WO2016140371A1

WO2016140371A1 - Radiation detection method and compton camera

Info

Publication number: WO2016140371A1
Application number: PCT/JP2016/057256
Authority: WO
Inventors: Toru Tanimori; Akira Nabetani; Hidetoshi Kubo
Original assignee: Canon Kabushiki Kaisha
Priority date: 2015-03-05
Filing date: 2016-03-02
Publication date: 2016-09-09
Also published as: JP2016161522A

Abstract

A radiation detection method for detecting an arrival point and energy of a radiation ray that has been scattered by Compton scattering, the method comprising: detecting a scattering point where a recoil electron is generated by the Compton scattering, and a recoil direction and energy of the recoil electron; and when an angle formed by the recoil direction and a scattering direction of the scattered radiation ray is set as a differential angle α, and when energy is detected that is components of the radiation ray scattered substantially simultaneously at a plurality of points in a radiation detector; using both a first differential angle α_kin calculated from the recoil electron energy and the radiation energy and a second differential angle α_geo calculated from a detection point of the scattered radiation ray, the scattering point, and the recoil direction to determine a first arrival point of the radiation ray.

Description

DESCRIPTION

Title of Invention : RADIATION DETECTION METHOD AND COMPTON CAMERA

Technical Field

[0001] The present invention relates to a method of detecting a radiation ray such as a gamma ray that is scattered by Compton scattering, and a Compton camera that utilizes this method to measure the incident direction of a radiation ray such as a gamma ray that is emitted from a radiation source.

Background Art

[0002] A Compton camera includes a scattering body configured to scatter a. photon of an incident gamma ray by Compton scattering, a gamma ray detector configured to detect a scattered gamma ray that is scattered by Compton scattering, and an electron detector configured to detect a recoil electron that is generated by Compton scattering. The gamma ray detector is configured to detect the energy and arrival point of the scattered gamma ray, and the electron detector is configured to detect the Compton scattering point and the energy of the recoil electron. From those pieces of detection information obtained in individual Compton scattering events, the incident direction of each photon of the incident gamma ray is calculated to further obtain information about an incident direction in which most photons have entered, thereby reconstructing the radiation distribution of the radiation source, as an image.

[0003] An advanced Compton camera has also been proposed (see NPL 1) in which, in addition to the pieces of detection information described above, the track of the recoil electron in the scattering body is detected by the electron detector, and this information is used in combination to calculate the incident direction of an incident gamma ray.

[0004] In the advanced Compton camera, a scattering direction vector g (unit vector) of the gamma ray is calculated from Compton scattering points and gamma ray arrival points that are detected in individual Compton scattering events. A recoil direction vector e (unit vector) of the recoil electron is calculated from the measured electron track information. An incident direction vector s of the incident gamma ray is further calculated based on the detected recoil electron energy and scattered gamma ray energy, which are given as Ke and Εγ, respectively, by Expression ( 1 ) ·:

s= (coscp-sincp/tana) g+ (sincp/sina) e (1).

[0005] The symbol φ represents an angle formed by the incident direction vector s and the scattering direction vector g, and a represents an angle formed by the recoil direction vector e and the scattering direction vector g. The angles φ and a can be calculated by Expressions (2) and (3), respectively, where m represents the rest mass of the electron and c represents the velocity of light.

cp=cos^_1 [l-{mc²/ (Εγ+Ke) } (Ke/Εγ) ] (2) =cos^_1 [ (l-mc²/EY) { Ke/ (Ke+2mc²) }^1/2] (3) [0006] Advanced Compton cameras as this have a feature of being generally higher in measurement precision than earlier Compton cameras, which do not use electron track information.

[0007] Gamma ray detectors used in Compton cameras are pixel-type detectors capable of detecting both the absorption point and energy of a gamma ray, and generally use a photoelectric absorption material (a scintillator) . When a scattered gamma ray that is scattered in a scattering body by Compton scattering is absorbed by photoelectric absorption inside the detector (photoelectric absorption material) , the whole energy of the gamma ray can be detected at a pixel that is the absorption point and the absorption point can be identified as well.

Citation List

Patent Literature

[0008] PTL 1: U.S. Patent No. 6512232

PTL 2: Japanese Patent Application Laid- Open No. 2010-02235

Non Patent Literature

[0009] NPL 1: S. Kabuki et al . , "Development of

Electron Tracking Compton Camera using micro pixel gas chamber for medical imaging", Nucl. Instr. and Meth, A 580 (2007) 1031

Summary of Invention

Technical Problem

[0010] Gamma rays that Compton cameras as those described above are designed to measure have energy of 100 keV to 10 MeV. Generally speaking, the ratio of Compton scattering/photoelectric absorption increases with increasing of energy of gamma ray in this energy region. A gamma ray, which loses part of its energy in the scattering body by Compton scattering but still retains fairly large energy, is again scattered by Compton scattering inside the gamma ray detector (i.e. "in-detector Compton scattering") at a high probability When this in-detector Compton scattering occurs, part of the energy of the gamma ray is detected at a pixel that is the scattering point. If the re-scattered gamma ray is absorbed by photoelectric absorption at another point, the remaining energy is detected at a pixel that is the absorption point, which is called multiple detection. Multiple detection occurs at a time interval shorter than the time resolution of the detector, and it is not easy to discriminate the substantially simultaneous detection results based on time difference. [0011] Further, accidental concurrent multiple detection may occur in which a scattered gamma ray that is caused by one Compton scattering event and another gamma ray that has no relevance to the scattering are accidentally detected substantially simultaneously at different pixels of the gamma ray detector. The irrelevant gamma ray may be one emitted from a measurement subject, or may be a cosmic ray and the like.

[0012] Calculating the incident direction vector s of the incident gamma ray in one Compton scattering event in a Compton camera with the use of Expressions (1), (2), and (3) requires the following. That is, detection information about the scattered gamma ray, which is specifically the energy Εγ and the scattering direction vector g, needs to be paired with the recoil electron energy Ke and the recoil direction vector e that are detected in the same Compton scattering event.

