WO2023204653A1 - Collimation-less dual mode radiation imager - Google Patents

Abstract

The present invention relates to a radiation imager, and the radiation imager according to an embodiment of the present invention may reconfigure an image over a wide energy area according to collimation-less imaging, Compton imaging, fusion imaging in which the imaging modalities are fused on the basis of reaction signals for each radiation source position of a semiconductor detector, the reaction signals being acquired through a negative electrode, a positive electrode, and four side electrodes, which are arranged in a prescribed mosaic pattern, of the semiconductor detector.

Description

Non-focused dual-mode radiation imaging equipment

The present invention relates to non-focused, dual-mode radiation imaging equipment, and more specifically, to radiation imaging equipment that can simultaneously use non-focused radiation imaging and a Compton camera.

Gamma-ray imaging techniques are broadly classified into mechanical focusing and electrical focusing. Mechanical focusing is a method of using a focuser made of high atomic number materials such as lead and tungsten, and in the industrial field, the arrangement of the shield is mathematically derived to improve sensitivity while maintaining high resolution like a needle hole focuser. The encoded aperture concentrator method is used. The principle of this method is to determine the location of the source by decoding the measured data (shadowgram) based on the detector response according to the source location and concentrator pattern. It is very suitable for imaging low-energy gamma rays that are easily shielded by the concentrator. It is advantageous. However, in high-energy areas, a thick shielding material is required, which reduces portability, and there is a limitation in that the viewing angle is fundamentally limited by the concentrator.

Electric focusing, also called Compton imaging, is a method of imaging a radiation source through scattering-absorption reactions within a detector without a separate focuser. The scattering angle is calculated based on the position and energy information of the scattering and absorption reactions, and the Compton ring is back-projected to the point where the two scattering angles become equal on the source plane. By repeating the process for several valid events, the point where the Compton rings overlap is identified as the source's location. Existing Compton cameras consisted of a scattering section and an absorbing section and had a limited viewing angle, but recently, gamma rays incident from all directions can be imaged by measuring the scattering-absorption response within a single 3D position-sensitive detector. However, this method cannot be applied to gamma rays below 300 keV, where Compton scattering rarely occurs.

Accordingly, there is an urgent need for a method to solve the problems of conventional gamma-ray imaging techniques.

The present invention is intended to solve the problems of the prior art described above. One aspect of the present invention is to detect the position of the semiconductor detector according to the position of the source obtained through the cathode, anode, and four side electrodes of the semiconductor detector arranged in a predetermined mosaic pattern. The goal is to provide non-focused, dual-modality radiation imaging equipment that reconstructs images for a wide energy range based on response signals using non-focused imaging techniques, Compton imaging techniques, and fusion imaging techniques that combine them.

Radiation imaging equipment according to an embodiment of the present invention is formed in the shape of a square pillar, the depth direction from one end to the other is arranged along the Z-axis direction of a virtual XYZ coordinate system, and includes a semiconductor element that reacts with radiation, and connected to one end of the semiconductor element A plurality of semiconductor detectors each including a cathode, an anode connected to the other end of the semiconductor element, and four side electrodes each connected to a side of the semiconductor element, and the plurality of semiconductor detectors are connected to the XYZ a detection unit encoded and arranged in a predetermined mosaic pattern on the XY plane of the coordinate system; a signal measurement unit that measures response signals from the cathode, the anode, and the side electrode when the radiation is incident on the detection unit; and an image processing unit that generates an image of the source emitting the radiation based on the response signal measured by the signal measurement unit.

Additionally, the radiation imaging equipment according to an embodiment of the present invention may further include a frame for fixing a plurality of semiconductor detectors so that the plurality of semiconductor detectors are encoded and arranged in a predetermined mosaic pattern.

Additionally, in the radiation imaging equipment according to an embodiment of the present invention, the image processing unit calculates reaction positions within a plurality of the semiconductor detectors that reacted with the incident radiation, and determines the radiation source for each preset reaction position within the semiconductor detector. A non-focused imaging technique that obtains location information of the source in comparison with pixel information on the source surface in a spherical coordinate system, and a scattering reaction caused by the radiation incident on any one of the plurality of semiconductor detectors. Based on the scattering response signal for the scattering reaction and the absorption response signal for the absorption reaction occurring in the other semiconductor detector by the radiation scattered by the scattering reaction, a plurality of the semiconductors in which the scattering reaction and the absorption reaction occurred. A Compton imaging technique that calculates the response position within the detector and obtains location information of the source, and a fusion imaging technique that obtains location information of the source by fusing the non-focused imaging technique and the Compton imaging technique. The video image is generated according to any one imaging technique, and the volume area of each semiconductor device is divided and voxelized into a plurality of voxels arranged along the X, Y, and Z axes of the XYZ coordinate system, and According to the reaction signal measured by the signal measurement unit, reaction photons can be distributed to the voxel of each semiconductor device to calculate the reaction position based on the voxel.

