JP2012042344A - Radiation tomography apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation tomography apparatus capable of accurately performing count missing correction even while radioactive rays are generated outside a detector ring.SOLUTION: The present invention is configured to perform count missing correction on a simultaneous event count value. A strength of the count missing correction is determined by counting accidental simultaneous events but the strength of the correction may be made inexact by radioactive rays generated from the outside of a detector ring 12. The present invention is configured to acquire, for each portion of the detector ring 12, a correction value to be used by a correction section 23 on the basis of the accidental simultaneous event count value individually acquired for each portion of the detector ring 12 and the accidental simultaneous event count value acquired for the overall detector ring 12. The two count values are combined to determine the strength of the correction (correction value R). Thus, exact count missing correction can be performed by canceling the excess or lack of value.

Description

  The present invention relates to a radiation tomography apparatus that images radiation emitted from a subject, and more particularly to a radiation tomography apparatus for 3D collection that can correct the counting-down of radiation caused by the limit of time resolution of radiation detection.

  A specific configuration of a conventional radiation tomography apparatus will be described. As shown in FIG. 9, the conventional radiation tomography apparatus 50 includes a top plate 52 on which the subject M is placed, and a detector ring 62 that detects an annihilation radiation pair. The opening of the detector ring 62 can insert the subject M together with the top plate 52.

  When it is intended to know the distribution of the radiopharmaceutical in the head of the subject M using the conventional radiation tomography apparatus 50, the head of the subject M is moved to a position existing inside the opening of the detector ring 62. The Then, the generation position of the annihilation radiation pair emitted from the head of the subject M is imaged to obtain a radiation tomographic image. Such a radiation tomography apparatus is called a PET (positron emission tomography) apparatus.

  In such a PET apparatus, as the frequency of radiation incident on the detector ring 62 increases, a phenomenon occurs in which the number of times the radiation is incidentally decreased. If two radiations are incident on the detector ring 62 immediately after two radiations are incident on the detector ring 62 at the same time, if the other radiation is incident on the detector ring 62, the PET device Cannot be distinguished. Therefore, the PET apparatus continues to count the radiation on the assumption that the first two incident radiations are not incident.

  Thus, if the number of annihilation radiation pairs is counted on the assumption that there are no annihilation radiation pairs that are actually incident, the number of annihilation radiation pairs detected by the detector ring 62 is reduced. Such a phenomenon is called counting off of annihilation radiation pairs. This count-down occurs more frequently as the radiation incident on the detector ring 62 increases.

  According to the conventional configuration, the occurrence frequency of the counting-down according to the counting of the annihilation radiation pairs incident on the detector ring 62 is acquired using a previously collected table, and the number of actually measured annihilation radiation pairs increases. So that the effects of counting down are eliminated. Such count correction is referred to as count-down correction (see Non-Patent Document 1).

Seiichi Yamamoto et al. Deadtime Correction Method Using Random Coincidence for PET. J. et al. Nucl. Med. 27: 1925-1928, 1996

  However, the conventional radiation tomography apparatus has the following problems. That is, in an inspection in which radiation comes from the outside of the detector ring, there is a problem in that the counting-off correction cannot be performed accurately in the case of 3D collection that does not include a septa that prevents the incidence of these radiations.

  In the head inspection PET apparatus as shown in FIG. 9, the inspection target head is inserted into the detector ring 62. Since the radiopharmaceutical that generates the annihilation radiation pair travels around the whole body of the subject, radiation is also generated from other than the head of the subject. Radiation generated outside the detector ring 62 enters the detector ring 62.

  In the conventional configuration, there is a method of obtaining a count-down correction value at each detector pair constituting the detector ring 62. In this method, when radiation is incident on the detector ring 62 from the outside, it is estimated that the number of annihilation radiation pairs is counted off. As a result, the count value of the annihilation radiation pair particularly at the top of the head is strongly corrected and excessively corrected.

  In addition, in the conventional configuration, there is another method for obtaining the count-down correction value for the entire detector ring 62. In this method, when radiation is incident on the detector ring 62 from the outside, it is estimated that the count of annihilation radiation pairs is not counted. As a result, the count value of the annihilation radiation pair particularly at the top of the head is corrected weakly, resulting in insufficient correction.