[0013] However, when gamma ray energy is detected at a plurality of points in the gamma ray detector simultaneously as in the multiple detection due to in- detector Compton scattering and. the accidental concurrent multiple detection which are described above, the camera cannot identify which of the plurality of detection values corresponds to the first arrival point of the scattered gamma ray. The incident direction vector s of the incident gamma ray therefore cannot be calculated. With an incident gamma ray that has energy of 500 keV or more, in particular, the multiple detection due to in-detector Compton scattering increases, thereby diminishing effective detection information. An accordingly longer measurement time is required to obtain enough pieces of data for the reconstruction of a radiation distribution image (in other words, detection sensitivity drops) .

[0014] In PTL 1, there is disclosed a method that addresses this issue, in which the first arrival point in the detector is estimated by using the following principle. The principle is that, when multiple detection is due to in-detector Compton scattering, probability dictates that the gamma ray energy lost by scattering (the energy detected by the detector) is smaller than the remaining energy of the scattered gamma ray. However, this method is premised on in- detector Compton scattering, and is not applicable to accidental concurrent multiple detection. In addition, when the scattered gamma ray after in-detector Compton scattering is not absorbed by photoelectric absorption in the detector in the end, the method is also incapable of figuring out that fact. The detected energy is consequently inaccurate, and inaccurate information about the gamma ray incident direction calculated by this energy information constitutes noise in data for reconstructing the radiation distribution.

[0015] In PTL 2, there is disclosed another method in which the first arrival point of a gamma ray is determined when multiple detection due to in-detector Compton scattering occurs in positron emission tomography (PET) . The premise of this method, however, is that the energy of a gamma ray for which the determination is made is 511 keV. The method therefore cannot be applied to the determination for a scattered gamma ray that has been scattered once in a scattering body by Compton scattering and consequently has indeterminate energy, such as the determination made in Compton cameras.

Solution to Problem

[0016] According to one embodiment of the present invention, there is provided a method of detecting a radiation ray such as a gamma ray, for detecting, by a radiation detector, an arrival point and energy of a radiation ray that has been scattered in a scattering body by Compton scattering. The method includes: detecting, by an electron detector, a scattering point at which a recoil electron is generated by the Compton scattering, a recoil direction of the recoil electron, and energy of the recoil electron; and when an angle that is formed by the recoil direction of the recoil electron and a scattering direction of the scattered radiation ray is set as a differential angle a, and when energy is detected that is components of the radiation ray scattered substantially simultaneously at a plurality of points in the radiation detector; using both a first differential angle a_kin and a second differential angle a_geo to determine a first arrival point at which the scattered radiation ray first reaches the radiation detector, the first differential angle a_kin being calculated from the detected recoil electron energy and the detected radiation energy, the second differential angle _geo being calculated from a detection point of the scattered radiation ray, the scattering point, and the recoil direction.

[0017] Further, according to one embodiment of the present invention, there is provided a Compton camera including a scattering body, an electron detector, and a radiation detector, . the Compton camera being configured to: detect, by the radiation detector, an arrival point and energy of a radiation ray. scattered in the scattering body by Compton scattering; detect, by the electron detector, a scattering point at which a recoil electron is generated by the Compton scattering, a recoil direction of the recoil electron, and energy of the recoil electron; and obtain an incident direction of the radiation ray from detection information about the arrival point and the energy of the radiation ray, and from information about the detected scattering point, the recoil direction of the recoil electron, and the energy of the recoil electron. When an angle formed by the recoil electron recoil direction and a scattering direction of the radiation ray is set as a differential angle a, and energy is detected that is components of the radiation ray scattered substantially simultaneously at a plurality of points in the radiation detector, a first differential angle a_ki_n and a second differential angle ageo are both used to determine a first arrival point at which the scattered radiation ray first reaches the radiation detector, the first differential angle a_kin being calculated from the detected recoil electron energy and the detected radiation energy, the second differential angle _ge0 being . calculated from a detection point of the scattered radiation ray, the detected scattering point, and the detected recoil direction.

[0018] Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings .

Brief Description of Drawings

[0019] FIG. 1 is a diagram for illustrating a configuration example of a Compton camera.

FIG. 2A is a diagram for illustrating a structural example of a μTPC.

FIG. 2B is a diagram for illustrating the structural example of the μΤΡΟ.

FIG. 3 is a diagram for illustrating the operation of the TPC.

FIG. 4A is a diagram for illustrating a structural example of a μΤΡΟ.

FIG. 4B is a diagram for illustrating the structural example of the μΤΡΟ.

FIG. 5 is a flow chart of an example of determination processing in a gamma ray detection method according to the present invention. FIG. 6A is a graph for showing the probability density distribution of a true value of recoil electron energy to a measurement value of recoil electron energy.

FIG. 6B is a graph for showing the probability density distribution of a true value of scattered gamma ray energy to a measurement value of scattered gamma ray energy.

FIG. 7A is a graph for illustrating the probability density distribution of a true value of a, and a determination method example.

FIG. 7B is a graph for illustrating the probability density distribution of the true value of a, and a determination method example.

FIG. 7C is a graph for illustrating the probability density distribution of the true value of a, and a determination method example.

Description of Embodiments

[0020] In an embodiment of the present invention, a Compton scattering point, the recoil direction of a recoil electron, and the energy of the recoil electron are detected and, when energy is detected that is components of radiation ray scattered substantially simultaneously at a plurality of points, in a radiation detector, the following action is taken. An angle formed by the recoil direction and the scattering direction is referred to as a differential angle a, and the first arrival point of a scattered radiation ray is determined with the use of a first differential angle a_kin , which is obtained from the energy of the recoil electron and the energy of the radiation ray, and a second differential angle a_geo^ which is obtained from the detection point and scattering point of the scattered radiation ray component and the recoil direction. Based on this determination, the differential angle a is obtained to be used in combination with the angle φ of Expression (2), the scattering direction vector g, and the recoil direction vector e in the calculation of the incident direction vector s of the incident radiation ray.

