Pet device and calibration method

Abstract

To enable calibration of DOI detectors.SOLUTION: A PET device according to an embodiment comprises an acquisition unit and calibration unit. The acquisition unit acquires detection datasets, each corresponding to a depth of gamma ray interaction in each of a plurality of DOI detectors. The calibration unit calibrates the DOI detectors based on the acquired detection datasets and an ideal distribution of the detection datasets.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The embodiments disclosed herein and in the drawings relate to a PET device and a calibration method. [Background technology]

[0002] In positron emission tomography (PET), a tracer agent is introduced into the patient, and the physical and biomolecular properties of the agent cause it to concentrate at a specific location in the patient's body. The tracer emits a positron, which collides with an electron, creating an annihilation event. This annihilation event produces two gamma rays (of 511 keV) that travel effectively 180 degrees apart.

[0003] PET devices use a PET scanner with multiple detectors arranged in a ring around the patient to detect coincidence pairs of gamma rays. The PET scanner measures the scintillation light pulses generated by the interaction of incident gamma rays with the detectors. That is, PET scanners consist of thousands of individual scintillator crystals arranged in a two-dimensional scintillator array packaged in a module with a photosensor. Gamma rays are then converted into scintillation light within the scintillator crystals and detected as scintillation events by one or more photosensors.

[0004] To reconstruct the spatiotemporal distribution of the tracer by the principles of tomographic reconstruction, each detected event is characterized by its energy (i.e. the amount of light produced), location, and timing. By detecting two gamma rays and drawing a line between them, i.e. the Line-of-Response (LOR), the location of the annihilation event can be estimated on the LOR. There is also a widely known time-of-flight (TOF) technique that measures the time difference between when two gamma rays enter the detector and estimates the location of the annihilation event to a certain range on the LOR based on the time difference.

[0005] In recent years, in addition to TOF technology, TOF-DOI detectors (hereinafter referred to as DOI detectors) that can utilize DOI (Depth of interaction) information corresponding to the depth at which gamma rays incident from the surface of a scintillator crystal interact have been known. With DOI detectors, for example, even if an annihilation event occurs at the edge of the image field of the detector ring and the generated gamma rays are incident on the scintillator crystal from an oblique direction, the depth at which the gamma rays interact can be estimated. Therefore, since the LOR can be limited to the range connecting the relevant depth among the ranges connecting the individual scintillator crystals that have interacted in each of the opposing DOI detectors, it is possible to suppress a decrease in estimation accuracy when estimating the occurrence position of the annihilation event at the edge of the image field on the LOR. Each scintillator crystal used in such a DOI detector can be calibrated by linking the DOI information with the signal output by irradiating each depth from the side with thin radiation focused by a collimator or the like.

[0006] However, the DOI detector cannot be calibrated after the detector ring is assembled because radiation cannot be irradiated to each depth from the side of each scintillator crystal. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2013-140160 A Summary of the Invention [Problem to be solved by the invention]

[0008] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to calibrate a DOI detector. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problem. Problems corresponding to the effects of each configuration shown in the embodiments described later can also be positioned as other problems. [Means for solving the problem]

[0009] The PET device according to the present embodiment includes an acquisition unit and a calibration unit. The acquisition unit acquires detection data corresponding to depths at which gamma rays interact in a plurality of DOI detectors. The calibration unit calibrates the DOI detectors based on each of the acquired detection data and an ideal distribution of the detection data. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the configuration of a PET apparatus according to the first embodiment. [Diagram 2] FIG. 2 is a schematic diagram showing the configuration of the DOI detector according to the first embodiment. [Diagram 3] FIG. 3 is a schematic diagram for explaining an ideal distribution according to the first embodiment. [Figure 4] FIG. 4 is a schematic diagram for explaining the speeds of gamma rays and photons in the scintillator crystal in the first embodiment. [Diagram 5] FIG. 5 is a schematic diagram for explaining the DOI information and the ideal distribution in the first embodiment. [Figure 6] FIG. 6 is a schematic diagram for explaining the count number and ideal distribution in the first embodiment. [Figure 7] FIG. 7 is a flowchart for explaining the operation in the first embodiment. [Figure 8] FIG. 8 is a flowchart for explaining step ST20 in FIG. [Figure 9] FIG. 9 is a schematic diagram for explaining step ST21 in FIG. [Figure 10] FIG. 10 is a schematic diagram for explaining step ST22 in FIG. [Figure 11] FIG. 11 is a schematic diagram for explaining the walk correction. [Figure 12] FIG. 12 is a schematic diagram for explaining an ideal distribution in a modified example of the first embodiment. [Figure 13]FIG. 13 is a schematic diagram for explaining Compton scattering in another modified example of the first embodiment. [Figure 14] FIG. 14 is a flowchart for explaining the operation in the second embodiment. [Figure 15] FIG. 15 is a schematic diagram for explaining the DOI information and the ideal distribution in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, each embodiment will be described with reference to the drawings. In the following description, the same reference numerals are used to designate substantially the same parts in different drawings, and redundant description will be omitted.

[0012] <First embodiment> FIG. 1 is a diagram showing the configuration of a PET (positron emission tomography) device according to a first embodiment, and FIG. 2 is a schematic diagram showing the configuration of a DOI (depth of interaction) detector. As shown in FIG. 1, the PET device 1 has a PET gantry 10, a bed 50, and a console 70. Typically, the PET gantry 10 and the bed 50 are installed in a common examination room. The console 70 is installed in a control room adjacent to the examination room. The PET gantry 10 is an imaging device that performs PET imaging of a subject P. The bed 50 movably supports a top plate 53 on which the subject P to be imaged is placed. The console 70 is a computer that controls the PET gantry 10 and the bed 50. The subject P is, for example, a phantom or a human body.

