Gamma camera based on gas detector and reconstruction method

Abstract

The invention provides a gamma camera based on a gas detector and a reconstruction method, the gamma camera comprises a gas time projection chamber detector and five groups of crystal energy meters, and the crystal energy meters are distributed on five side surfaces of the gas time projection chamber detector; after gamma incidence, Compton scattering occurs in working gas of the detector, particle tracks of scattered electrons are recorded by the detector, and the position of a Compton action vertex and the direction and energy of the scattered electrons can be reconstructed; the scattered photons are detected by a crystal energy meter on the side wall, the connecting line of the position detected by the crystal energy meter and a Compton action point is the direction of scattered photons, and energy collected by the crystal energy meter can reconstruct the momentum of the scattered photons; based on the momentum of scattered electrons and photons, the direction of incident gamma photons can be reconstructed. According to the gamma camera design, imaging information of five dimensions of three-dimensional position, time and energy is provided, and the imaging capability of the gamma camera is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of gamma ray imaging, and more specifically, particularly relates to a gamma camera based on a gas detector and a reconstruction method. Background Art

[0002] Gamma camera is an instrument used to detect and measure gamma rays. In recent years, with the increasing maturity of technology, Compton cameras are gradually being used in various fields such as nuclear energy industry, nuclear pollution treatment, astronomy, and medicine. Most traditional gamma-ray imaging devices are based on the principle that light propagates in a straight line, and use mechanical collimators (such as pinhole plates, coding plates, etc.) to image the radiation source. However, with the increase of gamma ray energy, the penetration of gamma rays increases. Therefore, in order to effectively intercept gamma rays, the thickness of collimators (such as coding plates, etc.) needs to be processed thicker and thicker. This inevitably leads to an increase in processing difficulty, a decrease in detection efficiency, spatial resolution, and signal-to-noise ratio. Therefore, high-energy gamma-ray imaging has always been a difficult problem in scientific research and practical applications. Compton camera is a relatively new type of gamma camera, generally composed of two layers of gamma-ray detectors. The emergence of Compton camera successfully solved the difficult problem of high-energy gamma-ray imaging and is suitable for imaging gamma rays with an energy range of 100 keV to more than a dozen MeV.

[0003] The image reconstruction scheme of a gamma camera is generally a back-projection method and maximum likelihood iteration. For most detection methods, the recoil electron track is short and tortuous, so it is difficult to determine the emission direction of the recoil electron. Therefore, we can only limit the direction of the incident photon to a conical surface with the opposite direction of the scattered photon emission direction as the axis and the Compton scattering angle as the semi-vertex angle based on the first two points of information. We call it the Compton cone or back-projection cone. The Compton cones obtained from multiple Compton scattering events are superimposed to determine the position of the incident photon.

[0004] Back-projection is a reconstruction technique used in medical imaging and nuclear physics research. It reconstructs the internal structure of an object or the position distribution of a radiation source by performing an inverse projection operation on the measured data on the detector. Specifically, the imaging space is first divided into a cubic grid space, with each cube as a voxel. The size of the voxel affects the resolution of the reconstructed position of the radiation source. The trajectory of the ray to the detector is a straight line, but according to the detection principle, what is actually determined is a conical surface with the straight line trajectory as the central axis. The radiation source is located on such a conical surface. The location of the radiation source is determined by calculating the intersection of the conical surfaces through multiple events. The back projection method reconstructs the image by projecting based on the weight of the voxel. Each voxel obtains a certain weight after calculation for the cone generated by each event. The weight is an approximate probability estimate of the event corresponding to the specific voxel and cone. Each event can obtain a weight for each voxel, and then all events are counted to obtain the estimated intensity of each voxel, so as to determine the location of the gamma-ray emitting nuclides distributed in the imaging space. The probability of the radiation source in each voxel is proportional to the sum of the weights of all events in this voxel. In the back projection method of the Compton camera, the location of the intersection of the cone surfaces generated by a large number of events is the location with the highest probability of the radiation source.

[0005] The weight of the voxels in the imaging space obtained by the back-projection method is only a preliminary result, and the result is subject to a lot of noise interference. Specifically speaking, the reconstruction result of a point source is an area. Therefore, further algorithms are needed to optimize this problem.

[0006] Maximum Likelihood Estimation (MLE) is a statistical method for estimating model parameters. It looks for the model parameter values ​​that are most likely to produce the observed data based on the observed data. The core idea of ​​the maximum likelihood method is to find the model parameters that maximize the probability of the observed data, that is, to find the model that is most likely to explain the data. Specifically, for a given probability distribution model and observed data, the goal of the maximum likelihood method is to find the model parameter values ​​that maximize the probability of the given data occurring. This usually involves maximizing the likelihood function, that is, finding the parameter value that maximizes the probability of the observed data occurring. Mathematically, this is usually achieved by taking the logarithm of the likelihood function and then taking the derivative of the parameter.

