular to an incremental area of the arcuate surface (68) at which the radiation strikes.

Description

SPECT DETECTOR ELIMINATING THE DEPTH-OF-INTERACTION

PROBLEM

DESCRIPTION

The present application relates to the diagnostic imaging systems and methods- It finds particular application in conjunction with the Single Photon Emission Computed Tomography (SPECT) systems and will be described with particular reference thereto. It will be appreciated that the following is also applicable to other like applications.

Nuclear medicine imaging employs a source of radioactivity to image a patient. Typically, a radiopharmaceutical is injected into the patient. Radiopharmaceutical compounds contain a radioisotope that undergoes gamma-ray decay at a predictable rate and characteristic energy. One or more radiation detectors are placed adjacent to the patient to monitor and record emitted radiation. The radiation detector is typically a large flat scintillation crystal, such as sodium iodide, having the property of emitting light when struck by gamma photons. Affixed to the rear of this crystal are photomultiplier tubes with associated circuitry to detect the light flashes and to locate their position within the scintillation crystal. Such detector provides a two-dimensional image of radiotracer distribution. To obtain a three-dimensional image, the detector is rotated or indexed around the patient to monitor the emitted radiation from a plurality of directions. Based on information such as detected position and energy, the radiopharmaceutical distribution in the body is determined. An image of the distribution is reconstructed to study the circulatory system, radiopharmaceutical uptake in selected organs or tissue, and the like.

Typically, in SPECT imaging, a collimator is positioned in front of the detector to control the direction of radiation and angular spread from which each element of the detector can receive the radiation. In some SPECT systems, in which the increased detector sensitivity and resolution are desired, the collimator for each detector defines a single pinhole or a slit which is positioned at some distance from both the object to be examined and the detector. The pinhole or slit functions analogous to an optical pinhole camera, which is equivalent in its performance to an optical lens to "focus" radiation for the region of interest onto the flat detector. The gamma rays which are transmitted through the pinhole or slit reach the center of the flat detector relatively perpendicular. But, at

. . i . . points progressively further from the center, the rays strike the detector at progressively more oblique angles. Because the gamma ray is not converted into a scintillation event until it has progressed some distance into the detector, the scintillations can occur at the locations which are offset from the point of incidence on the detector face. The detectors report the actual coordinates of the scintillation event on the detector that is offset from the point at which the radiation was received by the crystal. The absorption depth or the depth- of-interaction is a quantum mechanical event and not a consistent distance. The variation in the depth of interaction and the variation in incidence angle leads to a positioning error when the position of the gamma event is calculated, which, in turn, leads to image degradation.

The present application provides new and improved imaging apparatuses and methods which overcome the above-referenced problems and others.

In accordance with one aspect of the present application, an imaging system is disclosed. At least one radiation detection head is disposed adjacent a field of view to detect radiation from the field of view. The radiation detection head includes a collimator, including an opening, through which radiation rays emanating from the field of view pass. A detection system forms an arcuate surface focused on the collimator opening to detect radiation. Radiation rays received through the collimator opening strike the arcuate surface substantially perpendicular to an incremental area of the arcuate surface at which the radiation strikes.

In accordance with another aspect, a method of imaging is disclosed. Radiation, that has passed through the opening from a subject disposed in a field of view, is detected with a detection system forming an arcuate surface focused on an opening so that radiation quanta propagate the detection system in a direction substantially perpendicular to the arcuate surface. The detected radiation is converted into electrical signals with an array of light sensitive elements.

In accordance with another aspect, a diagnostic imaging apparatus is disclosed. A gantry assembly supports at least one detector head. Each detector head includes a collimator plate which defines an aperture and an array of detection modules, each module including a face which receives radiation that has passed through the opening and a major axis aligned with the opening. A reconstruction processor reconstructs signals from the detection modules into an image,

In accordance with another aspect, a detector is disclosed. At least one scintillator with a face receives radiation that has passed through an opening along a multiplicity of trajectories, the received radiation traveling along an associated radiation trajectory through the scintillator until the radiation interacts in a quantum mechanical interaction to generate a corresponding scintillation event, the quantum mechanical interactions occurring at non-constant distances along respective trajectories, whereby each scintillation event tends to be offset from a point of incidence of the respective trajectory on the scintillator face in accordance with an angle of the respective trajectory and the distance along the respective trajectory that the scintillation event occurs. Opto-electric transducers convert the scintillation events into electrical signals indicative of the point of incidence on the scintillator face of the respective trajectory, the at least one scintillator and the opto-clcctric transducers being geometrically configured so that the scintillation events are not indicated by the electrical signals as being offset from their respective points of incidence on the scintillator face.