  The present invention has been made in view of such circumstances, and the purpose thereof is radiation that can be accurately counted and corrected even when radiation is generated outside the detector ring. It is to provide a tomographic apparatus.

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 in a ring shape, and a phenomenon in which two radiations are simultaneously incident on the detector ring. The simultaneous counting means that counts simultaneous events and counting correction that corrects the decrease in the simultaneous event count that occurs when the efficiency of the simultaneous event count decreases as the dose of radiation incident on the detector ring increases. A correction means for performing correction, a correction value acquisition means for acquiring a correction value at the time of counting-off correction, and an incidental coincidence that is a phenomenon in which two radiations that are not a pair of annihilation radiation pairs are incident on the detector ring simultaneously And a coincidence count means, and the correction value acquisition means detects the coincidence coincidence event count value acquired individually for each part of the detector ring, and detects Based on the random coincidence events count acquired in entire ring, it is characterized in that to obtain for each portion of the detector ring a correction value correction unit employed.

  [Operation / Effect] According to the configuration of the present invention, the count-down correction is performed on the simultaneous event count value. The strength of this count-down correction is determined by counting the coincidence simultaneous events. Radiation emitted from the outside of the detector ring and incident on the detector ring fluctuates the coincidence coincidence event count value and makes the correction intensity inaccurate. According to the present invention, the correction value used by the correcting means is based on the coincidence coincidence event count value obtained individually for each part of the detector ring and the coincidence coincidence event count value obtained for the entire detector ring. It is configured to acquire each part of the detector ring. One of the random coincidence event count values obtained by the two measurement methods is larger than the actual value, and the other is smaller than the actual value. Therefore, if the intensity of correction (correction value) is determined by combining these two count values, the excess or deficiency of the value is offset and accurate counting correction can be performed.

  Further, in the above-described radiation tomography apparatus, the correction value acquisition unit obtains the coincidence simultaneous event count value acquired individually for each part of the detector ring and the random coincidence event count value acquired for the entire detector ring. It is more desirable to obtain a correction value based on a count value obtained by adding weights.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. In other words, the coincidence simultaneous event count values acquired by the two methods are weighted and added to acquire the correction value. In this way, accurate counting correction can be performed more reliably.

  Further, in the above-described radiation tomography apparatus, the coincidence coincidence unit compares the data indicating the timing at which the radiation is incident on the detector ring with the edit data obtained by shifting the data by a predetermined time, and between the two data. It is more desirable to count incidental coincidence by counting simultaneous events.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. The coincidence simultaneous event count is an effective index for knowing the frequency of simultaneous event count counting down. If simultaneous events are counted by comparing data with different times, simultaneous events due to annihilation radiation pairs are not counted, so it is possible to accurately know the frequency of occurrence of events in which irrelevant radiation accidentally enters the detector ring at the same time. be able to.

  Further, the above-described radiation tomography apparatus includes a storage unit that stores a table in which the coincidence coincidence event count value and the correction value are related, and the correction value acquisition unit is acquired individually for each part of the detector ring. If a correction value is obtained by obtaining a reference value based on the coincidence coincidence event count value and the coincidence coincidence event count value obtained for the entire detector ring, and obtaining a correction value corresponding to the reference value from the table. desirable.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. There is a certain relationship between the coincidence simultaneous event count value and the count-down correction value. However, if the correction value is obtained using the coincidence simultaneous event count value as it is from this relationship, a correct correction value cannot be obtained. This is because the relationship between the coincident coincidence event count value and the correction value does not take into account that radiation is generated from the outside of the detector ring. According to the above configuration, the reference value is obtained from the coincident coincidence event count value obtained by the two measurement methods, and the correction value corresponding to the reference value is obtained from the table in which the coincidence coincident event count value and the correction value are related. Therefore, more accurate counting correction can be made.

