JPH1172564A - Gamma camera system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure a sufficiently wide effective visual field even in PET shoot ing by preventing the visual field angle for simultaneously measuring a pair of gamma rays from changing much between a position near a rotary center and a position away from it. SOLUTION: Energy signals from a pair of detectors that are subjects each other are subjected to a waveheight analysis for screening only gamma rays with 511 kev caused by positron nuclide and for measuring whether or not the time deviation between the detectors of the timing signal of gamma rays is, for example, within a time window of 10-20 nsec, and then for outputting a signal for indicating that a coincidence occurred when the time deviation is within the time window to a three-dimensional direct nucleus reconfiguration unit 44 exclusively for PET nuclide. The unit 44 counts the coincidence for each combination of incidence positions (XY signals) according to detection signals from waveheight analysis/detection circuits 41-43. The effective visual field can be expanded if a visual field angle that is near or is equivalent to the visual field angle near a rotary center can be secured at a position away from a point near a rotary center.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gamma camera system for detecting gamma rays emitted from a radioisotope (RI) administered to a subject and imaging an RI concentration distribution in the body.

[0002]

2. Description of the Related Art In recent years, there have been many gamma camera systems capable of capturing not only a planar image obtained by projecting an RI density distribution in one direction but also an RI density distribution in a tomographic plane such as an X-ray computed tomography apparatus. It is commercially available. The technique of imaging the concentration distribution in the tomographic plane of RI is based on the SPECT (single photon emission comput
ed tomography) and PET (positron emission computed)
tomography).

[0003] SPECT is a technique in which a single photon nuclide is administered to a subject, gamma rays emitted from the decay are then measured from outside the body, and the concentration distribution of the nuclide in the tomographic plane is reconstructed. In addition, PET is configured to administer a positron nuclide to a subject, simultaneously count two photons generated in opposite directions when the positron emitted from the positron is combined with a nearby negative electron and annihilated, and the nuclide is counted. Is a technique of reconstructing the concentration distribution in the tomographic plane.

[0004] Recently, those which can share these two types of photographing techniques have appeared. The dual-purpose machine is based on a two-detector type SPECT machine. However, this dual-purpose machine has a problem that the effective visual field of PET is significantly narrower than the effective visual field of SPECT.

The reason for this is that, as shown in FIG. 6, a pair of photons can be picked up by two opposing detectors (viewing angles F,
G) becomes significantly narrower at a position (Q) deviating from the center of rotation than at a position (P) near the center of rotation, resulting in uneven sensitivity. In order to alleviate this problem, counting is performed only in a range near the rotation center where a certain or more viewing angle can be secured. For this reason, PET
However, there arises a problem that the effective field of view becomes extremely narrow.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide PE
An object of the present invention is to provide a gamma camera system capable of securing a sufficiently wide effective field of view even during T imaging.

[0007]

SUMMARY OF THE INVENTION A gamma camera system according to the present invention comprises a first gamma camera system dispersedly disposed near a subject.
To a third detector and a pair of gamma rays generated when the positron emitted from the positron nuclide administered to the subject decays, the first detector and the second detector are applied for a predetermined time. Means for detecting a first event of being incident with a time difference within a window, and said pair of gamma rays being incident on the first and third detectors with a time difference within the predetermined time window. Means for detecting a second event;
Means for detecting a third event in which the pair of gamma rays is incident on the second detector and the third detector with a time difference within the predetermined time window; and the first to third gamma rays. Means for counting events for each combination of incident positions and reconstructing a concentration distribution in the tomographic plane for the positron nuclide based on the counting result.

According to the present invention, since the viewing angle at which a pair of gamma rays can be measured simultaneously does not change so much between a position near the center of rotation and a position deviating therefrom, a sufficiently wide effective field of view can be ensured even during PET imaging. Become.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 shows a configuration of a gamma camera system according to the present embodiment. This gamma camera system is equipped with three detectors 1,2,3. As shown in FIG. 2, these three detectors 1, 2, and 3 are supported by a support mechanism so that they are 12 around each other around the subject.
It can be set to a triangle state deviated by 0 °, and it is possible to continuously orbit around the subject intermittently or at a constant speed while maintaining this state.

As shown in the perspective view of FIG. 3 and the cross-sectional view of FIG. 4, these detectors 1, 2, and 3 have CZTs for converting incident gamma rays into electric signals having wave heights corresponding to the energies.
(Cadmium Zinc Telluride) or CdTe (Cadmiu
m Telluride) semiconductor detection elements 11, 21, 31 are 2
An element array 6 arranged in a dimension is provided. The semiconductor detection elements 12 and 22 are, for example, 3 mm × 3 mm
CZT having an area of 7 mm and a thickness of 7 mm are densely arranged, and assuming a large visual field detector having an effective visual field of 50 cm × 40 cm, about 20,000 elements (500 × 400
/ 9 = 22222).

