JP2011232044A - Radiation tomographic photographing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation tomographic photographing apparatus with which the correction value of a radiation detection time can be obtained per scintillator crystal unit.SOLUTION: The correction value according to the invention is obtained by calculating the correction value per fragment unit in a scintillator 2. In so doing, a more accurate correction value can be determined compared with the correction value obtained per detection device unit. When the fragment is made adequately greater, the number of annihilation radiation pairs incident into the fragment per unit time becomes greater so that the detection of the annihilation radiation pairs necessary for obtaining the correction value can be ended in short time.

Description

  The present invention relates to a radiation tomography apparatus that detects annihilation radiation emitted from a subject and images a radiopharmaceutical distribution in the subject, and in particular, uses a time difference in detection of annihilation radiation pairs to eliminate annihilation radiation pairs. The present invention relates to a radiation tomography apparatus capable of specifying the occurrence position.

  In medical institutions, radiation tomography apparatuses that image the distribution of radiopharmaceuticals are deployed. A specific configuration of such a radiation tomography apparatus will be described. A conventional radiation tomography apparatus includes a detector ring in which radiation detectors that detect radiation are arranged in an annular shape. The detector ring detects a pair of radiations (an annihilation radiation pair) emitted in opposite directions from the radiopharmaceutical in the subject. Such a radiation tomography apparatus is called a PET (positron emission tomography) apparatus (see Patent Document 1).

  Among PET apparatuses, there is an object called a TOF (timing of flight) -PET apparatus that can specify the generation position of an annihilation radiation pair using a time difference of detection of the annihilation radiation pair. Assuming that an annihilation radiation pair is generated at a point p in FIG. 16, the radiation g jumped to the right side of the paper from the point p and the radiation h jumped to the left side of the paper are both in the radiation detector 51 constituting the detector ring. Detected. However, the radiation g has a shorter range from the point p to the radiation detector 51 than the radiation h, and reaches the radiation detector 51 at an earlier timing.

  This means that the point p, which is the generation position of the annihilation radiation pair, is on the line segment connecting the two radiation detectors 51 that detect the radiation g and the radiation h, and the radiation g detected at an earlier timing. It is on the detected radiation detector 51 side. By utilizing this, for example, when the annihilation radiation pair is detected by the two radiation detectors 51 at the same time, it can be seen that the generation position of the annihilation radiation pair is located at the midpoint of the above-mentioned line segment. Further, it is possible to determine where the generation position of the annihilation radiation pair existed based on the deviation of the annihilation radiation pair detected over time.

  The TOF-PET apparatus is configured to acquire the tomographic image indicating the distribution of the radiopharmaceutical in the subject by successively identifying the generation positions of such annihilation radiation pairs and imaging them.

  The configuration of the radiation detector 51 will be described. As shown in FIG. 17, the radiation detector 51 includes a scintillator 52 configured by arranging scintillator crystals C three-dimensionally. The radiation detector 51 can acquire the exact incident position of the radiation in the radiation detector 51 by determining which of the scintillator crystals C emits radiation-derived fluorescence.

  In such a radiation detector 51, the degree of fluorescence detection differs depending on each scintillator crystal C. That is, even when radiation is incident on a scintillator crystal C and fluorescence is generated there, the degree of generation of this fluorescence is different for each scintillator crystal C. Such a difference is caused by a difference in how the scintillator crystal C emits fluorescence, a position of the scintillator crystal C in the scintillator, and a difference in characteristics when the radiation detector detects fluorescence. Therefore, even if the annihilation radiation pair is incident on the two radiation detectors 51 at the same time, the annihilation radiation pair may be detected with a time difference depending on the two radiation detectors.

  In the conventional radiographic apparatus, the detection time information output from the radiation detector 51 is corrected in order to eliminate the influence of the apparent time difference depending on the apparatus. The correction value used at this time is obtained in advance using a phantom before imaging of the subject.

  The correction value acquisition method is as follows. First, a phantom containing a radiopharmaceutical is inserted into the detector ring. And the annihilation radiation pair which generate | occur | produced from the radiopharmaceutical is detected with a detection ring. If there is a time difference when two radiation detectors detect annihilation radiation pairs, this time difference is an apparent time difference depending on the apparatus, and the correction value is determined so that this time difference is eliminated.

  In order to measure the apparent time difference between two radiation detectors, the two radiation detectors can repeatedly detect annihilation radiation pairs and generate a timing histogram in which the frequency of detection is related to the time difference. Therefore, it is necessary to acquire the distribution of the time difference. To generate this timing histogram, it is necessary to detect a considerable number of annihilation radiation pairs.

JP 2008-51701 A

  However, the conventional radiation tomography apparatus has the following problems. That is, the conventional radiation tomography apparatus has a problem in that correction values are inaccurate because only correction values for each radiation detector are acquired.

  That is, since the method of shifting the radiation detection time is originally different for each scintillator crystal, it is better to acquire the correction value for each scintillator crystal. However, if an attempt is made to obtain a correction value for each scintillator crystal, it takes too much time to detect an annihilation radiation pair, which is not practical. In view of this, in the conventional configuration, acquisition of correction values in units of scintillator crystals is given up, and correction values in units of radiation detectors are acquired.

  When the correction value is to be obtained in the scintillator crystal unit by the conventional method, the timing histogram must be generated for two scintillator crystals to obtain the correction value. To generate a timing histogram, a considerable number of annihilation radiation pairs must be detected. Accordingly, when trying to obtain a correction value in units of scintillator crystals between two radiation detectors composed of 10 scintillator crystals and 6 scintillator crystals, one of the radiation detectors. When one scintillator crystal is selected from the other and one scintillator crystal is selected from the other radiation detectors, there are 60 × 60 combinations. It takes 3,600 times as long as the correction value to be obtained for each radiation detector.

  Furthermore, as shown in FIG. 17, if the scintillator has four layers of scintillator crystals, the scintillator crystal on the deepest side when viewed from the incident radiation is blocked by the shallow scintillator crystal, Almost no radiation is incident. In other words, when trying to obtain a correction value for the scintillator crystal in the deepest side layer, in order to obtain a sufficient number of annihilation radiation pairs to generate a timing histogram, the scintillator crystal in the shallow side layer is obtained. Takes more time than.