  Further, in the above-described radiation tomography apparatus, the table stored by the storage means is more preferably a table obtained by inserting a radioactive phantom into the detector ring.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. If the relationship between the coincidence coincident event count value and the correction value is known in a state where the phantom is inserted, more accurate counting correction can be performed.

  In the above-mentioned radiation tomography apparatus, it is more desirable to provide a ring-shaped absorber that absorbs radiation at a position covering the end of the detector ring in the central axis direction.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. If the detector ring is provided with an absorber, radiation generated from the outside can be prevented from entering the detector ring as much as possible.

  In the above-described radiation tomography apparatus, the correction value acquisition means performs correction when the coincidence coincident event count value individually obtained for each part of the detector ring is smaller than the coincident coincidence event count in the entire detector ring. If not, it is more desirable.

  [Operation / Effect] The above-described configuration shows a more specific configuration of the present invention. If comprised as mentioned above, a complicated correction | amendment operation | movement can be simplified.

  Further, in the head PET apparatus, the trunk of the subject is external to the detector ring. If the present invention is applied to a head PET apparatus, it is possible to accurately correct the counting-down correction even in a situation where radiation is likely to be generated from the outside of the detector ring.

  According to the configuration of the present invention, the count-down correction is performed on the simultaneous event count value. The intensity of this count-down correction is determined by counting incidental simultaneous events, but radiation emitted from outside the detector ring will cause the correction intensity to be inaccurate. According to the present invention, the correction value used by the correcting means is based on the coincidence coincidence event count value obtained individually for each part of the detector ring and the coincidence coincidence event count value obtained for the entire detector ring. It is configured to acquire each part of the detector ring. If the intensity of correction (correction value) is determined by combining these two count values, the excess or deficiency of the value is offset and accurate counting correction can be performed.

1 is a functional block diagram illustrating a configuration of a radiation tomography apparatus according to Embodiment 1. FIG. 1 is a perspective view illustrating a configuration of a radiation detector according to Embodiment 1. FIG. FIG. 3 is a plan view illustrating a configuration of a detector ring according to the first embodiment. 3 is a flowchart for explaining the operation of the radiation tomography apparatus according to Embodiment 1. FIG. 3 is a schematic diagram for explaining the operation of the radiation tomography apparatus according to the first embodiment. FIG. 3 is a schematic diagram for explaining the operation of the radiation tomography apparatus according to the first embodiment. FIG. 3 is a schematic diagram for explaining the operation of the radiation tomography apparatus according to the first embodiment. FIG. 3 is a schematic diagram for explaining the operation of the radiation tomography apparatus according to the first embodiment. It is sectional drawing explaining the structure of the radiation tomography apparatus of a conventional structure.

<Configuration of radiation tomography system>
Embodiments of a 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 gantry 11 is provided with an opening large enough to accommodate the head of the subject M. The head of the subject M is inserted into this opening. Therefore, the radiation imaging apparatus 9 is for head inspection.

  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 plate moving mechanism 15 is controlled by the top plate movement control unit 16. 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 area is located outside the detector ring 12 and is introduced into the opening of the detector ring 12 from one side thereof.

  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 15 cm to 26 cm. The ring-shaped absorbers 13a and 13b are provided so as to cover both ends of the detector ring 12 in the central axis direction (z direction). The absorbers 13a and 13b are made of a member that hardly transmits γ rays, and prevent the γ rays from entering the inside of the detector ring 12 from the outside. The absorbers 13a and 13b are provided for the purpose of removing γ-rays generated outside the detector ring 12 that obstruct the imaging of the tomographic image of the subject M. The inner diameters of the absorbers 13 a and 13 b are smaller than the inner diameter of the detector ring 12.

  The clock 19 sends time information as a serial number to the detector ring 12. The detection data output from the detector ring 12 is given time information indicating when the γ-ray is detected, and is input to a coincidence counting unit 20 and an accidental coincidence counting unit 21 described later. The coincidence counting unit 20 corresponds to the coincidence counting unit of the present invention, and the coincidence coincidence counting unit 21 corresponds to the accidental coincidence counting unit of the present invention.

