JPH08211154A - Positron tomographic apparatus - Google Patents

Abstract

PURPOSE: To eliminate the lack of uniformity of the spatial sensitivity of an apparatus by a method wherein the arrangement position of a part of detectors arranged on the circumference of a circle is dislocated to the radial direction and the spatial concentration of simultaneously counted beams near the center of a detector ring is avoided. CONSTITUTION: In order to avoid the spatial concentration near the center of a stacked detector ring of simultaneously counted beams formed by connecting, by means of a simultaneous counting method, respective detectors for the detection ring constituting a detection system for a positron tomographic apparatus, the arrangement direction of a part of the detectors arranged on the circumference of a circle is dislocated to the radial direction. For example, even-numbered detectors out of 50 detectors are arranged so as to be dislocated toward the radial direction by 1/2 of a ring interval. In addition, when the number of detectors is odd, e.g. 51 detectors are formed as sets of three pieces each, and two detectors out of every set are arranged so as to be dislocated toward the center in the radial direction by 1/2 and 1/4 of a ring interval. Thereby, sampling operations which are concentrated in a specific position on the central axis can be dispersed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positron tomography apparatus useful as a radiation-type computed tomography apparatus in the field of nuclear medicine diagnosis, which uses radioisotopes for image diagnosis.

[0002]

2. Description of the Related Art A positron tomography apparatus has a large number of small scintillation detectors arranged around a human body and measures the distribution and accumulated concentration of positron-emitting nuclides in the human body through a section passing through the human body. It is known that some isotopes emit positrons and decay, and these positrons lose energy by collision while traveling a distance of several mm or less in the human body, and combine with anions. Disappear. At that time, a pair of 511 keV γ rays corresponding to the rest masses of two electrons are 180
Emit in the opposite direction of °. If this is detected by the coincidence counting method at which position of the detectors placed around the human body, the position where the positron disappears is found to be on the straight line connecting the pair of detectors. . This straight line is called a coincidence counting line or sampling. The coincidence counting method here is a method of measuring and recording an event as a detection event only when γ rays are simultaneously detected by two opposing detectors.

As described above, in the positron tomography apparatus, detectors are arranged in a ring shape, the detectors in each ring and the detectors in adjacent detector rings are simultaneously counted, and projection data thereof is obtained. To reconstruct a tomographic image in the plane of the ring and in the plane between the adjacent rings. Then, a plurality of tomographic images are created by stacking several detector rings. Since the resolution of the tomographic image obtained by the positron tomography device by stacking such detector rings is determined by the size of the incident surface of the crystal of the detector, a small size detector is used to make a high resolution device. There is a need to. However, if the detector is made smaller, there is a problem that the detection sensitivity is lowered and at the same time, the value in each picture element of the image data is reduced, which is statistically insufficient. In order to overcome this problem of low sensitivity, recently, it has been considered to extend the range of the counter detectors for simultaneously counting the respective detectors not only to the adjacent detector rings but to all the detector rings.

[0004]

The positron tomography apparatus currently on the market is of a type in which detector rings are laminated. However, sensitivity is significantly reduced when trying to achieve high resolution with small detectors in this type of device. To overcome this problem, the coincidence of the detector rings can also be done with all the detector rings in the stack to increase the sensitivity several fold or more. This will be called a three-dimensional positron tomography apparatus. However, in this device, the sampling represented by the straight line connecting the detectors has a problem that the sampling is concentrated on the central axis of the detector system at an interval of exactly half the interval of the detector rings. Also, looking at the sampling in the plane of the detector ring with a stationary detector system, a space where no simultaneous counting line is passed due to simultaneous counting of detectors on detector rings other than the detector ring of interest Is systematically present. Therefore, the present invention solves these problems,
It is an object of the present invention to provide an improved positron tomography apparatus capable of eliminating non-uniformity in spatial sensitivity of the apparatus.

[0005]

In order to solve the above-mentioned problems, the present invention connects the detectors of the stacked detector rings constituting the detector system of the positron tomography apparatus by the coincidence counting method. It is characterized in that the spatial concentration of the coincidence lines formed by the above in the vicinity of the center of the detector ring is avoided by displacing part of the detectors arranged on the circumference in the radial direction. This disperses the sampling concentrated at a specific position on the central axis. At that time, the size to shift the position of the detector in the radial direction is related to the size of the detector and the interval between the detector rings, and 1/2 of the ring interval is a rough guideline. It may take different values for an even number, an odd number, or a mosaic detector in which a plurality of detectors are combined.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus of the present invention will be described below in more detail with reference to the drawings. 1, 2 and 3 are plan views showing an embodiment of the detector arrangement of the detector ring of the positron tomography apparatus according to the present invention. The detector array shown here is one in which detectors for detecting γ-rays are arrayed in a circle at equal intervals. In this arrangement, the number of detectors is reduced to facilitate understanding, but in an actual device, about 250 detectors are used per one detector ring.

In FIG. 1, even-numbered detectors out of 50 detectors are arranged so as to be displaced by 1/2 of the ring interval in the radial direction toward the center. In FIG. 2, 51 detectors are 3
Each set has two detectors with ring spacing
It is arranged to be displaced by 1/2 and 1/4 in the radial direction toward the center. Figure 3 is a mosaic-shaped 1 set of 4
Five sets of detectors are arranged on the circumference, and each set of detectors is centered so that three detector crystals are displaced by 3/6, 2/6, 1/6 of the ring interval in each set. They are arranged so that they are offset from each other.

