Radiation diagnosis device with a first detector detecting Cherenkov light and a second detector detecting energy information of radiation

A radiation diagnosis device according to an embodiment includes a first detector and a second detector. The first detector detects Cherenkov light generated when a radiation passes. The second detector is provided to face the first detector on a side farther from a source of generating the radiation and detects the energy information of the radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-014410, filed on Jan. 31, 2020; and Japanese Patent Application No. 2021-008503, filed on Jan. 22, 2021; and the entire contents of which are incorporated herein by reference.

FIELD

Embodiments disclosed herein relate to a radiation diagnosis device and a radiation diagnosis method.

BACKGROUND

As a radiation diagnosis device, a positron emission tomography (PET) device has been known. In the PET device, by detecting a scintillation light generated when a pair of annihilation gamma rays enter a scintillator, a pair-annihilation position of a positron is specified and by using this, a medical image is generated, the pair of annihilation gamma rays being generated in a pair when the positron emitted from a radiopharmaceutical marked with a positron emission radionuclide and electrons annihilate,

The scintillation light, however, is the light that is re-emitted in a transition process in which an excitation state produced by the pair annihilation gamma-rays returns to a ground state over time, and therefore, the response speed is relatively low and errors may occur in specifying the pair annihilation position of the positron.

DETAILED DESCRIPTION

A radiation diagnosis device according to one embodiment includes a first detector and a second detector. The first detector detects Cherenkov light that is generated when a radiation passes. The second detector is provided to face the first detector on a side farther from a source of generating the radiation and detects the energy information of the radiation.

The radiation diagnosis device, a PET device, and a radiation diagnosis method according to the embodiment are hereinafter described in detail with reference to the drawings.

Embodiment

First, regarding a structure of the radiation diagnosis device according to the embodiment, a PET device is described as one example with reference toFIG.1toFIG.6.

FIG.1is a diagram illustrating a structure of a PET device100according to the embodiment. As illustrated inFIG.1, the PET device100according to the embodiment includes a gantry10and a console device20.

The gantry10detects a pair of annihilation gamma-rays released from positrons in a subject P using a first detector1configured to detect Cherenkov light and a second detector2configured to detect scintillation light, the first detector1and the second detector2being provided in a ring-like form so as to encompass the subject P. In addition, the gantry10generates counting information from output signals of the first detector1and the second detector2by first timing information acquisition circuitry101and second timing information acquisition circuitry102.

More specifically, the gantry10includes a couchtop103, a couch104, a couch driving unit106, the first detector1that detects Cherenkov light, the second detector2that detects the scintillation light, the first timing information acquisition circuitry101that generates the counting information from the first detector1, and the second timing information acquisition circuitry102that generates the counting information from the second detector2.

First, the first detector1and the second detector2are described with reference toFIG.1andFIG.2.

The first detector1is a detector that acquires the counting information about pair annihilation gamma-rays released from the positrons in the subject P in a manner that the first detector1detects the Cherenkov light that is the light of a shock wave generated when a charged particle, which is generated from the interaction of the pair annihilation gamma-rays released from the positrons in the subject P with a light emitter (radiator) inside, moves faster than the phase velocity of the light in the medium. That is to say, the first detector1detects the Cherenkov light generated when the radiation passes.

The first detector1that is the detector of the pair annihilation gamma-rays using the Cherenkov light is inferior to the detector of the pair annihilation gamma-rays using the scintillation light in terms of sensitivity to energy. However, since the Cherenkov light is generated in a very short time compared to the scintillation light, the response characteristic is excellent. For this reason, the first detector1that employs a method of detecting the Cherenkov light has a characteristic of being superior to the detector that employs a method of detecting the scintillation light in terms of time resolution.

In other words, the first detector1that is the detector configured to detect the Cherenkov light has the characteristic being advantageous in terms of time resolution over the second detector2that is the detector configured to detect the scintillation light. On the other hand, the second detector2that is the detector configured to detect the scintillation light has a characteristic of being advantageous in terms of energy resolution over the first detector1that is the detector configured to detect the Cherenkov light.

Therefore, the radiation diagnosis device according to the embodiment generates the counting information using the first detector1and the second detector2. Thus, the counting information maintaining the high time resolution while keeping the energy resolution can be generated.

FIG.2is a schematic diagram of the arrangement of the first detector1and the second detector2. InFIG.2, a generation point Q of the pair annihilation gamma-rays is illustrated. Here, the first detector1is formed of a plurality of pixels1a,1b,1c, and the like.

Note that one pixel of the detector refers to the minimum separation unit of the position resolution of the detector. For example, in a case where the light detection elements detect the Cherenkov light generated at different positions, each of the light detection elements functions as a detector of one pixel. On the contrary, in a case where the light detection elements detect the Cherenkov light generated at the same position, the light detection elements collectively function as a detector of one pixel. InFIG.2, only three pixels1a,1b, and1care illustrated as the pixels of the first detector1; however, in fact, many pixels are arranged as these pixels of the detector are arranged in a ring-like form.FIG.2is merely a schematic diagram illustrating the arrangement of the first detector1and the second detector2, and the size of the pixels1a,1b, and1cof the detector is different from that of the actual detector.

In the embodiment, one light detection element may be configured as a light detection element of multiple pixels.

In the example illustrated inFIG.1, the detector1is formed by a plurality of detector blocks; however, one pixel inFIG.2means a pixel unit by which the position of the generation of the Cherenkov light can be separated, and may be a unit smaller than the detector block illustrated inFIG.1. That is to say, each detector block of the first detector1that is configured as the ring-like form inFIG.1may be formed of a plurality of pixels.

Back toFIG.2, the first detector1includes a light emitter50formed of a medium that generates the Cherenkov light by the interaction with the pair annihilation gamma-rays that are the radiation released from the positrons in the subject P, and a light detection element51that detects the generated Cherenkov light. That is to say, the pixels1a,1b, and1cincluded in the first detector1include light emitters (radiators)50a,50b, and50c, and light detection elements51a,51b, and51cthat detect the generated Cherenkov light, respectively.