[0013] Here, the PET gantry 10 includes a detector ring 11 , a signal processing circuit 13 , and a coincidence circuit 15 .

[0014] The detector ring 11 has a plurality of DOI detectors 17 arranged on a circumference around the central axis Z. An image field of view (FOV) is set at the opening of the detector ring 11. The subject P is positioned so that the imaging area of ​​the subject P is included in the image field of view. If the subject P is a human body, a drug labeled with a positron-emitting nuclide is administered to the subject P, and positrons are emitted from the positron-emitting nuclide. If the subject P is a phantom, positrons are emitted from a radiation source arranged at a predetermined position within the phantom. The emitted positrons annihilate with surrounding electrons, generating a pair of gamma rays.

[0015] The DOI detector 17 detects gamma rays emitted from the subject P and generates an electrical signal according to the amount of light of the detected gamma rays. The DOI detector 17 is preferably configured such that, for example, a photosensor is provided on only one side of a scintillator crystal, and DOI information can be acquired without reducing time resolution. For example, as shown in FIG. 2, the DOI detector 17 includes a plurality of scintillator crystals 171 and a plurality of photosensors 172. For example, the structure of the DOI detector 17 uses a many-to-many connection (m1:m2 connection) in which m1 scintillator crystals 171 and m2 photosensors 172 are connected. However, m1 and m2 are different natural numbers, and one of them is larger than the other. The many-to-many connection may be a many-to-one connection, but from the viewpoint of easily performing center of gravity calculation, a many-to-many connection or a one-to-many connection is preferable. Moreover, the many-to-many connection does not include a one-to-one connection from the viewpoint of performing center of gravity calculation. The DOI detector may be called a TOF-DOI detector.

[0016] The scintillation counter crystal 171 generates light (scintillation photons) upon receiving gamma rays emitted from within the subject P. That is, the scintillation counter crystal 171 converts the incident gamma rays into photons.

[0017] The photosensor 172 is provided on one side of the scintillator crystal 171 and generates an electrical signal according to the amount of light generated by the scintillator crystal 171. As the photosensor 172, for example, a photomultiplier tube (PMT) or a silicon photomultiplier tube (SiPM) can be appropriately used. The generated electrical signal is supplied to the signal processing circuit 13.

[0018] The signal processing circuit 13 generates single event data based on the electrical signal of the DOI detector 17. Specifically, the signal processing circuit 13 performs detection time measurement processing, position calculation processing, and energy calculation processing. The signal processing circuit 13 is realized by an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), another complex programmable logic device (CPLD), or a simple programmable logic device (SPLD) configured to be able to execute the detection time measurement processing, position calculation processing, and energy calculation processing.

[0019] In the detection time measurement process, the signal processing circuit 13 measures the detection time of the gamma ray by the DOI detector 17. Specifically, the signal processing circuit 13 monitors the peak value of the electric signal from the DOI detector 17, and measures the time when the peak value exceeds a preset threshold as the detection time. In other words, the signal processing circuit 13 detects the gamma ray electrically by detecting that the peak value exceeds the threshold. In the position calculation process, the signal processing circuit 13 calculates the incident position of the pair annihilation gamma ray based on the electric signal from the DOI detector 17. The incident position of the gamma ray corresponds to the position coordinate of the scintillator on which the gamma ray is incident. In the energy calculation process, the signal processing circuit 13 calculates the energy value of the detected gamma ray based on the electric signal from the DOI detector 17. The detection time data, position coordinate data, and energy value data related to the single event are associated with each other. The combination of the energy value data, position coordinate data, and detection time data related to the single event is called single event data. The single event data is generated one after another every time a gamma ray is detected. The generated single event data is supplied to the coincidence counting circuit 15 .

[0020] The coincidence circuit 15 performs coincidence processing on the single event data from the signal processing circuit 13. As hardware resources, the coincidence circuit 15 is realized by an ASIC, FPGA, CPLD, or SPLD configured to be able to perform coincidence processing. In the coincidence processing, the coincidence circuit 15 repeatedly identifies single event data related to two single events that fall within a predetermined time frame from the repeatedly supplied single event data. This pair of single events is presumed to be derived from a gamma ray generated in an annihilation event. The pair of single events are collectively called coincidence events. A line connecting the pair of DOI detectors 17 (more specifically, the depth within the scintillator crystal) that detected this gamma ray is called an LOR (line of response). Event data related to the pair of events that make up the LOR is called coincidence event data. The coincidence event data and single event data are transmitted to the console 70. When there is no particular distinction between the coincidence event data and the single event data, they will be called PET event data. Each of the coincidence event data, single event data, and PET event data is an example of detection data. Note that "gamma rays emitted from a radiation source administered to the subject P" may be called "gamma rays emitted from the subject P." Furthermore, the "radiation source" may be called a "positron emission source," "positron emitting nuclide," "radioisotope," etc. The detector ring 11, the signal processing circuit 13, and the coincidence circuit 15 may be called a detection data acquisition unit that acquires detection data.

[0021] In the above configuration, the signal processing circuit 13 and the coincidence circuit 15 are included in the PET gantry 10, but this embodiment is not limited to this. For example, the coincidence circuit 15, or both the signal processing circuit 13 and the coincidence circuit 15, may be included in a device separate from the PET gantry 10. Also, one coincidence circuit 15 may be provided for the multiple signal processing circuits 13 mounted on the PET gantry 10, or the multiple signal processing circuits 13 mounted on the PET gantry 10 may be divided into multiple groups and one coincidence circuit 15 may be provided for each group.