  Assume that the detection number of radiation detected by the deepest scintillator crystal is 1/10 compared to the shallowest scintillator crystal when viewed from the incident radiation. As shown by the hatched lines in FIG. 18, when it is attempted to obtain correction values for the two scintillator crystal pairs existing at the deepest positions in both radiation detectors, both of the annihilation radiation pairs are paired with the hatched scintillator crystals. Therefore, it takes 100 times as long as the correction value of the shallowest scintillator crystal pair.

  Therefore, if the correction value is to be obtained for all of the scintillator crystals of 10 × 6 × 4 layers, it takes 360,000 times the time required to obtain the correction value for each radiation detector. Taking acquisition of correction values in units of actual radiation detectors in the prior art as an example, this time is about 250 days, and cannot be acquired realistically.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a radiation tomography apparatus capable of acquiring a correction value of radiation detection time in units of scintillator crystals.

The present invention has the following configuration in order to solve the above-described problems.
That is, the radiation tomography apparatus according to the present invention includes a detector ring for detecting an annihilation radiation pair configured by arranging radiation detectors having a scintillator in an annular shape, and a clock number indicating a predetermined time unit. A clock for obtaining data indicating the time based on the gantry covering the detector ring, a correction means for correcting the time information indicating the detection time of the radiation output based on the clock from the radiation detector based on the correction value, Based on the corrected time information, find the time difference at which the annihilation radiation pair was detected, obtain the generation position of the annihilation radiation pair from this time difference, and the generation position of the annihilation radiation pair specified by the position specification means An image generation means for imaging as an image, a storage means for storing a correction value, and a correction value acquisition means for acquiring a correction value are provided. Stage is characterized in that for obtaining the correction value of the compartment units in the scintillator by using the detection data of the annihilation radiation pairs detector ring outputs.

  [Operation / Effect] According to the present invention, the correction means for correcting the time information output from the radiation detector is provided. As a result, time information that is inaccurate due to variations in radiation detection of the radiation detector can be corrected correctly. The correction value used for correction by the correction means is obtained by obtaining a correction value for each partition in the scintillator. In this way, it is possible to obtain a more accurate correction value than to obtain a correction value for each detector. If the section is made sufficiently large, the number of annihilation radiation pairs incident on the section per unit time increases, so that detection of the annihilation radiation pairs necessary for obtaining the correction value is completed in a short time.

  In the radiation tomography apparatus described above, the scintillator of the radiation detector has a scintillator crystal three-dimensionally formed by laminating scintillator crystal layers configured by two-dimensionally arranging scintillator crystals in the radiation incident direction. If the scintillator is arranged, the correction value acquisition means obtains a correction value for each scintillator crystal based on the correction value for each section, and the correction means corrects the time information based on the correction value for each scintillator crystal. desirable.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. That is, the scintillator of the radiation detector is configured by scintillator crystals arranged three-dimensionally, and the correction value acquisition means obtains the individual scintillator crystal correction value using the correction value for each section. As a result, even if the scintillator crystals have different temporal radiation detection characteristics, the time information can be corrected while changing the correction value according to the individual scintillator crystals.

  Further, in the above-described radiation tomography apparatus, the correction value acquisition unit obtains a correction value in units of radiation detectors, and the correction value acquisition unit has units of columnar sections in which scintillator crystals are arranged in a line in the radiation incident direction. It is more desirable if the correction value is obtained, the correction value acquisition means obtains the correction value in units of scintillator crystal layers, and the correction value acquisition means obtains the correction value for each scintillator crystal based on the correction values obtained thereby.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. In other words, the correction value acquisition means is configured to obtain a correction value for each column-shaped section, obtain a correction value for each scintillator crystal layer, and finally obtain a correction value for each scintillator crystal. As described above, when the division unit for obtaining the correction value is determined based on the arrangement of the scintillator crystal, it is considered that the temporal radiation detection characteristic of the scintillator crystal varies depending on where the scintillator crystal occupies the scintillator. A correction value can be acquired.

  Further, once the correction value is obtained for each column-shaped section unit and scintillator crystal layer unit, this correction value hardly changes over time. Therefore, when the radiation detector changes over time and the temporal radiation detection characteristics of the radiation detector change, it is possible to cope with this aging by simply obtaining a correction value for each radiation detector. . Since the number of annihilation radiation pairs required to determine the correction value for each radiation detector is smaller than that for determining the correction value for each unit described above, the correction value for each radiation detector should be determined promptly. Can do.

  In the above-mentioned radiation tomography apparatus, it is more desirable that the correction means is provided in the gantry.

  In the above-described radiation tomography apparatus, it is more desirable that the correction means is provided as a console outside the gantry.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. The correction means of the present invention may be provided inside the gantry or may be provided outside. When the correction means is provided inside the gantry, the configuration of the apparatus becomes simple. When the correction means is provided outside the gantry, more accurate calculation can be performed to correct the time information accurately. .

  Further, in the above-described radiation tomography apparatus, correction means are provided in a distributed manner in the gantry and outside the gantry, and the correction means in the gantry shifts the time information over time by a time equal to or more than the clock time unit. It is more preferable that the correction means outside the gantry corrects the time information by correcting the time information corrected in the gantry so that the time information is further shifted over time by a time less than the clock time unit.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. That is, the correction means are distributed in the gantry and outside the gantry, the correction means in the gantry corrects simple time information, and the correction means outside the gantry corrects complex time information. . If each correction means is in charge of the correction calculation in this way, the configuration of the correction means in the gantry is simplified, the calculation load of the correction means outside the gantry is reduced, and accurate time information is obtained. A radiation tomography apparatus capable of performing correction can be provided.

