JP2010266234A - Radiation tomograph - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radiation tomograph which is improved in a detection sensitivity characteristic without increasing manufacturing cost by devising a radiation detector composing a detector ring. <P>SOLUTION: A first unit ring 12a and a third unit ring 12c are positioned at both ends in the direction z of the detector ring 12, and therefore, in the rings, it is more difficult to detect an annihilation radiation pair than for a second unit ring 12b positioned in the central part of the detector ring 12. However, the radiation detection sensitivity of a first radiation detector constituting the first unit ring 12a and the third unit ring 12c is made higher than that of a second radiation detector. Thus, this radiation tomograph suppresses positional nonuniformity in the radiation sensitivity of the detector ring 12 and can provide a more homogeneous tomographic image over the range of a tomographic field of the detector ring 12. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a radiation tomography apparatus that detects an annihilation radiation pair emitted from a subject and images a distribution of a radiopharmaceutical in the subject, and in particular, the radiation detectors are arranged in a C shape and an annular shape. The present invention relates to a radiation tomography apparatus having a detector ring.

  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. As shown in FIG. 9A, the conventional radiation tomography apparatus includes a detector ring 62 in which radiation detectors 51 (see Patent Document 1) for detecting radiation are arranged in an annular shape. . The detector ring 62 detects a pair of radiations (an annihilation radiation pair) in opposite directions that are irradiated from the radiopharmaceutical in the subject.

  The configuration of the detector ring 62 will be described. As shown in FIG. 9A, the detector ring 62 has a thickness corresponding to three radiation detectors 51 in the z direction. The detector rings 62 are all composed of the same radiation detector 51.

  An inspection method using a radiation tomography apparatus will be described. A subject to which a radiopharmaceutical is injected is introduced into the detector ring 62. The radiation tomography apparatus images the distribution of the radiopharmaceutical with respect to the portion of the subject introduced inside the detector ring 62. In this way, the space inside the detector ring 62 is an imaging field of view of the radiation tomography apparatus.

  By the way, the radiation tomography apparatus includes a mammography apparatus that can examine a breast of a subject. In such a mammography apparatus, the breast of the subject is introduced into the detector ring 62 so that the breast of the subject is within the field of view of the radiation tomography apparatus, and radiopharmaceutical distribution imaging is performed. Do.

JP 2004-279057 A

However, the conventional configuration of the radiation tomography apparatus has the following problems.
In the conventional radiation tomography apparatus, the detection sensitivity of the annihilation radiation pair is uneven due to the detector ring 62 portion. The conventional radiation tomography apparatus is configured ignoring such unevenness, resulting in deterioration in detection sensitivity of the radiation tomography apparatus and unnecessary cost increase.

  Such problems will be specifically described. FIG. 9 shows a state in which the subject M is actually introduced into the detector ring 62. An annihilation radiation pair generated near the nipple portion of the subject M is easily detected by the detector ring 62. In order to detect the annihilation radiation pair, it is necessary to detect both of the two radiations constituting the annihilation radiation pair by the detector ring 62. As shown in FIG. 9A, the annihilation radiation pair generated near the nipple portion P <b> 1 is highly likely to enter the detector ring 62 even when the traveling direction is inclined in the z direction with respect to the detector ring 62. An annihilation radiation pair as indicated by an arrow in FIG. 9A can also be detected.

  However, as shown in FIG. 9B, the annihilation radiation pair generated near the chest wall (near the base of the breast) P2 of the subject M is difficult for the detector ring 62 to detect. Although one of the annihilation radiation pairs generated near the chest wall P2 as shown in FIG. 9B is incident on the detector ring 62, the other is not incident on the detector ring 62, and the detector ring 62 I have jumped away outside. Such an annihilation radiation pair cannot be detected by the detector ring 62. Actually, the annihilation radiation pairs shown in FIGS. 9A and 9B are intended to enter the detector ring 62 at the same angle. Nevertheless, in the case of FIG. 9A, detection is possible, and in the case of FIG. 9B, detection is impossible.