[0022] The bed 50 places the subject P to be scanned on it and moves the subject P placed on it. The bed 50 includes a base 51, a support frame 52, a top plate 53, and a bed drive device 54. The base 51 is placed on the floor surface. The base 51 is a housing that supports the support frame 52 so as to be movable in a direction perpendicular to the floor surface (Y-axis direction). The support frame 52 is a frame provided on the upper part of the base 51. The support frame 52 supports the top plate 53 so as to be slidable along the central axis Z. The top plate 53 is a flexible plate on which the subject P is placed. In this embodiment, the longitudinal direction of the top plate 53 of the bed 50 is defined as the Z-axis direction, the axial direction perpendicular to the Z-axis direction and horizontal to the floor surface is defined as the X-axis direction, and the axial direction perpendicular to the Z-axis direction and perpendicular to the floor surface is defined as the Y-axis direction. The bed 50 is disposed so that the longitudinal axis of the top plate 53 is parallel to the central axis Z of the opening of the PET gantry 10.

[0023] The bed driving device 54 is housed in the housing of the bed 50. The bed driving device 54 is a motor or actuator that generates power for moving the support frame 52 on which the subject P is placed and the tabletop 53. The bed driving device 54 operates under the control of the console 70 or the like.

[0024] The console 70 is a device including a PET data memory 71, a processing circuit 73, a display 74, a memory 75, and an input interface 76. For example, data communication between the PET data memory 71, the processing circuit 73, the display 74, the memory 75, and the input interface 76 is performed via a bus.

[0025] The PET data memory 71 is a storage device that stores the single event data and coincidence event data transmitted from the PET gantry 10. The PET data memory 71 is a storage device such as a hard disk drive (HDD), a solid state drive (SSD), or an integrated circuit storage device.

[0026] The processing circuitry 73 controls the operation of the entire PET device 1 in response to an electric signal of an input operation output from the input interface 76. For example, the processing circuitry 73 has a processor such as a CPU, MPU, or GPU (Graphics Processing Unit) and a memory such as a ROM or RAM as hardware resources. The processing circuitry 73 executes various programs read from the memory to realize an imaging control function 731, an acquisition function 732, a calibration function 733, a reconstruction function 734, and a display control function 735. The imaging control function 731, the acquisition function 732, the calibration function 733, the reconstruction function 734, and the display control function 735 may be implemented by the processing circuitry 73 of one board, or may be distributed and implemented by the processing circuits 73 of multiple boards. In addition, the various programs may include, for example, a program for causing a computer to realize the acquisition function 732 and the calibration function 733. In addition, for example, the various programs may include a program for causing the PET device 1 to execute a calibration method. For example, the calibration method may include acquiring detection data corresponding to depths at which gamma rays interact in the multiple DOI detectors, and calibrating the DOI detector 17 based on each of the acquired detection data and an ideal distribution of the detection data. The program may also be read from memory 75. The memory in processing circuit 73 and memory 75 are each an example of a non-transitory computer-readable storage medium.

[0027] In the imaging control function 731, the processing circuitry 73 synchronously controls the PET gantry 10 and the bed 50 to perform PET imaging. The processing circuitry 73 can also execute a positioning scan using the PET gantry 10.

[0028] The acquisition function 732 acquires each detection data corresponding to a depth at which the gamma ray interacts in the multiple DOI detectors 17. Each detection data is, for example, single event data. The acquisition function 732 and the processing circuit 73 are an example of an acquisition unit.

[0029] The calibration function 733 calibrates the DOI detector 17 based on each of the acquired detection data and the ideal distribution of the detection data. For example, the calibration function 733 corrects the depth corresponding to the detection data in the DOI detector 17 to be calibrated so as to approach the ideal distribution. Note that the calibration function 733 may correct the detection data for a time shift due to a difference in rise time of the output signal from the DOI detector 17 prior to the correction based on the ideal distribution. The calibration function 733 and the processing circuit 73 are an example of a calibration unit.

[0030] Here, the ideal distribution is information showing a correlation between ideal time difference information and DOI information. For example, the ideal distribution is a distribution that expresses the depth at which gamma rays interact in the calibrated DOI detector 17 and the time difference at which each detection data is detected by the multiple DOI detectors 17 in association with each other. In the example shown in Fig. 3, the ideal distribution expresses measurement time difference information that expresses the difference in measurement time at which a pair of gamma rays are detected in association with DOI information that expresses the depth at which interaction occurs between the surface of the scintillator crystal 171 and the optical sensor 172. Note that the measurement time difference information may be called timing jitter information.

[0031] The depth represented by the ideal distribution is the depth at which gamma rays incident on one of the multiple DOI detectors 17 arranged opposite each other interact with each other. In other words, the depth represented by the ideal distribution is the depth at which one of a pair of gamma rays interacts with each other within the scintillator crystal 171. This ideal distribution is a distribution in which the time difference decreases as the depth increases, and has, for example, a linear or curved shape.

[0032] The time difference represented by the ideal distribution may include the time difference Δt in the measured time difference (TOF information) of gamma rays between a pair of DOI detectors 17, as shown in Figure 4, between the gamma ray 30 and the photon 40 generated by the interaction 34 of the gamma ray 30 traveling within the scintillator crystal 171.