Additionally, in the radiation imaging equipment according to an embodiment of the present invention, when the energy band of the radiation is less than 250 keV, the image processing unit uses the non-focused imaging technique to detect the radiation when the energy band of the radiation is greater than 600 keV. In this case, the video image can be generated by the Compton imaging technique, and when the energy band of the radiation is 250 to 600 keV, the video image can be generated by the fusion imaging technique.

Additionally, in the radiation imaging equipment according to an embodiment of the present invention, the non-focused imaging technique calculates the response position in the Z-axis direction based on the ratio of response signal magnitudes measured from the anode and the cathode. can do.

Additionally, in the radiation imaging equipment according to an embodiment of the present invention, the non-focused imaging technique applies the center of gravity method based on the size of the response signal measured from the side electrode, The reaction position in each of the X-axis and Y-axis directions can be calculated.

Additionally, in the radiation imaging equipment according to an embodiment of the present invention, the fusion imaging technique can generate the image according to the following [Equation 1] based on MLEM (maximum likelihood expectation maximization).

[Equation 1]

Here, λ _j ⁿ , λ _j ^{n +1} are the estimates of the source surface pixel j (j is a natural number greater than 1) after the n, n+1 (n is a natural number greater than 1) th iteration, M is the total number of source surface pixels, Y _i ^a is the number of response photons measured in voxel i (i is a natural number greater than 1) of the semiconductor detector in the non-focused imaging technique, N is the total number of response photons measured in the semiconductor detector in the non-focused imaging technique, C _i,j ^a is the probability that a photon generated from source plane pixel j will be measured at voxel i of the semiconductor detector in the non-focused imaging technique, and Y _i' ^b is measured as event i'(i' is a natural number greater than 1) in the Compton imaging technique. The number of response photons, N', is the type of event i' in the Compton imaging technique, and C _{i ', j} ^b are the probability that a photon generated from the source plane pixel j is measured as event i' in the Compton imaging technique.

The features and advantages of the present invention will become more apparent from the following detailed description based on the accompanying drawings.

Prior to this, terms or words used in this specification and claims should not be construed in their usual, dictionary meaning, and the inventor may appropriately define the concept of the term in order to explain his or her invention in the best way. It must be interpreted with meaning and concept consistent with the technical idea of the present invention based on the principle that it is.

According to the present invention, a large-area detector can be constructed, has excellent energy and spatial resolution, and can image radiation in the entire energy range. In particular, it is advantageous for imaging low-energy gamma rays, which have large attenuation depending on the penetration depth of the gamma rays, making imaging possible in energy regions where imaging using the Compton imaging technique was impossible.

In addition, because it does not require a separate focuser, omnidirectional (4π) imaging is possible and it is highly portable.

Figure 1 is a diagram schematically showing the configuration of a non-focused dual-mode radiation imaging equipment according to an embodiment of the present invention.

FIG. 2 is a diagram showing the arrangement of the semiconductor detector shown in FIG. 1.

3 to 6 are diagrams illustrating an arrangement pattern of a semiconductor detector according to an embodiment of the present invention.

Figure 7 is a diagram explaining the source surface and pixels.

FIG. 8 shows the anode and cathode signal sizes depending on the depth of the semiconductor detector shown in FIG. 1.

FIG. 9 shows the waveform of an induced signal of an electrode according to electron movement inside the semiconductor detector shown in FIG. 1.

Figure 10 is a photograph showing the non-focused dual-mode radiation imaging equipment used in the experimental example.

Figures 11 to 15 show the results of reconstructing ⁵⁷ Co at various positions using a non-focused imaging technique in experimental examples.

16 to 22 are images generated according to the non-focused imaging technique, Compton imaging technique, and fusion imaging technique for each energy when the source is located in front of the semiconductor detector in the experimental example.

Figure 23 is a graph showing quantitative indices evaluated according to energy when the source is located in front of the semiconductor detector in the experimental example.

Figure 24 shows the results of reconstruction of ⁵⁷ Co, ¹³³ Ba, and ¹³⁷ Cs at various positions in an experimental example using a fusion imaging technique.

The objectives, specific advantages and novel features of the present invention will become more apparent from the following detailed description and preferred embodiments taken in conjunction with the accompanying drawings. In this specification, when adding reference numbers to components in each drawing, it should be noted that identical components are given the same number as much as possible even if they are shown in different drawings. Additionally, terms such as “first” and “second” are used to distinguish one component from another component, and the components are not limited by these terms. Hereinafter, in describing the present invention, detailed descriptions of related known technologies that may unnecessarily obscure the gist of the present invention will be omitted.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

Figure 1 is a diagram schematically showing the configuration of a non-focused dual-mode radiation imaging equipment according to an embodiment of the present invention, Figure 2 is a diagram showing the arrangement of the semiconductor detector shown in Figure 1, and Figures 3 to 3 6 is a diagram illustrating an arrangement pattern of a semiconductor detector according to an embodiment of the present invention.