Here, the light emitter (radiator)50in the first detector1is formed of, for example, a medium containing an atom with a large atomic number having a property of easily causing a photoelectric effect by the interaction with the incident radiation and while not easily generating the scintillation light that becomes noise; for example, bismuth germanium oxide (BGO), and lead compounds such as lead glass (SiO2+PbO), lead fluoride (PbF2), and PWO (PbWO4) are usable. In other words, the light emitter50in the first detector1is formed of a medium that easily causes the photoelectric effect but suppresses the scintillation due to the radiation, for example.

The light detection element51detects the generated Cherenkov light. The light detection element51is a silicon photomultiplier (SiPM) that is an avalanche photo diode (APD) array in which the size of the respective pixels is reduced to about several tens of micrometers, and operates in a Geiger mode. In another example, the light detection element51is formed of a plurality of pixels that perform the photoelectric conversion, and each pixel is formed of a single photon avalanche diode (SPAD).

Here, the thickness of the light emitter50in the first detector1may be designed to be smaller than the thickness of a scintillator60provided in the second detector2, for example, in order to prevent the gamma-rays from losing its entire energy in the light emitter. Thus, the pair annihilation gamma-rays that are produced as a pair enter the second detector2with most of the energy kept, while the Cherenkov light is generated in the first detector1, and the scintillation light can be generated in the second detector2.

Moreover, the pixel size of the light emitter50in the first detector1can be made smaller than the pixel size of the scintillator60in the second detector2, for example. Thus, the positional resolution of the data obtained from the first detector1can be increased relatively. Furthermore, the detector column length of the first detector may be shorter than that of the second detector.

Back toFIG.1, the second detector2is the detector that detects the radiation by detecting the scintillation light (fluorescence) that is the light released again when the material that has become excited by the interaction of the annihilation gamma-rays released from the positrons in the subject P with the light emitter (scintillator) transits to the ground state again. The second detector2is also a detector that detects the energy information of the radiation of the annihilation gamma-rays released from the positrons in the subject P.

The second detector2is provided to face the first detector1that detects the Cherenkov light on a side farther from a source of generating the radiation, that is, the annihilation gamma-rays released from the positrons in the subject P. In another example, the second detector2is provided in a manner that a plurality of detector blocks are disposed so as to encompass, in the ring-like form, the first detector1that is provided to encompass the subject P in the ring-like form. In still another example, the second detector2is the detector with a ring-like shape, which is similar to the first detector1, and the diameter of the second detector2is larger than that of the first detector1.

As described above, the generation of the scintillation light is a slower process than the generation of the Cherenkov light. On the other hand, most part of the energy of the annihilation gamma-rays is converted into the scintillation light; therefore, from the viewpoint of measuring the energy of the annihilation gamma-rays, the second detector2using the scintillation light is advantageous over the first detector1using the Cherenkov light.

Back toFIG.2again, the second detector2is formed of a plurality of pixels2a,2b,2c, and the like.

Note that one pixel of the detector refers to the minimum separation unit of the positional resolution of the detector, which is similar to the case of the first detector1. For example, in a case where the light detection elements detect the scintillation light generated at different positions, each of the light detection elements serves as a detector of one pixel. On the contrary, in a case where the light detection elements detect the scintillation light generated at the same position, the light detection elements collectively serve as a detector of one pixel. Similar to the case of the first detector1, only three pixels2a,2b, and2care illustrated as the pixels of the second detector2; however, in reality, many pixels are arranged in the ring-like form as the pixels of the detector.

In the embodiment, in a manner similar to the first detector1, one light detection element included in the second detector2may be configured as light detection elements of multiple pixels.

Note that inFIG.1, the second detector2is formed of detector blocks; however, one pixel inFIG.2means a pixel unit by which a position of generation of the Cherenkov light can be separated, and one pixel may be a unit smaller than a unit of the detector block inFIG.1. That is to say, each detector block of the second detector2in the ring-like form illustrated inFIG.1may be formed by a plurality of pixels.

Subsequently, as one example of a specific structure of the second detector2, a photon counting type or Anger type detector is given. For example, the second detector2includes the scintillator60and a light detection element61that are illustrated inFIG.2, and a light guide that is not illustrated. That is to say, the pixels2a,2b, and2cin the second detector2include scintillators60a,60b, and60cand light detection elements61a,61b, and61cthat detect the generated scintillation light, respectively.

The scintillator60converts the incident annihilation gamma-rays released from the positrons in the subject P into the scintillation light (scintillation photons, optical photons), and outputs the resulting light. The scintillator is formed by, for example, scintillator crystal suitable for TOF measurement or energy measurement, such as lanthanum bromide (LaBr3), lutetium yttrium oxyorthosilicate (LYSO), lutetium oxyorthosilicate (LSO), lutetium gadolinium oxyorthosilicate (LGSO), or BGO, and is arranged two-dimensionally, for example.

As the light detection element61, for example, the aforementioned silicon photomultiplier (SiPM) or a multiplier phototube is used. The multiplier phototube includes a photoelectric cathode that receives the scintillation light and generates photoelectrons, a multistage dynode that applies an electric field that accelerates the generated photoelectrons, and an anode corresponding to an outlet from which the electrons flow out. The multiplier phototube multiplies the scintillation light output from the scintillator and converts the scintillation light into electric signals.

The light guide is formed of a plastic material with the excellent light-transmitting property or the like, and sends the scintillation light output from the scintillator to the light detection element, for example the SiPM or the multiplier phototube.

Note that the thickness of the scintillator60provided to the second detector2can be made larger than the thickness of the light emitter50provided to the first detector1.

In addition, the pixel size of the scintillator60provided to the second detector2can be made larger than the pixel size of the light emitter50provided to the first detector1.

Subsequently, back toFIG.1, other structures are described.

The first timing information acquisition circuitry101generates the counting information from the output signal of the first detector1, and stores the generated counting information in a memory142in the console device20. Note that although not illustrated inFIG.1, the first detector1is sectioned into a plurality of blocks and includes the first timing information acquisition circuitry101for each block.

The first timing information acquisition circuitry101converts the output signal of the first detector1into digital data and generates the counting information. This counting information includes a detection position and a detection time of the annihilation gamma-rays. For example, the first timing information acquisition circuitry101specifies the light detection elements that have converted the Cherenkov light into electric signals at the same timing. Then, the first timing information acquisition circuitry101calculates the position of center of gravity using the position of each of the specified light detection elements and the intensity of the electric signal, and specifies a detection element number (P) expressing the position of the radiator on which the annihilation gamma-rays have been incident.