[0033] In the first path shown in the upper part of FIG. 4, the depth where the interaction 34 occurs is located approximately in the center of the scintillator crystal 171. In addition, in the second path shown in the lower part, the depth where the interaction 34 occurs is located on the surface side of the scintillator crystal 171. Here, the distance between the depth of the interaction 34 in the first path and the depth of the interaction in the second path is Δd, and the speed of the gamma ray 30 in the scintillator crystal 171 is vγ(=c) (where c represents the speed of light). In addition, the speed of the photon 40 in the scintillator crystal 171 is vp(=c / n) (where n represents the refractive index of the scintillator crystal 171). At this time, the time difference Δt traveling in the scintillator crystal 171 is expressed by the formula Δt=Δd(vγ-cp).

[0034] That is, the velocity vγ of the gamma ray 30 hardly changes even in the scintillator crystal 171, but the velocity vp of the photon 40 generated after the interaction 34 is slowed down by the refraction of the scintillator crystal 171. Therefore, the farther the depth position of the interaction 34 is from the optical sensor 172, the slower the time until the gamma ray 30 is detected by the optical sensor 172 (second path in the lower part). In other words, the closer the depth position of the interaction 34 is to the optical sensor 172, the faster the time until the gamma ray 30 is detected (first path in the upper part). In this way, a difference occurs in the measurement time obtained from the signal output of the optical sensor 172 depending on the depth of the interaction 34. Therefore, as shown in FIG. 3, a proportional relationship is established as an ideal correlation between the depth represented by the ideal distribution (DOI information) and the measurement time difference (TOF information).

[0035] However, there are variations in the time resolution of the PET gantry 10 and the accuracy of the time information by the optical sensor 172. Therefore, in reality, the relationship between the DOI information and the measurement time difference does not have the linear shape shown in FIG. 3, but has a shape in which multiple gentle distributions overlap at the ends as shown in FIG. 5. Supplementally, FIG. 5(a) shows the actual DOI information when the scintillator crystal 171 is divided into three in the depth direction. However, the division in the depth direction is not limited to three divisions. In the figure, the DOI information representing the depth located on the crystal surface side is DOI1, the DOI information representing the depth located approximately in the center is DOI2, and the DOI information representing the depth located on the optical sensor 172 side is DOI3. FIG. 5(b) shows the relationship between the DOI information and the measurement time difference when divided into three. In the figure, the relationship between the actual DOI information DOI1 to DOI3 and the measurement time difference is shown as a convex distribution of the proportional relationship represented by each DOI information DOI1, ..., DOI3, which overlaps at the end. When the ideal distribution 60 representing the proportional relationship described above is superimposed on this actual relationship, the difference between each DOI information and the ideal distribution 60 is large at the overlapping end. For this reason, the calibration function 733 corrects the DOI information in the calibrated DOI detector 17 so as to approach the ideal distribution 60.

[0036] In addition, the ideal distribution may use the correlation between the ideal event count and the DOI information to improve accuracy. For example, as shown in FIG. 6, the ideal distribution may express the relationship between the count number of photons 40 converted from gamma rays 30 and the depth represented by the DOI information in the form of a curve. In the figure, the ideal distribution 60a is a distribution that expresses the relationship between the depth at which the gamma rays 30 cause an interaction 34 in the DOI detector 17 to be calibrated and the count number of photons 40 generated by the interaction 34 with the gamma rays 30 in the DOI detector 17 to be calibrated. The ideal distribution 60a is a distribution in which the count number decreases as the depth increases. For example, the ideal distribution 60a may be a distribution that follows the formula I=Io exp(-μt), where t is the depth, I is the count number at the depth t, Io is the count number near the surface of the DOI detector 17, and μ is the attenuation coefficient of the gamma rays 30 in the DOI detector 17. The DOI detector 17 may include multiple types of scintillator crystals 171 along the depth direction between the crystal surface and the optical sensor 172. For example, materials having different reaction cross sections can be used as the multiple types of scintillator crystals 171. In this case, an ideal distribution reflecting the different reaction cross sections may be used. The ideal distribution 60a shown in FIG. 9 corresponds to the case where the scintillator crystal 171 is a single crystal. In the figure, the relationship between the actual DOI information DOI1 to DOI3 and the count number is shown as a mountain-shaped distribution represented by each of the DOI information DOI1, . . . , DOI3 overlapping at the end. When the ideal distribution 60a represented by the above-mentioned exponential function formula is superimposed on this actual relationship, the error between each of the DOI information and the ideal distribution 60 is large at the end where they overlap. For this reason, the calibration function 733 corrects the DOI information in the DOI detector 17 to be calibrated so as to approach the ideal distribution 60a.

[0037] In the reconstruction function 734 , the processing circuitry 73 reconstructs a PET image showing the distribution of positron-emitting nuclides within the subject P based on the coincidence event data transmitted from the PET gantry 10 .

[0038] In the display control function 735, the processing circuitry 73 displays various information on the display 74. For example, the processing circuitry 73 displays the PET image reconstructed by the reconstruction function 734.

[0039] The display 74 displays various information under the control of the processing circuit 73 in the display control function 735. As the display 74, for example, a CRT (Cathode Ray Tube) display, a liquid crystal display, an organic EL (Electro-Luminescence) display, an LED (Light Emitting Diode) display, a plasma display, or any other display known in the art can be appropriately used.

[0040] The memory 75 is a storage device such as an HDD, SSD, integrated circuit storage device, etc., that stores various information. The memory 75 may also be a drive device that reads and writes various information to and from a portable storage medium such as a CD-ROM (Compact Disk Read Only Memory) drive, a DVD (Digital Versatile Disc) drive, or a flash memory.