  Detection data output from the detector ring 12 is sent to the coincidence unit 20. The two gamma rays simultaneously incident on the detector ring 12 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 the scintillator crystals constituting the detector ring 12, and sends the result to the correction unit 23. The operation of counting the number of annihilation γ-ray pairs is called simultaneous event counting, and the value obtained by the counting is called simultaneous event counting value. 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 coincidence count may be an analog count that is determined by an AND circuit using a pulse signal generated when radiation is detected without using the time information by the clock 19 described above.

  When the coincidence counting unit 20 performs coincidence event counting, the counting of annihilation γ-ray pairs is generated. That is, as the number of γ rays incident on the detector ring 12 increases, the number of incidents of γ rays irrelevant to the detector ring 12 increases simultaneously with the incidence of the annihilation γ ray pair. When such an event occurs, the detection data of γ rays overlap with time, and it becomes impossible to detect an annihilation γ ray pair. This event occurs more frequently as the gamma rays incident on the detector ring 12 increase. Counting down means a decrease in the coincidence event count value caused by a decrease in the efficiency of coincidence event counting due to the above-described event as the dose of γ rays incident on the detector ring 12 increases.

  The random coincidence counting unit 21 is provided for the purpose of correcting the influence of counting off the simultaneous event count. The random coincidence counting unit 21 counts coincident coincidence events, which is a phenomenon in which two γ rays that are not a pair of annihilation γ rays are incident on the detector ring 12 at the same time. The value obtained by counting will be referred to as an accidental simultaneous event count value. The coincidence simultaneous event count value is an index indicating how frequently the simultaneous event count is counted down. The coincidence coincidence event count value is acquired by the coincidence coincidence counting unit 21. That is, the coincidence coincidence unit 21 compares the data indicating the timing at which the γ-rays are incident on the detector ring 12 with the edit data obtained by shifting the data by a certain time (30 nsec to 100 nsec), and between the two data Count the coincidence coincidence by counting the simultaneous events. Specifically, the coincidence coincidence counting unit 21 counts the simultaneous events, one of which is γ-ray belonging to the data before shifting the time, and the other is the γ-ray belonging to the editing data after shifting the time. There are only pairs. Therefore, the coincidence coincidence unit 21 does not count the pair of γ-rays belonging to the data before both of which are shifted in time, and similarly, both of the γ-rays belonging to the edited data after being shifted in time. The pair is not counted. The pair of γ rays counted by the coincidence coincidence unit 21 is detected by the detector ring 12 with a time interval of 30 nsec to 100 nsec, and is not an annihilation γ ray pair.

  If the simultaneous event count is performed without the operation of shifting the time in order to acquire the coincidence coincidence count, the simultaneous event counts counted include those derived from the annihilation γ-ray pairs, and these cannot be distinguished. Accordingly, the coincidence coincidence unit 21 performs the coincidence event counting by shifting the time, thereby removing the influence of the incident of the annihilation gamma ray pair and reliably performing the coincidence coincidence event counting.

  The correction value acquisition unit 22 acquires a correction value H used for counting-off correction based on the coincidence coincidence event count value output from the coincidence coincidence counting unit 21, and the correction unit 23 uses the correction value to calculate the simultaneous event counter. Correct the counting of the numerical value. The specific configuration of each part will be described later. The correction unit 23 corresponds to the correction unit of the present invention.

  The image generation unit 24 acquires a tomographic image D in which the generation positions of annihilation γ-ray pairs are mapped based on the output of the correction unit 23. The correction unit 23 corrects the number of annihilation gamma ray detections output from the coincidence unit 20 and outputs the corrected number to the image generation unit 24. The image generation unit 24 generates a tomographic image D by mapping the generation intensity of the annihilation γ-ray pair inside the subject M from these pieces of information.

  The configuration of the radiation detector 1 constituting the detector ring 12 will be briefly described. FIG. 2 is a perspective view illustrating the configuration of the radiation detector according to the first embodiment. As shown in FIG. 2, 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 is configured by arranging scintillator crystals two-dimensionally. The scintillator crystal 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 emits fluorescence, and can also specify the intensity of fluorescence and the time when the fluorescence is generated. it can. 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 configuration of the detector ring 12 will be described. According to the first embodiment, as shown in FIG. 3, one unit ring 12b is formed by arranging around eight radiation detectors 1 in a virtual circle on a plane perpendicular to the z direction. The unit rings 12b are arranged in the central axis direction (z direction) to constitute the detector ring 12.