  That is, the detection sensitivity of the annihilation radiation pair varies depending on the part of the detector ring 62. FIG. 10A illustrates detection of an annihilation radiation pair at a part Pa having the highest detection sensitivity in the detector ring 62. FIG. The widths W1 and W2 in the z direction from the end of the detector ring 62 to the part Pa are sufficiently wide. Therefore, the traveling direction of the annihilation radiation pair that can be detected by the detector ring 62 may be various as indicated by the arrows in FIG.

  FIG. 10B illustrates detection of an annihilation radiation pair at a portion Pb where the detection sensitivity is close to the lowest in the detector ring 62. A width W3 in the z direction from one end of the detector ring 62 to the site Pb is sufficiently wide. However, the width W4 in the z direction from the other end of the detector ring 62 to the site Pb is narrow. Since the traveling direction of the annihilation radiation pair that can be detected by the detector ring 62 depends on the narrower width W4, the traveling direction of the annihilation radiation pair that can be detected by the detector ring 62 is shown in FIG. It is limited as shown by the path of the arrow. The characteristics of the detection sensitivity of the detector ring 62 will be described generally. The detection sensitivity at the center of the detector ring 62 in the z direction is the highest, and the detection sensitivity decreases as the distance from the center in the z direction increases. That is.

  In the actual examination, it is not particularly necessary that the center of the detector ring 62 has the highest detection sensitivity. In the case of breast cancer screening, rather, the area around the opening of the detector ring 62 such as the part Pb in FIG. Higher detection sensitivity is often desirable. However, according to the conventional configuration, the detector ring 62 is configured using the same radiation detector. Therefore, if it is desired to increase the sensitivity of the part Pb, detection is performed by arranging expensive radiation detectors with high detection sensitivity. There is no choice but to configure the vessel ring 62. In other words, according to the conventional configuration, the only way to increase the detection sensitivity is to increase the manufacturing cost accordingly.

  The present invention has been made in view of such circumstances, and its purpose is to devise a radiation detector that constitutes the detector ring, so that the detection sensitivity characteristic can be improved without increasing the manufacturing cost. It is to provide an improved radiation tomography apparatus.

The present invention has the following configuration in order to solve the above-described problems.
That is, the radiation tomography apparatus according to claim 1 includes a first array in which first radiation detectors for detecting radiation are arranged in an arc shape along a virtual circle, and a second radiation detector for detecting radiation in a virtual manner. And a second array arranged in an arc along the circle, the first array and the second array being orthogonal to each virtual circle and having an axis passing through the center of each virtual circle as the central axis. By sharing the central axes of each other and arranging them in the direction of the central axis, a detector ring having an opening for introducing the subject from the first array side is configured, and the radiation detection sensitivity of the first radiation detector is 2 It is characterized by being higher than that of the radiation detector.

  [Operation / Effect] The detector ring for detecting radiation according to the present invention comprises a first array and a second array. Since the first array is located on the introduction side of the subject, it is difficult to detect the annihilation radiation pair as compared with the second array located in the center of the detector ring. However, the radiation detection sensitivity of the first radiation detector constituting the first array is higher than that of the second radiation detector. As described above, the radiation tomography apparatus according to the present invention can suppress unevenness in the positional radiation sensitivity of the detector ring, and can provide a more uniform tomographic image over the imaging field of view of the detector ring.

  According to the present invention, since the detection sensitivity of the first array is intensively increased, the above-described effects can be achieved without increasing the manufacturing cost of the second array.

  The invention according to claim 2 is the radiation tomography apparatus according to claim 1, wherein the first radiation detector includes a first scintillator that converts radiation into light, and a photodetector that detects light, The second radiation detector includes a second scintillator that converts radiation into light and a light detector that detects light, and the first radiation detector in the first array is arranged such that the first scintillator faces the center of the virtual circle. The second radiation detectors in the second array are arranged so that the second scintillator faces the center of the virtual circle, and the first scintillator in the direction from the central axis toward the first radiation detector is arranged. The thickness is characterized by being thicker than the thickness of the second scintillator in the direction from the central axis toward the second radiation detector.