[0007] In the Compton camera, after obtaining the back-projection reconstruction result, the maximum likelihood method can be used for optimization. The iterative optimization process of the maximum likelihood method is shown in formula 1-1: ; In the formula Represents that at the nth iteration, the voxel The calculation weight of Representative voxel middle The probability of a photon being detected at any location; Represents the weighted probability of an event originating from a voxel. These data can be obtained through direct measurement or data processing. The data obtained through Monte Carlo simulation, according to the proportion or weight ratio of each event in the total events, obtain the weighted probability of a certain imaging voxel in a certain event in the maximum likelihood method and the probability of a certain event being emitted from a certain voxel and observed, including , The values ​​of probability, etc. are included. In this way, the parameters and data required in the maximum likelihood method can be obtained by extraction and calculation. Substituting all parameters into the calculation, the maximum likelihood method can be expected to optimize the processing of the data obtained by the back-projection algorithm.

[0008] The current array-type Compton gamma camera does not have the ability to image particle tracks. Its imaging accuracy is 10°@662keV, and it is unable to reconstruct the incident direction of a single gamma photon. With the development of detector technology, fields such as radiomedicine and astronomical observation have higher requirements for the imaging accuracy of gamma cameras. The present invention proposes a gamma detector based on gas time projection chamber technology, which can record five-dimensional information such as energy deposition of particles in the detector, three-dimensional particle tracks and time, which will comprehensively improve the imaging accuracy of gamma cameras. In order to give full play to the advantages of the five-dimensional gamma camera, this patent invents a new event reconstruction method. Summary of the invention

[0009] In order to solve the above technical problems, the present invention provides a gamma camera based on a gas detector and a reconstruction method, which are achieved by the following specific technical means: A gamma camera based on a gas detector comprises a gas time projection chamber detector and a crystal calorimeter, wherein the crystal calorimeter is provided in five groups and is distributedly installed on five sides of the gas time projection chamber detector; after gamma is incident, Compton scattering occurs in the working gas of the detector, and the particle track of the scattered electron is recorded by the detector, so that the position of the Compton action vertex and the direction and energy of the scattered electron can be reconstructed; the scattered photons are detected by the crystal calorimeter on the side wall, and the line connecting the position detected by the crystal calorimeter and the Compton action point is the scattered photon direction, and the energy collected by the crystal calorimeter can reconstruct the scattered photon momentum; based on the momentum of the scattered electron and the photon, the direction of the incident gamma photon can be reconstructed.

[0010] A reconstruction method of a gamma camera based on a gas detector comprises the following steps: S1: Using Geant4 to simulate the interaction of gamma rays with different energies and incident directions in a five-dimensional gamma camera; S2: Reconstruction of single gamma incident direction: The incident direction of a single incident gamma photon is restored through the reconstructed Compton action vertex, the scattered electron emission direction, the scattered electron energy reconstructed by the gas detector, and the scattered photon position and energy recorded by the crystal calorimeter; S3: The angular deviation ARM between the reconstructed Compton cone and the true radiation source direction is used to measure the angular resolution of Compton imaging; S4: Reconstruction of Compton action vertex and scattered electron direction: A combination of multi-level clustering and Kalman filtering is used to reconstruct the Compton action vertex and scattered electron direction. A multi-level clustering method is used to outline the skeleton of the electron track. On this basis, the Kalman filtering method is used to reconstruct the particle track and restore the vertex position of the scattered electron action point and the direction at that position.

[0011] Further, in step S1, the electron track detection accuracy of the gas detector is 1 mm, the particle size of the crystal calorimeter is 25 mm, and the incident gamma energy range is 662 keV-5000 keV.

[0012] Further, in step S3, the angle deviation ARM is equal to the measured deflection angle and the Compton scattering angle calculated from the measured energy The difference, among which The calculation formula is as follows: ; A gamma photon with energy E0 is incident from the direction of r0 and undergoes Compton scattering at position P, generating scattered electrons with energy E1 and scattered photons with energy E2. The position where the first Compton scattering occurs is the core of the reconstruction, and the reconstruction cone has P as its vertex.

[0013] Furthermore, in step S1, the structure of the gas detector includes a detector tank body, a field cage is fixedly installed inside the detector tank body, and a readout plane and a cathode grid are respectively installed at the upper and lower ends of the field cage.

[0014] Furthermore, a crystal calorimeter is installed on the outside of the field cage, and the inside of the field cage is filled with filling gas.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The gamma camera design of the present invention has imaging information in five dimensions: three-dimensional position, time, and energy, which greatly improves the imaging capability of the gamma camera; and a method based on the combination of multi-level clustering and Kalman filtering is proposed in a targeted manner, which improves the reconstruction accuracy of the Compton action vertex and the scattered electron emission direction, and can also realize the direction reconstruction of a single incident gamma. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the working principle of the five-dimensional gamma camera of the present invention.