Moreover, the first timing information acquisition circuit101specifies the detection time (T) when the first detector1has detected the Cherenkov light generated by the annihilation gamma-rays. Note that the detection time (T) may be either the absolute time or elapsed time since the start of the image capture. In this manner, the first timing information acquisition circuitry101generates the counting information including the detection element number (P) and the detection time (T).

The second timing information acquisition circuitry102generates the counting information from the output signal of the second detector2, and stores the generated counting information in the memory142in the console device20. Note that, in a manner similar to the first detector1, the second detector2is sectioned into a plurality of blocks, and includes the second timing information acquisition circuitry102for each block.

The second timing information acquisition circuitry102converts the output signal of the second detector2into digital data, and generates the counting information. This counting information includes the detection position, the energy value, and the detection time of the annihilation gamma-rays. For example, the second timing information acquisition circuitry102specifies the light detection elements that have converted the scintillation light into electric signals at the same timing. Then, the second timing information acquisition circuitry102specifies the scintillator number (P) expressing the position of the scintillator on which the annihilation gamma-rays have been incident. The position of the scintillator on which the annihilation gamma-rays have been incident may be specified by the calculation of center of gravity on the basis of the position of each light detection element and the intensity of the electric signal. In the case where each element size of the scintillator and the light detection element corresponds to each other, the scintillator corresponding to the light detection element from which the output is obtained may be specified as the position of the scintillator on which the annihilation gamma-rays have been incident.

The second timing information acquisition circuitry102specifies the energy value (E) of the annihilation gamma-rays incident into the second detector2by the integral calculation of the intensity of the electric signal output from each light detection element. In addition, the second timing information acquisition circuitry102specifies the detection time (T) when the second detector2has detected the scintillation light by the annihilation gamma-rays. Note that the detection time (T) may be either the absolute time or elapsed time since the start of the image capture. In this manner, the second timing information acquisition circuitry102generates the counting information including the scintillator number (P), the energy value (E), and the detection time (T).

Here, P is a position of detector on which the annihilation gamma-rays have been incident.

Note that the first timing information acquisition circuitry101and the second timing information acquisition circuitry102may be formed by a circuit, for example, a central processing unit (CPU), a graphical processing unit (GPU), an application specific integrated circuit (ASIC), a programmable logic device (such as a simple programmable logic device: SPLD), a complex programmable logic device (CPLD), a field programmable gate array (FPGA), or the like. The first timing information acquisition circuitry101and the second timing information acquisition circuitry102are examples of the first information acquisition unit and the second timing information acquisition unit, respectively.

The couchtop103is a bed on which the subject P is placed, and is disposed on the couch104. The couch driving unit106moves the couchtop103under the control of a couch controlling function105dof the processing circuitry105. For example, the couch driving unit106moves the couchtop103so that the subject P moves into an image capturing port of the gantry10.

The console device20receives the operator's operation of the PET device100and controls the capture of the PET image, and moreover reconfigures the PET image using the counting information collected by the gantry10. As illustrated inFIG.1, the console device20includes the processing circuitry105, an input device140, a display141, and the memory142. Note that each part of the console device20is connected through a bus.

In the embodiment, each processing function performed by a specifying function (simultaneous counting information generating function)105a, an image generating function105b, a system controlling function105c, and the couch controlling function105dis stored in the memory142in the format of computer programs that are executable by the computer. The processing circuitry105is a processor that reads out the computer program from the memory142and executes the computer program so as to achieve the function corresponding to the computer program. In other words, the processing circuitry105that has read each computer program has each function illustrated in the processing circuitry105inFIG.1. InFIG.1, the processing functions that are performed in the specifying function (simultaneous counting information generating function)105a, the image generating function105b, the system controlling function105c, and the couch controlling function105dare achieved in one processing circuitry105; however, the processing circuitry105may alternatively be configured by combining a plurality of independent processors and by executing the computer program in each processor, the processing circuitry105may achieve the functions. In other words, each of the aforementioned functions may be configured as the computer program and one processing circuitry105may execute each computer program. In another example, a particular function may be mounted in a dedicated independent computer program executing circuitry.

Note that inFIG.1, the specifying function105a, the image generating function105b, the system controlling function105c, and the couch controlling function105dare examples of a specifying unit, an image generation unit, a system control unit, and a couch control unit, respectively.

The term “processor” used in the above description refers to a circuit such as a central processing unit (CPU), a graphical processing unit (GPU), an application specific integrated circuit (ASIC), a programmable logic device (such as a simple programmable logic device: SPLD), a complex programmable logic device (CPLD), or a field programmable gate array (FPGA). The processor achieves the function by reading out and executing the computer program saved in the memory142.

The processing circuitry105causes the specifying function (simultaneous counting information generating function)105ato generate the simultaneous counting information on the basis of the counting information about the first detector1that is acquired by the first timing information acquisition circuitry101and the counting information about the second detector2that is acquired by the second timing information acquisition circuitry102, and stores the generated simultaneous counting information in the memory142. The detailed process of the specifying function105ais described below.

The processing circuitry105causes the image generating function105bto reconfigure the PET image. Specifically, the processing circuitry105causes the image generating function105bto read out the time-series list of the simultaneous counting information stored in the memory142, and reconfigures the PET image using the read time-series list. In addition, the processing circuitry105stores the reconfigured PET image in the memory142.

The processing circuitry105causes the system controlling function105cto control the gantry10and the console device20, thereby controlling the entire PET device100. For example, the processing circuitry105causes the system control unit105cto control the image capture in the PET device100.

The processing circuitry105causes the couch controlling function105dto control the couch driving unit106.

The input device140is a mouse or a keyboard, for example, that is used to input various instructions or settings by the operator of the PET device100, and transfers the input various instructions and settings to the processing circuitry105. For example, the input device140is used to input the image capture start instruction.

The display141is a monitor or the like that is seen by the operator, and under the control of the processing circuitry105, displays the respiration waveform or the PET image of the subject, and displays a graphical user interface (GUI) for receiving various instructions or settings from the operator.

The memory142stores various pieces of data used in the PET device100. The memory142is, for example, a semiconductor memory element such as a random access memory (RAM) or a flash memory, a hard disk, or an optical disk. The memory142stores therein, the counting information corresponding to the information in which the scintillator number (P), the energy value (E), and the detection time (T) are associated with each other, the simultaneous counting information in which the coincidence number corresponding to the serial number of the simultaneous counting information is associated with a set of counting information, the reconfigured PET image, and the like.