  [Operation / Effect] According to the above-described configuration, the thickness of the first scintillator provided in the first array is larger than the thickness of the second scintillator provided in the second array. The greater the thickness of the scintillator, the greater the chance that the scintillator will convert radiation into light, since there are more opportunities for radiation to be converted into light. According to the above-described configuration, the detection sensitivity of the first radiation detector having the thick first scintillator is reliably higher than the sensitivity of the second radiation detector having the thin second scintillator, and the first radiation The radiation detection sensitivity of the detector can surely be higher than that of the second radiation detector.

  The invention according to claim 3 is the radiation tomography apparatus according to claim 1, further comprising a third array in which third radiation detectors for detecting radiation are arranged in an arc shape along a virtual circle, In addition to the first array and the second array, the third array shares the respective central axes, and each array is arranged in the direction of the central axis in this order, thereby forming a detector ring. The radiation detection sensitivity is higher than that of the second radiation detector.

  [Operation / Effect] The detector ring for detecting radiation according to the present invention includes a first array, a second array, and a third array. Since the third array is located at the end of the detector ring, it is difficult to detect annihilation radiation pairs as compared to the second array located at the center of the detector ring. However, the radiation detection sensitivity of the third radiation detector constituting the third array is higher than that of the second radiation detector. As described above, the radiation tomography apparatus according to the present invention can suppress unevenness in the positional radiation sensitivity of the detector ring, and can provide a more uniform tomographic image over the imaging field of view of the detector ring.

  Further, the invention according to claim 4 is the radiation tomography apparatus according to claim 3, wherein the second radiation detector includes a second scintillator that converts radiation into light, and a photodetector that detects light, The third radiation detector includes a third scintillator that converts radiation into light and a light detector that detects light, and the second radiation detector in the second array is arranged such that the second scintillator faces the center of the virtual circle. The third radiation detectors in the third array are arranged so that the third scintillator faces the center of the virtual circle, and the third scintillator in the direction from the central axis toward the third radiation detector is arranged. The thickness is characterized by being thicker than the thickness of the second scintillator in the direction from the central axis toward the second radiation detector.

  [Operation and Effect] According to the above-described configuration, the third scintillator provided in the third array is thicker than the second scintillator provided in the second array. The greater the thickness of the scintillator, the greater the chance that the scintillator will convert radiation into light, since there are more opportunities for radiation to be converted into light. According to the above-described configuration, the detection sensitivity of the third radiation detector having the thick third scintillator is surely higher than the sensitivity of the second radiation detector having the thin second scintillator. The radiation detection sensitivity of the detector can surely be higher than that of the second radiation detector.

  The invention according to claim 5 is the radiation tomography apparatus according to any one of claims 1 to 4, wherein each of the arrays has a C shape along each virtual circle. To do.

  [Operation / Effect] According to the above-described configuration, it is possible to provide a radiation tomography apparatus capable of introducing the breast of the subject to the back of the detector ring. This is because if each of the arrays is C-shaped, the arm of the subject can be passed through the defect portion of the detector ring.

  The invention according to claim 6 is the radiation tomography apparatus according to any one of claims 1 to 4, wherein each of the arrays is O-shaped along each virtual circle. To do.

  [Operation / Effect] With the above-described configuration, a radiation tomography apparatus with high detection sensitivity can be provided. If the region of interest of the subject is surrounded by the detector ring without blind spots, the amount of annihilation radiation that can be detected increases. Therefore, it is possible to provide a radiation tomography apparatus that can acquire a clearer tomographic image.

  The invention according to claim 7 is the radiation tomography apparatus according to claim 5 or 6, wherein the apparatus is a mammography apparatus for examining the breast of a subject.

  [Operation / Effect] According to the above-described configuration, the radiation tomography apparatus is a mammography apparatus for examining the breast of a subject. If there is uneven detection sensitivity in the detector ring, it may seem that the subject may be moved deep inside the detector ring. However, in the mammography apparatus for breast examination, during the examination, the part other than the breast of the subject exists outside the detector ring, and the breast of the subject cannot be introduced deep inside the detector ring. . If the configuration of the present invention is applied to a mammography apparatus, a tomographic image suitable for diagnosis can be provided by an inexpensive apparatus.