As shown in Figures 1 and 2, the radiation imaging equipment according to an embodiment of the present invention is formed in the shape of a square pillar, the depth direction from one end to the other is arranged along the Z-axis direction of a virtual XYZ coordinate system, and reacts with radiation. A semiconductor element 11, a cathode 13 connected to one end of the semiconductor element 11, an anode 15 connected to the other end of the semiconductor element 11, and a side surface of the semiconductor element 11, respectively. A detection unit (100) comprising a plurality of semiconductor detectors (10) each having four side electrodes (17), and in which the plurality of semiconductor detectors (10) are encoded and arranged in a predetermined mosaic pattern on the XY plane of the XYZ coordinate system. ), when radiation is incident on the detection unit 100, the signal measurement unit 200 measures the response signal from the cathode 13, the anode 15, and the side electrode 17, and the signal measurement unit 200 measures It includes an image processing unit 300 that generates an image of a source emitting radiation based on a response signal.

The present invention relates to radiation imaging equipment that can simultaneously use non-focused radiation imaging and a Compton camera. Radiation imaging methods are divided into mechanical focusing using a concentrator and electrical focusing called control imaging. The mechanical focusing method requires a thick shield in high energy areas, making it less portable, and the viewing angle is fundamentally limited by the concentrator. There are limitations and the electrical focusing method cannot be applied to radiation with an energy of less than 300 keV, in which Compton scattering rarely occurs. Accordingly, the present invention was developed as a means to solve the problems of conventional radiological imaging methods.

Specifically, the radiation imaging equipment according to an embodiment of the present invention includes a detection unit 100, a signal measurement unit 200, and an image processing unit 300.

The detection unit 100 includes a plurality of semiconductor detectors 10 that detect radiation. The semiconductor detector 10 is a device that detects radiation using a semiconductor and includes a semiconductor element 11, a cathode 13, an anode 15, and a side electrode 17.

The semiconductor element 11 is an element that reacts with incident radiation. The semiconductor element 11 is formed in the shape of a square pillar, and the depth direction from one end to the other is arranged along the Z-axis direction of a virtual XYZ coordinate system. An insulator may be disposed on the side of the semiconductor device 11. When radiation is incident on the semiconductor detector 10, photons react inside the semiconductor element 11. Hereinafter, photons that react with the semiconductor device 11 are referred to as reaction photons. This semiconductor element 11 is made of a semiconductor such as CdTe, CdZnTe (CZT) crystal, etc. There is no particular limitation on its type, and any semiconductor can be used as long as it can react with radiation and output a reaction signal. .

The cathode (13) is connected to one end of the semiconductor element (11), and the anode (15) is connected to the other end of the semiconductor element (11). The side electrodes 17 are disposed on the side surfaces of the semiconductor device 11. Since the semiconductor device 11 is formed in the shape of a square pillar, the side electrodes 17 are connected one to each of the four sides. That is, the side electrodes 17 are placed on each of the two sides (X1 and X2) of the semiconductor elements 11 disposed on the One side electrode 17 is also disposed on each of the two sides (Y1 and Y2).

When radiation reacts within the semiconductor element 11, the cathode 13, anode 15, and side electrode 17 output a reaction signal, that is, an electrical signal. According to this semiconductor detector 10, whenever a reaction signal is generated by a photon of radiation, photons with energy equal to or higher than the threshold energy can be counted. Here, the radiation may be gamma rays.

The detection unit 100 includes a plurality of semiconductor detectors 10, where the plurality of semiconductor detectors 10 are encoded and arranged in a predetermined mosaic pattern on the XY plane of the XYZ coordinate system. Referring to FIG. 3, it can be seen that the semiconductor detector 10 according to the present invention has a predetermined mosaic pattern arranged in a grid in the form of a 21×21 matrix. However, the arrangement pattern of the semiconductor detector 10 does not necessarily have to be arranged as shown in FIG. 3, and may be arranged in various patterns, which will be described later. According to the arrangement of the encoded semiconductor detector 10, the response of the semiconductor detector 10 varies depending on the position of the source emitting radiation, similar to the conventional mechanical focusing-based encoded aperture method. In other words, the distribution of reaction positions where reaction photons react within each semiconductor element 11 varies depending on the position of the source of radiation, and by calculating the position of the source through this distribution of reaction positions, an image for the source is created. can be created.

The pattern in which the semiconductor detectors 10 are arranged allows the direction of radiation incident from the source to be easily determined. The response of the semiconductor detector 10 varies depending on the direction of the incident radiation. Since the incident direction of the radiation is clearly distinguished by the encoded pattern of the semiconductor detector 10, the response information of the semiconductor detector 10 can be used to more accurately identify the source. The location can be determined.