[0041] The input interface 76 inputs various commands from a user. Specifically, the input interface 76 is connected to an input device. Examples of the input device that can be used include a keyboard, a mouse, a trackball, a joystick, a touchpad, a touch screen in which a display screen and a touchpad are integrated, a non-contact input circuit using an optical sensor, a voice input circuit, and various switches. The input interface 76 supplies an output signal from the input device to the processing circuit 73 via a bus. Note that in this specification, the input interface 76 is not limited to an interface having physical operation parts such as a mouse and a keyboard. For example, an example of the input interface 76 also includes an electrical signal processing circuit that receives an electrical signal corresponding to an input operation from an external input device provided separately from the device and outputs the electrical signal to the processing circuit 73.

[0042] Next, the calibration operation of the DOI detector 17 by the PET apparatus 1 configured as above will be described with reference to the flow charts of FIGS. 7 and 8 and the schematic diagrams of FIGS.

[0043] First, the processing circuitry 73 of the PET device 1 controls the PET gantry 10 to perform PET imaging. Through this PET imaging, coincidence counting event data based on the signal output of the DOI detector 17 in the detector ring 11 is collected in the PET data memory 71 in the console 70. Each of the coincidence counting event data includes a pair of single event data (respective detection data).

[0044] Next, in step ST10, the processing circuitry 73 acquires from the PET data memory 71 each piece of detection data corresponding to the depth at which the gamma ray interacted in the multiple DOI detectors 17.

[0045] After step ST10, in step ST20, the processing circuit 73 calibrates the DOI detector 17 based on each of the acquired detection data and the ideal distribution of the detection data. Such step ST20 includes steps ST21 to ST24, as shown in FIG.

[0046] In step ST21, the processing circuit 73 acquires the measurement time difference between the opposing DOI detectors 17, 17A as shown in FIG. 9. First, the processing circuit 73 counts the measurement time difference between the two DOI detectors 17, 17A as a timing histogram without considering the depth position of the interaction 34. The timing histogram has a mixture of many DOI information (depth positions) and has a mountain-like shape including a shift before and after the time difference with the largest number of interactions 34 at the center. This shift is caused, for example, by a relative delay between the signal path from the optical sensor 172 that detects light and outputs a signal to the signal processing circuit 13 that performs detection time measurement processing, and is preferably calibrated. This calibration may be called timing calibration.

[0047] A supplementary explanation of timing calibration is provided. In the figure, an LOR including the first and second paths corresponding to the trajectories of the gamma rays 30, 30A detected by the DOI detectors 17, 17A is drawn between the positions of the DOI detectors 17 and 17A. The LOR includes the first and second paths propagated by the gamma rays 30, 30A. The difference (L1-L2) between the length L1 of the first path corresponding to the trajectory of the gamma rays 30 and the length L2 of the second path corresponding to the trajectory of the gamma rays 30A corresponds to the distance between the positions of the DOI detectors 17, 17A and the position of the radiation source. This difference (L1-L2), together with the speed of light c, is used to calculate the measured time difference Δt_TOF between the two detection events. The measured time difference Δt_TOF is expressed by the formula Δt_TOF=t1-t2=(L1-L2) / c. where times t1 and t2 are the measurement times at which the respective interactions 34 and 34A are detected. Thus, the obtained measurement time difference is calibrated (timing calibration) by comparing it with the calculated measurement time difference. This calibration is improved by performing statistical analysis on the results of repeated measurements to minimize statistical uncertainty.

[0048] After step ST21, in step ST22, the processing circuit 73 acquires DOI information by a center of gravity calculation based on the signal output of the optical sensor 172 that detected the photons 40 converted from the gamma rays 30. The center of gravity calculation utilizes the fact that, as shown in Fig. 10, the farther the depth position of the interaction 34 is from the optical sensor 172, the wider the range in which the photons 40 are detected, and the closer the depth position of the interaction 34 is to the optical sensor 172, the narrower the range in which the photons 40 are detected. By such a center of gravity calculation, DOI information indicating the depth of the interaction 34 is acquired based on the spread of the photons 40.

[0049] After step ST22, in step ST23, the processing circuit 73 plots the DOI information acquired in step ST22 and the measurement time difference acquired in step ST21 on a graph with the DOI information on the horizontal axis and the measurement time difference on the vertical axis. At this time, it is preferable that the processing circuit 73 performs walk correction for correcting a time shift caused by a slower rise of a signal pulse as the energy of the detected gamma ray becomes lower.

[0050] To supplement, the walk correction is a technique for correcting the time shift Tw in a situation where two signal pulses S1 and S2 arriving at the same time have a time shift Tw caused by the difference in rise times ts1 and ts2 exceeding a threshold Th, as shown in FIG. 11. In the figure, the horizontal axis indicates time, and the vertical axis indicates voltage amplitude. The two signal pulses S1 and S2 have the same shape with different magnitudes. One signal pulse S2 has a signal peak height that is lower than the signal peak height of the other signal pulse S1. As a result of the different magnitudes, the two signal pulses S1 and S2 are recorded as occurring at different times despite arriving at the same time. The difference in the recorded arrival times arises from the rising edges of the signal amplitudes of the signal pulses S1 and S2 exceeding the threshold Th at different times. For example, the signal pulse S1 reaches the threshold Th at rise time ts1. The signal pulse S2 reaches the threshold Th at rise time ts2. Since the rise time ts2 of the signal pulse S2 is larger (slower) than the rise time ts1 of the signal pulse S1, the recorded arrival time includes a time shift (Tw) due to the time walk. Note that since the rise time of the signal pulse is, for example, shorter for higher energies, it is preferable to perform the walk correction according to the magnitude of the energy. For example, the time shift due to the time walk is calculated as a function of the recorded energy of the gamma ray. For example, the gamma ray energy (e.g., voltage amplitude) for the signal pulse may be based on the integrated area under the signal curve, the peak height of the signal pulse, the amount of time above a threshold, or other measured quantity that is monotonically related to the energy of the gamma ray. The walk correction may be performed based on curve fitting a predetermined functional form, for example, by selecting coefficients that minimize an error function (e.g., root mean square error (RMS)) between the functional form and the empirically derived quantity (i.e., the measured time shift and energy of the pulse). Alternatively, the walk correction may be performed using a look-up table of empirically measured values ​​from which the time shift due to the time walk value can be interpolated / extrapolated.