Subsequently, a process of generating the simultaneous counting information in the PET device100according to the embodiment is described with reference toFIG.3AtoFIG.6.

Description is hereinafter made of a case in which, as illustrated inFIG.3A, the Cherenkov light derived from a pair of gamma-rays produced as a pair from an annihilation point Q is detected by a detector1xand a detector1ythat correspond to the first detector1configured to detect the Cherenkov light and the scintillation light of a pair of gamma-rays produced as a pair from the annihilation point Q is detected by a detector2xand a detector2ythat correspond to the second detector2configured to detect the scintillation light. That is to say, the processing circuitry105causes the specifying function105ato generate the simultaneous counting information using the data obtained by at least two pairs of detectors: a pair of detectors that is the first detector1and a pair of detectors that is the second detector2.

Note that, in fact, as illustrated inFIG.3B, when Compton scattering occurs in the first detector1that detects the Cherenkov light and a recoil electron4is generated thereby, the trajectory of a pair of gamma-rays produced as a pair from the pair annihilation Q is deviated a little from the trajectory of the gamma-rays inside the first detector1as expressed by a gamma-ray trajectory3after Compton scattering.

Subsequently, the generation of the simultaneous counting information of the gamma-rays is described with reference toFIG.4. As the method of generating the simultaneous counting information, for example, the following two methods are considered: a method as illustrated inFIG.4in which the simultaneous counting is performed in the first detector1that is the detector configured to detect the Cherenkov light and then, by referring to the data obtained in the second detector2that is the detector configured to detect the scintillation light, the final simultaneous counting information is generated; and a method in which the simultaneous counting is performed in the second detector2that is the detector configured to detect the scintillation light and then, by referring to the data obtained in the first detector1that is the detector configured to detect the Cherenkov light, the final simultaneous counting information is generated. The former method is described with reference toFIG.4andFIG.5, and the latter method is described with reference toFIG.6.

First, as the method of generating the simultaneous counting information, the method of performing the simultaneous counting in the first detector1that is the detector configured to detect the Cherenkov light first is described with reference toFIG.4andFIG.5. In this method, a process of generating the simultaneous counting information is performed using the data obtained from the first detector1with the high time resolution first. Specifically, as illustrated inFIG.4, the processing circuitry105performs the process using the detector1xand the detector1ythat are the first detector, and after that, performs the process using the detector2xand the detector2ythat are the second detector, and specifies whether the event is the pair annihilation event and if it is the pair annihilation event, specifies the pair annihilation position and time.FIG.5is a flowchart expressing this process.

At step5100, the first timing information acquisition circuitry101acquires the first timing information of the pair annihilation gamma-rays in the first detector1. In addition, the first timing information acquisition circuitry101transmits the acquired first timing information to the processing circuitry105in the console device20. Here, the first timing information of the pair annihilation gamma-rays in the first detector corresponds to, for example, a list of the detection element number (P) and the detection time (T).

Note that one typical example of the detection time (T) is the time when the light detection element51of the first detector1has observed the Cherenkov light. However, the embodiment is not limited to this example, and the detection time may be the estimated value of the time when the Cherenkov light is generated by the interaction between the pair annihilation gamma-rays and the light emitter50of the first detector1that is estimated based on the time when the light detection element51of the first detector1has observed the Cherenkov light, for example. In the first detector1, the estimated time of the time when the Cherenkov light is generated by the interaction between the pair annihilation gamma-rays and the light emitter50of the first detector1and the time when the light detection element51of the first detector1has observed the Cherenkov light are approximately the same. As described above, the detection time (T) may be either the absolute time or elapsed time since the start of the image capture.

As illustrated inFIG.4, the first timing information acquisition circuitry101acquires a pair of pieces of first timing information of the pair annihilation gamma-rays in the first detector1from the detector1xthat is the first detector in a certain direction and the detector1ythat is the first detector on the approximately opposite side of the detector1x. Thus, the processing circuitry105can cause the specifying function105ato specify the generation position of the pair annihilation gamma-rays.

Here, description is made of the procedure of estimating the generation position of the pair annihilation gamma-rays from the pair of pieces of first timing information.

The pair annihilation gamma-rays are released to the opposite side due to the relation of momentum conservation along with the pair annihilation of positrons and electrons, and therefore, when Compton scattering or the like is ignored, it is considered that the generation position of the pair annihilation gamma-rays exists on a line connecting the detector1xand the detector1y.

Next, when it is assumed that the distance between the detector1xand the generation position of the pair annihilation gamma-rays is x1, the distance between the detector1yand the generation position of the pair annihilation gamma-rays is y1, the detection time when the detector1xhas observed the Cherenkov light is t1, the detection time when the detector1yhas observed the Cherenkov light is t2, and the speed of light is c, the difference in detection time between the detector1xand the detector1yis the difference in time by which the light advances the difference of distance from the generation position. Thus, x1−y1=c(t1−t2) is satisfied. In addition, the distance L1between the detector1xand the detector1yis known and the expression x1+y1=L2is satisfied. By setting up these two expressions simultaneously, it is possible to calculate the distance x1between the detector1xand the generation position of the pair annihilation gamma-rays, and the distance y1between the detector1yand the generation position of the pair annihilation gamma-rays. Therefore, the processing circuitry105can cause the specifying function105ato estimate the generation position and the generation time of the pair annihilation gamma-rays on the basis of the pair of pieces of timing information.

As is understood from the above expressions, when the generation position of the pair annihilation gamma-rays is near the center of the image capture range, the detection time is substantially the same in the detector1xand the detector1y, and as the generation position of the pair annihilation gamma-rays is away from near the center of the image capture range, the difference in detection time between the detector1xand the detector1yincreases. Therefore, the processing circuitry105can cause the specifying function105ato extract the event of the pair annihilation gamma-rays generated within a distance R from the center of the image capture range by extracting the pair annihilation gamma-rays generation event using the trigger that the difference in detection time between the detector1xand the detector1yis less than a threshold T.