Explaining the pattern in which the semiconductor detector 10 is arranged, the semiconductor detector 10 has N (N is a natural number of 1 or more) rows and M (M is a natural number of 1 or more) columns in the form of an N × M matrix. Within the grid, they can be arranged in a reference pattern.

Referring to FIG. 3, in a grid in the form of a 5×5 matrix, all columns ((1,1) to (1,5)) of the first row, third and fourth columns of the second row ((2) ,3), (2,4)), 2nd and 5th columns of the 3rd row ((3,2), (3,5)), 2nd and 5th columns of the 4th row ((4,2) ), (4,5)), semiconductor detectors 10 are arranged one by one at the positions of the third and fourth columns ((5,3), (5,4)) of the fifth row to form a first reference pattern. It can be. Additionally, an extended pattern in which the first reference pattern is expanded may be formed by arranging the first reference pattern side by side along the row and/or column direction. Figure 3 illustrates a case where the first reference pattern in a 5×5 matrix is expanded to a 9×9 matrix. Here, when the four first reference patterns (indicated by dotted lines) are arranged side by side, they are expanded into a 10×10 matrix, which can be expanded into a 9×9 matrix by removing the outermost columns and rows.

In the same manner, a second reference pattern is formed in a grid in the form of a 7 × 7 matrix as shown in FIG. 4, and the second reference pattern is expanded in the form of a 13 × 13 matrix, or a grid in the form of a 11 × 11 matrix as shown in FIG. 5. A third reference pattern is formed in the grid and the third reference pattern is expanded in the form of a 13 × 13 matrix, or a fourth reference pattern is formed in the grid in the form of a 13 × 13 matrix as shown in FIG. 6, and the third reference pattern is expanded in the form of a 25 × 25 matrix. 4 The reference pattern can be extended.

In this way, the semiconductor detector 10 may be arranged in various mosaic patterns, but the patterns are not necessarily limited to FIGS. 3 to 6.

Meanwhile, the radiation imaging equipment according to an embodiment of the present invention may further include a frame (not shown). The frame is a means of fixing the semiconductor detectors 10 so that the plurality of semiconductor detectors 10 can be arranged in a predetermined mosaic pattern. For example, the frame is provided with a plurality of hollow grids into which the semiconductor detectors 10 can be inserted, so that the semiconductor detectors 10 can be inserted and arranged in a predetermined pattern within the grids.

When radiation is incident on the detection unit 100, the signal measurement unit 200 measures response signals from the cathode 13, anode 15, and side electrode 17 of each semiconductor detector 10. The response signal may be an electrical signal. In order to measure the response signal from a plurality of semiconductor detectors 10, the signal measurement unit 200 includes a cathode 13 substrate electrically connected to the cathode 13 of each of the plurality of semiconductor detectors 10, and each anode ( It may include an anode 15 substrate electrically connected to 15) and a grid substrate electrically connected to each side electrode 17.

The image processing unit 300 may generate a video image of the source based on the response signal measured by the signal measurement unit 200. This image processing unit 300 may be implemented as hardware that performs the image processing algorithm of the semiconductor detector 10.

Below, the video image generation operation of the image processing unit 300 will be described.

FIG. 7 is a diagram explaining the source surface and pixels, FIG. 8 shows the anode and cathode signal sizes according to the depth of the semiconductor detector shown in FIG. 1, and FIG. 9 shows the size of the anode and cathode signals according to electron movement inside the semiconductor detector shown in FIG. 1. Shows the induced signal waveform of the electrode.

The image processing unit 300 generates a video image by acquiring the location information of the source according to any one of the non-focused imaging technique, the Compton imaging technique, and the fusion imaging technique that combines the non-focused imaging technique and the Compton imaging technique. do. Here, when the energy band of radiation is less than 250 keV, the non-focused imaging technique is used, when the energy band of radiation is more than 600 keV, the Compton imaging technique is used, and when the energy band of radiation is between 250 and 600 keV, the fusion imaging technique is used. A video image can be created by .

First, the source surface and pixels will be described with reference to FIG. 7. The source surface may be set to a spherical coordinate system as shown on the left side of FIG. 7. In the coordinate system, a specific point P is expressed as a length r, ϕ, which is the angle formed by the straight line connecting the origin and P with the Z-axis, and the angle θ formed between the X-axis and the straight line projected from the origin and P onto the XY plane. Substituting this into the radiation image, r refers to the distance between the source and the semiconductor detector 10, and ϕ and θ refer to the direction in which the source is incident based on the semiconductor detector 10. Referring to the right side of FIG. 7, the spherical coordinate system is divided based on ϕ and θ. When ϕ is in the range of 0 to 180 degrees, θ has a range of 0 to 359 degrees, and pixels can be divided at regular intervals. For example, if pixels are divided into 1-degree intervals, there can be a total of 181 × 360 = 65,160 pixels in the line surface.