  The radiation tomography apparatus 9 includes a main control unit 41 that controls each unit in an integrated manner, and a display unit 36 that displays a radiation tomographic image. The main control unit 41 is constituted by a CPU, and realizes the respective units 16, 19, 20, 21, 22, 23, and 24 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 storage unit 37 stores a table T described later. The storage unit 37 corresponds to the storage unit of the present invention.

<Operation of radiation tomography system>
Next, the operation of the radiation tomography apparatus 9 according to the first embodiment will be described. In order to examine the head of the subject M using the radiation tomography apparatus 9, as shown in FIG. 4, the subject M is placed on the top board 10 (placement step S1). Detection of annihilation gamma ray pairs emitted from the head is started, and simultaneous event counting is started (simultaneous event counting start step S2). Then, the coincidence coincidence event count is started from the detection data output from the detector ring 12 (incident coincidence event count start step S3), and the correction value H is obtained using the obtained coincidence coincidence event count value (correction value). Acquisition step S4). Subsequently, the simultaneous event count value is corrected based on the acquired correction value H (counting correction step S5), and a tomographic image of the head of the subject M is generated using the corrected simultaneous event count value. (Tomographic image generation step S6). Hereinafter, these steps will be described in order.

<Installation step S1>
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 board 10, and then the head of the subject M is inserted into the detector ring 12. FIG. 1 shows a state in which this operation is completed.

<Simultaneous event counting start step S2>
When the surgeon gives an instruction to detect the annihilation gamma ray pair through the console 35, the detector ring 12 starts sending detection data to the coincidence counting unit 20. The detection data transmitted at this time is a data set in which the incident position of the γ-ray detector ring 12, energy, and incident time are related. The coincidence unit 20 performs coincidence event counting of the detected data.

<Contingent coincidence event counting start step S3>
Detection data output from the detector ring 12 is also sent to the coincidence coincidence unit 21. The coincidence coincidence counting unit 21 can know in real time the frequency of coincidence coincidents occurring in the detector ring 12 during the examination.

<Regarding the coincidence coincidence count required individually in the radiation detector>
The coincidence coincidence counting unit 21 outputs the coincidence coincidence event count value in two ways. One of them is a method for obtaining the coincidence simultaneous event count value between the two radiation detectors 1 constituting the detector ring 12. According to this method, the correction value for the count-down correction is obtained using the random coincidence event count value obtained individually. In other words, the simultaneous event count output from the simultaneous counting unit 20 is corrected by counting down by applying different correction values depending on the combination of the radiation detectors 1 that counted the simultaneous events. There are a plurality of coincidence coincidence event count values obtained individually, and these are represented by count values A1, A2,... An.

  If the count values A1, A2,... An are used, accurate counting correction cannot be performed. The reason for this will be described. FIG. 5 shows the state of the detector ring 12 during simultaneous event counting. In this state, the annihilation gamma ray pair emitted from the head of the subject M is detected by the detector ring 12. By the way, since the radiopharmaceutical that emits the annihilation γ-ray pair travels around the whole body of the subject M, the annihilation γ-ray pair is also emitted from the trunk of the subject M located outside the detector ring 12, and this is the detector ring. 12 to enter.

  A portion on the top of the subject M on the detector ring 12 is denoted by r1, and a portion on the trunk side is denoted by rn. Since the portion rn is adjacent to the absorber 13a on the trunk side, most of the γ rays generated by the trunk are absorbed by the absorber 13a before entering the portion rn. However, since the portion r1 is separated from the absorber 13a, the gamma rays generated by the trunk are not sufficiently blocked by the absorber 13a. Therefore, the portion r1 is more likely to receive gamma rays generated by the trunk that disturb the head examination compared to the portion rn.