[0021] A Compton camera according to the embodiment of the present invention is described below with reference to the drawings. Radiation rays in the following description are gamma rays. However, the same measurement principle applies to other types of radiation that have enough energy for Compton scattering, such as X-rays. The present invention is also not limited to the following embodiment, and various modifications and changes can be made without departing from the spirit of the present invention. . ,

[0022] FIG. 1 is a diagram for illustrating a configuration example of a Compton camera. This Compton camera includes, as illustrated in FIG. 1, a micro time projection chamber (yTPC) 101, which is an electron detector, and a gamma . ray detector 102, which is a radiation detector. Also illustrated in FIG. 1 are a radiation source 103, which is a measurement subject, and an incident gamma ray 104, which is emitted from the radiation source and enters the TPC 101. The configuration of the TPC 101. is illustrated in perspective view in FIG. 2A and in sectional view in FIG. 2B. The interior of the . μTPC 101 is filled with an argon gas or the like that serves as a scattering body 202, and a drift plane 201 made from a conductive material is provided on a top surface of the μTPC 101. The μTPC 101 includes a micro pixel gas chamber (μΡΙΟ , which is an electron track detecting unit configured to detect the track of a recoil electron that results from Compton scattering by amplifying, through an electron avalanche phenomenon, ionized electrons caused by the recoil electron. The μΡ^ is described later. The μTPC 101 also includes a an electric field applying unit configured to apply an electric field by giving the drift plane 201 an electric potential that is negative with respect to the electric potential of an electrode of the electron track detecting unit (a first electrode) . The drift plane 201 is a second electrode, which faces the first electrode across the scattering body such as gas. Conductor lines 205 are wound around the sides of the μΤΡΟ 101 in parallel to the drift plane 201 at equal intervals in a multistage manner. A resistor 204 is connected between every two conductor lines 205, and between the drift plane 201 and the topmost conductor line 205. A power supply 206 applies a DC voltage between the drift plane 201. and the lowermost conductor line 205, and the DC voltage is divided by the resistors 204 into voltages that are applied to the conductor lines 205 separately. A uniform electric field is thus generated inside the μTPC 101 in a direction indicated by the arrow E. The micro pixel gas chamber (yPIC) is arranged at the bottom of the TPC 101 to serve as an electron detector 203.

[0023] The operation of the μTPC 101 is described next with reference to FIG. 3. A photon of the incident gamma ray 104 emitted from the radiation source 103 and entering the μTPC 101 interacts with an electron of a gas molecule 302 contained in the scattering body 202 in the form of Compton scattering. A scattered gamma ray 301 and an ionized recoil electron 304 are generated as a result. The recoil electron 304 travels through the scattering body 202 while ionizing electrons from other molecules one after another. The large number of ionized electrons thus generated along the track of the recoil electron 304 form an electron cloud 303 (a cotton-like mass of electrons) . The electron cloud 303, under the influence of the uniform electric field inside the μTPC 101 which is indicated by the arrow E, drifts to the electron detector 203 at a uniform velocity while maintaining the cloud-shape.

[0024] FIG. 4A is a top view of the pPIC, which is the electron detector 203, and FIG. 4B is a partially enlarged perspective view of the μΡΙ<2. The μΡΙΟ includes a substrate 406 made from a dielectric material, a large number of anode strips 401 formed on a rear surface of the substrate 406 at equal intervals, and a large number of cathode strips 402 formed on a front surface of the substrate 406 at equal intervals in a direction orthogonal to the anode strips 401. Circular openings are formed in the cathode strips 402 at equal intervals, and peripheral portions of the openings serve as cathode electrodes 404. Thin, column-shaped anode electrodes 403 rise from the anode strips 401 on the rear surface at equal intervals so as to pierce the insulating substrate, and are exposed at the centers of the openings formed in the cathode strips 402. A power supply 405 applies a high DC voltage between the anode strips 401 and the cathode strips 402, with the result that an intense electric field is generated between the anode electrodes 403 and the cathode electrodes 404.

[0025] The ionized electrons forming the electron cloud 303 that has drifted and reached the electron detector 203 (the μΡΙ^ are rapidly accelerated by the intense electric field between the anode electrodes 403 and the cathode electrodes 404 to generate a large number of ionized electrons and cations from gas molecules (a gas amplification effect) in an avalanche manner. The generated ionized electrons concentrate on and are absorbed by the anode electrodes 403, and the cations concentrate on the cathode electrodes 404, thereby neutralizing the electric charges.

[0026] In this case, the anode strips .401 and the cathode strips 402 are formed so as to be orthogonal to each other. Thus, the two-dimensional position of the electron cloud 303, namely, the track of the recoil electron having the Compton scattering point as one end thereof, in a plane parallel to the electron detector 203 can be detected from the position of the anode strip 401 where negative electric charges due to the ionized electrons are detected and the position of the cathode strip 402 where positive electric charges due to the cations are detected.

[0027] The drift distance of the electron cloud

303, namely, the distance in a direction perpendicular to the electron detector 203, can also be detected from the drift velocity of the electron cloud 303, and from a time difference between the detection of the scattered gamma ray by the gamma ray detector 102 and the detection of the electron cloud 303 by the electron detector 203. Three-dimensional position information of the Compton scattering point and the recoil electron track is obtained in this manner.

[0028] The drift velocity is a constant value that is in proportion to the product of a mean time interval of collisions between an electron and a gas molecule and the magnitude of the electric field. The drift times of the large number of ionized electrons generated by many Compton scattering events that occur in the μ PC 101 are measured and expressed in the form of a frequency distribution as follows. That is, the top end of the distribution (the maximum drift time) corresponds to a drift time required to travel the maximum drift distance, which is a distance from the drift plane 201 to the electron detector 203. When the distance from the drift plane 201 to the electron detector 203 is given as L and the maximum drift time is given as . Tmax, the drift velocity can therefore be calculated by L/Tmax. [0029] The recoil electron energy Ke is the total of energy given to ionized electrons that have been generated until the recoil electron has come to a stop, and can be calculated from the total amount of electric charge detected in the electron cloud 303 by the electron detector 203. A proportionality coefficient used for the calculation can be obtained in advance by calibration from a relation between the energy and the detected electric charge amount.