[0017] Figure 2 It is a flow chart of the overall reconstruction scheme of the present invention.

[0018] Figure 3 This is the data simulation of the present invention. (Left picture) Geant4 simulates the original scattered electron track; (Middle picture) the electron track considering the electron diffusion effect; (Right picture) the electron track considering the detector readout.

[0019] Figure 4 It is a schematic diagram of single gamma direction reconstruction of the present invention.

[0020] Figure 5 It is a schematic diagram of ARM reconstruction of the present invention.

[0021] Figure 6 It is a schematic diagram of electron track reconstruction of the present invention.

[0022] Figure 7 This is a schematic diagram of the vertex and direction reconstruction accuracy versus energy distribution of the present invention: (upper figure) scattered electron direction and (lower figure) vertex reconstruction accuracy versus energy distribution.

[0023] Figure 8 It is a schematic diagram of the gamma-ray incident direction reconstruction accuracy versus energy distribution according to the present invention.

[0024] Fig. 9 It is a schematic diagram of energy reconstruction of the gamma camera of the present invention.

[0025] Fig.10 It is a structural schematic diagram of the gas detector of the present invention. DETAILED DESCRIPTION

[0026] The following embodiments of the present invention are described in further detail in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0027] In the description of the present invention, unless otherwise specified, "plurality" means two or more than two; the orientations or positional relationships indicated by the terms "upper", "lower", "left", "right", "inner", "outer", "front end", "rear end", "head", "tail", etc. are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", "third", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0028] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "connected" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances. Example

[0029] As attached Figure 1 and Fig.10 As shown, the present invention provides a gamma camera based on a gas detector, which is composed of a central gas time projection chamber detector and a crystal calorimeter surrounded by five sides. After gamma is incident, Compton scattering occurs in the working gas of the detector, and the particle track of the scattered electron is recorded by the detector, which can accurately reconstruct the position of the Compton action vertex and the direction and energy of the scattered electron. The scattered photons are detected by the crystal calorimeter on the side wall. The line connecting the position detected by the crystal calorimeter and the Compton action point is the direction of the scattered photon, and the energy collected by the crystal calorimeter can reconstruct the momentum of the scattered photon. Based on the momentum of the scattered electrons and photons, the direction of the incident gamma photons can be accurately reconstructed. For the reconstruction method, the reconstruction accuracy of the position of the Compton action point and the reconstruction accuracy of the scattered electron direction will be the key to affecting the angular resolution of the gamma camera.

[0030] like Figure 2 As shown, a reconstruction method of a gamma camera based on a gas detector comprises the following steps: S1: Using Geant4 to simulate the interaction of gamma rays with different energies and incident directions in a five-dimensional gamma camera ( Figure 3 :Data simulation). The gas detector electron track detection accuracy is 1mm, the crystal calorimeter particle size is 25mm, and the incident gamma energy range is 662keV-5000keV. The simulation process simulates the broadening effect caused by diffusion in the electron track collection process through Garfield.

[0031] S2: Single gamma incident direction reconstruction ( Figure 4 : Schematic diagram of reconstruction of a single gamma direction): The incident direction of a single incident gamma photon is restored through the reconstructed Compton action vertex, the scattered electron emission direction, the scattered electron energy reconstructed by the gas detector, and the scattered photon position and energy recorded by the crystal calorimeter.

[0032] S3: The angular resolution of Compton imaging can be measured by the angular deviation between the reconstructed Compton cone and the true direction of the radiation source. This angular deviation is defined as ARM (Angular Resolution Measurement). ARM reflects the deviation between the reconstructed direction of the radiation source and the true direction, such as Figure 5 As stated, it is equal to the measured deflection angle and the Compton scattering angle calculated from the measured energy The difference, among which The calculation formula is as follows:

[0033] Gamma photons with energy E0 are incident from the direction of r0, and Compton scattering occurs at position P, generating scattered electrons with energy E1 and scattered photons with energy E2. The position where the first Compton scattering occurs is the core of reconstruction. The reconstruction cone takes P as the vertex. Whether it is measurement deviation or the approximation of the vertex during reconstruction, the direction of the cone passing through the real position of the radiation source during reconstruction will have a certain deviation from the real direction of the radiation source. Measurement deviation can be achieved by improving hardware conditions, while the reconstruction of the vertex needs to rely on the scattered electron direction reconstruction algorithm.