1 is a functional block diagram illustrating a configuration of a radiation tomography apparatus according to Embodiment 1. FIG. It is a schematic diagram explaining the acquisition method of the generation | occurrence | production position of the annihilation gamma ray based on Example 1. FIG. 1 is a perspective view illustrating a radiation detector according to Embodiment 1. FIG. FIG. 3 is a schematic diagram illustrating a configuration of a detector ring according to the first embodiment. FIG. 3 is a schematic diagram illustrating a configuration of a detector ring according to the first embodiment. 6 is a flowchart illustrating the operation of a correction value acquisition unit according to the first embodiment. It is a perspective view explaining operation | movement of the correction value acquisition means which concerns on Example 1. FIG. FIG. 6 is a schematic diagram for explaining an operation of a correction value acquisition unit according to the first embodiment. It is a perspective view explaining operation | movement of the correction value acquisition means which concerns on Example 1. FIG. It is a perspective view explaining operation | movement of the correction value acquisition means which concerns on Example 1. FIG. FIG. 6 is a schematic diagram for explaining an operation of a correction value acquisition unit according to the first embodiment. It is a perspective view explaining the structure concerning one modification of the present invention. It is a perspective view explaining the structure concerning one modification of the present invention. It is a perspective view explaining the structure concerning one modification of the present invention. It is a schematic diagram explaining the structure which concerns on 1 modification of this invention. It is a schematic diagram explaining the structure of the radiation tomography apparatus of a conventional structure. It is a perspective view explaining the structure of the radiation tomography apparatus of a conventional structure. It is a perspective view explaining the structure of the radiation tomography apparatus of a conventional structure.

<Configuration of radiation tomography system>
Embodiments of the radiation tomography apparatus according to the present invention will be described below with reference to the drawings. The gamma rays in Example 1 are an example of the radiation of the present invention. FIG. 1 is a functional block diagram illustrating the configuration of the radiation tomography apparatus according to the first embodiment. The radiation tomography apparatus 9 according to the first embodiment includes a top plate 10 on which the subject M is placed, a gantry 11 having an opening for introducing the top plate 10 from the longitudinal direction (z direction), and a gantry 11 inside. And a ring-shaped detector ring 12 for introducing the provided top plate 10 in the z direction. The opening provided in the detector ring 12 has a cylindrical shape extending in the z direction (the longitudinal direction of the top 10 and the body axis direction of the subject M). Therefore, the detector ring 12 itself extends in the z direction.

  The top plate 10 is provided so as to penetrate the opening of the gantry 11 (detector ring 12) from the z direction, and is movable back and forth along the z direction. Such sliding of the top plate 10 is realized by the top plate moving mechanism 15. The top board movement control unit 16 is a top board movement control means for controlling the top board movement mechanism 15. The top plate 10 slides from a position where the entire region is located outside the detector ring 12 and is introduced from one side into the opening of the detector ring 12 and penetrates the inside of the detector ring 12. Thus, it can protrude from the other side of the opening of the detector ring 12.

  Inside the gantry 11 is provided a detector ring 12 for detecting an annihilation gamma ray pair emitted from the subject M. The detector ring 12 has a cylindrical shape extending in the body axis direction of the subject M, and the length in the z direction is about 26 cm. The clock 19 sends time data having a serial number to the detector ring 12. The detection data output from the detector ring 12 is given time data indicating when the γ-ray was detected, and is input to the coincidence counting unit 20 described later. The time data assigned to the detection data is referred to as time information in distinction from the time data sequentially output by the clock 19. The clock 19 recognizes the flow of time in increments of 64 ps, for example. A predetermined number of clocks is set in the clock 19, and a serial number indicating the time is output to the detector ring 12 for each time unit (64 ps) indicated by the number of clocks.

  Detection data output from the detector ring 12 is sent to the coincidence unit 20. The two gamma rays incident on the detector ring 12 almost simultaneously are annihilation gamma ray pairs caused by the radiopharmaceutical in the subject. The coincidence counting unit 20 counts the number of times the annihilation γ-ray pair is detected for every two combinations of scintillator crystals constituting the detector ring 12, and sends the result to the correction unit 21. The determination of the coincidence of the detected data by the coincidence unit 20 uses time information given to the detected data by the clock 19. The correction unit 21 corresponds to the correction unit of the present invention.

  The correction unit 21 corrects the time information given to the detection data. There is a variation over time in the time data given to the detection data depending on where the γ rays are incident on the detector ring 12. Specifically, the time data given to the detection data is deviated from the actual time by a degree determined at each location of the detector ring 12. Accordingly, the correction unit 21 refers to a table in which the position of the detector ring 12 and the correction value of the time are related, corrects the time information given to the detection data, and detects the detector ring 12 as described above. Eliminate variations in detection time at different locations. For example, when correcting the detection data when a certain γ-ray is detected, the correction unit 21 acquires a correction value corresponding to the location of the detector ring 12 that detected the γ-ray from the table. And the correction | amendment part 21 correct | amends time information by making the correction value act on the time information provided to detection data.

  The position specifying unit 22 specifies the generation position of the annihilation γ-ray pair using the detected time of the annihilation γ-ray pair. As shown in FIG. 2, it is assumed that an annihilation gamma ray pair is detected at points p and q of the detector ring 12. The annihilation γ-ray pair is emitted at any point on the line segment connecting the point p and the point q, and reaches the points p and q. The position specifying unit 22 specifies where the annihilation gamma ray pair is generated on this line segment using the corrected time information given to the detection data at the points p and q. That is, the position specifying unit 22 specifies whether the generation point r of the annihilation γ-ray pair is near the point p or the point q from the time difference of the γ-ray detection at the points p and q. If γ rays are detected at points p and q at the same time, the generation point r exists at the midpoint of the line segment, and if the γ rays at the point p are detected earlier, they are only as early as possible. The generation point r is located on the point p side. Thus, the position specifying unit 22 specifies the generation point of the annihilation γ-ray.

  The image generation unit 23 acquires a tomographic image in which the generation positions of annihilation γ-ray pairs are mapped based on the position information output from the position specifying unit 22. The correction value acquisition unit 24 is provided for the purpose of generating a table in which the correction value stored in the setting value storage unit 37 is associated with the position of the detector ring 12. The correction value acquisition unit 24 corresponds to the correction value acquisition unit of the present invention, and the set value storage unit 37 corresponds to the storage unit of the present invention.