On the back side of the element array 6, a plurality of preamplifiers 12, 22, 32 and readout circuits 14, 24, 34 provided one for each of the plurality of semiconductor detection elements 11, 21, 31 are provided. Further, the circuit board 5 on which the coincidence detection / removal circuits 13, 23, 33 are formed is overlaid. These element array 6 and circuit board 5
It is housed in a shield case 7 to prevent erroneous detection of disturbance lines from the back and side surfaces.

Each time a gamma ray is incident on such a detector 1, 2, 3, a signal (Z signal) corresponding to the energy of the gamma ray and a signal indicating the incident position of the gamma ray on the array are provided. (XY signal) and a timing signal indicating the timing of gamma ray incidence are output to the PET imaging unit 4.

As described above, when a positron emitted from a positron nuclide administered to a subject is annihilated by binding to a nearby negative electron, a pair of gamma rays are simultaneously generated in opposite directions. A part of such a pair of gamma rays enters the two detectors (1 and 2, 2 and 3, 1 and 3) with a very short time difference. In order to pick up such an event that two gamma rays are incident on the two detectors with a short time difference (hereinafter referred to as coincidence), the PET imaging unit 4 includes three systems of wave height analysis / coincidence detection circuits 41. , 42, 43 are provided.

A wave height analysis / coincidence detection circuit 41 detects the above-mentioned coincidence between the first detector 1 and the second detector 2, and a wave height analysis / coincidence detection circuit 42 detects the coincidence between the second detector 2 and the third detector 2. A coincidence detection circuit 43 is provided for detecting the coincidence with the detector 3 and detecting the coincidence between the first detector 3 and the third detector 2. .

Specifically, first, energy signals from a pair of detectors of interest are subjected to wave height analysis to select only gamma rays of 511 keV caused by the positron nuclide, and detection of a timing signal of the selected gamma rays For example, the time difference between the instruments is 10 ns.
It is measured whether or not the time is within a time window of c to 20 nsec. If the time is within the time window, a coincidence detection signal indicating that the coincidence has occurred is output to the PET nuclide dedicated three-dimensional direct reconstruction unit 44.

Such coincidence between the detectors cannot be sorted out when two gamma rays are incident on the detectors 1, 2, 3 with a time difference within the above-mentioned time window. In order to remove, the coincidence detection / removal circuits 13, 23, 33
It is provided in each of 2 and 3.

The PET nuclide-dedicated three-dimensional direct reconstruction unit 44 comprises a peak height analysis / coincidence detection circuit 41,4.
The coincidence is counted for each combination of the incident position (XY signal) in accordance with the coincidence detection signal from 2, 43. Then, such counting is repeated while changing the direction with respect to the subject, and the concentration distribution (PET image) of the positron nuclide in the tomographic plane is reconstructed based on the counting data (projection data set) obtained thereby. Note that the window analysis unit 45 is provided in order to perform more detailed sorting using the same energy as that used in the wave height analysis first performed by the wave height analysis / coincidence detection circuits 41, 42, and 43 to improve the sorting accuracy. .

As described above, each of the semiconductor detecting elements 11 and
1, 31 respectively have preamplifiers 12, 22,.
32 are connected. The functions of the preamplifiers 12, 22, 32 include: an integration function for integrating charges generated due to gamma rays in the semiconductor detection elements 11, 21, 31; a shaping function for shaping the output (usually 2 μsec); A peak gate function for detecting the peak of the shaped pulse. This peak signal corresponds to the Z signal proportional to the energy. In a normal Anger-type scintillation camera, only one or two or three preamplifiers are provided for one detector.
00Kcps, which is a rate sufficient for performing simultaneous measurement processing for PET imaging (generally, when no collimator is attached, the true coincidence buried in the random coincidence is about 1%, for example, In order to simultaneously measure about 25 Kcps of positron nuclides, a maximum counting capacity of about 2500 Kcps was required), but in this embodiment employing an array of semiconductor detection elements, one semiconductor detection element was used. Even if the counting capability of one channel having a minimum unit is low, approximately 20,000 channels can be processed in parallel, and it is relatively easy to realize the count rate required for PET imaging.

Next, the effect of enlarging the effective visual field during PET imaging according to the present embodiment will be described. As described in the description of the related art, the field narrowing is caused by a large difference in the viewing angle between the vicinity of the rotation center of the detector and a position deviated from the center, thereby causing sensitivity unevenness. Therefore, if a viewing angle close to or equal to the viewing angle near the rotation center can be secured at a position deviated from the vicinity of the rotation center, the effective field of view can be expanded as compared with the related art. The viewing angle is the maximum angle range in which a pair of gamma rays generated simultaneously in the opposite directions can be detected by two detectors.