[0030] The scattered gamma ray 301, on the other hand, is scattered by Compton scattering toward a direction different from that of the incident gamma ray 104, is transmitted . through the electron detector 203, and is detected by the gamma ray detector 102 as illustrated in FIG. 3. The gamma ray detector 102 is a pixel-type detector capable of detecting both the absorption point and energy of the scattered gamma ray 301. A detector of this kind that is generally used combines a photoelectric absorption material (a scintillator) arranged in a grid pattern with a pixel- type photoelectric converter (a photomultiplier tube or an avalanche photodiode) . The scintillator is configured to convert the energy of a gamma ray into visible light or ultraviolet light, and the photoelectric converter is configured to convert this light into electric energy. Other than this kind of detector, a semiconductor element configured to covert gamma ray energy directly into electric energy may be used. Gamma rays to be detected by the Compton camera have relatively high energy, and the Compton camera can therefore use an absorption material that is high in gamma ray absorption capacity. When the scattered gamma ray 301 is absorbed by photoelectric absorption inside the gamma ray detector 102 (a photoelectric absorption material) , the whole energy of the gamma ray can be detected at a pixel that is the absorption point and the absorption point can be identified as well.

[0031] The scattering direction vector g (a unit vector) is calculated from the Compton scattering point and the arrival point of the scattered gamma ray 301 that are measured for the Compton scattering event of each photon of the incident gamma ray 104 in this manner. The recoil direction vector e (a unit vector) of the recoil electron 304 is also calculated from information on the measured electron track. The incident direction vector s (a unit vector) of the incident gamma ray 104 is further calculated based on the energy Ke of the recoil electron 304 and the energy Εγ of the scattered gamma ray 301 that are measured, with the use of Expressions. (1), (2), and (3). Calculating the incident direction vector s correctly requires pairing accurate detection information about the scattered gamma ray 301, namely, the energy Εγ and the scattered direction vector g, with the energy Ke and recoil direction vector e of the recoil electron 304 that are detected in the same Compton scattering event where the energy Εγ and the vector g are detected

[0032] In practice, however, there are cases where energy is detected at a plurality of points in the gamma ray detector substantially simultaneously (in other words, at a time interval equal to or shorter than the time resolution of the detector) as in the multiple detection due to in-detector Compton scattering and the accidental concurrent multiple detection which are described above. Examples thereof are given below.

(1) The incident gamma ray 104, which is emitted from the radiation source, is scattered by a scattering body by Compton scattering, and the scattered gamma ray 301 is further scattered by Compton scattering inside the gamma ray detector 102 once, twice, or more at different points to be ultimately absorbed by photoelectric absorption at another point in the detector. This is multiple detection due to in- detector Compton scattering. To illustrate this phenomenon, the gamma ray scattered by Compton scattering inside the gamma ray detector 102 is drawn in the broken line in FIG. 3.

(2) At substantially the same time as the detection of the scattered gamma ray 301, which is a detection subject, another gamma ray that has no relevance (a gamma ray transmitted without being scattered by the scattering body by Compton scattering, a cosmic ray, or the like) is detected accidentally at another point in the gamma ray detector 102. This is accidental concurrent multiple detection.

(3) A plurality of incident gamma rays 104 (photons), which are detection subjects, are scattered by the scattering body by Compton scattering substantially simultaneously, and a plurality of scattered gamma rays 301 are detected separately at different points in the gamma ray detector 102. This is accidental concurrent multiple detection of a plurality of incident gamma rays that are separate detection subjects.

[0033] There can also be cases where two or more of (1), (2), and (3) occur in combination. In such cases of multiple detection, which of the plurality of pieces of detection information about detection point and energy is to be paired with the detection information of the recoil, electron is determined in this embodiment. To that end, the angle a formed by the recoil direction vector e and the scattering direction vector g is calculated with the use of two computational expressions, and results thereof are compared to each other.

[0034] One of the computational expressions is

Expression (3) described above. With Expression (3), the angle a can be calculated kinetically from the recoil electron energy Ke and the scattered gamma ray energy Εγ that are detected. The angle a that is obtained kinetically by Expression (3) is called herein as a first differential angle ci_kin -

[0035] The other computational expression is

Expression (4) given below.

=cos^_1(g*e) (4) [0036] With this expression, the angle a can be calculated geometrically from the recoil direction vector e and the scattering direction vector g that are detected. The angle a that is obtained geometrically by Expression (4) is called herein as a second differential angle α_5βο·

[0037] When the scattering direction vector g, the recoil direction vector e, the recoil electron energy Ke, and the scattered gamma ray energy Εγ are all results of the same Compton scattering event, the first differential angle _kin calculated by Expression (3) and the second differential angle a_ge0 calculated by Expression (4) match.

[0038] Ά description is given as an example on how the determination is made when a photon having energy Εγι and a photon . having energy Εγ₂ are detected substantially simultaneously at two different points (a pixel Pgi and a pixel Pg₂) in the gamma ray detector, whereas only one recoil electron having energy Ke is detected at the same time as the photons. One of the two gamma rays detected in this case can possibly be an irrelevant gamma ray (accidental concurrent multiple detection) . Accordingly, Expression (3) is used first to calculate a first differential angle oi_kini by setting Εγ to Εγι, and to calculate a first differential angle Ci_kin2 by setting Εγ to Εγ₂. Two scattering direction vectors, a scattering direction vector gi running from the Compton scattering point toward the pixel Pgi and a scattering direction vector g₂ running from the Compton scattering point toward the pixel Pg₂, are assumed next, and a second differential angle a_geoi and a second differential angle _ge02 are calculated by Expression (4) for the vector g_x and the vector g₂, respectively. The first differential angle a_kj_.ni is compared to the second differential angle u_geoi, and the first differential angle a_kj_.n2 is compared to the second differential angle a_ge02 · The first differential angle a_kin and the second differential angle a_geo are supposed to match when the scattered gamma ray is one that has been generated by the same Compton scattering event that has generated the detected recoil electron. The pixel that is the base of one of those angles a for which the first differential angle and the second differential angle match is therefore treated as the arrival point of the scattered gamma ray.