Still further advantages and benefits of the present application will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.

The following may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the following.

FIGURE 1 is a diagrammatic illustration of an imaging system; FIGURE 2 is a diagrammatic illustration of one arrangement of detector assembly and a collimator; and

FIGURE 3 is a diagrammatic illustration of another arrangement of detector assembly and a collimator. With reference to FIGURE 1, a nuclear imaging system 10 typically includes a stationary gantry 12 that supports a rotatable gantry 14, One or more detector heads 16 are carried by the rotatable gantry 14 to detect radiation events emanating from a region of interest or examination region or field of view (FOV) 20. The detection heads 16 are arranged generally circumferentially about the field of view 20.

Typically, an object or subject to be imaged is injected with one or more radiopharmaceuticals or radioisotopes and placed in the examination region 20 supported by a couch 28. A few examples of such isotopes include Tc-99m, 1-131, Ga-67, and In- 1 1 1. The presence of the radiopharmaceuticals within the object produces emission radiation from the object. Radiation is detected by the detection heads 16 which surround or arc able to be angularly indexed or rotated around the examination region 20 to collect the projection emission data at one or more selected projection directions. The projection emission data, e.g. the location (x, y), energy (z), and an angular position (θ) of each detection head 16 around the examination region 20 (e.g., obtained from an angular position resolver 30) define the trajectory and energy of each radiation event and are stored in a data memory 32. A reconstruction processor 36 processes the emission data from the data memory 32 into a volumetric image representation. The image representation is stored in an image memory 38 for manipulation by a video processor 40 and display on an image display 42, such as a video monitor, printer, or the like.

With continuing reference to FIGURE I and further reference to FIGURE 2, each detection head 16 includes a detector assembly or detection assembly or system 44 comprising a first or detection array 46 which includes a detector or detector elements or scintillator elements 50, such as one or more scintillator plates, individual scintillation crystals, and the like, and a second array 52 which includes an array of light sensitive elements 54, e.g. photomultiplier tubes, photodiodes, opto-elcctric transducers, and the like. Direct photon to electrical converters, such as semiconductor crystals, CZT elements, and the like, are also contemplated. In the illustrated pixellated embodiment, the detector array includes an array of scintillator elements and diode (or other opto-eiectric element) pairs, each of which defines one detector pixel. The detector pixel that receives radiation defines its location (x, y) on a detector face 56 or trajectory and the strength, e.g. brightness of the scintillation, defines its energy (z). In each detector head 16, a collimator 60 controls the direction and angular spread, from which each element of the detector 50 via an opening or openings or aperture 62, can receive radiation, i.e., the detector 50 receives radiation along known rays or trajectories. Thus, the determined location on the detector 50 at. which radiation is detected, the position of the determined location within the detection head 16, and the angular position of the detector head define the nominal ray along which each radiation event occurred. The examples of the geometries of the collimator 60 are a pinhole collimator with a pinhole opening and a slit collimator with an opening extending in the axial direction Z.

With continuing reference to FIGURE 2, the detector elements 50 are positioned along a piecewise continuous arc 68 which is equidistant from the aperture 62. For example, for a pinhole aperture, the detector elements 50 are positioned on a spherical segment whose center coincides with a center point of the collimator opening 62. As another example, for a slit aperture, the detector elements 50 are positioned on a circular cylindrical segment whose central axis extends along the slit opening 62 of the slit collimator. E.g., the arc 68 is focused on the collimator opening 62. Optionally, vertical collimator vanes are disposed along and perpendicular to the slit such that each circular arc segment of detectors receives radiation from a single plane. The detectors is each displaced from the aperture 62 by a common arc radius R. Each detector element 50 of an illustrated pixilated embodiment is shaped as a rectangular prism which is oriented so that a first or longitudinal axis 72 of each detector element 50 is substantially aligned with a ray 74 which passes through the collimator opening 62 and strikes this detector. For example, the face 56 of the detector element 50, which is perpendicular to an associated ray 74, is a square with a width w equal to about 2.5mm, while a depth d of the detector element 50 is equal to from about 3mm to about 10mm. In this embodiment, each detector element 50 comprises a pixel which is linked to a corresponding individual light sensitive element 54 of the detector second array 52. Each detector element 50 together with a corresponding light sensitive element 54 comprises a detector or detection module 78. In one embodiment, the detection module 78 includes a semi-conductor detector element for conversion of radiation into electrical signals. A second axis 76 of each detector element 50 is substantially parallel to the arc 68 so that the rays 74 transmitted through the collimator opening 62 strike a corresponding detector element at an angle α equal to about 90° with respect to the second axis 76. That is, the face 56 of each detector is tangent to the arc 68. Such geometries eliminate the parallax errors.