The non-focused imaging technique calculates the reaction positions within a plurality of semiconductor detectors (10) that reacted with the incident radiation, and compares them with pixel information on the source surface in a spherical coordinate system where the radiation source for each reaction position within the preset semiconductor detector (10) is located, You can obtain crew member location information. Here, in order to calculate the reaction position, the volume area of each semiconductor device 11 is divided and voxelized into a plurality of voxels arranged along the X-, Y-, and Z-axis directions of the XYZ coordinate system, and the signal measurement unit According to the reaction signal measured at 200, reaction photons can be distributed to the voxels of each semiconductor device 11. Accordingly, a response location based on a voxel can be calculated.

The reaction position of the reaction photon along the Z-axis direction can be calculated based on the ratio (C/A ratio) of the reaction signal magnitudes measured from the anode 15 and the cathode 13. With reference to FIG. 8 showing the magnitude of the anode (15) signal and the cathode (13) signal for each reaction location, the remaining area (side electrode (17)) excluding the anode (15) and side electrode (virtual Frisch-grid, 17) areas and cathode 13), the anode 15 signal remains constant, while the cathode 13 signal increases linearly. Here, the reaction occurring in the area between the anode 15 and the side electrode 17 is removed, and only the reaction occurring between the side electrode 17 and the cathode 13 is used, so that the cathode 13 signal and the anode 15 signal are used. Response position information in the depth direction can be inferred through the ratio of . However, the response position information in the depth direction is not necessarily obtained through the ratio of the signal sizes of the anode 15 and the cathode 13, and can also be calculated using drift time, etc.

The reaction position of the reaction photon along the X-axis and Y-axis can be generated based on the signal size of the side electrode 17. In Figure 9, which shows the waveforms of the induced signals of the electrodes according to the movement of electrons inside the semiconductor detector 10, the waveforms are the result of inverting the original signal. Referring to FIG. 9, since electrons generated by reaction with radiation move away from the cathode 13 and are collected at the anode 15, the two electrodes exhibit opposite polarities. In the case of the side electrode 17, electrons approach but cannot be collected due to the insulator disposed between the semiconductor element 11 and the side electrode 17 and move to the anode 15, so the signal increases and then decreases. Here, the part where the signal increases is called a positive step, and the part where the signal decreases is called a negative step. By applying the center of gravity method to this negative step, the response position on the X and Y axes can be calculated as shown in [Equation 1] below.

[Equation 1]

(Here, A _x1 is the negative step size of the side electrode ( ₁₇ ) in _the X1 direction, A _x2 is the negative step size of the side electrode (17) in the is the negative step size of the side electrode (17) in the Y2 direction)

Radiation emitted from a source located a predetermined distance away from the semiconductor element 11 reacts inside the semiconductor element 11, at this time through the cathode 13, the anode 15, and the four side electrodes 17. Since the electrical signal is measured, and the measured electrical signal corresponds to the number (count) of reaction photons with which radiation reacted inside the semiconductor device 11, the position distribution and reaction number of reaction photons can be calculated through this.

Here, since the information about the source surface pixels in the spherical coordinate system where the source is located according to the reaction position in the semiconductor detector 10 is preset, by comparing the calculated response position distribution with the source surface pixel information for each preset response position distribution, the source surface is Location information can be obtained and video images can be generated.

The Compton imaging technique is a method of imaging using a Compton event, a reaction in which Compton scattering and photoelectric absorption occur continuously. In other words, the location of the source can be determined by generating a Compton cone based on the reaction location and energy information of each scattering reaction and absorption reaction. In actual sources, numerous radiations are emitted, and for each Compton event, the Compton cone varies depending on the combination of reaction position and energy, so multiple Compton cones are generated, and the number of Compton cones accumulated at each location is calculated as relative brightness. By converting, a video image can be created.

In the radiation imaging equipment according to the present invention, a scattering reaction occurs due to radiation incident on one of the plurality of semiconductor detectors 10, and the radiation scattered by the scattering reaction causes An absorption reaction occurs in the semiconductor detector 10. Here, the signal measurement unit 200 measures the scattering response signal and absorption response signal according to the scattering reaction and absorption reaction, and the image processing unit 300 obtains the location information of the source according to the Confton imaging technique based on the signal. And you can create a video image. At this time, the reaction positions within the plurality of semiconductor detectors 10 where scattering and absorption reactions occur, as described above in the non-focused imaging technique, voxelize the semiconductor device 11 and calculate the reaction positions based on the voxels. In this case, the reaction position along the axis direction is calculated based on the ratio (C/A ratio) of the reaction signal size measured from the anode 15 and the cathode 13, and the reaction along the X-axis and Y-axis The photon reaction position can be calculated based on the signal size of the side electrode 17.

The fusion imaging technique fuses the non-focused imaging technique and the Compton imaging technique to obtain location information of the source and generate a video image. Here, the fusion imaging technique can generate an image according to the following [Equation 2] based on MLEM (maximum likelihood expectation maximization).