  The configuration of the radiation detector 1 constituting the detector ring 12 will be briefly described. FIG. 3 is a perspective view illustrating the configuration of the radiation detector according to the first embodiment. As shown in FIG. 3, the radiation detector 1 includes a scintillator 2 that converts γ-rays into fluorescence, and a photodetector 3 that detects fluorescence. A light guide 4 for transmitting and receiving fluorescence is provided at a position where the scintillator 2 and the photodetector 3 are interposed. The scintillator 2, the light guide 4, and the photodetector 3 are stacked in the z direction shown in FIG. It can also be seen that the scintillator crystals C are two-dimensionally arranged vertically and horizontally in the radiation detector 1 and this array is arranged in the z direction (incident direction of γ rays). The two-dimensional scintillator crystal C array at this time is called a scintillator crystal layer. In FIG. 3, four scintillator crystal layers are laminated in the z direction to constitute the scintillator 2.

The scintillator 2 is configured by arranging scintillator crystals C three-dimensionally. The scintillator crystal C is composed of Lu _{2 (1-X)} Y _2X SiO ₅ (hereinafter referred to as LYSO ₎ in which Ce is diffused. The photodetector 3 can specify the fluorescence generation position indicating which scintillator crystal C emits fluorescence, and also specifies the intensity of fluorescence and the time when the fluorescence is generated. Can do. The scintillator 2 having the configuration of the first embodiment is merely an example of an aspect that can be adopted. Therefore, the configuration of the present invention is not limited to this. The radiation detector 1 can identify which of the scintillator crystals C it has emitted fluorescence. The table in which the correction value stored in the setting value storage unit 37 and the position of the detector ring 12 are associated has a correction value for each of the scintillator crystals C. When the correction unit 21 detects γ rays, Based on the positional information that identifies the scintillator crystal C in which the γ-rays included in the detected data are converted into fluorescence, a correction value corresponding to the scintillator crystal C is acquired from the table, and time information correction processing is performed.

  The deviation of the detection time will be described. When the actual time when fluorescence is emitted from the scintillator 2 in the radiation detector 1 and the time information given to the detection data by the radiation detector 1 are compared, there is a slight difference between the times. Further, when this difference is compared between the radiation detectors 1 constituting the detector ring 12, the magnitude of the difference varies depending on the radiation detector 1. As described above, the magnitude of temporal deviation regarding the detection of γ-rays differs among the radiation detectors 1.

  The same can be said between the scintillator crystals C provided in one radiation detector 1. That is, when the actual time when the fluorescence is emitted and the time information given to the detection data by the radiation detector 1 are compared with a single scintillator crystal C, there is a slight difference between the times, and one radiation detector 1 When this difference is compared among the scintillator crystals C provided in the scintillator crystal C, the magnitude of the difference varies depending on the scintillator crystal C. As described above, the temporal deviation regarding the detection of γ-rays differs among the scintillator crystals C.

  Thus, the factors that cause the time information to vary depending on the location of the detector ring 12 that detects γ rays are due to the difference in the radiation detector 1 that detects γ rays, and the scintillator crystal C that converts γ rays into fluorescence. There is a difference. The correction unit 21 applies the correction value to the time information, thereby erasing time fluctuation due to any factor at a time.

  The configuration of the detector ring 12 will be described. According to the first embodiment, as shown in FIG. 4, about 100 radiation detectors 1 are arranged in a virtual circle on a plane perpendicular to the z direction, so that one unit ring 12b is formed. The unit ring 12b is arranged in the z direction as shown in FIG.

  The radiation tomography apparatus 9 includes a main control unit 41 that comprehensively controls each unit. The main control unit 41 is constituted by a CPU, and realizes the units 16, 19, 20, 21, 22, and 23 by executing various programs. In addition, each above-mentioned part may be divided | segmented and implement | achieved by the control apparatus which takes charge of them. The console 35 is provided for the purpose of inputting various instructions regarding the operator's radiation tomography apparatus. The display unit 36 is provided for the purpose of displaying the tomographic image generated by the image generation unit 23. The setting storage unit 37 stores all parameters such as a table related to the position of the detector ring 12 and the correction value related to the control of the radiation tomography apparatus 9.

<Operation of correction acquisition unit>
Next, the operation of the correction value acquisition unit 24 will be described. This operation also explains how to obtain the correction value referred to by the correction unit 21. In order to obtain the correction value, as shown in FIG. 6, first, a radioactive phantom is introduced into the detector ring 12 (phantom introduction step S1), and detection of the annihilation gamma ray pair is started (detection start step). S2). The correction value acquisition unit 24 first obtains a detector unit correction value Tb1 that is a correction value for each radiation detector (detector unit correction value acquisition step S3), and based on this, the columnar scintillator crystal C section A columnar unit correction value Tp, which is a correction value in units, is obtained (columnar unit correction value acquisition step S4). Subsequently, the correction value acquisition unit 24 obtains a layer unit correction value Td that is a correction value for each scintillator crystal C layer based on these (layer unit correction value acquisition step S5), and the acquired columnar unit correction value Tp. The detector unit correction value Tb2 is obtained again based on the layer unit correction value Td (detector unit correction value reacquisition step S6). Finally, based on the detector unit correction value Tb2, the acquired columnar unit correction value Tp and the layer unit correction value Td, an individual correction value (crystal individual correction value Tc) of the scintillator crystal C is obtained (crystal individual correction value acquisition). Step S7), a table is created (table creation step S8). Hereinafter, the details of each step will be described in order.

<Phantom introduction step S1, detection start step S2>
First, a phantom that emits an annihilation gamma ray pair instead of the subject is placed on the top plate 10, and the phantom is introduced into the detector ring 12. This phantom has a cylindrical shape extending in the longitudinal direction of the top plate 10. When the surgeon instructs the start of detection of annihilation γ-ray pairs through the console 35, detection of γ-rays is started, and the coincidence counting unit 20 detects the annihilation γ-ray pairs. When the number of annihilation γ-rays is sufficient for the following steps, the process proceeds to the next step.

<Detector unit correction value acquisition step S3>
The detection data of the annihilation gamma ray pair generated by the coincidence unit 20 is output to the correction value acquisition unit 24. Since the time information given to the detection data is before correction, it is slightly different from the time when the γ-ray is actually detected. Moreover, the degree of difference varies depending on the position of the detector ring 12 that detects the γ-ray (the pair of scintillator crystals C in which the annihilation γ-ray pair is converted to fluorescence).