With reference to FIGURE 3, each detector element 50 is larger than the detector element of the embodiment of FIGURE 2. For example, the width w of the detector element face 56 can be equal from about lmm to about 20mm. In one preferred embodiment, the width w of the detector clement face 56 is equal from about 2.5mm to about 10mm. In another preferred embodiment, the width w of the detector face is equal from about 2.5mm to about 5mm. Such larger detector element comprises pixels 80 which each is linked to a corresponding individual light sensitive element or array of light sensitive elements by direct optical coupling or by use of a light guide.

Of course, it is contemplated that the detector clement 50 can be split into larger number of pixels, e.g. smaller pixels, in the manner described above, to increase spatial resolution. This can be done forming a two-dimensional matrix of pixels. In the example of FIGURE 3, the detector element 50 comprises 3 by 3 matrix of pixels 80 which each is linked to a corresponding individual light sensitive element 82, 84, 86. Each detector element 50 together with associated light sensitive elements 82, 84, 86 comprises the detector module 78. The manufacture of the detector assembly 44 with the larger detector modules 78 is substantially simplified as the detector assembly 44 requires fewer detector modules. In one embodiment, the first array 46 of the detector assembly 44 includes a continuous detector 50 such as an arcuate sodium iodide plate. The detector 50 includes an array of photomultiplier tubes or other light sensitive elements 54 optically coupled to the scintillator. Signals from the photomultiplier tubes or other light sensitive elements are converted to indications of detection location or radiation trajectory using Anger-logic or the like.

Of course, other geometries of the detection system 44 and/or detection head 16 arc also contemplated.

The above has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the application be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

1. An imaging system (10) comprising: at least one radiation detection head (16) disposed adjacent a field of view (20) to detect radiation from the field of view (20), the radiation detection head (16) including: a collimator (60), including an opening (62), through which radiation rays emanating from the field of view (20) pass, and a detection system (44) forming an arcuate surface (68) focused on the collimator opening (62) for detecting radiation so that radiation rays received through the collimator opening (62) strike the arcuate surface (68) substantially perpendicular to an incremental area of the arcuate surface (68) at which the radiation strikes.

2. The system as set forth in claim 1, wherein the detection system (44) includes a plurality of a detection modules (78) which each has a face (56) tangential to the arcuate surface and a major axis (72) perpendicular to the detection module face (56).

3. The system as set forth in claim 1, wherein the arcuate surface (68) is disposed at a constant radius (R) from the collimator opening (62).

4. The system as set forth in claim 1, wherein the collimator opening (62) is a slit and the arcuate surface (68) is a circular cylindrical segment with a radius (R) around a central axis which extends along the slit opening (62),

5. The system as set forth in claim 1, wherein the collimator opening (62) is a pinhole and the arcuate surface (68) is a spherical arc segment whose center coincides with the collimator pinhole opening (62).

6. The system as set forth in claim 1, wherein the detection system (44) includes a multiplicity of detection modules (78) which convert received radiation into electrical signals.

7. The system as set forth in claim 6, wherein each detection module (78) includes: a scintillator element (50) which receives the radiation from the field of view (20) through the collimator opening (62) and converts the radiation to light; and a light sensitive element (54) which converts the light from the radiation events into electrical signals, each scintillator element (50) being linked to the corresponding light sensitive element (54).