  Because of such circumstances, the counting correction at the portion r1 where the gamma rays generated by the trunk are likely to be incident is not accurate. The count value A1 in the portion r1 is actually measured including the case where gamma rays generated in the trunk are counted as accidental simultaneous events. Therefore, the incidental event count value is increased as compared with the case where gamma rays generated by the trunk are not incident. Accordingly, the count value A1 is inaccurate, and this affects the correction of counting the simultaneous event count value.

  The random coincidence event count value is an index for knowing the frequency of counting down occurring in the simultaneous event counting. That is, there is a relationship in which the larger the accidental simultaneous event count value, the greater the number of countdowns. As shown in FIG. Is determined. In the portion r1, it is assumed that the coincidence simultaneous event count value when the gamma ray generated by the trunk does not enter is a1. Accordingly, an appropriate correction value for the portion r1 is c1 corresponding to a1. However, the actual coincidence event count value actually acquired is A1 larger than a1 due to the influence of gamma rays generated by the trunk. Then, C1, which is a correction value larger than the appropriate correction value c1, is applied to the simultaneous event count value for the portion r1. The countdown is not determined by the count of a part of the device, but is also affected by the event count of the entire device, so a correct correction value exists between c1 and C1 when only a part of the count is increased. Therefore, when C1 is a correction value, the count-down correction is an overcorrection.

<Regarding the coincidence event count value required for the entire detector ring>
Another method of coincidence coincidence event counting is to obtain a single coincidence coincidence event count value for the entire detector ring 12. According to this method, the correction value for the count-down correction is obtained using the coincidence coincidence event count value obtained in a lump. That is, the correction value is obtained for each coincidence count pair, but the coincidence coincident event count for obtaining the correction value uses the same coincident coincidence event count value regardless of the coincidence count pair. The coincidence simultaneous event count value obtained in a lump is represented by a count value B.

  Even if the count value B obtained in a lump is used, accurate counting correction cannot be performed. The reason for this will be described. As shown in FIG. 7, the count value B is a random coincidence event count value when the count-off correction frequency is assumed to be a certain value Ave for the entire detector ring 12. Since extra γ-rays generated by the trunk are incident on the portion r1, the count-off frequency is higher than the value Ave as shown in the bar graph of FIG. That is, the simultaneous event count value is corrected for the portion r1 where the count-down correction frequently occurs, assuming that the count-down correction has occurred about the value Ave. Then, the count-down correction is insufficient for the simultaneous event count value in the portion r1.

  According to the configuration of the first embodiment, the reference values R1, R2... Rn are obtained using the count values A1, A2... An and the count value B obtained by the above two methods, and the reference values R1, R2 are obtained. ...... The correction value is obtained from Rn. As a result, the overcorrection and the undercorrection that are problems in the two methods are offset, and a more accurate correction value can be acquired.

  On the other hand, in the part rn in FIG. 7, the count-down frequency is smaller than the value Ave. In such a case, the correction unit 23 described later does not perform count-down correction. That is, the correction unit 23 corrects the incident simultaneous event count values A1, A2,... An obtained individually for each part of the detector ring 12 when the incident simultaneous event count value B in the entire detector ring 12 is smaller. Not performed.

<Correction value acquisition step S4>
Count values A1, A2,... An and count value B output from the coincidence coincidence unit 21 are sent to the correction value acquisition unit 22. The correction value acquisition unit 22 adds the count values A1, A2,... An and the count value B with a certain weight, and acquires reference values R1, R2,. The reference value is obtained individually for each of the count values. For example, when the correction value H1 is obtained assuming that the reference value R1 is the coincidence coincidence event count value, the correction value H1 is obtained by counting the count value A1 and the count value B alone as shown in FIG. A value between two correction values. In this way, the correction value acquisition unit 22 acquires correction values H1, H2,... Hn based on the count values A1, A2,. The correction value is obtained individually for each reference value. In this way, correction values are obtained for each portion of the detector ring 12. The relationship between the coincidence coincident event count value and the correction value shown in FIGS. 6 and 8 is acquired in advance by inserting a radioactive phantom into the detector ring 12 instead of the subject M, Both values are stored in the storage unit 37 as a related table T. As described above, the correction value acquisition unit 22 acquires the correction value H corresponding to the reference value R from the table T.