  The correction value acquisition unit 24 first acquires a detector unit correction value Tb1 (hereinafter referred to as a value Tb1). When obtaining a value Tb1 in a certain radiation detector, first, as shown in FIG. 7, a timing histogram H in which the frequency and time difference of annihilation γ-ray detection are related between two pairs of radiation detectors 1 is generated (FIG. 7). 8). The correction value acquisition unit 24 obtains a value such that the average value of the time differences becomes 0, as indicated by an arrow in FIG. 8, and obtains a value Tb1 for each radiation detector 1 based on this value. Specifically, the correction value acquisition unit 24 fits the timing histogram H to a smooth function, and obtains the value Tb1 based on this function.

  In addition, since the radiation detector 1 has many scintillator crystals C, it can be understood in which part of the radiation detector 1 the γ-rays are converted into fluorescence. However, when obtaining the value Tb1, it is not distinguished which scintillator crystal C the radiation detector is incident on, and the timing histogram H is simply obtained by using the detection data when the annihilation γ-ray pair is observed with the two scintillators 2. Desired. Accordingly, the number of detected annihilation γ-ray pairs necessary for obtaining the value Tb1 is quickly completed, and it does not take much time to obtain the value Tb1.

<Columnar unit correction value acquisition step S4>
Next, the correction value acquisition unit 24 acquires a columnar unit correction value Tp (hereinafter referred to as a value Tp). The value Tp is a correction value for each columnar section P in the scintillator 2. The columnar section P will be described. As shown in FIG. 9, the columnar section P refers to a region in which four scintillator crystals C are arranged in a line in the z direction (incident direction of γ rays) shown in FIG. 9 (and FIG. 2). If the scintillator crystal layer 2a is composed of 60 scintillator crystals C, there are 60 columnar sections P in one radiation detector 1.

  Before obtaining the value Tp, the correction unit 21 causes the detector unit correction value Tb1 to act on the detection data of the annihilation γ-ray pair generated by the coincidence counting unit 20. Thereby, the element resulting from a difference in the temporal characteristics of detection for each radiation detector 1 which is a part of the cause of the variation in the detection time difference is deleted from the detection data.

  When obtaining the value Tp in a certain columnar section P, first, as shown in FIG. 9, the frequency of annihilation γ-ray detection detected between a pair of columnar section P and another radiation detector 1 that does not include the columnar section P. And a timing histogram H in which the time difference is related. The correction value acquisition unit 24 calculates the value Tp so that the average value of the time differences becomes zero. Specifically, the correction value acquisition unit 24 fits the timing histogram H to a smooth function, and obtains the value Tp based on this function.

  Since the columnar section P has four scintillator crystals C, it can be seen in which part of the columnar section P the γ-rays are actually converted into fluorescence. However, when obtaining the value Tp, it is not distinguished which scintillator crystal C the radiation detector is incident on, and the detection data obtained by simply observing the annihilation γ-ray pair with the columnar section P and another scintillator 2 is used. Thus, a timing histogram H is obtained. Therefore, it takes more time to collect the number of detected annihilation γ-ray pairs necessary when obtaining the value Tp than when obtaining the value Tb1. Assuming that the scintillator 2 is composed of 60 columnar sections P, in order to detect a sufficient number of annihilation γ-rays for obtaining the values Tp for all the columnar sections, it is 60 than the value Tb1. It takes twice as long. However, this number of detections has already been acquired in the above-described detection start step S2.

<Layer Unit Correction Value Acquisition Step S5>
Next, the correction value acquisition unit 24 acquires a layer unit correction value Td (hereinafter referred to as a value Td). The value Td is a correction value for each scintillator crystal layer 2 a constituting the scintillator 2. This columnar section will be described. As shown in FIG. 10, the scintillator crystal layer 2a is an area in which a plurality of scintillator crystals C are two-dimensionally arranged on a plane orthogonal to the z direction (incident direction of γ rays) shown in FIG. 10 (and FIG. 2). Say. The scintillator 2 is provided with four scintillator crystal layers 2a.

  Before obtaining the value Td, the correction unit 21 causes the detector unit correction value Tb1 and the columnar unit correction value Tp to act on the detection data of the annihilation γ-ray pair generated by the coincidence unit 20. Thereby, the element resulting from the difference in detection time characteristics for each radiation detector 1 that is a part of the cause of the variation in the detection time difference, and the element resulting from different detection time characteristics for each columnar section Is deleted from the detection data.

  When obtaining a value Td in a scintillator crystal layer 2a, first, as shown in FIG. 10, annihilation γ rays detected between the scintillator crystal layer 2a and another pair of radiation detectors 1 not including the scintillator crystal layer 2a. A timing histogram H in which the detection frequency and the time difference are related is generated. The correction value acquisition unit 24 calculates the value Td so that the average value of the time differences becomes zero. Specifically, the correction value acquisition unit 24 fits the timing histogram H to a smooth function, and obtains the value Td based on this function.

  In addition, since the scintillator crystal layer 2a has 60 scintillator crystals C, it can be known in which part of the scintillator crystal layer 2a the γ rays are converted into fluorescence. However, when obtaining the value Td, it is not distinguished which scintillator crystal C the radiation detector is incident on, and the detection data obtained when the annihilation γ-ray pair is simply observed with the scintillator crystal layer 2a and the other scintillator 2 is used. The timing histogram H is obtained using this. Therefore, it takes more time to collect the number of detected annihilation γ-ray pairs necessary for obtaining the value Td than when obtaining the value Tb1. When the value Td is to be obtained for the scintillator crystal layer 2a on the side close to the light guide 4, the γ rays incident on the scintillator crystal layer 2a are blocked by other scintillator crystal layers 2a and are less. Therefore, obtaining the value Td takes the longest time between the scintillator crystal layers 2a, and takes, for example, 10 times longer than the value Tb1. However, this number of detections has already been acquired in the above-described detection start step S2.