[0034] S4: Compton action vertex reconstruction and scattered electron direction reconstruction ( Figure 6 : Schematic diagram of vertex reconstruction): A combination of multi-level clustering and Kalman filtering is used to reconstruct the Compton action vertex and the direction of scattered electrons. Since scattered electrons in gas detectors are affected by multiple scatterings during their flight, their track bending makes it difficult to directly restore the direction of the scattered electrons. In addition, the scattered track is affected by diffusion and becomes diffuse, which seriously affects the accuracy of Compton vertex reconstruction. We use a multi-level clustering method to outline the skeleton of the electron track, and on this basis, we use the Kalman filtering method to reconstruct the particle track, restoring the vertex position of the scattered electron action point (that is, the Compton action vertex) and the direction at that position (that is, the direction of the scattered electron emission). Taking 1MeV scattered electrons as an example, the accuracy of Compton vertex reconstruction is 0.9 mm, and the accuracy of scattered electron emission direction reconstruction is 10.1° ( Figure 7 : Vertex and orientation reconstruction accuracy varies with energy distribution).

[0035] Incident gamma ARM reconstruction and energy reconstruction: Using the above reconstruction method, the ARM accuracy of the reconstructed scattered electron vertex, direction, gamma position in the calorimeter and energy is as follows: Figure 8 As shown in the figure, taking 1MeV as an example, the reconstruction accuracy is 3.8°. In addition, the energy of the incident gamma ray is the sum of the scattered electron energy and the scattered gamma ray energy. Fig. 9 The gamma camera's energy spectrum reconstruction and nuclide identification capabilities are demonstrated using 60Co and 137Cs as examples.

[0036] The embodiments of the present invention are given for the purpose of illustration and description, and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are selected and described in order to better illustrate the principles and practical applications of the present invention and to enable those of ordinary skill in the art to understand the present invention and thereby design various embodiments with various modifications suitable for specific uses.

Claims

1. A gamma camera based on a gas detector, characterized in that: The invention comprises a gas time projection chamber detector and a crystal calorimeter, wherein the crystal calorimeter is provided with five groups and is installed on five sides of the gas time projection chamber detector. After gamma is incident, Compton scattering occurs in the working gas of the detector, and the particle track of the scattered electron is recorded by the detector, so that the position of the Compton action vertex and the direction and energy of the scattered electron can be reconstructed. The scattered photons are detected by the crystal calorimeter on the side wall, and the line connecting the position detected by the crystal calorimeter and the Compton action point is the direction of the scattered photons. The energy collected by the crystal calorimeter can reconstruct the momentum of the scattered photons. Based on the momentum of the scattered electrons and photons, the direction of the incident gamma photons can be reconstructed.

2. A reconstruction method of a gamma camera based on a gas detector, characterized in that: The steps include: S1: Using Geant4 to simulate the interaction of gamma rays with different energies and incident directions in a five-dimensional gamma camera; S2: Reconstruction of single gamma incident direction: The incident direction of a single incident gamma photon is restored through the reconstructed Compton action vertex, the scattered electron emission direction, the scattered electron energy reconstructed by the gas detector, and the scattered photon position and energy recorded by the crystal calorimeter; S3: The angular deviation ARM between the reconstructed Compton cone and the true radiation source direction is used to measure the angular resolution of Compton imaging; S4: Reconstruction of Compton action vertex and scattered electron direction: A combination of multi-level clustering and Kalman filtering is used to reconstruct the Compton action vertex and scattered electron direction. A multi-level clustering method is used to outline the skeleton of the electron track. On this basis, the Kalman filtering method is used to reconstruct the particle track and restore the vertex position of the scattered electron action point and the direction at that position.

3. The reconstruction method of a gamma camera based on a gas detector as claimed in claim 2, characterized in that: In step S1, the electron track detection accuracy of the gas detector is 1 mm, the particle size of the crystal calorimeter is 25 mm, and the incident gamma energy range is 662 keV-5000 keV.

4. The reconstruction method of a gamma camera based on a gas detector as claimed in claim 2, characterized in that: In step S3, the angle deviation ARM is equal to the measured deflection angle and the Compton scattering angle calculated from the measured energy The difference, among which The calculation formula is as follows: ； A gamma photon with energy E0 is incident from the direction of r0 and undergoes Compton scattering at position P, generating scattered electrons with energy E1 and scattered photons with energy E2. The position where the first Compton scattering occurs is the core of the reconstruction, and the reconstruction cone has P as its vertex.

5. The reconstruction method of a gamma camera based on a gas detector as claimed in claim 2, characterized in that: In step S1, the structure of the gas detector includes a detector tank body, a field cage is fixedly installed inside the detector tank body, and a readout plane and a cathode grid are respectively installed at the upper and lower ends of the field cage.

6. The reconstruction method of a gamma camera based on a gas detector as claimed in claim 5, characterized in that: A crystal calorimeter is installed on the outer side of the field cage, and the interior of the field cage is filled with filling gas.

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