3 is a functional block diagram illustrating a configuration of a radiation tomography layer according to Embodiment 1. FIG. 1 is a perspective view illustrating a configuration of a radiation detector according to Embodiment 1. FIG. It is a figure explaining the structure of the detector ring which concerns on Example 1. FIG. FIG. 6 is a schematic diagram for explaining the effect of the configuration according to the first embodiment. FIG. 6 is a schematic diagram for explaining the effect of the configuration according to the first embodiment. FIG. 6 is a schematic diagram for explaining the effect of the configuration according to the first embodiment. It is sectional drawing explaining the operation | movement explaining the structure which concerns on 1 modification of this invention. It is a top view explaining the structure which concerns on 1 modification of this invention. Explain the operation to explain the conventional configuration It is a schematic diagram explaining 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. The configuration of the first embodiment is a mammography apparatus for breast examination. FIG. 1 is a functional block diagram illustrating a specific configuration of the radiation tomography apparatus according to the first embodiment. The radiation tomography apparatus 9 according to the first embodiment includes a gantry 11 that introduces a subject's breast from the z direction, and a ring-shaped detector ring that introduces the subject's breast provided inside the gantry 11 from the z direction. 12. The inner hole provided in the detector ring 12 has a cylindrical shape (exactly an octagonal prism) extending in the z direction. Therefore, the detector ring 12 itself extends in the z direction. Note that the area of the inner hole of the detector ring 12 is a field of view in which a tomographic image of the radiation tomography apparatus 9 can be generated.

  The shielding plate 13 is made of tungsten or the like. Since the radiopharmaceutical is also present in parts other than the breast B of the subject, annihilation γ-ray pairs are also generated from there. However, when an annihilation gamma ray pair generated from a region other than such a region of interest enters the detector ring 12, it interferes with tomographic imaging. Therefore, a ring-shaped shielding plate 13 is provided so as to cover one end of the detector ring 12 on the side close to the subject M in the z direction.

  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 the filter unit 20 described later.

  The configuration of the detector ring 12 will be described. The detector ring 12 is formed by arranging eight radiation detectors 1 in a virtual circle on a plane perpendicular to the z direction (center axis direction) to form one unit ring. Three unit rings are arranged in the z direction to form a detector ring 12. Among the unit rings, the unit ring that is on the side where the subject's breast B is inserted and is adjacent to the shielding plate 13 is referred to as a first unit ring 12a, and the unit ring adjacent to the first unit ring 12a is the second unit ring. Called 12b. A unit ring adjacent to the second unit ring 12b is referred to as a third unit ring 12c [see FIG. 3B]. Thus, the detector ring 12 is configured by arranging the first unit ring 12a, the second unit ring 12b, and the third unit ring 12c in this order in the z direction. In addition, the radiation detector which comprises the 1st unit ring 12a is called the 1st radiation detector 1a, and the radiation detector which comprises the 2nd unit ring 12b is called the 2nd radiation detector 1b. And the radiation detector which comprises the 3rd unit ring 12c is called the 3rd radiation detector 1c. The sensitivity of radiation detection of the first radiation detector 1a is higher than the sensitivity of radiation detection of the second radiation detector 1b. Similarly, the radiation detection sensitivity of the third radiation detector 1c is higher than the sensitivity of the second radiation detector 1b. The first unit ring 12a corresponds to the first array of the present invention, and the second unit ring 12b corresponds to the second array of the present invention. The third unit ring 12c corresponds to the third array of the present invention.

  The configuration of the radiation detector 1 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 radiation into light, and a photodetector 3 that detects light. A light guide 4 for transmitting and receiving light is provided at a position where the scintillator 2 and the photodetector 3 are interposed.

The scintillator 2 is configured by scintillator crystals arranged three-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 light detector 3 can specify the light generation position of which scintillator crystal emits light, and also specifies the light intensity and the time when the light 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.