[0039] When the first differential angle and the second differential angle do not match for both of the angles a, however, it may be multiple detection in which Compton scattering happens at one of the pixels first and then photoelectric absorption occurs at the other pixel, instead of accidental concurrent multiple detection. Then, the sum of the energy Εγι and energy Εγ₂ detected at two points is the scattered gamma ray energy Εγ. The first, differential angle a_kj_.n is accordingly calculated by Expression (3) by setting Εγ as Εγχ+ . Εγ₂. The calculated first differential angle a_kin is compared to the second differential angle a_geoi and the second differential angle 0i_geO2 calculated by Expression (4), and the pixel that is the base of one of those angles a for which the first differential angle and the second differential angle match is treated as the arrival point of the scattered gamma ray.

[0040] This procedure can be expanded to be applied to the case where simultaneous detection is made at three or more different points in the gamma ray detector. A determination method that covers such cases as well is described. A flow chart of the determination processing is given in FIG. 5.

[0041] A case is considered in which gamma ray energy is detected at N points (pixels Pgi, Pg2 ... Pgt^ N≥2) substantially simultaneously. It is assumed in this case that scattered gamma rays that have been generated by the same Compton scattering event that has generated the detected recoil electron are detected at n points (l≤n≤N) out of the N points, and the rest are regarded as detection points of irrelevant gamma rays (accidental concurrent multiple detection) . First, n is set to 1. In other words, it is assumed that the scattered gamma ray is absorbed by photoelectric absorption, at the first arrival, point. Next, one combination of n pixels (here, one pixel) out of the pixels Pgi, Pg₂ ... Pg_N is selected. One pixel, for example, Pgi, is selected because n is 1 at first. The following Steps 1 to 3 are subsequently executed. In FIG. 5, Step 1 is indicated by 501, Step 2 is indicated by 502, and Step 3 is indicated by 503.

[0042] [Step 1 (First Round)]

The total of energy that is detected at n pixels included in the selected combination (here, one pixel) is calculated and set as Εγ. The energy Εγι detected at the pixel Pgi is set as Εγ at first. This energy Εγ is used in Expression (3) to calculate the first differential angle oikini ·

[0043] [Step 2 (First Round)]

One pixel is selected out of n pixels (here, one pixel) included in the selected combination. The pixel Pgi is selected first. The scattering direction vector gi that runs from the Compton scattering point toward the selected pixel Pgi is then obtained. This scattering direction vector g_x is used in Expression (4) to calculate the second differential angle a_geoi ·

[0044] [Step 3 (First Round)]

The first differential angle a_kj_.ni obtained in Step 1 and the second differential angle a_geoi obtained in Step 2 are compared to each other. When the two match, the pixel Pgi, which is the base of this angle a, is determined as the first arrival point of the scattered gamma ray. When the two do not match, it is determined that the pixel Pg_x is not the first arrival point of the scattered gamma ray. When it is determined in Step 3 that the selected pixel is not the first arrival point of the scattered gamma ray, whether the determination has been finished for every pixel that is included in the selected combination of n pixels is checked. When the determination has not been finished for every included pixel, another pixel is selected out of the selected combination of pixels to repeat Step 2 and Step 3. In the first round where n is 1, the determination processing ends after executing Steps 1 to 3 once.

[0045] When the determination has been finished for all of the n pixels, whether the determination has been finished for every combination of n pixels is checked first. When the determination has not been finished for every combination of n pixels, another combination of n pixels is selected to repeat Steps 1 to 3. When the determination has been finished for every combination of n pixels, n is increased by 1 and one combination of n pixels is newly selected to repeat Steps 1 to 3. In the first round where n is 1, the determination processing ends after executing Steps 1 to 3 once.

[0046] When it is determined that the pixel Pgi selected when n is set to 1 is the first arrival point of the scattered gamma ray, as opposed to the scattered gamma ray being absorbed by photoelectric absorption at the pixel Pgi, n is set to 2. In other words, it is assumed that the scattered gamma ray has been scattered by Compton scattering at the first arrival point and then absorbed by photoelectric absorption at another pixel. Accordingly, a combination of two pixels out of the pixels Pgi, Pg₂ · ·■ Pgt for example, a combination of the pixels Pgi and Pg₂, is selected and the following Steps 1 to 3 are further executed.

[0047] [Step 1 (Second Round)]

The total of energy that is detected at n pixels included in the selected combination (here, two pixels) is calculated and set as Εγ. The total of energy Εγι+Εγ₂ detected at the pixels Pgi and Pg₂ is set as Εγ in this case. This energy Εγ is used in Expression (3) to calculate a first differential angle α_¾ίηι+2.

[0048] [Step 2 (Second Round)]

One pixel is selected out of n pixels (here, two pixels) included in the selected combination. The pixel Pgi is selected first. The scattering direction vector gi that runs from the Compton scattering point toward the selected pixel Pgi is then obtained. This scattering direction vector gi is used in Expression (4) to calculate the second differential angle _geoi · [0049] [Step 3 (Second Round)]

The first differential angle Oikini+2 obtained in Step 1 and the second differential angle oi_geoi obtained in Step 2 (the second round) are compared to each other When the two match, the pixel Pgi, which is the base of this angle a, is determined as the first arrival point of the scattered gamma ray. When the two do not match, it is determined that the pixel Pgi is not the first arrival point of the scattered gamma ray. When it is determined in Step 3 that the selected pixel is not the first arrival point of the scattered gamma ray, Step 2 is repeated.

[0050] [Step 2 (Third Round)] Another pixel, specifically the pixel Pg₂, is selected out of the n pixels that are included in the selected combination (here, two pixels) . The scattering direction vector g₂ that runs from the Compton scattering point toward the selected pixel Pg₂ is obtained next. This scattering direction vector g₂ is used in Expression (4) to calculate the second differential angle a_ge02 ·

[0051] [Step 3 (Third Round)]

The first differential angle _ki_ni₊₂ obtained in Step 1 and the second differential angle _9θθ2 obtained in Step 2 (the third round) are compared to each other. When the two match, the pixel Pg₂, which is the base of this angle a, is determined as the first arrival point of the scattered gamma ray. When the two do not match, it is determined that the pixel Pg₂ is not the first arrival point of the scattered gamma ray.