Incidentally, in the process of extracting the pair annihilation gamma-rays event or the process of estimating the generation position of the pair annihilation gamma-rays in the specifying function105aof the processing circuitry105, the response speed of the detector or the variation thereof results in the occurrence of an error. However, it takes shorter after the Cherenkov light is generated from the interaction of the pair annihilation gamma-rays with the light emitter50and before the light detection element51detects the Cherenkov light in the first detector1configured to detect the Cherenkov light than in the second detector2configured to detect the scintillation light; therefore, the response speed of the first detector1is shorter and the extraction of the pair annihilation gamma-rays event and the estimation of the generation position of the pair annihilation gamma-rays are performed with high accuracy.

Subsequently, at step S110, the second timing information acquisition circuitry102acquires the second timing information of the pair annihilation gamma-rays in the second detector2in order to specify the event of the pair annihilation gamma-rays in which the first timing information is acquired on the basis of the first timing information acquired at step S100. The processing circuitry105causes the specifying function105ato generate the simultaneous counting information using both the first timing information acquired by the first detector1and the second timing information acquired by the second detector2.

In addition, the second timing information acquisition circuitry102transmits the acquired second timing information to the processing circuitry105of the console device20. Here, the second timing information of the pair annihilation gamma-rays in the second detector is the counting information including the scintillator number (P) and the detection time (T), for example. In addition, the second timing information of the pair annihilation gamma-rays in the second detector may be the counting information including the energy value (E) of the pair annihilation gamma-rays having entered the scintillator in addition to those above.

Note that at step S100and step S110, the first timing information acquisition circuitry101and the second timing information acquisition circuitry102usually perform the processes at step S100and step S110at the same time in parallel instead of performing the processes sequentially.

Note that one example of the detection time (T) is the time when the light detection element61of the second detector2has observed the scintillation light; however, the embodiment is not limited to this example. In another example, the detection time (T) may be the estimated time of the time when the Cherenkov light is generated by the interaction between the pair annihilation gamma-rays and the scintillator60of the second detector2, the estimated time being estimated based on the time when the light detection element61of the second detector2has observed the scintillation light. In the second detector2, the time when the pair annihilation gamma-rays are observed and the second detector2observes the scintillation light is a little delayed from the time when the pair annihilation gamma-rays interact with the scintillator60of the second detector2and the excited state is generated. As described above, the detection time (T) may be either the absolute time or elapsed time since the start of the image capture.

Back toFIG.4, the second timing information acquisition circuitry102acquires a pair of pieces of second timing information of the pair annihilation gamma-rays in the second detector2from the detector2xthat is the second detector and the detector2ythat is the second detector on the side substantially opposite to the detector2x. Thus, in a manner similar to the case of the first detector1, the processing circuitry105can cause the specifying function105ato estimate the generation position of the pair annihilation gamma-rays.

That is to say, when it is assumed that the distance between the detector2xand the generation position of the pair annihilation gamma-rays is x2, the distance between the detector2yand the generation position of the pair annihilation gamma-rays is y2, the detection time when the detector2xhas observed the scintillation light is t3, the detection time when the detector2yhas observed the scintillation light is t4, and the speed of light is c, the difference in detection time between the detector2xand the detector2yis the difference in time by which the light advances the difference of distance from the generation position. Thus, x2−y2=c(t3−t4) is satisfied. In addition, the distance L2between the detector2xand the detector2yis known and the expression x2+y2=L2is satisfied. By setting up these two expressions simultaneously, it is possible to calculate the distance x2between the detector2xand the generation position of the pair annihilation gamma-rays, and the distance y2between the detector2yand the generation position of the pair annihilation gamma-rays. Therefore, the processing circuitry105can cause the specifying function105ato estimate the generation position and the generation time of the pair annihilation gamma-rays on the basis of the pair of pieces of timing information.

Similarly, as is understood from the above expressions, when the generation position of the pair annihilation gamma-rays is near the center of the image capture range, the detection time is substantially the same in the detector2xand the detector2y, and as the generation position of the pair annihilation gamma-rays is away from near the center of the image capture range, the difference in detection time between the detector2xand the detector2yincreases. Therefore, the processing circuitry105can cause the specifying function105ato extract the event of the pair annihilation gamma-rays generated within the distance R from the center of the image capture range by extracting the pair annihilation gamma-rays generation event using the trigger that the difference in detection time between the detector2xand the detector2yis less than the threshold T.

In the process of extracting the pair annihilation gamma-rays event or the process of estimating the generation position of the pair annihilation gamma-rays in the specifying function105aof the processing circuitry105, the response speed of the detector or the variation thereof results in the occurrence of an error. Here, in the second detector2, it takes relatively long after the scintillation light is generated by the interaction of the pair annihilation gamma-rays with the scintillator60and before the light detection element61detects the generated scintillation light.

However, in the second detector2, after the pair annihilation gamma-rays interact with the scintillator by the photoelectric effect and before the system returns to the ground state, most part of the energy of the pair annihilation gamma-rays is released again as the scintillation light. Therefore, by counting the number of scintillation light released again, the second timing information acquisition circuitry102can acquire the information about the energy of the pair annihilation gamma-rays. At step5120, the second timing information acquisition circuitry102further acquires the energy information of the pair annihilation gamma-rays detected in the second detector2.

Since the energy of the pair annihilation gamma-rays is 511 keV, which is a predetermined energy corresponding to the rest mass of the positron, if the energy of the observed gamma-rays is largely deviated from the predetermined energy, it can be presumed that the observed gamma-rays are the gamma-rays generated due to Compton scattering or the like. Therefore, the processing circuitry105can cause the specifying function105ato eliminate the scattering event such as Compton scattering using the energy information of the observed gamma-rays.

Subsequently, at step S130, the processing circuitry105causes the specifying function105to determine whether the first timing information and the second timing information are included in a predetermined time window, thereby determining whether the first timing information and the second timing information are about the same pair annihilation gamma-rays.

Here, in the case where the first timing information and the second timing information are included in the predetermined window, the processing circuitry105causes the specifying function105to specify that the first timing information and the second timing information are the timing information acquired based on the same pair annihilation gamma-rays and the process advances to step S135.

In one example, in the case where the difference between the generation time of the pair annihilation gamma-rays that is calculated based on the first timing information acquired at step S100and the generation time of the pair annihilation gamma-rays that is calculated based on the second timing information acquired at step S110is less than a predetermined threshold and is included in a predetermined time window, the processing circuitry105causes the specifying function105to specify that the first timing information and the second timing information are acquired based on the same pair annihilation gamma-rays.