4 and 5 are sectional views of the detector ring taken along a plane passing through the central axis of the multilayer detector ring. For ease of understanding, the detector ring is shown as a total of 7 layers. In actual devices, fewer or more detector rings are used. The manner in which the detectors on each layer of the detector ring form coincidence counts with other opposing detectors is shown by the straight line connecting the detectors. This is called a coincidence counting line or a sampling line.

FIG. 4 is the distribution of samplings that are generally made when the detector ring is stationary. The coincidence lines are shown as solid lines. Sampling is concentrated on the central axis of the detector ring at points at half the ring spacing. This means that the annihilation radiation generated in the absence of the sampling line between points where sampling is concentrated cannot be detected.

FIG. 5 is due to the detector array of the present invention. Within each detector ring, the position of a particular detector is offset by half the detector ring spacing toward the center. The staggered detectors and the coincidence lines they create are shown as dashed lines. It can be understood that the coincidence sampling in the detector ring is almost the same, but the sampling between the detector rings is scattered in the central point and the interval between the adjacent sampling lines is small.

FIG. 6 shows the plane formed by the central detector ring (the 11th ring) in the case of a cylindrical detector ring having 50 detectors per ring and consisting of 21 layers of detector rings. Is a frequency distribution along the straight line passing through the central axis of the detector ring. As is clear from the figure, it is understood that there is an extreme concentration of sampling in the center and a non-sampling region around it.

FIG. 7 shows a similar sampling frequency distribution obtained in the detector system when even-numbered detectors are shifted toward the center by 3/4 of the distance between the detector rings with respect to the same detector ring. is there. It can be seen that the extreme concentration of sampling and the non-sampling area have been eliminated. The staggered detectors in the plane of each detector ring have little effect on the sampling within that ring and provide for finer sampling along the central axis of the detector ring stack.

[0013]

As described above in detail, the positron tomography apparatus according to the present invention has a detector system in which a large number of detectors are arranged in a ring shape, and these rings are stacked, and each detector has a detector system. Simultaneous counting of outputs is performed, and the simultaneously counted detector addresses are converted into projection data and stored,
In a positron tomography apparatus having a data processing device for reconstructing a three-dimensional distribution image of a positron using these data, the positions of specific detectors in each detector ring in the detector system are displaced in the radial direction. Place it.
For this reason, even if the detector is not scanned, it is possible to sample the region where the sampling of the conventional three-dimensional positron tomography apparatus is missing, and the spatial unevenness of the detection sensitivity is greatly improved compared to the conventional apparatus. can do. Further, since it is not necessary to scan the detector system, the mechanical structure is simplified, the generation of mechanical noise due to the detector scanning is eliminated, and there is no restriction on the data acquisition for obtaining the temporal change of the distribution image. .

[Brief description of drawings]

FIG. 1 shows a detector ring in which even-numbered detectors are arranged in the detector ring according to the present invention, and the even-numbered detectors are shifted toward the center of a circle by 1/2 of the distance between the detector rings. FIG.

[Fig. 2] Detection in which three detector rings are set as one set according to the present invention, and two detectors in each set are shifted in the center direction by 1/2 and 1/4 of the interval of the detector rings. It is a layout of a container ring.

FIG. 3 shows three detectors in each set of mosaic detectors each having four detector rings according to the present invention. It is a layout diagram of the detector ring in which the orientation of each set of detectors is displaced from the center so as to be shifted by 1/6.

FIG. 4 is a cross-sectional view showing the coincidence lines of stacked cylindrical detector rings, taken along a cross section passing through the central axis of the detector rings.

FIG. 5: Centers on coincidence lines of the detectors in which even-numbered detectors in the laminated cylindrical detector rings according to the present invention are shifted by 1/2 of the ring interval in the direction of the central axis of the detector rings. It is sectional drawing cut and shown by the cross section which passes along an axis. The detectors displaced in the direction of the center and the coincidence lines caused thereby are indicated by broken lines.

FIG. 6 is a frequency distribution chart of sampling when there is no staggered detector in the cylindrical detector ring.

FIG. 7 is a sampling frequency distribution diagram on the plane of the detector ring located at the center when the even-numbered detectors in each ring of the cylindrical detector ring are shifted in the central direction.

Claims (4)

[Claims] 1. A plurality of detectors are arranged in a circular ring shape,
Further, it has a cylindrical, spherical, and / or intermediate shape detector system constructed by stacking several layers of such rings, and simultaneously counting the outputs of the detectors,
In a positron tomography apparatus having a data processing device that converts the detector addresses that have been simultaneously counted into projection data and accumulates the data, and reconstructs a distribution image using this data,
The arrangement of a part of the detectors is such that samplings concentrated at specific positions on the central axis are dispersed by changing the distance from the central axis by a certain ratio of the detector ring spacing. And positron tomography equipment. 2. The positron tomography apparatus according to claim 1, wherein the data processing apparatus includes a preprocessing apparatus for interpolating and rearranging data. 3. Arranging a large number of detectors in a circular ring shape,
Further, it has a cylindrical, spherical, and / or intermediate shape detector system constructed by stacking several layers of such rings, and simultaneously counting the outputs of the detectors,
In a positron tomography apparatus having a data processing device that converts the detector addresses that have been simultaneously counted into projection data and accumulates the data, and reconstructs a distribution image using this data,
When each detector is composed of mosaic crystals arranged in one or two dimensions, one end of these detectors is changed by changing the distance from the center axis by a certain percentage of the crystal spacing in the center axis direction. A positron tomography apparatus characterized in that sampling concentrated at a specific position on the axis is dispersed. 4. The positron tomography apparatus according to claim 3, wherein the data processing apparatus includes a preprocessing apparatus for interpolating and rearranging data.