[0052] Whether the determination has been finished for every pixel that is included in the selected combination of n pixels (here, two pixels) is checked at this point. When the determination has not been finished for every included pixel, another pixel is selected out of the selected combination of pixels to repeat Step 2 and Step 3 again. When the determination has been finished for all of the n pixels (here, two pixels) , whether the determination has been finished for every combination of n pixels (here, two pixels) is checked first. When the determination has not been finished for every combination of n pixels, another combination of n pixels (here, two pixels) is selected to repeat Steps 1 to 3 again.

[0053] When the determination has been finished for every combination of n pixels (here, two pixels) , n is increased by 1, and one combination of n pixels is newly selected to repeat Steps 1 to 3 again. When the first differential angle a_ki_n and the second differential angle a_ge0 do not match at any combination of two pixels when n is set to 2, n is set to 3. In other words, it is assumed that the scattered gamma ray has been scattered by Compton scattering at the first arrival point, and, after having been scattered by Compton scattering once again at another pixel, has ultimately been absorbed by photoelectric absorption at a still another pixel. A combination of three pixels is accordingly selected out of the pixels Pgi, Pg₂ ... Pg_N, and the determination is executed repeatedly as when n is 2. In this manner, the determination is repeatedly executed while increasing n by 1 from 1 to N, until the first differential angle a_kin and the second differential angle a_geo match.

[0054] When the first differential angle a_ki_n and the second differential angle a_geo do not match at any combination of n pixels at any value of n from 1 to N, the energy Εγ of the scattered gamma ray is unknown and Expressions (1) to (3) therefore cannot be used to obtain the incident direction vector s of the gamma ray. The detection results are therefore discarded as invalid data. This way, inaccurate information about the scattered gamma ray can be eliminated that is created when the scattered gamma ray is not absorbed by photoelectric absorption inside the gamma ray detector in the end or in similar cases. Noise can accordingly be reduced in data for configuring the radiation distribution .

[0055] The angle a for which the first differential angle and the second differential angle match is obtained in the manner described above, and is used in combination with the angle cp, which is calculated by Expression (2), the scattering direction vector g, and the recoil direction vector e from the Compton scattering point to calculate the incident direction vector s of the incident gamma ray by Expression (1). The scattering direction vector g can be calculated from the Compton scattering point and the first arrival point of the scattered gamma ray that are obtained as described above.

[0056] Described next is how a match between the first differential angle a_ki_n and the second differential angle a_geo is determined taking measurement errors into consideration. Measurement errors in recoil electron energy, recoil electron detection point, scattered gamma ray energy, and scattered gamma ray detection point can be actually measured beforehand as characteristics unique to the apparatus. It is assumed here that measurement errors in recoil electron energy form a normal distribution having a standard deviation σ_Εβ while measurement errors in track direction form a normal distribution having a standard deviation a_Ae , and that those facts are confirmed in advance by actual measurement. It is also assumed that measurement errors in gamma ray energy form a normal distribution having a standard deviation o_Eg, while measurement errors in gamma ray detection point form a normal distribution having a standard deviation o_Ag, and that those facts are confirmed in advance. In this case, it is considered that the probability density distribution of a true value to a measurement value takes after a normal distribution that has the standard deviation given above when the measurement value is set as a mean value .

[0057] For example, the probability density distribution of a true value to a certain measurement value of the recoil electron energy Ke is as shown in FIG. 6A, and the probability density distribution of a true value to a certain measurement value of the scattered gamma ray energy Εγ is as shown in FIG. 6B. The probability density distribution of a true value to the first differential angle oi_ki_n obtained kinetically from those measurement values by Expression (3) is as shown in FIG. 7A. A standard deviation a_kj_.n of this probability density distribution can be obtained based on the properties of the measurement instrument or the like. On the other hand, the probability density distribution of a true value to the second differential angle oi_geo obtained geometrically is a normal distribution that has the calculation value of Expression (4) as a mean value and has o_geo= (o_Ae ²+o_Ag ²) ^1/2 as a standard deviation, and is as shown in FIG. 7B.

[0058] Whether the first differential angle a_ki_n and the second differential angle 0i_geo match can be determined by methods A to C given below. When it is determined as a result of the match determination that the two match, one of the first differential angle a_ki_n and the second differential angle a_ge0i or a mean value of the two, or the like is set as the angle a formed by the recoil direction vector e and the scattering direction vector g.

[0059] A: A true value probability density distribution that has the first differential angle a_ki_n as a mean value and has o_ki_n as a standard deviation like the one in FIG. 7A is used, and it is determined that the first differential angle _kin and the second differential angle a_geo match when the second differential angle a_geo to be compared is within a given range, for example, _ki_n±o_ki_n. When the second differential angle a_ge0 is outside the given range, it is determined that the two do not match. The width of a determination section used for the determination can be determined based on the properties of the measurement instrument and experience as described above.

[0060] B: A true value probability density distribution that has the second differential angle a_geo as a mean value and has o_qeo as a standard deviation like the one in FIG. 7B is used, and it is determined that the first differential angle a_kin and the second differential angle _geo match when the first differential angle _kin to be compared is within a given range, for example, α_9ΘΟ±σ_9εο· When the first differential angle a_ki_n s outside the given range, it is determined that the two do not match.

[0061] C: A true value probability density distribution that has the first differential angle a_kin as a mean value and a true value probability density distribution that has the second differential angle a_geo as a mean value like the ones in FIG. 7C are used. It is determined that the first differential angle a_ki_n and the second differential angle a_geo match when an overlapping portion of the probability density distributions (the hatched area in FIG. 7C) is equal to or more than a threshold. When the overlapping portion is less than the threshold, it is determined that the two do not match.