  Detection data output from the detector ring 12 is sent to the coincidence counting unit 21 (see FIG. 1) via the filter 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 21 counts the number of times that an annihilation γ-ray pair is detected for every two combinations of scintillator crystals constituting the detector ring 12, and sends the result to the tomographic image generation unit 22. The positional relationship of the scintillator crystals in the coincidence count indicates the position where the annihilation γ-ray pair is incident on the detector ring 12 and the direction in which the annihilation γ-ray pair is incident, and is information necessary for radiopharmaceutical mapping. The number of annihilation γ-ray pairs detected and the energy intensity of annihilation radiation stored for each combination of scintillator crystals indicates the variation in the generation of annihilation γ-ray pairs in the subject, and is necessary for mapping radiopharmaceuticals. It is. The coincidence of the detected data by the coincidence counting unit 21 uses time information given to the detected data by the clock 19.

  The filter unit 20 is provided for the purpose of preventing unnecessary data in the detector ring 12 from being sent to the coincidence unit 21. Since the coincidence counting unit 21 has to handle a large amount of data, it is likely to be loaded. The filter unit 20 can thin out the detection data so as to reduce the load on the coincidence counting unit 21. For example, all the detection data when the subject M is not inserted into the detector ring 12 is discarded by the filter unit 20 and is not input to the coincidence counting unit 21.

  The tomographic image generation unit 22 receives data relating to the generation position and radiation intensity of the annihilation γ-ray pair output from the coincidence counting unit 21, and generates a tomographic image in which the generation position of the annihilation radiation is spatially mapped. The tomographic image at this time is, for example, an axial image in which the subject's breast B is cut in a circle.

  The display unit 36 displays the tomographic image generated by the tomographic image generation unit 22, and the console 35 allows the operator to input various operations performed on the radiation tomography apparatus 9. The storage unit 37 stores the detection data output from the detector ring 12, the coincidence counting data generated by the coincidence counting unit 21, data generated by the operation of each unit, such as tomographic images, and all parameters referred to during the operation of each unit. To do.

  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 19, 20, 21, and 22 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.

<Specific configuration of detector ring>
Next, a specific configuration of the detector ring 12 which is the most characteristic configuration in the present invention will be described. FIG. 3 is a plan view and a cross-sectional view illustrating the configuration of the detector ring 12 according to the first embodiment. FIG. 3A is a plan view when the detector ring 12 is viewed from the z direction. The detector ring 12 is configured by arranging eight radiation detectors 1 in an annular shape. The inner hole of the detector ring 12 is a regular octagonal prism. Moreover, the inner wall of the detector ring 12 is all covered with the scintillator 2. Assuming that the center axis along the z direction of the detector ring 12 (the axis of rotational symmetry of the inner wall of the detector ring 12) is A, the center axis A is located at the center point of the inner hole of the detector ring 12. When each radiation detector 1 is viewed from the central axis A, all the scintillators 2 of the radiation detector 1 are directed to the central axis A. The radiation detector 1 includes a light guide 4 and a photodetector 3 on the back side of the scintillator 2 when viewed from the central axis A.

  The scintillator included in the first radiation detector 1a is referred to as a first scintillator 2a, and the scintillator included in the second radiation detector 1b is referred to as a second scintillator 2b. Similarly, the scintillator having the third radiation detector 1c is referred to as a third scintillator 2c.

  FIG. 3B shows a sectional view of the detector ring 12. As shown in FIG. 3B, in the detector ring 12, a first unit ring 12a, a second unit ring 12b, and a third unit ring 12c are arranged in this order in the z direction. The shape of the inner hole is the same over each unit ring 12a, 12b, 12c, and the width 16a, 16b, 16c of the inner hole of each unit ring 12a, 12b, 12c is the same. Accordingly, the front faces of the scintillators 2a, 2b, 2c of the radiation detectors 1a, 1b, 1c arranged in the z direction belong to a single plane, and the scintillators 2a, 2b , 2 c constitute one surface of the inner wall of the detector ring 12. With this configuration, the detector ring 12 that does not interfere with the entrance of the subject's breast B can be provided. In this way, the unit rings 12a, 12b, and 12c share the respective central axes (the rotational symmetry axes of the inner walls of the unit rings) and are arranged in this order in the central axis direction. The subject's breast B is inserted from the first unit ring 12a side.