8. The system as set forth in claim 6, wherein each detection module (78) includes: a scintillator clement (50) which converts the radiation received from the field of view (20) through the collimator opening (62) into light; and light sensitive elements (54) which convert light from the radiation events into electrical signals, each scintillator element (50) being optically linked to more than one light sensitive element (82, 84, 86),

9. The system as set forth in claim 6, wherein each detection module (78) includes: a semiconductor detector element which converts the radiation received from the field of view (20) through the collimator opening (62) into electrical signals.

10. The system as set forth in claim 1, wherein the system (44) includes an array of pixellated detection modules (78) configured to at least piccewisc continuously define the arcuate surface.

1 1. The system as set forth in claim 1, wherein the detection system (44) includes: a continuous or piecewise continuous scintillator extending along the arcuate surface (68); and a plurality of light sensitive elements (54) optically coupled to the scintillator.

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12. The system as set forth in claim 1, wherein the arcuate surface (68) is one of a spherical surface segment and a circularly cylindrical surface segment with a constant radius (R) that is defined by faces (56) of a detection array (46), the constant radius (R) being defined between the collimator opening (62) and the surface segment, the constant radius (R) being shorter than a distance from a center of the field of view (20) to the detector head (16).

13. The system as set forth in claim 1, wherein the arcuate surface (68) is one of a spherical surface segment and a circularly cylindrical surface segment with a constant radius (R) that is defined by faces (56) of a detection array (46), the constant radius (R) being defined between the collimator opening (62) and the surface segment, the constant radius (R) being equal to or larger than a distance from a center of the field of view (20) to the detector head (16).

14. An imaging method comprising: detecting radiation, that has passed through an opening (62) from a subject disposed in a field of view (20) with a detection system (44) forming an arcuate surface (68) focused on an opening (62), radiation quanta propagating in the detection system in a direction substantially perpendicular to the arcuate surface; and converting the detected radiation into electrical signals with an array of light sensitive elements.

15. The method as set forth in claim 14, wherein the arcuate surface (68) lies along a radius (R), which radius is substantially equal to a distance between a face (56) of the detection system and the opening.

16. The method as set forth in claim 14, wherein the opening (62) is a slit and the arcuate surface (68) lies along a circularly cylindrical section which has a central axis which substantially coincides with a longitudinal axis of the slit opening.

17. The method as set forth in claim 14, wherein the opening (62) is a pinhole and the arcuate surface is a spherical surface whose center coincides with the pinhole opening.

18. The method as set forth in claim 14, wherein the detection system (44) includes one of: a multiplicity of semiconductor detector elements (78), a multiplicity of scintillator elements (50), and a continuous detector plate (50).

19. The method as set forth in claim 14, wherein the detection system (44) includes a plurality of detection modules (78) each having a face (56) at the arcuate surface and a major axis (72) aligned with the opening (62) and, further including: passing each received ray of radiation through the opening, through the face of one of the detection modules, and substantially parallel to the major axis of the one detection module.

20. A diagnostic imaging apparatus including means for performing the steps of claim 14.

21. A diagnostic imaging apparatus comprising: a gantry assembly (12, 14) which supports at least one detector head (16) which includes: a collimator plate (60) which defines an aperture (62); and detection modules (7S), each module including a face (56) which receives radiation that has passed through the opening (62) and a major axis (72) aligned with the opening (62); and a reconstruction processor (36) which reconstructs signals from the detection modules (78) into an image.

22. The apparatus as set forth in claim 21 , wherein the detection module faces (56) define a continuous or piecewise continuous spherical surface segment with a geometric center at the opening (62) or a circularly cylindrical surface segment with a central axis along the opening (62).

23. A detector (50) including: at least one scintillator (50) with a face (56) which receives radiation that has passed through an opening (62) along a multiplicity of trajectories, the received radiation traveling along an associated radiation trajectory through the scintillator (50) until the radiation interacts in a quantum mechanical interaction to generate a corresponding scintillation event, the quantum mechanical interactions occurring at non- constant distances along respective trajectories, whereby each scintillation event tends to be offset from a point of incidence of the respective trajectory on the scintillator face (56) in accordance with an angle of the respective trajectory and the distance along the respective trajectory that the scintillation event occurs; and opto-electric transducers (54) which convert the scintillation events into electrical signals indicative of the point of incidence on the scintillator face (56) of the respective trajectory, the at least one scintillator (50) and the opto-electric transducers (54) being geometrically configured so that the scintillation events are not indicated by the electrical signals as being offset from their respective points of incidence on the scintillator face (56).

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