[0062] The method C resembles the concept of a t- test, and whether the first differential angle _ki_n and the second differential angle a_geo match may be determined by a t-test when the first differential angle a_kin and the second differential angle a_geo form normal distributions, or normal distributions, in effect Besides those methods, other stochastic methods for testing a match between mean values of two probability density distributions (for example, a parametric test) may be used to determine whether the two match.

[0063] When the Compton camera is used for a nuclear medicine test or similar cases, energy E₀ of a gamma ray emitted by the radiation source is foreknown. The energy E₀ here satisfies Κε+Εγ>Ε₀ when a detected recoil electron energy is Ke and the total of scattered gamma ray energy detected at a plurality of points in the gamma ray detector substantially simultaneously is Εγ. If it is assumed that those are energy components of a scattered gamma ray that are generated from the same Compton scattering event and detected at a plurality of points, the hypothesis is contrary to the principle of energy conservation. Such detection values can thus be excluded from subjects of the determination processing as events for which the determination cannot be made, before the determination processing of FIG. 5 is executed. This reduces pointless processing, and accordingly lightens the load required for the processing.

[0064] It is understood from Expression (3) that there is a lower limit to values that the angle a can take when the recoil electron energy Ke is varied with respect to the energy Eo of one incident gamma ray. For example, the lower limit value of the angle a is 90° when E₀ is 511 keV. When the second differential angle a_geo calculated in the determination processing of FIG. 5 is smaller than the lower limit value for each of a plurality of points in the gamma ray detector where energy has been detected substantially simultaneously, the following measures can be taken. That each of the detection points is at least not the first arrival point of a scattered gamma ray that has been generated by the same Compton scattering event as the detected recoil electron can be determined before comparison to the first differential angle α^_η is executed, which means that a comparison to the first differential angle a_kin can be omitted.

[0065] The determination processing described above is applicable to the following, even when different gamma rays (photons) are scattered in the scattering body by Compton scattering in succession, if the electron detector is capable of separating recoil electrons time-wise so that the recoil electrons are detected as separate Compton scattering events. First, the determination processing is of course applicable to each of the Compton scattering events. The determination processing is also applicable to each of Compton scattering events of different gamma rays (photons) that occur substantially simultaneously if the electron detector is capable of separating the successive Compton scattering space-wise as separate Compton scattering events.

[0066] As has been described above, the gamma ray detector of the Compton camera according to this embodiment can obtain accurate information about the first arrival point and energy of a gamma ray even when energy is detected at a plurality of points substantially simultaneously due to multiple detection that is caused by in-detector Compton scattering, accidental concurrent multiple detection, or the like. The Compton camera according to this embodiment is consequently capable of increasing accurate gamma ray incident direction information that is used to reconstruct a radiation distribution as an image, and can be improved in sensitivity. In addition, noise is reduced in the data for reconstructing the radiation distribution because inaccurate information about a scattered gamma ray can be eliminated.

[0067] The radiation detection technology according to the present invention can be used in gamma cameras with which an environmental radiation measurement, a nuclear medicine diagnosis, and the like are made.

[0068] According to the present invention, more accurate information about the first arrival point of a radiation ray can be obtained even when energy is detected at a plurality of points in the radiation detector substantially simultaneously due to multiple detection that is caused by in-detector Compton scattering, accidental concurrent multiple detection, or the like.

[0069] While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

[0070] This application claims the benefit of

Japanese Patent Application No .2015-043067 , filed March 5, 2015, which is hereby incorporated by reference herein in its entirety.

Reference Signs List

[0071] 101-- TPC (electron detector), 102· -gamma ray detector (radiation detector), 103 ·· radiation source, 104^■· incident gamma ray, 202·· scattering body, 203·· electron detector, 301·· scattered gamma ray, 30 ^{• •}recoil electron

Claims

[Claim 1] A radiation detection method for detecting, by a radiation detector, an arrival point and energy of a radiation ray that has been scattered in a scattering body by Compton scattering, the radiation detection method comprising:

detecting, by an electron detector, a scattering point at which a recoil electron is generated by the Compton scattering, a recoil direction of the recoil electron, and energy of the recoil electron; and

when an angle that is formed by the recoil direction of the recoil electron and a scattering direction of the scattered radiation ray is set as a differential angle a, and

when energy is detected that is components of the radiation ray scattered substantially simultaneously at a plurality of points in the radiation detector;

using both a first differential angle a_kin and a second differential angle a_geo to determine a first arrival point at which the scattered radiation ray first reaches the radiation detector, the first differential angle a_ki_n being calculated from the detected recoil electron energy and the detected radiation energy, the second differential angle a_geo being calculated from a detection point of the scattered radiation ray, the scattering point, and the recoil direction.

[Claim 2] A radiation detection method according to claim 1, wherein, when the detected recoil electron energy is given as Ke, the detected energy of the scattered radiation ray is given as Εγ, a rest mass of the electron is given as m, and a light velocity is given as c, the first differential angle a^n is set as the differential angle a that is calculated by a=cos^_1 [ (l-mc²/EY) {Ke/ (Ke+2mc²) }^1/2] .

[Claim 3] A radiation detection method according to claim 2, wherein the first differential angle _kin is calculated by setting as Εγ a total of energy detected at two or more points that are selected out of the plurality of points.

[Claim 4] A radiation detection method according to claim 3, wherein the two or more points correspond to all points at which the scattered radiation ray is scattered by Compton scattering inside the radiation detector, and a point at which the radiation ray is absorbed by photoelectric absorption.

[Claim 5] A radiation detection method according to claim 1, wherein, for one point selected out of the plurality of points, or for each of at least two points selected out of the plurality of points, the first differential angle a_ki_n and the second differential angle _geo are obtained from the detection point of the scattered radiation ray and from the detected energy, and, when the first differential angle a_kin and the second differential angle a_geo match, or when a difference between the first differential angle a_kin and the second differential angle a_geo is within a range of measurement errors, one of the selected points is determined as the first arrival point at which the scattered radiation ray first reaches the radiation detector .