  The characteristic configuration of the present invention is the width of each scintillator 2a, 2b, 2c. That is, the thicknesses of the scintillators 2a, 2b, and 2c in the direction from the central axis A toward the scintillators 2a, 2b, and 2c (radiation detectors 1a, 1b, and 1c) are different. The first scintillator 2a constituting the first unit ring 12a located on the breast B insertion side of the subject is thick in the direction away from the central axis A and has high radiation detection sensitivity. The material constituting the scintillator 2 is expensive. Therefore, the thicker the scintillator 2 is, the higher the price of the radiation detector 1 is. The first radiation detector 1a constituting the first unit ring 12a is expensive.

  On the other hand, the second scintillator 2b constituting the second unit ring 12b located at the center in the z direction of the detector ring 12 is thin in the direction away from the central axis A, and the radiation detection sensitivity is relatively low. . Instead, the second radiation detector 1b constituting the second unit ring 12b is inexpensive.

  And the 3rd scintillator 2c which comprises the 3rd unit ring 12c located in the furthest position from the breast B of a subject is thick in the direction away from the central axis A, and has high radiation detection sensitivity. Instead, the third radiation detector 1c constituting the third unit ring 12c is expensive. In addition, the thickness of the 3rd scintillator 2c is below the thickness of the 1st scintillator 2a. That is, the thickness of each scintillator in the direction away from the central axis A has a relationship of second scintillator 2b <third scintillator 2c ≦ first scintillator 2a.

  An effect in which the thickness of the scintillator 2 is configured as described above will be described. FIG. 4 is a schematic diagram of the detector ring 12. The annihilation gamma ray pair generated from the annihilation point P is incident on two different places of the detector ring 12. The longer the detector ring 12 is, the easier it is for the annihilation γ-ray pair to enter the detector ring 12, and the detection sensitivity of the annihilation γ-ray pair of the detector ring 12 becomes higher. By the way, among the both ends of the detector ring 12 in the z direction, the side close to the vanishing point P is defined as a first end 12p, and the far side is defined as a second end 12q. A distance from the vanishing point P to the first end portion 12p is a first distance R1, and a distance from the vanishing point P to the second end portion 12q is a second distance R2.

  One of the annihilation γ-ray pairs generated from the annihilation point P is directed to the first end 12p, and the other is directed to the second end 12q. If none of the annihilation γ-ray pairs is detected by the detector ring 12, simultaneous counting cannot be performed. Therefore, either of the γ-rays toward the first end 12p and the γ-rays toward the second end 12q. Also needs to be detected by the detector ring 12. Since the γ ray toward the second end 12q has a long R2, detection of the γ ray is easy. However, since the γ-ray directed toward the first end portion 12p needs to be detected by a part of the detector ring 12 having a length of R1, it is difficult to detect. Therefore, the detector ring 12 has a characteristic that the more the annihilation γ-ray pair whose generation position is near the end in the z direction, the harder it is to detect.

  The range of the detector ring 12 that can detect an annihilation gamma ray pair generated from a certain annihilation point P is determined by the first distance R1. Specifically, this range is a range R3 having a length twice the first distance R1 of the detector ring 12 (see FIG. 4). The center of the range R3 in the z direction coincides with the position of the vanishing point P in the z direction. The narrower the range R3, the more difficult it is to detect annihilation gamma ray pairs.

  Consider a vanishing point Pa close to the insertion side of the subject's breast B in the z direction of the detector ring 12 (see FIG. 5). In the detector ring 12, the range R3 in which the annihilation gamma ray pair emitted from the annihilation point Pa can be detected is narrow. Therefore, it is difficult to detect such annihilation γ-ray pairs. However, according to the configuration of the first embodiment, the thickness of the first scintillator 2a is thicker in the direction away from the central axis A, and the detection sensitivity of the first unit ring 12a is high. Therefore, the annihilation γ-ray pair emitted at the annihilation point Pa is detected with high sensitivity. Such a situation is the same when there is a vanishing point near the insertion side of the breast B of the subject in the z direction of the detector ring 12. However, since the breast B of the subject has a tapered shape as it goes from the first detector ring 12a to the third detector ring 12c, the dose of γ rays detected by the third unit ring 12c is The detection sensitivity of γ rays in the third unit ring 12c is not necessarily as high as that of the first unit ring 12a.