  As described above, the correction value acquisition unit 24 includes the detector unit correction value Tb1, the columnar unit correction value Tp, and the layer unit correction value Td, which are the target section for obtaining the correction value and another radiation detector 1. By detecting the annihilation γ-ray pairs converted into fluorescence by the above, the structure is obtained one after another. At this time, even if a plurality of scintillator crystals C exist in the target section or another radiation detector 1, the scintillator crystals C are not distinguished from each other and are simply detected between the section and another radiation detector 1. The timing histogram H is generated by counting the number of annihilation γ-ray pairs.

  Thus, when obtaining correction values for a certain section, annihilation γ-ray pairs are detected in any one of the plurality of scintillator crystals C, rather than continuing to detect annihilation γ-ray pairs individually for each scintillator crystal C. If the annihilation γ-ray pair is detected as described above, the time taken to detect the annihilation γ-ray pair is significantly shortened. Of the annihilation γ-ray pairs incident on the two radiation detectors 1, very few annihilation γ-ray pairs are converted into fluorescence by a specific scintillator crystal C. However, among the annihilation γ-ray pairs incident on the two radiation detectors 1, annihilation γ-ray pairs converted into fluorescence in a specific columnar section P should be about 1/60 of the total number. is there. As described above, according to the configuration of the first embodiment, each correction value is obtained with a larger division than that of the scintillator crystal C. Therefore, the time required for measuring the annihilation γ-ray pair is shortened compared to the conventional case.

<Detector unit correction value reacquisition step S6>
Next, the correction value acquisition unit 24 applies the columnar unit correction value Tp and the layer unit correction value Td to the time information before correction in the detection data generated by the coincidence counting unit 20. Then, a timing histogram H is generated based on the corrected detection data. As indicated by the arrows in FIG. 11, the correction value acquisition unit 24 obtains a value such that the average value of the time difference becomes 0, and based on this, the detector unit correction value Tb2 for each radiation detector. Ask for.

  This operation will be described. The correction value for correcting the variation in temporal characteristics of the detection of different γ-rays in the scintillator crystal C is the sum of the columnar unit correction value Tp, the layer unit correction value Td, and the detector correction value. Therefore, the variation of the time difference observed after the columnar unit correction value Tp and the layer unit correction value Td are applied to the time information before correction in the detection data is a state in which factors other than the factors that differ for each detector are removed. It is. Therefore, if the correction value is obtained so as to correct the difference in the time difference of the timing histogram H generated by applying the columnar unit correction value Tp and the layer unit correction value Td to the time information before correction in the detection data, it is detected. The unit correction value is acquired.

  The detector unit correction value Tb2 obtained in the detector unit correction value reacquisition step S6 is more accurate than the detector unit correction value Tb1 obtained in the detector unit correction value acquisition step S3. The timing histogram H that is the basis of the detector unit correction value Tb1 has a considerable spread in terms of time difference as shown in FIG. In comparison, the timing histogram H that is the basis of the detector unit correction value Tb2 is generated after the cause of the variation in the time difference regarding the columnar unit correction value Tp and the layer unit correction value Td is eliminated. As shown in FIG. 11, the reliability of the average value is increased with a narrower spread over time. If the detector unit correction value is obtained again using such a timing histogram H, a more accurate detector unit correction value can be obtained.

  Further, when the radiation detector 1 changes over time, the temporal radiation detection characteristics of the radiation detector 1 may change. In this case, a new correction value must be obtained. Even so, it is not necessary to repeat all of the above steps S1 to S6. This is because the columnar unit correction value Tp and the layer unit correction value Td related to the scintillator 2 do not vary, and the detector unit correction value Tb2 can be obtained using these values. Therefore, when a new correction value is obtained, a detector unit correction value reacquisition step using the previously acquired columnar unit correction value Tp and layer unit correction value Td after the phantom introduction step S1 and detection start step S2. It is only necessary to perform S6. In this case, in the detection start step S2, the number of detected annihilation γ-ray pairs need only be such that the detector unit correction value Tb1 can be acquired, and thus the detection of the annihilation γ-ray pairs is quickly completed. That is, it is not necessary to spend 60 times as much time for detection as the value Tb1 as in the columnar unit correction value acquisition step S4.

<Individual crystal correction value acquisition step S7>
Next, the correction value acquisition unit 24 calculates a crystal individual correction value Tc, which is a correction value for each scintillator crystal C, using the columnar unit correction value Tp, the layer unit correction value Td, and the detector unit correction value Tb2. The scintillator crystal C belongs to any one of the radiation detectors 1 arranged in the detector ring 12, belongs to any columnar section P, and belongs to any scintillator crystal layer 2a. . When obtaining the individual crystal correction value Tc in a scintillator crystal C, the correction value acquisition unit 24 detects the radiation detector 1, the columnar section P to which the scintillator crystal C belongs, the detector unit correction value Tb2 in the scintillator crystal layer 2a, and the columnar unit. The correction value Tp and the layer unit correction value Td are added together.

  The correction value acquisition unit 24 calculates the individual crystal correction value Tc for all scintillator crystals C in the detector ring 12.

<Table creation step S8>
Finally, the correction value acquisition unit 24 associates the individual crystal correction value Tc with the position of each scintillator crystal C, and generates a table in which the position of the detector ring 12 is associated with the correction value. The table is stored in the set value storage unit 37.

<Operation of radiation tomography system>
Next, the operation of the radiation tomography apparatus according to Embodiment 1 will be described. First, a radiopharmaceutical is injected into the subject M. When a predetermined time has elapsed from this point, the subject M is placed on the top plate 10 and the subject M is inserted into the inner hole of the detector ring 12. When the surgeon instructs the extinction gamma ray pair to be detected through the console 35, the detector ring 12 starts sending detection data to the coincidence counting unit 20. The coincidence counting unit 20 performs coincidence of detection data, and the correction unit 21 corrects time information given to the detection data based on a table stored in the setting storage unit 37. The position specifying unit 22 determines the generation position of the annihilation γ-ray pair based on the corrected time information. The image generation unit 23 images the identified generation position of the annihilation γ-ray pair to generate a tomographic image. The tomographic image is displayed on the display unit 36, and the operation of the radiation tomography apparatus according to the first embodiment is finished.