Furthermore, only in the case where the difference between the generation position of the pair annihilation gamma-rays that is calculated based on the first timing information acquired at step S100and the generation position of the pair annihilation gamma-rays that is calculated based on the second timing information acquired at step S110is less than the predetermined threshold, the processing circuitry105may cause the specifying function105to specify that the first timing information and the second timing information are acquired based on the same pair annihilation gamma-rays.

In another example, as illustrated inFIG.4, in the case where the difference between the detection time in the first detector1and the detection time in the second detector2is included in a predetermined time window, the processing circuitry105causes the specifying function105to specify that the first timing information and the second timing information are the timing information acquired based on the same pair annihilation gamma-rays. For example, inFIG.4, in a case where the distance between the detector1xthat is the first detector1and the detector2xthat is the second detector2is L, the difference t3−t1between the detection time t1in the detector1xand the detection time t3in the detector2xis expected to be about L/c, in which c is the speed of light. Therefore, in a case where the difference between the difference t3−t1between the detection time in the first detector1and the detection time in the second detector2and the time L/c estimated from the distance between the detectors is included in a predetermined time window, the processing circuitry105may cause the specifying function105to specify that the first timing information and the second timing information are the timing information acquired based on the same pair annihilation gamma-rays.

Subsequently, at step S135, the processing circuitry105causes the specifying function105to determine whether the acquired energy information is included in the predetermined energy window. Here, the energy of the pair annihilation gamma-rays is always 511 keV regardless of the nuclide at the time of generation. Therefore, in a case where the energy of the observed gamma-rays is much lower than 511 keV, it is possible to determine that the observed gamma-rays are influenced by Compton scattering or the like.

Therefore, by determining whether the acquired energy information is included in the predetermined energy window, the processing circuitry105can cause the specifying function105to exclude the data influenced by the scattering from the object to be reconfigured.

For example, the processing circuitry105can cause the specifying function105to determine whether the difference between the estimated energy of the pair annihilation gamma-rays detected in the second detector at step S120and 511 keV, which is the energy when the pair annihilation gamma-rays are generated, is less than the predetermined threshold and is included in the predetermined energy window.

If the acquired energy information is included in the predetermined energy window (Yes at step S135), the process advances to step S140and the processing circuitry105causes the specifying function105to specify that the first timing information and the second timing information are acquired based on the same pair annihilation gamma-rays. On the other hand, if the acquired energy information is not included in the predetermined energy window (No at step S135), the processing circuitry105causes the specifying function105to determine that the data is influenced by scattering, for example, and excludes this data from the object to be reconfigured.

Thus, when the processing circuitry105causes the specifying function105to specify the first timing information and the second timing information that are acquired based on the same pair annihilation gamma-rays at step S140, the processing circuitry105causes the image generating function105bto calculate the line of response (LOR) of the pair annihilation gamma-rays on the basis of these pieces of information. For example, in consideration of a possibility that the trajectory of the annihilation gamma-rays is a little displaced on the outside of the first detector1compared to the inside of the first detector1due to Compton scattering in the first detector1as described with reference toFIG.3B, the processing circuitry105causes the image generating function105bto estimate the line of response (LOR) of the pair annihilation gamma-rays on the basis of the first timing information.

In another example, the processing circuitry105may cause the image generating function105bto estimate the LOR of the pair annihilation gamma-rays on the basis of both the first timing information and the second timing information. Here, the processing circuitry105may estimate the scattering angle of Compton scattering in the first detector1on the basis of the number of photons of the Cherenkov light detected in the first detector1, for example, and after correcting the effect of Compton scattering in the first detector1, estimate the LOR of the pair annihilation gamma-rays on the basis of the first timing information and the second timing information.

Subsequently, the processing circuitry105causes the image generating function105bto generate the medical image on the basis of the LOR estimated for each of the pair annihilation gamma-rays.

Subsequently, as another example, with reference toFIG.6, description is made of a case in which the processing circuitry105causes the specifying function105ato determine the coincidence using the second timing information first, and then generate the final counting information using the first timing information. The process inFIG.6is similar to that inFIG.5but the order of steps is different. The process common to that inFIG.5is not described below. InFIG.5andFIG.6, the contents of the final process are substantially the same but the order of steps is different, so that the time required in the calculating process may be different, for example. Therefore, whether the process inFIG.5is used or the process inFIG.6is used is selected depending on the number of events in the first detector1and the second detector2or the number of scattering events, for example.

First, at step S200, the second timing information acquisition circuitry102acquires the second timing information of the pair annihilation gamma-rays in the second detector2. At step S210, the second timing information acquisition circuitry120further acquires the energy information of the pair annihilation gamma-rays detected in the second detector2. The process at step S200and S210is similar to the process at S110and S120inFIG.5.

Subsequently, at step S220, the processing circuitry105causes the specifying function105ato determine the coincidence on the basis of the energy information of the pair annihilation gamma-rays detected in the second detector2. For example, the processing circuitry105causes the specifying function105ato determine whether the acquired energy information is included in a predetermined energy window, in a manner similar to the process at step S135. In another example, the processing circuitry105causes the specifying function105ato determine whether the difference between the estimated energy of the pair annihilation gamma-rays detected in the second detector and 511 keV, which is the energy when the pair annihilation gamma-rays are generated is less than the predetermined threshold and is included in the predetermined energy window. The processing circuitry105determines that the data not included in the predetermined energy window is the data influenced by the scattering and excludes such data from the object to be reconfigured.

On the other hand, if the acquired energy information is included in the predetermined energy window, the processing circuitry105causes the specifying function105ato estimate the generation position and the generation time of the pair annihilation gamma-rays on the basis of the scintillator number (P) and the detection time (T) using the procedure described above with reference toFIG.5, for example. In addition, for example, the processing circuitry105causes the specifying function105ato extract, as the simultaneous counting information, the event of the pair annihilation gamma-rays generated within the distance R from the center of the image capture range by extracting the pair annihilation gamma-rays generation event using the trigger that the difference in detection time between the opposite detectors in the detector2is less than the threshold T.

Subsequently, the first timing information acquisition circuitry101acquires the first timing information of the pair annihilation gamma-rays in the first detector1at step S230. In addition, the first timing information acquisition circuitry101transmits the acquired first timing information to the processing circuitry105in the console device20. Here, the first timing information of the pair annihilation gamma-rays in the first detector is, for example, the detection element number (P) and the detection time (T). The process at step S230is similar to the process at step S100inFIG.5.