[0051] After step ST23, in step ST24, the processing circuit 73 calibrates the DOI information based on the ideal distribution. That is, the processing circuit 73 corrects the DOI information in the DOI detector 17 to be calibrated so as to approach the ideal distribution 60. This ends step ST20, which includes steps ST21 to ST24.

[0052] 7, after step ST20, in step ST30, the processing circuit 73 determines whether or not the calibration is completed, and if not, returns to step ST10 and repeats the processing of steps ST10 to ST30. For example, the processing circuit 73 stores ID information for identifying all DOI detectors 17 of the detector ring 11 in association with a calibration completion flag in the memory 75, and determines that there is an uncalibrated DOI detector 17 based on the stored contents. On the other hand, if the determination results in the calibration being completed, the processing circuit 73 ends the processing.

[0053] As described above, according to the first embodiment, the processing circuitry 73 acquires each piece of detection data corresponding to the depth at which the gamma rays interacted in the multiple DOI detectors 17, 17A. The processing circuitry 73 calibrates the DOI detector 17 based on each piece of acquired detection data and the ideal distribution of detection data 60. This makes it possible to calibrate the DOI detector by using the ideal distribution of detection data, unlike the conventional method in which radiation is irradiated from the side to each depth.

[0054] According to the first embodiment, the ideal distribution 60 is a distribution that expresses the depth at which gamma rays interact in the DOI detector 17 to be calibrated and the time difference at which each detection data is detected by the multiple DOI detectors 17 and 17A in association with each other. Therefore, in addition to the above-mentioned effects, the DOI detector can be calibrated based on the depth at which gamma rays interact and the time difference at which each detection data is detected. For example, the DOI detector can be calibrated based on DOI information representing the depth and TOF information representing the time difference. In addition, even a large-scale PET device 1 can easily provide highly accurate DOI information and TOF information. For example, the PET device 1 acquires DOI information from the spread of photons 40 by the optical sensor 172 provided on one side of the DOI detector 17, and acquires measurement time difference information between a pair of opposing DOI detectors 17 and 17A. Such a PET device 1 can be realized with a simple configuration, and can acquire highly accurate DOI information and TOF information by calibration based on the ideal distribution.

[0055] Furthermore, according to the first embodiment, the ideal distribution 60 is a distribution in which the time difference decreases as the depth increases. Therefore, in addition to the above-mentioned effects, the DOI detector can be calibrated with high accuracy as the depth at which the gamma rays interact increases.

[0056] Moreover, according to the first embodiment, the depth represented by the ideal distribution 60 is the depth at which gamma rays incident on one DOI detector 17 of the multiple DOI detectors 17, 17A arranged opposite each other interact with each other. Therefore, the same effect as that described above can be obtained.

[0057] Furthermore, according to the first embodiment, the processing circuitry 73 corrects the depth corresponding to the detection data in the calibrated DOI detector 17 so as to approach the ideal distribution 60. Therefore, in addition to the above-mentioned effects, for example, when the spatial resolution is low between the spatial resolution regarding the depth and the time resolution regarding the time difference, the spatial resolution can be improved.

[0058] Furthermore, according to the first embodiment, the processing circuit 73 corrects the detection data for a time shift due to a difference in rise time of the output signal from the DOI detector 17, prior to the correction based on the ideal distribution 60. Therefore, in addition to the above-mentioned effects, it is possible to improve the time resolution regarding the time at which the detection data is detected.

[0059] According to the first embodiment, the ideal distribution 60a may be a distribution that correlates the depth at which a gamma ray interacts in the DOI detector to be calibrated with the number of photon counts generated by the interaction with the gamma ray in the DOI detector to be calibrated. In this case, the DOI detector 17 can be calibrated based on the number of counts in addition to the above-mentioned effects.

[0060] According to the first embodiment, the ideal distribution 60a may be a distribution in which the count number decreases with increasing depth. In this case, in addition to the above-mentioned effects, the DOI detector can be calibrated with higher accuracy as the depth at which the gamma rays interact increases.

[0061] Furthermore, according to the first embodiment, ideal distribution 60a may be a distribution that follows the formula I=Io exp(-μt), where I is the depth, t is the number of counts at depth t, Io is the number of counts near the surface of DOI detector 17, and μ is the attenuation coefficient of gamma rays in the scintillator crystal. In this case, a more accurate ideal distribution can be expressed, and therefore the DOI detector can be calibrated with higher accuracy.

[0062] (Modification of the first embodiment) The following modifications can be similarly applied to the following embodiments.