  As described above, according to the configuration of the first embodiment, the correction unit 21 that corrects the time information output from the radiation detector 1 is provided. Thereby, the time information which is inaccurate due to variations in the γ-ray detection of the radiation detector 1 can be corrected correctly. The individual crystal correction value Tc used for correction by the correction unit 21 is obtained by obtaining the columnar unit and layer unit correction values Tp and Td in the scintillator 2, respectively. In this way, it is possible to obtain a more accurate correction value than to obtain a correction value for each detector. If the section is made sufficiently large, the number of annihilation γ-ray pairs incident on the section per unit time increases, so that detection of the annihilation γ-ray pairs necessary for obtaining the correction value is completed in a short time.

  Further, the scintillator 2 of the radiation detector 1 is configured by three-dimensionally arranging the scintillator crystals C, and the correction value acquisition unit 24 obtains individual correction values of the scintillator crystals C using the correction values in units of sections. ing. Thereby, even if each scintillator crystal C has different temporal γ-ray detection characteristics, the time information can be corrected while changing the crystal individual correction value Tc according to the individual scintillator crystal C.

  The above configuration shows a more specific configuration of the configuration of the first embodiment. That is, the correction value acquisition unit 24 obtains the columnar unit correction value Tp in a columnar unit (columnar partition unit), obtains the layer unit correction value Td in the scintillator crystal layer 2a unit, and finally the individual crystal of the scintillator crystal C individually. The correction value Tc is obtained. As described above, when the division unit for obtaining the correction value is determined based on the arrangement of the scintillator crystal C, the temporal γ-ray detection characteristic of the scintillator crystal C varies depending on the location of the scintillator 2 in which the scintillator crystal C occupies. In consideration of this, the individual crystal correction value Tc can be acquired.

  Further, once the correction value is obtained from the columnar unit correction value Tp and the layer unit correction value Td, the correction value hardly changes over time. Therefore, when the radiation detector 1 changes over time and the characteristics of the temporal γ-ray detection of the radiation detector 1 change, it is possible to cope with this change over time only by obtaining a correction value for each radiation detector. be able to. Since the number of annihilation gamma ray pairs necessary for obtaining the correction value for each radiation detector is smaller than that for obtaining the correction value for each unit described above, the correction value for the radiation detector unit is promptly obtained. be able to.

  The present invention is not limited to the above-described configuration, and can be modified as follows.

  (1) In the above-described columnar unit correction value acquisition step S4, the annihilation γ-ray pair is detected between the columnar section P and the scintillator 2, and the columnar unit correction value Tp is obtained. An annihilation gamma ray pair may be detected between Ps to obtain a columnar unit correction value Tp. That is, in the present modification, the correction value acquisition unit 24 detects the annihilation γ-ray pair between the columnar section P1 and the columnar section P2 existing in each of the pair of scintillators 2, as shown in FIG. H is created, and the magnitude of the detection time deviation for the set of columnar sections P1 and P2 is acquired. The correction value acquisition unit 24 obtains the amount of time deviation for all combinations when the columnar sections P are selected one by one from the two scintillators 2. Since each of the two scintillators 2 has 60 columnar sections P, there are 3,600 combinations of columnar sections P to be obtained. Accordingly, in order to detect a sufficient number of annihilation γ-rays for obtaining the amount of time deviation for all the columnar sections P, the time is 3,600 times as long as the detector unit correction value Tb1. Take it.

  The correction value acquisition unit 24 obtains a columnar unit correction value Tp for each columnar section P using the magnitude of time deviation in each combination. If the columnar unit correction value Tp is obtained in this way, a more accurate correction value can be obtained.

  (2) In the above-mentioned layer unit correction value acquisition step S5, the annihilation γ-ray pair is detected between the scintillator crystal layer 2a and the scintillator 2, and the layer unit correction value Td is obtained. An annihilation gamma ray pair may be detected between the crystal layers 2a to obtain a layer unit correction value Td. That is, in the present modification, the correction value acquisition unit 24 detects annihilation γ-ray pairs between the scintillator crystal layer 2a1 and the scintillator crystal layer 2a2 that exist in each of the pair of scintillators 2, as shown in FIG. A timing histogram H is created, and the magnitude of the detection time shift between the pair of scintillator crystal layers 2a1 and scintillator crystal layers 2a2 is acquired. The correction value acquisition unit 24 obtains the amount of time shift for all combinations when the scintillator crystal layers 2a are selected one by one from the two scintillators 2. Since there are four scintillator crystal layers 2a in each of the two scintillators 2, there are 16 combinations of scintillator crystal layers 2a to be obtained.

  The correction value acquisition unit 24 obtains the layer unit correction value Td for each of the scintillator crystal layers 2a using the magnitude of time deviation in each combination. Thus, if the layer unit correction value Td is obtained, a more accurate correction value can be obtained.

  (3) In the individual crystal correction value acquisition step S7, the crystal individual correction value Tc in the scintillator crystal C is obtained, but instead of this, four scintillators arranged in the vertical and horizontal directions in the scintillator crystal layer 2a. A correction value may be obtained for each of the scintillator crystal groups composed of the crystals C. In this way, since it is not necessary to obtain a correction value for each crystal, it is easy to obtain the correction value. Further, the number of scintillator crystals C belonging to the scintillator crystal group can be freely changed.

  (4) According to the above-described configuration, the scintillator 2 is configured by the scintillator crystals C arranged three-dimensionally, but the present invention is not limited to such a configuration. The present invention can also be applied to a radiation tomography apparatus employing a monolithic scintillator in which the scintillator 2 is composed of one scintillator crystal. In this case, as shown in FIG. 14, a correction value may be obtained for each virtual section Q, and a correction value for each part of the scintillator 2 may be obtained.

  (5) As a specific configuration of the correction unit 21, the correction unit 21 may be realized by an information processing device provided in the gantry 11.

  (6) As a specific configuration of the correction unit 21, the correction unit 21 may be realized by a console provided outside the gantry 11.

  (7) As a specific configuration of the correction unit 21, the correction unit 21 may be realized by being distributed by an information processing device provided in the gantry 11 and a console provided outside the gantry 11. In this case, the time data given to the detection data is subjected to rough correction within the gantry 11 and then subjected to detailed correction outside the gantry 11.