  Next, consider the vanishing point Pb located at the center of the detector ring 12 in the z direction (see FIG. 6). In the detector ring 12, the range R3 in which the annihilation gamma ray pair emitted from the annihilation point Pb can be detected is wide. This is because the vanishing point Pb is far from both ends 12p and 12q of the detector ring 12 in the z direction. Therefore, detection of such annihilation γ-ray pairs is easy, and annihilation γ-ray pairs emitted at the annihilation point Pb are detected with high sensitivity. Moreover, according to the configuration of the first embodiment, the thickness of the second scintillator 2b is thin, and the manufacturing cost of the second unit ring 12b is suppressed.

<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's breast B is inserted into the detector ring 12. When the surgeon instructs the detection of the annihilation gamma ray pair through the console 35, the detector ring 12 starts sending detection data to the filter unit 20. The detection data transmitted at this time is a data set in which the incident position, energy, and incident time of the radiation detector ring 12 are linked.

  The coincidence unit 21 performs coincidence on the detected data and sends the result to the tomographic image generation unit 22. The tomographic image generation unit 22 generates a tomographic image such that the breast B of the subject is cut into circles, and this is displayed on the display unit 36. Thus, the operation of the radiation tomography apparatus in Embodiment 1 is completed.

  As described above, the detector ring 12 for detecting radiation according to the first embodiment includes the first unit ring 12a, the second unit ring 12b, and the third unit ring 12c. Since the first unit ring 12a and the third unit ring 12c are located at both ends of the detector ring 12 in the z direction, the γ disappears in comparison with the second unit ring 12b located at the center of the detector ring 12. Detection of line pairs is difficult. However, the radiation detection sensitivity of the first radiation detector 1a and the third radiation detector 1c constituting the first unit ring 12a and the third unit ring 12c is higher than that of the second radiation detector 1b. As described above, the radiation tomography apparatus 9 according to the first embodiment suppresses unevenness of the positional radiation sensitivity of the detector ring 12, and a more uniform tomographic image can be obtained over the imaging field range of the detector ring 12. Can be provided.

  In addition, according to the first embodiment, since the detection sensitivity of the first unit ring 12a and the third unit ring 12c is mainly increased, the manufacturing cost of the second unit ring 12b is not increased. The effect of Example 1 can be achieved.

  Further, according to the configuration of the first embodiment, the thickness of the first scintillator 2a and the third scintillator 2c provided in the first unit ring 12a and the third unit ring 12c is the second scintillator provided in the second unit ring 12b. It is thicker than 2b. As the thickness of the scintillator 2 increases, the chance that the radiation is converted into light increases, so the ability of the scintillator 2 to convert the radiation into light is improved. According to the configuration of the first embodiment, the first radiation detector 1a having the thick first scintillator 2a and the third scintillator 2c and the detection sensitivity of the third radiation detector 1c are the first having the thin second scintillator 2b. The radiation detection sensitivity of the first radiation detector 1a and the third radiation detector 1c can surely be higher than that of the second radiation detector 1b. .

  And according to the structure of Example 1, the radiation tomography apparatus 9 with high detection sensitivity can be provided. If the region of interest of the subject is surrounded by the detector ring 12 without a blind spot, more annihilation radiation can be detected. Therefore, the radiation tomography apparatus 9 which can acquire a clearer tomographic image can be provided.

  According to the configuration of the first embodiment, the radiation tomography apparatus 9 is a mammography apparatus that examines the breast B of the subject. If there is uneven detection sensitivity in the detector ring 12, it seems at first glance that the subject should be moved deep inside the detector ring 12. However, in the mammography apparatus for breast examination, a part other than the breast B of the subject is present outside the detector ring 12 during the examination, and the breast of the subject is introduced deep inside the detector ring 12. I can't. If the configuration of the first embodiment is applied to a mammography apparatus, a tomographic image suitable for diagnosis can be provided by an inexpensive apparatus.