FIG. 4 shows the viewing angle near the rotation center, and FIG. 5 shows the viewing angle at a position deviated from the vicinity of the rotation center. First, near the center of rotation, the viewing angle of the pair of detectors 1 and 2 is 2C, the viewing angle of the pair of detectors 2 and 3 is 2A, and the viewing angle of the pair of detectors 1 and 3 is 2C.
B, and the total viewing angle is 2A + 2B + 2C
Becomes

On the other hand, at a position far from the center of rotation (here, it is assumed that the viewing angle will be narrowest so that coincidence cannot be detected with the pair of detectors 1 and 2). The viewing angle by the pair of and 3 is 2
E, the viewing angle of the pair of detectors 1 and 3 is 2D, and the total viewing angle is 2D + 2E.

Here, a comparison will be made with the viewing angle at the time of PET photographing using the conventional two-detector type with reference to FIG. In the case of the present embodiment, the central viewing angle (2A + 2B + 2)
It can be understood that the difference between C) and the peripheral viewing angle (2D + 2E) is significantly smaller than the conventional difference between the central viewing angle (2F) and the peripheral viewing angle (2G), and is greatly improved. There will be. Therefore, the effective viewing angle can be greatly increased as compared with the related art.

For example, according to the verification by the inventor, the turning radius is 2
In the case of 2 cm, the effective field of view is 1.7 times the conventional one, and when the turning radius is a general 27 cm, the effective field of view is 1.4 in the conventional case.
The effective field of view could be enlarged by a factor of about 1.5 on average.

As described above, according to the present embodiment, the viewing angle at which a pair of gamma rays can be measured simultaneously does not change so much between a position near the center of rotation and a position outside the rotation center.
A sufficiently wide effective field of view can be ensured even when PET imaging is performed by a combined SPECT and PET machine. The present invention is not limited to the embodiments described above, and can be implemented with various modifications.

[0025]

According to the present invention, since the viewing angle at which a pair of gamma rays can be measured simultaneously does not change so much between a position near the center of rotation and a position deviating therefrom, a sufficiently wide effective field of view can be secured even during PET imaging. Become like

[Brief description of the drawings]

FIG. 1 is a block diagram of a gamma camera system according to an embodiment of the present invention.

FIG. 2 is a diagram showing an arrangement relationship between three detectors in FIG. 1;

FIG. 3 is a perspective view of the detector of FIG. 1;

FIG. 4 is a diagram showing a viewing angle when a photon generation point is near a rotation center.

FIG. 5 is a diagram illustrating an effective angle when a photon generation point is at a position deviated from a rotation center.

FIG. 6 is a diagram showing a state in which a viewing angle at the time of PET imaging by a conventional combined SPECT / PET machine changes depending on the position of a photon generation point.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... 1st detector, 2 ... 2nd detector, 3 ... 3rd detector, 4 ... PET imaging unit, 5 ... Circuit board, 6 ... Element array, 7 ... Shield case, 11 ... Semiconductor detection element, 12 ... Preamplifier, 13 ... Coincidence detection / removal circuit, 14 ... Readout circuit, 21 ... Semiconductor detection element, 22 ... Preamplifier, 23: coincidence detection / removal circuit, 24: readout circuit, 31: semiconductor detection element, 32: preamplifier, 33: coincidence detection / removal circuit, 34: readout Circuit 41: Wave height analysis / coincidence detection circuit 42: Wave height analysis / coincidence detection circuit 43: Wave height analysis / coincidence detection circuit 44: PET imaging specialty 3D direct reconstruction unit, 45 ... window analysis unit.

Claims (5)

[Claims] 1. A first device dispersedly disposed near a subject.
To a third detector, and a pair of gamma rays generated when the positron emitted from the positron nuclide administered to the subject decays, the first detector and the second detector are provided for a predetermined time. Means for detecting a first event of being incident at a time difference within a window, wherein said pair of gamma rays are incident on said first and third detectors at a time difference within said predetermined time window. Means for detecting a second event; and detecting a third event in which the pair of gamma rays enters the second detector and the third detector with a time difference within the predetermined time window. Means for counting the first to third events for each combination of incident positions, and reconstructing a concentration distribution in a tomographic plane for the positron nuclide based on the counting result. Gun characterized by Camera system. 2. The gamma camera according to claim 1, wherein each of the first to third detectors includes a plurality of semiconductor elements that convert gamma rays into electric signals in accordance with the energy of the gamma rays. system. 3. The semiconductor device according to claim 1, wherein the semiconductor element is CdZnTe or CdZn.
The gamma camera system according to claim 2, wherein the gamma camera system is Te. 4. When each of the first to third detectors receives a gamma ray within a time difference within the time window in each of the first to third detectors, the first to third detectors have means for excluding the event from counting targets. The gamma camera system according to claim 1, wherein 5. A method in which three detectors having a structure in which semiconductor elements are arranged two-dimensionally are arranged in a triangular shape in contact with each other to enable fan beam SPECT imaging. Gamma camera system.