Subsequently, at step S240, the processing circuitry105causes the specifying function105ato determine whether the first timing information and the second timing information are included in the predetermined time window, and in a case where the first timing information that is the timing information in the first detector1and the timing information in the second detector are included in the predetermined time window (Yes at step S240), the process advances to step S250, and the processing circuitry105causes the specifying function105ato specify that the first timing information that is the timing information in the first detector1and the second timing information that is the timing information in the second detector2are acquired based on the same pair annihilation gamma-rays. Note that the process at step S240is similar to the process at step S130.

In this manner, the processing circuitry105causes the specifying function105ato specify the timing information in the first detector corresponding to the event of the pair annihilation gamma-rays detected in the second detector on the basis of the determined coincidence.

In this manner, the processing circuitry105causes the specifying function105ato generate the simultaneous counting information using the first timing information in the first detector1and the second timing information in the second detector2. Here, the simultaneous counting information is the information containing the time and position where the pair annihilation gamma-rays are generated, for example. The processing circuitry105causes the image generating function105bto generate the PET image on the basis of this simultaneous counting information.

In the case ofFIG.6, similar to the case inFIG.5, in consideration of Compton scattering in the first detector1, the processing circuitry105may cause the image generating function105bto estimate the line of response (LOR) of the pair annihilation gamma-rays on the basis of the first timing information, or the LOR of the pair annihilation gamma-rays on the basis of both the first timing information and the second timing information.

Subsequently, description is made of the arrangement of the pixels of the first detector1and the pixels of the second detector2.

In the embodiment, the first detector1and the second detector2may be disposed so that the center lines of a plurality of pixels forming the first detector1are displaced from the center lines of a plurality of pixels of the second detector. Here, the center line of each pixel in the first detector1means a line connecting centers of pixels corresponding to pixels at symmetric positions with respect to the center of the ring among the pixels forming the first detector1. Moreover, the center line of each pixel in the second detector means a line connecting centers of pixels corresponding to pixels at symmetric positions with respect to the center of the ring among the pixels forming the second detector2. Thus, the line of response (LOR) can be estimated more accurately.

Such a circumstance is described with reference toFIG.7andFIG.8.

FIG.7is a diagram for describing the estimation of the LOR in a case where the center lines of the pixels1a,1b,1c,1d,1e, and if in the first detector1coincide respectively with the center lines of the pixels2a,2b,2c,2d,2e, and2fin the second detector2as a comparative example. For example, inFIG.7, the center lines of the pixels1band1ein the first detector1coincide with the center lines of the pixels2band2ein the second detector2.

In this case, it is assumed that a pair of annihilation gamma-rays generated from a generation point Q radiate on a line70, and along with this, the Cherenkov light is observed in the pixels1band1ein the first detector1and the scintillation light is observed in the pixels2band2ein the second detector2. In this case, the pixels1band1eof the first detector1and the pixels2band2eof the second detector2exist at the symmetric position with respect to the center of the ring, and are the corresponding pixels.

Here, the center line of the pixels1band1eof the first detector1, that is, the line connecting the center of the pixel1band the center of the pixel1eand the center line of the pixels2band2eof the second detector2, that is, the line connecting the center of the pixel2band the center of the pixel2ecoincide. Therefore, the direction of the LOR estimated based on the information acquired by the first detector1and the direction of the LOR estimated based on the information acquired by the second detector2coincide. As a result, the range where the generation of the pair annihilation gamma-rays is estimated based on the information acquired by the first detector1is a range40. In addition, the range where the generation of the pair annihilation gamma-rays is estimated based on the information acquired by the second detector2is a range41. Therefore, both ranges coincide.

Accordingly, a range42corresponding to a common part of the range40and the range41is the range where the generation of the pair annihilation gamma-rays is estimated and the range42is relatively wide.

On the other hand,FIG.8is a diagram for describing the estimation of the LOR in a case where the center lines of the pixels1a,1b,1c,1d,1e, and if of the first detector1are displaced from the center lines of the pixels2a,2b,2c,2d,2e, and2fof the second detector2. For example, inFIG.8, the center lines of the pixels1band1ein the first detector1are displaced from the center lines of the pixels2band2ein the second detector2.

In this case, it is assumed that a pair of annihilation gamma-rays generated from the generation point Q radiate on the line70, and along with this, the Cherenkov light is observed in the pixels1band1ein the first detector1and the scintillation light is observed in the pixels2band2ein the second detector2. In this case, the pixels1band1eof the first detector1and the pixels2band2eof the second detector2exist at the symmetric position with respect to the center of the ring, and are the corresponding pixels.

Here, the center line of the pixels1band1eof the first detector1, that is, the line connecting the center of the pixel1band the center of the pixel1eand the center line of the pixels2band2eof the second detector2, that is, the line connecting the center of the pixel2band the center of the pixel2eare in the different directions and displaced from each other. Therefore, the direction of the LOR estimated based on the information acquired by the first detector1and the direction of the LOR estimated based on the information acquired by the second detector2are different from each other. As a result, the range where the generation of the pair annihilation gamma-rays is estimated based on the information acquired by the first detector1is the range40. In addition, the range where the generation of the pair annihilation gamma-rays is estimated based on the information acquired by the second detector2is the range41. Thus, both ranges cover different ranges.

Accordingly, the range42corresponding to a common part of the range40and the range41is the range where the generation of the pair annihilation gamma-rays is estimated and the range42is smaller than that in the case ofFIG.7. As a result, the positional resolution of the PET device100is improved.

The summary of the embodiment is as below: the radiation diagnosis device according to the embodiment described above performs PET imaging using both the first detector1that is the detector configured to detect the Cherenkov light and the second detector2that is the detector configured to detect the scintillation light. The detector employing the method of detecting the Cherenkov light has a characteristic of being advantageous in terms of time resolution over the detector employing the method of detecting the scintillation light. On the other hand, the second detector2that is the detector configured to detect the scintillation light is inferior to the first detector1that is the detector configured to detect the Cherenkov light in terms of the response speed but is advantageous in terms of the energy resolution, and has a characteristic of being able to efficiently remove the scattering event or the like. Therefore, the radiation diagnosis device according to the embodiment generates the counting information using the first detector1and the second detector2. Thus, the counting information in which the high time resolution is maintained while the energy resolution is kept can be generated and therefore, the image quality can be improved.