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

  (1) According to the configuration of the first embodiment, the configuration has the third unit ring 12c. However, as shown in FIG. In breast examination, if it is expected that lesions frequently occur in the chest wall (the base of the breast) of the subject, it is not always necessary to increase the detection sensitivity of the whole breast. According to this modification, it is possible to provide a radiation tomography apparatus in which the detection sensitivity of the annihilation gamma ray pair in the chest wall of the subject is enhanced and the manufacturing cost is further suppressed.

  (2) According to the configuration of the first embodiment, the detector ring 12 has a ring shape, but instead of this, as shown in FIG. 8, a C-shaped detector ring 12 having a defective portion may be used. Good. Each unit ring 12a, 12b, 12c is also provided with a defect portion, and these are stacked in the z direction to constitute the detector ring 12. The gantry 11 and the shielding plate 13 are also C-shaped in accordance with the shape of the detector ring 12. By configuring in this way, it is possible to provide the radiation tomography apparatus 9 that can introduce the subject's breast B to the back of the detector ring 12. This is because the arm of the subject can be passed through the defect portion of the detector ring 12.

(3) The scintillator crystal referred to in each of the above embodiments was composed of LYSO. However, in the present invention, the scintillator crystal is composed of other materials such as GSO (Gd ₂ SiO ₅ ) instead. Also good. According to this modification, it is possible to provide a method of manufacturing a radiation detector that can provide a cheaper radiation detector.

  (4) In each of the embodiments 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.

A central axis 1a first radiation detector 1b second radiation detector 1c third radiation detector 2a first scintillator 2b second scintillator 2c third scintillator 9 radiation tomography apparatus 12 detector ring 12a first unit ring (first unit ring) array)
12b Second unit ring (second array)
12c Third unit ring (third array)

Claims (7)

A first array in which first radiation detectors for detecting radiation are arranged in an arc along a virtual circle;
A second radiation detector for detecting radiation comprises a second array arranged in an arc along a virtual circle;
When the axis that is orthogonal to each virtual circle and passes through the center of each virtual circle is the central axis,
The first array and the second array share the center axis of each other and are arranged in the direction of the center axis, thereby forming a detector ring having an opening for introducing the subject from the first array side. ,
The radiation tomography apparatus according to claim 1, wherein the radiation detection sensitivity of the first radiation detector is higher than that of the second radiation detector. The radiation tomography apparatus according to claim 1,
The first radiation detector includes a first scintillator that converts radiation into light, and a photodetector that detects light,
The second radiation detector includes a second scintillator that converts radiation into light, and the photodetector that detects light,
The first radiation detectors in the first array are arranged so that the first scintillator faces the center of a virtual circle, and the second radiation detectors in the second array have a virtual second scintillator. Arranged to face the center of the circle,
The thickness of the first scintillator in the direction from the central axis toward the first radiation detector is thicker than the thickness of the second scintillator in the direction from the central axis toward the second radiation detector. A radiation tomography apparatus. The radiation tomography apparatus according to claim 1,
A third radiation detector for detecting radiation further comprising a third array arranged in an arc along a virtual circle;
In addition to the first array and the second array, the third array shares the central axis, and the arrays are arranged in the direction of the central axis in this order, thereby configuring the detector ring.
The radiation tomography apparatus according to claim 3, wherein the radiation detection sensitivity of the third radiation detector is higher than that of the second radiation detector. The radiation tomography apparatus according to claim 3,
The second radiation detector includes the second scintillator that converts radiation into light, and a photodetector that detects light,
The third radiation detector includes a third scintillator that converts radiation into light, and the photodetector that detects light,
The second radiation detectors in the second array are arranged so that the second scintillator faces the center of the virtual circle, and the third radiation detectors in the third array are virtual in the third scintillator. Arranged to face the center of the circle,
The thickness of the third scintillator in the direction from the central axis toward the third radiation detector is thicker than the thickness of the second scintillator in the direction from the central axis toward the second radiation detector. A radiation tomography apparatus. The radiation tomography apparatus according to any one of claims 1 to 4,
Each of the arrays has a C shape along each virtual circle. The radiation tomography apparatus according to any one of claims 1 to 4,
Each of the arrays is O-shaped along each virtual circle. The radiation tomography apparatus according to claim 5 or 6,
A radiation tomography apparatus characterized by being a mammography apparatus for examining a breast of a subject.

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