[Equation 2]

(Here, λ _j ⁿ , λ _j ^{n +1} is the estimate of the line surface pixel j (j is a natural number of 1 or more) after n, n+1 (n is a natural number of 1 or more), M is the total number of line surface pixels, Y _i ^a is the number of reaction photons measured in voxel i (i is a natural number of 1 or more) of the semiconductor detector 10 in the non-focused imaging technique, and N is the total reaction photons measured in the semiconductor detector 10 in the non-focused imaging technique. Number, C _i,j ^a is the probability that a photon generated from source plane pixel j is measured at voxel i of the semiconductor detector 10 in the non-focused imaging technique, and Y _i' ^b is the event (event) i' in the Compton imaging technique. (i' is a natural number greater than or equal to 1), which is the number of response photons measured, N' is the type of event i' in the Compton imaging technique, and C _{i ', j} ^b are the photons generated from source plane pixel j in the Compton imaging technique. ' is the probability measured as ')

λ _j ⁿ , λ _j ^{n +1} are the estimates of the source surface pixel j after the n, n+1th repetition, where λ _j means the estimate at a specific pixel j among all pixels, and the higher this estimate, the probability that there is a source. This means it is high. The initial estimate must be a value between 0 and 1, and when this value is substituted into the equation, the next estimate is derived from that solution. The accuracy of the estimate can be increased by repeating the process several times, but if it exceeds a certain level, performance deteriorates, such as amplification of noise in the image, so it is repeated an appropriate number of times. By arranging these estimates in a two-dimensional matrix, a radiological image can be obtained.

Y _i ^a means the number of responses measured at voxel location i of a specific semiconductor detector 10 when measuring an unknown source at a specific location, and i varies depending on the structure or voxel of the semiconductor detector 10. . For example, when the semiconductor detectors 10 are arranged in a 2×2 matrix and the individual semiconductor detectors 10 are divided into 3×3×10 voxels, i is 3×3×10×4 = 360.

C _i,j ^a is the response function of the semiconductor detector 10 and means the probability that the photon generated when the source is located at a specific pixel j of the source plane in the non-focused imaging technique is measured in voxel i of the semiconductor detector 10. do. It is derived through the distribution of reaction positions within the semiconductor detector 10 according to each position by placing the source in all possible positions within the spherical coordinate system. Since it is generally difficult to implement through experiment, computer simulation can be used. As the amount of data increases, a more accurate response function can be obtained.

Y _i' ^b is the number of reaction photons measured as Compton event i' in the Compton imaging technique, and means the number of reactions measured as a specific event i' when measuring an unknown source at a specific location.

C _{i ', j} ^b refers to the probability that a photon generated from pixel j of the source plane will be measured as event i' in the Compton imaging technique, that is, the probability of being measured as event i' when the source is located at a specific pixel j.

In the MLEM equation, the sum of all possible events i' is required for image reconstruction, but since the combination of reaction positions and scattered photons is very diverse, it is very difficult to obtain probabilities for all reactions, which requires enormous computational time and memory. do. Therefore, the list-mode method can be used to solve this problem. Because there are so many types of event i', the probability that the measured events are the same is very low, so it can be assumed that the measured data occurred uniformly once for all types of reactions. Therefore, all reaction types are replaced by the measured reaction types, and Y _i' ^b is set to 1 only for the measured reactions, and the probability of occurrence of the reaction can be theoretically calculated by considering the Compton scattering angle and attenuation within the detector. there is.

Hereinafter, the present invention will be described in more detail through experimental examples.

Experiment method: computer simulation-based experiment

To confirm the effectiveness of the method proposed in the present invention, the Geant4 Application for Emission Tomography (GATE) v7.0 computational simulation tool was used, and the modeling conditions were as follows. Figure 10 is a photograph showing the non-focused dual-mode radiation imaging equipment used in the experimental example, and the semiconductor detector was modeled as a 2 × 2 array type CZT detector. The equipment is an array of four 6 mm × 6 mm × 19 mm CZT detectors using the VFG (virtual Frisch-grid) method, and the space between detectors is 6 mm.

Each detector is voxelized into 3 × 3 × 10, and the spatial resolution in the X, Y, and Z directions is 2, 2, and 1.9 mm, respectively. The source plane was set as a sphere with a radius of 1 m from the center of the detector array, divided into 1 degree intervals and consisting of 181 × 360 pixels. 2% was reflected at 662 keV, the energy resolution measured through experiment, and two types of computer simulations were performed. The first is a computational simulation that obtains the system matrix of the non-focused imaging technique. The system matrix denoted by C _i,j ^a in [Equation 2] above refers to the detector response according to the source location, and a sufficient amount of data is required to reduce statistical errors. Therefore, long-term data were acquired by modeling a spherical source. The second is a computer simulation that acquires events to reconstruct the image, and the measurement time is set to 1 hour. The source was located 1 m away from the center of the detector, and the performance was compared by reconstructing non-focused, Compton, and fused images based on the acquired data. The computerized sources are ⁵⁷ Co (122 keV), ¹³³ Ba (356 keV), and ¹³⁷ Cs (662 keV), and gamma rays of 150 to 300 keV, where the main reaction of gamma rays changes from photoelectric absorption to Compton scattering, were also measured. It has been done.