[0063] (First Modification) In the first embodiment, the depth represented by the ideal distribution 60, 60a is the depth at which the gamma ray incident on one DOI detector 17 of the multiple DOI detectors 17, 17A arranged opposite each other interacts, but is not limited thereto. That is, in order to increase the amount of statistics and complete the calibration in a shorter time, an ideal correlation with a DOI detector other than the opposing DOI detector 17A may be used. For example, the depth represented by the ideal distribution 60, 60a may include, in addition to the above-mentioned depth, the depth at which the gamma ray incident obliquely on the DOI detector adjacent to the one DOI detector 17 interacts. In this case, the DOI information representing the depth at which the obliquely incident gamma ray 30 interacts 34 represents a shifted depth compared to the DOI information described in the first embodiment. This is because the photon 40 does not reach the optical sensor 172 unless it travels a greater distance due to the oblique incidence. Therefore, when the deviation due to oblique incidence is included, the ideal distribution 60b is not a simple straight line, but has a peak and is uneven, as shown in Fig. 12. With this first modification, the same effect as that described above can be obtained, with only the shapes of the ideal distributions 60, 60a being different. Furthermore, according to the first modification, by including the detection data for oblique incidence in addition to the detection data used in the first embodiment, the amount of statistics for calibration can be increased, and the calibration can be completed in a shorter time.

[0064] (Second Modification) In the first embodiment, the depth represented by the ideal distributions 60, 60a, and 60b is the depth at which the gamma rays incident on the DOI detector 17 interact, but is not limited thereto. That is, in order to increase the amount of statistics and complete the calibration in a shorter time, an ideal distribution that also includes events due to Compton scattering may be used. For example, the depth represented by the ideal distribution 60 may include the depth at which the gamma rays incident on the DOI detector interact and the depth at which the scattered gamma rays generated by Compton scattering of the incident gamma rays interact. In this case, if the energy is fixed when Compton scattering occurs, the scattering angle is also fixed. For example, as shown in FIG. 13, the incident gamma rays 30 are partly converted into photons 40 by the interaction 34 and detected by the optical sensor 172, and the remaining part is Compton scattered and travels as scattered gamma rays 301 along a path according to the scattering angle. The scattered gamma rays 301 are converted into photons 40 by the interaction 341 and detected by the optical sensor 172. The path of the scattered gamma rays 301 due to Compton scattering can be estimated by a physical simulation using the energy and scattering angle of the gamma rays 30. In addition, in FIG. 13, the depth at which the incident gamma rays 30 cause interaction 34 can be obtained as DOI information DOI1 from the spread of the photons 40, as described above. Similarly, in FIG. 13, the depth at which the scattered gamma rays 301 cause interaction 341 can be obtained as DOI information DOI2 from the spread of the photons 40, as described above. According to the second modification, by including detection data of the scattered gamma rays 301 due to Compton scattering in addition to the detection data used in the first embodiment and the first modification, the amount of statistics for calibration can be increased, and the calibration can be completed in a shorter time.

[0065] (Third Modification) In the first embodiment, the number of divisions of the DOI information is three, but is not limited to this. For example, the number of divisions of the DOI information may be two, or any number of divisions of four or more. In addition, since overlap of the DOI information occurs depending on the degree of the temporal resolution, the number of divisions may be changed according to the performance of the temporal resolution, and the width in the depth direction may be made different between the divided units. For example, when the temporal resolution is high, it is preferable that the DOI information has a large number of divisions and a small width in the depth direction. This is also true for the depth where the DOI information has good accuracy and the depth where there is a lot of interaction. In addition, when the temporal resolution is low, it is preferable that the DOI information has a small number of divisions and a large width in the depth direction. According to the third modified example, in addition to the above-mentioned effects, the spatial resolution can be improved according to the degree of the temporal resolution.

[0066] <Second embodiment> The second embodiment is different from the first embodiment in that the spatial resolution is improved by correcting the DOI information, and is an embodiment in which the time resolution is improved by correcting the measured time difference information. That is, the second embodiment is configured to correct the time difference information using the DOI information when the accuracy of the DOI information is better than the accuracy of the time difference information.

[0067] Accordingly, the calibration function 733 of the processing circuit 73 corrects the time difference at which the detection data in the DOI detector 17 to be calibrated is detected so as to approach the ideal distribution. The calibration function 733 and the processing circuit 73 are an example of a calibration unit.

[0068] The other configurations are similar to those of the first embodiment.

[0069] Next, the operation of the PET device 1 configured as above will be described with reference to the flow chart of FIG. 14 and the schematic diagram of FIG.

[0070] First, step ST10 is executed in the same manner as described above.

[0071] After step ST10, in step ST20, the processing circuit 73 calibrates the DOI detector 17 based on each of the acquired detection data and the ideal distribution of the detection data. Here, when the time resolution of the PET gantry 10 is low, the relationship between the DOI information and the measurement time difference is a shape in which multiple gentle distributions are smeared into the ideal distribution at the top, as shown in FIG. 15. Supplementally, FIG. 15(a) shows the case where the scintillator crystal 171 is divided into three in the depth direction for the actual DOI information, as described above. FIG. 15(b) shows the relationship between the DOI information and the measurement time difference when the scintillator crystal 171 is divided into three. In the figure, the relationship between the actual DOI information DOI1 to DOI3 and the measurement time difference is shown as a convex distribution of proportional relationship represented by each DOI information DOI1, . . . , DOI3, which protrudes from the ideal distribution 60 at the center of the convex shape, while the end of the convex shape does not overlap with the adjacent end. In this actual relationship, since the time resolution is low, each piece of DOI information exceeds the ideal distribution in the center, and since the spatial resolution is high, each piece of DOI information does not overlap at the end. Therefore, when the ideal distribution 60 representing the proportional relationship described above is superimposed on this actual relationship, each piece of DOI information has a large error from the ideal distribution 60 in the center of the convex shape. Therefore, the calibration function 733 corrects the measurement time difference information in the DOI detector 17 to be calibrated so as to approach the ideal distribution 60. Returning to FIG. 14, such step ST20 includes steps ST21 to ST23 and ST24a.