  Although the specific method of weighting the count value performed by the correction value acquisition unit 22 is not particularly limited, the more accurate reference value R can be acquired when the count value B is weighted more heavily. Further, if the specific method of weighting is a method for obtaining an intermediate value between the count values A1, A2,... An and the count value B, the effect of this embodiment can be realized.

<Counting-off correction step S5>
The correction value acquisition unit 22 sends the correction values H1, H2,... Hn to the correction unit 23. The correction unit 23 obtains the simultaneous event count value from the simultaneous counting unit 20, and applies correction values H1, H2,. The correction values H1, H2,... Hn are individual values depending on the position of the detector ring 12. Accordingly, different correction values H1, H2,... Hn are applied to the simultaneous event count value depending on the combination of the two radiation detectors 1 that have detected the annihilation gamma ray pair. Thereby, the count-down correction of the simultaneous event count value is completed. Since the correction value acquisition unit 22 repeats the acquisition of the reference value R and the correction value H in real time during the inspection, the strength of the counting correction performed by the correction unit 23 changes with time. Further, as described above, the correction unit 23 determines that the coincidence coincident event count values A1, A2,... An obtained individually for each portion of the detector ring 12 are larger than the coincidence coincident event count value B in the entire detector ring 12. When it is small, no correction is performed.

<Tomographic image generation step S6>
The corrected simultaneous event count value is sent to the image generation unit 24 together with position information indicating the detection location in the detector ring 12. The image generation unit 24 generates a tomographic image D indicating the distribution of the radiopharmaceutical based on the position information indicated by the simultaneous event count value. The tomographic image D is displayed on the display unit 36, and the inspection ends.

  As described above, according to the configuration of the first embodiment, the count-down correction is performed on the simultaneous event count value. The strength of this count-down correction is determined by counting the coincidence simultaneous events. Gamma rays emitted from the outside of the detector ring 12 and incident on the detector ring 12 fluctuate the accidental coincidence event count value and make the correction intensity inaccurate. According to the first embodiment, the correction unit 23 uses the random coincidence event count value acquired individually for each part of the detector ring 12 and the random coincidence event count value acquired for the entire detector ring. The correction value is obtained for each part of the detector ring 12. One of the random coincidence event count values obtained by the two measurement methods is larger than the actual value, and the other is smaller than the actual value. Therefore, if the intensity of correction (correction value) is determined by combining these two count values, the excess or deficiency of the value is offset and accurate counting correction can be performed.

  In addition, the coincidence simultaneous event count values acquired by the two methods in the above-described configuration are weighted and added to acquire a correction value. In this way, accurate counting correction can be performed more reliably.

  The counting of coincidence simultaneous events in the above configuration is an effective index for knowing the frequency of simultaneous event counting. If the simultaneous event counting is performed by comparing the data shifted in time, the simultaneous event due to the annihilation gamma ray pair is not counted. Therefore, the frequency of occurrence of an event in which irrelevant gamma rays accidentally enter the detector ring 12 at the same time is calculated. Know exactly.

  In addition, there is a certain relationship between the coincidence simultaneous event count value and the count-down correction value in the above-described configuration. However, if the correction value is obtained using the coincidence simultaneous event count value as it is from this relationship, a correct correction value cannot be obtained. This is because the relationship between the coincident coincidence event count value and the correction value does not take into consideration that γ rays are generated from the outside of the detector ring 12. According to the above-described configuration, the reference value R is obtained from the coincidence simultaneous event count values obtained by the two measurement methods, and the correction value corresponding to the reference value R is the table T in which the coincidence coincident event count value and the correction value are associated. Therefore, it is possible to correct the counting loss more correctly.

  If the relationship between the coincidence coincident event count value and the correction value is known in a state where the phantom is inserted, more accurate counting correction can be performed.