Evaluation 1: Results according to detector structure

In addition to the array-type detector structure of Figure 10, the structure of a widely commercialized CZT detector was also computer simulated to compare and analyze its performance. The commercial instrument modeled is IDEAS' SRE-3021 instrument, which has a single CZT detector of 20 mm × 20 mm × 15 mm inside, and the measured energy resolution is approximately 2.1% at 662 keV. This equipment has the same effect as the VFG method by using an 11 × 11 pixelated anode structure instead of the side electrode, and X, Y coordinates can be acquired through this pixel. The pixel pitch is approximately 1.72 mm, and this value is reflected in the spatial resolution. The Z coordinate can be calculated based on the anode and cathode signals, just like the previous VFG detector, and the spatial resolution was set to 1.5 mm. Other conditions were set the same.

Figures 11 to 15 show the results of reconstructing ⁵⁷ Co at various positions using a non-focused imaging technique in experimental examples. Since the structure of a single CZT detector is closer to symmetry than an array detector, the radiation incident perpendicular to the detector surface, such as (180°, 0°) and (270°, 0°), appears circular. As a result, an angle occurs where the detector response characteristics depending on the direction become ambiguous, and the distribution of the radiation incident in the 45° direction is reconstructed to be very wide. On the other hand, array-type detectors have a non-circular source shape due to their asymmetrical structure, but the response characteristics according to direction are clearly distinguished, allowing the location to be determined relatively accurately. In conclusion, it is important for the detector to have an asymmetric structure according to the gamma ray incident direction, and the encoded array type is suitable for this.

Evaluation 2: Results by energy

Figures 16 to 22 are images generated according to the non-focused imaging technique, Compton imaging technique, and fusion imaging technique for each energy when the source is located in front of the semiconductor detector in the experimental example, and Figure 23 shows the image generated by the source in the experimental example. This is a graph showing quantitative indicators evaluated according to energy when placed in front of a semiconductor detector.

(1) Number of video reactions

The reaction number used for imaging decreases exponentially as energy increases in the non-focused method, whereas in the Compton method it increases up to 250 keV and then decreases thereafter. In all energy bands, the number of responses from non-focused images is overwhelmingly high, and fusion images have the largest number because they use the responses of both images.

(2) Angular resolution

Angular resolution is an indicator that evaluates the extent to which the distribution of the source is spread in the image. It is expressed as the full width at half maximum (FWHM) of the distribution around the highest point of the image, and the lower the value, the better. The non-focused image increases exponentially as the energy increases, and the increase becomes gradual from 356 keV, but in reality, the distribution appears too wide from 300 keV or higher, making it impossible to accurately determine the location of the source. Compton images showed very good performance in all energy bands except for results below 200 keV, where image reconstruction is difficult due to almost no Compton reaction, and decreased as energy increased, but increased slightly at 662 keV. The fusion image showed values close to the middle of the two images and close to the good side, and showed values of approximately 10 to 20° in all energy bands.

(3) Normalized standard deviation of the source portion.

The normalized standard deviation of the source portion represents the statistical fluctuation in the pixel corresponding to the reconstructed source, and the lower the value, the better. The source portion is defined as pixels whose distance from the highest point in the image is smaller than the FWHM. The non-focused image decreased as the energy increased and was almost saturated starting from 300 keV, but this was not because the image performance was good and the value was low, but because the source was not properly reconstructed. The Compton image increased and became almost constant starting from 356 keV, and the fusion image showed constant values in almost all energy regions. This appears to have offset the changes in the two imaging techniques.

(d) figure-of-merit

Figure-of-merit (FOM) is an indicator that simultaneously considers efficiency and angular resolution and is defined as efficiency divided by FWHM ³ . In this experiment, efficiency was replaced by number of reactions. In the non-focused image, the FOM decreases very quickly and becomes lower than the Compton image starting at 250 keV. The Compton image increases up to 356 keV and then tends to decrease at 662 keV due to decreased efficiency. When one of the two imaging techniques is overwhelmingly superior (122 keV or 662 keV), the fusion image shows the same value as the corresponding image, and is equal to or higher than the other two imaging techniques in all energy bands.

Assessment 3: Results by crew location

Figure 24 shows the results of reconstruction of ⁵⁷ Co, ¹³³ Ba, and ¹³⁷ Cs at various positions in an experimental example using a fusion imaging technique.

Figure 24 shows a fused image reconstructed from sources located at various (θ, ϕ), and each row is the result of ⁵⁷ Co, ¹³³ Ba, and ¹³⁷ Cs in that order. All results confirmed that the source was reconstructed in the correct position, and the difference between the source position determined through the highest point in the image and the actual position was very small, with an average and maximum value of 2.5° each.