[0072] Steps ST21 to ST23 are executed in the same manner as described above.

[0073] After step ST23, in step ST24, the processing circuit 73 calibrates the measurement time difference based on the ideal distribution. That is, the processing circuit 73 corrects the measurement time difference information in the DOI detector 17 to be calibrated so as to approach the ideal distribution 60. This completes step ST20 including steps ST21 to ST23 and ST24a.

[0074] Thereafter, the processes from step ST30 onwards are executed in the same manner as described above.

[0075] As described above, according to the second embodiment, the processing circuitry 73 corrects the time difference at which the detection data in the DOI detector 17 to be calibrated is detected so as to approach the ideal distribution. Therefore, the DOI detector can be calibrated in the same manner as in the first embodiment. In detail, according to the second embodiment, when the time resolution is low between the spatial resolution regarding the depth and the time resolution regarding the measurement time difference, the measurement time difference information is corrected so as to approach the ideal distribution 60, thereby improving the time resolution. According to the second embodiment, the effects other than the correction of the DOI information can be obtained in the same manner as in the first embodiment.

[0076] According to at least one embodiment described above, the DOI detector can be calibrated.

[0077] The term "processor" used in the above description means a circuit such as a central processing unit (CPU), a graphics processing unit (GPU), an application specific integrated circuit (ASIC), a programmable logic device (e.g., a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a field programmable gate array (FPGA)). When the processor is a CPU, for example, the processor realizes a function by reading and executing a program stored in a memory circuit. On the other hand, when the processor is an ASIC, for example, instead of storing the program in a memory circuit, the function is directly incorporated as a logic circuit in the circuit of the processor. Note that each processor in this embodiment is not limited to being configured as a single circuit for each processor, and may be configured as a single processor by combining multiple independent circuits to realize its function. Furthermore, the multiple components in FIG. 1 may be integrated into a single processor to realize its function.

[0078] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope of the invention and its equivalents described in the claims, as well as in the scope and spirit of the invention. [Explanation of symbols]

[0079] 1 PET device 10 PET Gantry 11 Detector Ring 13 Signal Processing Circuit 15 Coincidence circuit 17 DOI Detector 171 Scintillator Crystals 172 Optical Sensor 50 berths 51 Foundation 52 Support Frame 53 Tabletop 54 Bed Drive Unit 70 Console 71 PET data memory 73 Processing Circuit 74 Display 75 Memory 76 Input Interface 731 Imaging control function 732 Acquisition Function 733 Calibration Function 734 Reconfiguration function 735 Display Control Function

Claims

1. an acquisition unit that acquires each detection data corresponding to a depth at which the gamma ray interacts in the multiple DOI detectors; a calibration unit that calibrates the DOI detector based on each of the acquired detection data and an ideal distribution of the detection data; A PET device equipped with the above.

2. The ideal distribution is a distribution that represents an association between a depth at which a gamma ray interacts in the DOI detector to be calibrated and a time difference at which each detection data is detected by the multiple DOI detectors.

2. The PET device according to claim 1.

3. The ideal distribution is a distribution in which the time difference becomes smaller as the depth increases. The PET device according to claim 2.

4. The depth represented by the ideal distribution is a depth at which gamma rays incident on one of the multiple DOI detectors arranged opposite each other interact with each other. The PET device according to claim 2.

5. The depth represented by the ideal distribution is a depth at which gamma rays obliquely incident on a DOI detector adjacent to one of the multiple DOI detectors arranged opposite each other interact with each other. The PET device according to claim 2.

6. The depth represented by the ideal distribution includes a depth at which the gamma rays incident on the DOI detector interact with each other, and a depth at which the scattered gamma rays generated by Compton scattering of the incident gamma rays interact with each other. The PET device according to claim 2.

7. The calibration unit corrects a depth corresponding to the detection data in the DOI detector to be calibrated so as to approach the ideal distribution.

7. A PET device according to any one of claims 2 to 6.

8. The calibration unit corrects the detection data for a time shift due to a difference in rise time of the output signal from the DOI detector prior to the correction based on the ideal distribution.

8. The PET device according to claim 7.

9. The calibration unit corrects a time difference at which the detection data is detected in the DOI detector to be calibrated so as to approach the ideal distribution.

7. A PET device according to any one of claims 2 to 6.

10. The ideal distribution is a distribution that correlates the depth at which a gamma ray interacts in the calibrated DOI detector with the number of photon counts generated by the interaction with the gamma ray in the calibrated DOI detector.

2. The PET device according to claim 1.

11. The ideal distribution is a distribution in which the number of counts decreases as the depth increases.

11. The PET device according to claim 10.

12. The ideal distribution is a distribution that follows the formula I=Io exp(-μt), where t is the depth, I is the number of counts at the depth t, Io is the number of counts in the vicinity of the surface of the DOI detector, and μ is the attenuation coefficient of gamma rays in the DOI detector.

11. The PET device according to claim 10.

13. acquiring respective detection data corresponding to depths at which the gamma rays interacted within the plurality of DOI detectors; calibrating the DOI detector based on each of the acquired detection data and an ideal distribution of the detection data; A calibration method comprising:

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