  In the head PET apparatus, the trunk of the subject M is external to the detector ring 12. If Example 1 is applied to a head PET apparatus, it is possible to accurately correct the counting-down correction even in a situation where γ rays are likely to be generated from the outside of the detector ring 12.

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

  (1) The present invention is not limited to the head examination, but can be applied to all examinations for diagnosing the whole body of the subject M.

(2) Although the scintillator crystal referred to in the above-described embodiments is composed of LYSO, in the present invention, instead of LGSO (Lu _{2 (1-X)} G _2X SiO ₅ ) or GSO (Gd ₂ SiO _The scintillator crystal may be composed of other materials such as ₅ ). According to this modification, it is possible to provide a method of manufacturing a radiation detector that can provide a cheaper radiation detector.

  (3) In the embodiment described above, the photodetector is composed of a photomultiplier tube, but the present invention is not limited to this. Instead of the photomultiplier tube, a photodiode, an avalanche photodiode, a semiconductor detector, or the like may be used.

  (4) In the above-described embodiments, the count values A1, A2,... An are obtained for each radiation detector 1, but the present invention is not limited to this. The detector ring 12 may be divided into arbitrary intervals, and the count value may be obtained individually for each divided section.

12 detector rings 13a and 13b absorber 20 coincidence counting unit (simultaneous counting means)
21 Accidental coincidence counting unit (accidental coincidence counting means)
22 Correction value acquisition unit (correction value acquisition means)
23 Correction unit (correction means)
37 storage unit (storage means)
R reference value T table

Claims (8)

A detector ring for detecting an annihilation radiation pair configured by arranging the radiation detectors in a ring shape; and
Coincidence counting means for counting simultaneous events, which is a phenomenon in which two radiations are simultaneously incident on the detector ring;
Correction means for performing counting correction to correct a decrease in the simultaneous event count value caused by a decrease in the efficiency of the simultaneous event count as the dose of radiation incident on the detector ring increases;
Correction value acquisition means for acquiring a correction value when performing count correction,
A coincidence coincidence means for counting coincident coincidence events, which is a phenomenon in which two radiations that are not a pair of annihilation radiation pairs are incident on the detector ring simultaneously;
The correction value acquisition means is used by the correction means based on the coincidence simultaneous event count value acquired individually for each part of the detector ring and the random coincidence event count value acquired for the entire detector ring. A radiation tomography apparatus, wherein correction values are acquired for each portion of the detector ring. The radiation tomography apparatus according to claim 1,
The correction value acquisition means weights and adds the coincidence simultaneous event count value acquired individually for each part of the detector ring and the random coincidence event count value acquired for the entire detector ring. A radiation tomography apparatus characterized in that a correction value is acquired based on a numerical value. The radiation tomography apparatus according to claim 1 or 2,
The coincidence coincidence means compares data indicating the timing at which radiation is incident on the detector ring with edit data obtained by shifting the data by a predetermined time, and performs coincidence counting between the two data. A radiation tomography apparatus for counting simultaneous counting. The radiation tomography apparatus according to claim 3,
A storage means for storing a table in which the coincidence coincidence event count value and the correction value are associated;
The correction value acquisition means obtains a reference value based on the coincident coincidence event count value obtained individually for each part of the detector ring and the coincidence coincidence event count value obtained for the entire detector ring, A radiation tomography apparatus, wherein a correction value is acquired by acquiring a correction value corresponding to the reference value from the table. The radiation tomography apparatus according to claim 4,
The radiation tomography apparatus according to claim 1, wherein the table stored in the storage means is obtained by inserting a radioactive phantom into the detector ring. The radiation tomography apparatus according to any one of claims 1 to 5,
A radiation tomography apparatus comprising a ring-shaped absorber that absorbs radiation at a position covering an end of the detector ring in the central axis direction. The radiation tomography apparatus according to any one of claims 1 to 6,
A radiation tomography apparatus for head examination. The radiation tomography apparatus according to any one of claims 1 to 7,
The correction value acquisition means does not perform correction when the coincidence coincidence event count value individually obtained for each part of the detector ring is smaller than the coincidence coincidence event count in the entire detector ring. Shooting device.

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