Although the present invention has been described in detail through specific examples, this is for detailed explanation of the present invention, and the present invention is not limited thereto, and can be understood by those skilled in the art within the technical spirit of the present invention. It is clear that modifications and improvements are possible.

All simple modifications or changes of the present invention fall within the scope of the present invention, and the specific scope of protection of the present invention will be made clear by the appended claims.

The non-focused dual radiation imaging equipment according to the present invention is based on the response signal of the semiconductor detector for each position of the source obtained through the cathode, anode, and four side electrodes of the semiconductor detector arranged in a predetermined mosaic pattern. Since it is characterized by reconstructing images over a wide energy range according to imaging techniques, Compton imaging techniques, and fusion imaging techniques that combine them, its industrial applicability is recognized.

Claims (7)

1. A semiconductor element formed in the shape of a square pillar and the depth direction from one end to the other end is disposed along the Z-axis direction of a virtual XYZ coordinate system and reacts with radiation, a cathode connected to one end of the semiconductor element, and It includes a plurality of semiconductor detectors each having a connected anode and four side electrodes each connected to a side of the semiconductor element, wherein the plurality of semiconductor detectors are arranged in a predetermined mosaic pattern on the XY plane of the XYZ coordinate system. A detection unit that is encoded and arranged;

   a signal measurement unit that measures response signals from the cathode, the anode, and the side electrode when the radiation is incident on the detection unit; and

   Radiation imaging equipment including; an image processing unit that generates an image of the source emitting the radiation based on the response signal measured by the signal measurement unit.
2. In claim 1,

   Radiation imaging equipment further comprising a frame for fixing a plurality of semiconductor detectors so that the plurality of semiconductor detectors are encoded and arranged in a predetermined mosaic pattern.
3. In claim 1,

   The image processing unit,

   Calculate reaction positions within the plurality of semiconductor detectors that reacted with the incident radiation, and obtain location information of the source by comparing it with pixel information on the source surface of a spherical coordinate system where the source is located for each preset reaction position within the semiconductor detector. Non-focused imaging techniques,

   A scattering response signal for a scattering reaction generated by the radiation incident on one of the plurality of semiconductor detectors, and a scattering response signal generated in the other semiconductor detector by the radiation scattered by the scattering reaction. Based on the absorption reaction signal for the absorption reaction, the Compton imaging technique calculates reaction positions within the plurality of semiconductor detectors where the scattering reaction and the absorption reaction occur, and obtains location information of the source, and

   The video image is generated according to any one of the following imaging techniques: a fusion imaging technique that obtains location information of the source by fusing the non-focused imaging technique and the Compton imaging technique,

   The volume area of each semiconductor device is divided into voxels with a plurality of voxels arranged along the X-, Y-, and Z-axes of the XYZ coordinate system,

   Radiation imaging equipment for calculating the reaction position based on the voxel by distributing reaction photons to the voxel of each semiconductor device according to the reaction signal measured by the signal measurement unit.
4. In claim 3,

   The image processing unit,

   When the energy band of the radiation is less than 250 keV, using the non-focused imaging technique,

   When the energy band of the radiation exceeds 600 keV, the Compton imaging technique is used,

   Radiation imaging equipment that generates the video image by the fusion imaging technique when the energy band of the radiation is 250 to 600 keV.
5. In claim 3,

   Radiation imaging equipment that calculates the response position in the Z-axis direction based on the ratio of response signal magnitudes measured from the anode and the cathode.
6. In claim 3,

   Radiation imaging equipment that calculates the response positions in each of the X-axis and Y-axis directions by applying a center of gravity method, based on the size of the response signal measured from the side electrode.
7. In claim 3,

   The fusion imaging technique is,

   Radiation imaging equipment that generates the video image according to the following [Equation 1] based on MLEM (maximum likelihood expectation maximization).

   [Equation 1]

   

   here,

   λ _j ⁿ , λ _j ^{n +1} is the estimate of line surface pixel j (j is a natural number greater than 1) after the n, n+1 (n is a natural number greater than 1) th iteration,

   M is the total number of line surface pixels,

   Y _i ^a is the number of response photons measured in voxel i (i is a natural number greater than or equal to 1) of the semiconductor detector in the non-focused imaging technique,

   N is the total number of response photons measured at the semiconductor detector in the non-focused imaging technique,

   C _i,j ^a is the probability that a photon generated from source plane pixel j in a non-focused imaging technique will be measured at voxel i of the semiconductor detector,

   Y _i' ^b is the number of response photons measured as event i'(i' is a natural number greater than 1) in the Compton imaging technique,

   N' is the type of event i' in Compton imaging technique,

   C _{i ', j} ^b is the probability that a photon generated from source plane pixel j will be measured as event i' in the Compton imaging technique.

Images