[Claim 6] A radiation detection method according to claim 5, further comprising:

a first step of calculating the first differential angle a_kin from the detected recoil electron energy and the detected scattered radiation ray energy;

a second step of calculating the second differential angle a_geo from the detection point of the scattered radiation ray, the detected scattering point, and the detected recoil direction; and a third step of comparing the first differential angle _ki_n and the second differential angle oi_geo to each other, to thereby determine whether or not one of the selected points is the first arrival point at which the scattered radiation ray first reaches the radiation detector.

[Claim 7] A radiation detection method according to claim 6, wherein, when it is determined in the third step that the selected point is not the first arrival point at which the scattered radiation ray first reaches the radiation detector, another point or another combination of two or more points is selected out of the plurality of points to execute the first step, the second step, and the third step again.

[Claim 8] A radiation detection method according to claim 1, wherein the first arrival point at which the scattered radiation ray first reaches the radiation detector is determined by determining that the first differential angle a_ki_n and the second differential angle _geo match when the calculated second differential angle a_geo is within a range of a_ki_n±o_ki_n in a true value probability density distribution that has the calculated first differential angle a_kin as a mean value and that has o_kin as a standard deviation, and by determining that the first differential angle _ki_n and the second differential angle a_geo do not match when the calculated second differential angle a_geo is outside of the range of a_kin±o_kin.

[Claim 9] A radiation detection method according to claim 1, wherein, when measurement errors in the detected recoil direction of the recoil electron form a normal distribution having a standard deviation o_Aer and measurement errors in the detection point of the detected the scattered radiation ray form a normal distribution having a standard deviation o_ftg, the first arrival point at which the scattered radiation ray first reaches the radiation detector is determined by determining that the first differential angle a_ki_n and the second differential angle a_geo match when the calculated first differential angle _ki_n is within a range of a_geo±a_geo in a true value probability density distribution that has the calculated second differential angle o!geo as a mean value and that has Ogeo⁼ (o^"Ae²+o_Ag ²) ^{1 2} as a standard deviation, and by determining that the first differential angle a_ki_n and the second differential angle a_geo do not match when the calculated first differential angle a_kin is outside of the range of oi_geo±a_geo.

[Claim 10] A radiation detection method according to claim 1, wherein, when measurement errors in the detected recoil electron energy form a normal distribution having a standard deviation o_Ee, measurement errors in the detected recoil direction of the recoil electron form a normal distribution having a standard deviation σ_Άβ , measurement errors in the detected scattered radiation ray energy form a normal distribution having a standard deviation o_Eg, and measurement errors in the detection point of the detected radiation ray form a normal distribution having a standard deviation o_Ag, the first arrival point at which the scattered radiation ray first reaches the radiation detector is determined by determining that the first differential angle a_kj._n and the second differential angle a_geo match when an overlapping portion between a true value probability density distribution that has the calculated first differential angle _kin as a mean value and a true value probability density distribution that has the calculated second differential angle _geo as a mean value is equal to or more than a given threshold, and by determining that the first differential angle a_kj._n and the second differential angle _geo do not match when the overlapping portion is less than the given threshold.

[Claim 11] A radiation detection method according to claim 1, wherein, when the radiation ray has energy Eo that is a known value, the detected recoil electron energy is Ke, a total of scattered radiation ray energy detected at a plurality of points in the radiation detector is Εγ, and Ke+Ey>E₀ is true, this case is excluded from a subject of the determination as an event that is undeterminable.

[Claim 12] A radiation detection method according to claim 1, wherein the second differential angle a_geo that is smaller than a lower limit, value that recoil electron energy Ke takes when varied with respect to energy E₀ of the radiation ray is excluded from a subject of the determination by omitting a comparison between the second differential angle a_geo and the calculated first differential angle _ki_n.

[Claim 13] A radiation detection method according to claim 1, wherein the radiation ray comprises a gamma ray .

[Claim 14] A Compton camera comprising a scattering body, an electron detector, and a radiation detector, the Compton camera being configured to:

detect, by the . radiation . detector, an arrival point and energy of a radiation ray scattered in the scattering body by Compton scattering;

detect, by the electron detector, a scattering point at which a recoil electron is generated by the Compton scattering, a recoil direction of the recoil electron, and energy of the recoil electron; and

obtain an incident direction of the radiation ray from information about the detected arrival point and the detected energy of the scattered radiation ray, and from information about the detected scattering point, the detected recoil direction of recoil electron, and the detected recoil electron energy,

wherein, when an angle formed by the recoil direction of the recoil electron and a scattering direction of the scattered radiation ray is set as a differential angle a, and energy is detected that is components of the radiation ray scattered substantially simultaneously at a . plurality of points in the radiation detector, a first differential angle a_ki_n and a second differential angle a_geo are both used to determine a first arrival point at which the scattered radiation ray first reaches the radiation detector, the first differential .angle a_kin. being calculated from the detected recoil electron energy and the detected radiation energy, the second differential angle a_qeo being calculated from a detection point of the scattered radiation ray, the detected scattering point, and the detected recoil direction.

[Claim 15] A Compton camera according to claim 14, wherein the electron detector comprises an electron track detector and an electric field applying unit, the electron track detector amplifying, through an electron avalanche phenomenon, ionized electrons that are caused by the recoil electron generated by the Compton scattering to detect a track of the recoil . electron, the electric field applying unit applying an electric field while giving an electrode an electric potential that is negative with respect to an electric potential of an electrode of the electron track detector, the negative potential electrode facing the electrode of the electron track detector across the scattering body.

[Claim 16] A Compton camera according to claim 14,

wherein the scattering body comprises gas, wherein a part of the electron track detector forms a first electrode, and a second electrode of the electric field applying unit is arranged so as to face the electron track detector across the gas, and

wherein the electric field applying unit is configured to apply an electric field for causing ionized electrons to drift in a direction toward the first electrode, by giving the second electrode an electric potential that is negative with respect to an electric potential of the first electrode.

[Claim 17] A Compton camera according to claim 14, wherein the radiation ray comprises a gamma ray.