Other Embodiment

The embodiment is not limited to the aforementioned examples. In the above embodiment, the PET device is described, that is, a pair of annihilation gamma-rays is detected using the first detector1and the second detector2; however, the embodiment is not limited to the example of detecting a pair of annihilation gamma-rays and one gamma-ray may be detected. In addition, the embodiment is not limited to the gamma-rays and is applicable to other radiation such as X-rays. Other examples of the radiation diagnosis device than the PET device to which the embodiment is similarly applicable include single photon emission computed tomography (SPECT) or a Compton camera.

In addition, as illustrated inFIG.9, in the radiation diagnosis device according to the embodiment, the first detector1that is the detector configured to detect the Cherenkov light and the second detector2that is the detector configured to detect the scintillation light may be movable detectors. For example, as illustrated inFIG.9, the first detector1may be the movable detector that is movable in a z-axis direction that is a body axis direction perpendicular to the slice plane. Thus, the position of the detector can be moved in accordance with the image capture object, and the image quality can be improved.

In addition, the first detector1that is the detector configured to detect the Cherenkov light does not acquire the energy information in the above description; however, the embodiment is not limited to this example and the first detector1that is the detector configured to detect the Cherenkov light may acquire the energy information. In one example, the first timing information acquisition circuitry101may estimate the energy information of the annihilation gamma-rays on the basis of the angle of the Cherenkov ring obtained by the first detector1that is the detector configured to detect the Cherenkov light.

In the above embodiment, the first detector and the second detector are arranged so that the center lines of the pixels forming the first detector1are displaced from the center lines of the pixels of the second detector2; however, the embodiment is not limited to this example. The first detector and the second detector may be arranged so that the center lines of the detector blocks in the first detector1are displaced from the center lines of the detector blocks of the second detector2.

According to at least one embodiment described above, the image quality can be improved.

Regarding the embodiments described above, the following notes are disclosed as aspects and selective characteristics of the invention.Note 1. A radiation diagnosis device comprising:a first detector configured to detect Cherenkov light generated when a radiation passes; anda second detector provided to face the first detector on a side farther from a source of generating the radiation and configured to detect energy information of the radiation.Note 2. The first detector and the second detector are ring-shaped detectors, and a diameter of the first detector is smaller than a diameter of the second detector.Note 3. The first detector includes a light emitter configured to generate the Cherenkov light as the radiation passes, andthe light emitter is thinner than a scintillator provided to the second detector.Note 4. A pixel size of the light emitter is smaller than a pixel size of the scintillator.

The pixel is the minimum separation unit of the positional resolution of the detector.Note 5. The radiation is pair annihilation gamma-rays, and the radiation diagnosis device further comprises:a first timing information acquisition unit configured to acquire first timing information of the pair annihilation gamma-rays in the first detector; anda second timing information acquisition unit configured to acquire second timing information of the pair annihilation gamma-rays in the second detector in order to specify an event of the pair annihilation gamma-rays in which the first timing information is acquired, based on the first timing information.Note 6. The radiation diagnosis device further comprises a specifying unit configured to specify that the first timing information and the second timing information are acquired based on the same pair annihilation gamma-rays in a case where the first timing information and the second timing information are included in a predetermined time window.Note 7. The specifying unit is configured to estimate a line of response (LOR) of the radiation, based on the first timing information.Note 8. The specifying unit estimates an LOR of the radiation by correcting, based on the estimation result at the first detector, the LOR estimated based on the detection result at the second detector.Note 9. The specifying unit estimates an LOR of the radiation taking Compton scattering at the first detector into consideration.Note 10. The specifying unit estimates an LOR of the radiation based on the number of photons of Cherenkov light detected at the first detector.Note 11. The specifying unit estimates a scattering angle of Compton scattering at the first detectors and estimates an LOR of the radiation based on the scattering angle.Note 12. The second timing information acquisition unit is configured to further acquire energy information of the pair annihilation gamma-rays in the second detector, andthe specifying unit is configured to specify that the first timing information and the second timing information are acquired based on the same pair annihilation gamma-rays in a case where the energy information is included in a predetermined energy window.Note 13. The radiation is pair annihilation gamma-rays, and the radiation diagnosis device further comprises a specifying unit configured to determine coincidence, based on the energy information and timing information of the pair annihilation gamma-rays detected in the second detector, and specify the timing information in the first detector corresponding to an event of the pair annihilation gamma-rays detected in the second detector, based on the determined coincidence.Note 14. The specifying unit is configured to specify that the timing information in the first detector and the timing information in the second detector are acquired based on the same pair annihilation gamma-rays in a case where the timing information in the first detector and the timing information in the second detector are included in a predetermined time window.Note 15. A detector column length of the first detector is shorter than a detector column length of the second detector.Note 16. The light emitter is formed of a medium that suppresses scintillation by the radiation compared to the scintillator of the second detector.Note 17. The medium is bismuth germanium oxide (BGO) or a lead compound.Note 18. The first detector and the second detector are arranged so that center lines of a plurality of pixels forming the first detector are displaced from center lines of a plurality of pixels forming the second detector.

Here, the center line of each pixel in the first detector1means a line connecting centers of pixels corresponding to pixels at symmetric positions with respect to the center of the ring among the pixels forming the first detector1. Moreover, the center line of each pixel in the second detector means a line connecting centers of pixels corresponding to pixels at symmetric positions with respect to the center of the ring among the pixels forming the second detector1.Note 19. The first detector is a movable detector.Note 20. The first detector is a detector that is movable in a z-axis direction corresponding to a body axis direction that is perpendicular to a slice plane.Note 21. A positron emission tomography (PET) device comprising:a first detector configured to detect Cherenkov light generated when a radiation passes; anda second detector provided to face the first detector on a side farther from a source of generating the radiation and configured to detect energy information of the radiation.Note 18. A radiation diagnosis method to be performed by a radiation diagnosis device, the radiation diagnosis method comprising:detecting, with a first detector, Cherenkov light that is generated when a radiation passes; anddetecting, with a second detector, energy information of the radiation, the second detector being provided to face the first detector on a side farther from a source of generating the radiation.