  This correction will be specifically described. A predetermined number of clocks is set in the clock 19, and a serial number indicating the time is output to the detector ring 12 for each time unit (64 ps) indicated by the number of clocks. The detector ring 12 recognizes the difference in the incident time of the γ rays in the detector ring 12 in increments of 64 ps. For example, if two γ rays are incident on the detector ring 12 with a certain time difference, the detector ring 12 can recognize that the timing of incidence of the γ rays is shifted by a time that is a multiple of 64 ps. . However, the detector ring 12 cannot detect the time difference of incidence finer than 64 ps.

  According to this modification, first, the time data given to the detection data is corrected in the gantry 11 so as to shift with time by a time that is a multiple of 64 ps. In the gantry 11, it is only necessary to make corrections by considering how many times of time data should be shifted, so that the calculation for correction is not so large.

  Outside the gantry 11, the time data roughly corrected in the gantry 11 is shifted over time by a time less than 64 ps. The reason why the individual crystal correction value Tc can be obtained with a fineness of less than 64 ps although the detector ring 12 cannot detect the time difference of incidence finer than 64 ps will be described. FIG. 15 shows the vicinity of the average value of the timing histogram H when obtaining the correction value. Since the pre-correction time data given to the detection data indicates the γ-ray incident timing in increments of 64 ps, the time difference in detection of the annihilation γ-ray pair is a multiple of 64 ps. Therefore, the timing histogram H can be represented as a bar graph in which detection frequencies are arranged at a pitch of 64 ps.

  The correction value acquisition unit 24 fits the timing histogram H to a smooth function, obtains values Tb1, Tb2, Tp, and Td based on this function, and finally obtains the individual crystal correction value Tc. Therefore, a function used for fitting at this time is indicated by S in FIG. In the figure, D indicates an average value of time differences obtained based on the function S. Thus, unlike the timing histogram H that can be represented by a bar graph, the function S is a smooth function, so the average value D obtained by the function S does not become a multiple of 64 ps. In addition, since the average value obtained at this time is obtained based on a sufficient number of detected annihilation γ-ray pairs, the reliability is high.

  That is, the correction value acquisition unit 24 can obtain the values Tb1, Tb2, Tp, and Td with a fineness of less than 64 ps that cannot be detected originally by fitting the timing histogram H to a smooth function. Because of such circumstances, the crystal individual correction value Tc can be obtained with a fineness of less than 64 ps. The individual crystal correction value Tc can be expressed as a number obtained by adding a value less than 64 ps to a multiple of 64 ps.

  According to this modification, the individual crystal correction value Tc is considered separately for a portion that is a multiple of 64 ps and a portion that is less than 64 ps. Then, the correction for the multiple portion is performed in the gantry 11 and the correction for the portion of less than 64 ps is performed outside the gantry 11. The correction for the portion below 64 ps is complicated in calculation. If a complicated correction calculation is performed outside the gantry 11 as in this modification, the calculation efficiency of the correction is further improved.

  In the above-described configuration, the correction unit 21 is provided in a distributed manner inside and outside the gantry. The correction unit 21 in the gantry corrects simple time information, and the correction unit 21 outside the gantry is complicated. Correct the time information. If the correction unit 21 is in charge of the correction calculation in this way, the configuration of the correction unit 21 in the gantry is simplified, the calculation load of the correction unit 21 outside the gantry is lightened, and the calculation is accurate. A radiation tomography apparatus 9 capable of correcting time information can be provided.

DESCRIPTION OF SYMBOLS 1 Radiation detector 2 Scintillator 11 Gantry 12 Detector ring 19 Clock 21 Correction | amendment part (correction means)
22 position specifying means 23 image generating means 24 correction value acquisition unit (correction value acquisition means)
37 Setting value storage section (storage means)

Claims (6)

A detector ring for detecting an annihilation radiation pair configured by arranging a radiation detector having a scintillator in an annular shape; and
A clock for acquiring data indicating time based on the number of clocks indicating a predetermined time unit;
A gantry covering the detector ring;
Correction means for correcting time information indicating a detection time of radiation output based on the clock by the radiation detector based on a correction value;
Finding the time difference when the annihilation radiation pair was detected based on the time information after correction, and a position specifying means for acquiring the generation position of the annihilation radiation pair from this time difference;
Image generating means for imaging the generation position of the annihilation radiation pair specified by the position specifying means as an image;
Storage means for storing the correction value;
Correction value acquisition means for acquiring the correction value,
The radiation tomography apparatus according to claim 1, wherein the correction value acquisition means obtains the correction value for each section in the scintillator using detection data of the annihilation radiation pair output from the detector ring. The radiation tomography apparatus according to claim 1,
The scintillator of the radiation detector constitutes the scintillator in which scintillator crystals are arranged in a three-dimensional manner by laminating scintillator crystal layers constituted by scintillator crystals in a two-dimensional arrangement in the radiation incident direction,
The correction value acquisition means obtains the correction value for each scintillator crystal based on the correction value in units of sections,
The radiation tomography apparatus according to claim 1, wherein the correction unit corrects the time information based on the correction value of each scintillator crystal. The radiation tomography apparatus according to claim 2,
The correction value acquisition means obtains the correction value in units of the radiation detector,
The correction value acquisition means obtains the correction value in a columnar section unit in which scintillator crystals are arranged in a line in the radiation incident direction,
The correction value acquisition means determines the correction value in units of scintillator crystal layers,
The radiation tomography apparatus according to claim 1, wherein the correction value acquisition means calculates the correction value for each scintillator crystal based on the correction value obtained by these. The radiation tomography apparatus according to any one of claims 1 to 3,
A radiation tomography apparatus, wherein the correction means is provided in a gantry. The radiation tomography apparatus according to any one of claims 1 to 3,
A radiation tomography apparatus wherein the correction means is provided as a console outside the gantry. The radiation tomography apparatus according to any one of claims 1 to 3,
The correction means is provided distributed in the gantry and outside the gantry,
The correction means in the gantry corrects the time information so as to shift over time by a time equal to or more than the time unit of the clock,
The correction means outside the gantry corrects the time information by correcting the time information corrected in the gantry so that the time information is further shifted over time by a time less than the time unit of the clock. Tomography equipment.

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