Gamma imaging noise reduction detector based on depth information

Abstract

The invention provides a gamma imaging noise reduction detector based on depth information, which comprises a layered probe, a host and an acquisition circuit, and is characterized in that the layered probe is of a multi-layer structure formed by arranging single-layer position sensitive probes front and back; the acquisition circuit acquires timestamps, positions and amplitude information of the electric pulse signals; the host is responsible for collecting the number of the collection circuit and generating a detector image, forming the same particle event by using the electric pulse signals with the same timestamp, and determining thickness direction information of activated pixel data corresponding to each electric pulse signal through the number of the collection circuit. And only the data of the foremost activation pixel in the thickness direction is accumulated into the detector image. According to the method, a thick detector is divided into layered probes to obtain depth information of particle energy deposition, so that the noise influence caused by scattering in a detector crystal in medium-high energy gamma ray imaging is reduced, a first scattering point is speculated to eliminate secondary scattering noise of medium-high energy gamma rays, and the contradiction between the imaging speed and the imaging quality is solved.

Description

Technical Field

[0001] This technology belongs to the field of nuclear radiation detection and imaging, and specifically relates to a gamma imaging noise reduction detector based on depth information. Background Art

[0002] The field of nuclear science and technology in China has developed rapidly, and radioactive isotopes and related irradiation devices and technologies are widely used in industrial, agricultural, medical, and geological exploration fields. Medium and high-energy gamma-ray imaging has important applications in isotope localization detection and prompt gamma measurement in proton therapy.

[0003] At present, the position-sensitive probe of a medium and high-energy gamma imaging system usually consists of a scintillator coupled with a photoelectric conversion device. The electrical signal generated is then processed and reconstructed by the backend electronics system to obtain the radiation source image. For example, a GAGG:Ce crystal with high light yield, fast attenuation rate, and strong radiation resistance is coupled with an SiPM array using a silica optical plate and optical silicone oil to achieve high-gain, low operating voltage, and temperature-stable electrical signal output.

[0004] In terms of the methods of medium and high-energy gamma imaging, the imaging system mainly has two technical routes: coded aperture imaging and Compton imaging.

[0005] When the scintillator-photoelectric conversion device is irradiated by medium and high-energy gamma rays, multiple internal scatterings will occur, activating multiple pixels simultaneously. This phenomenon will cause the following three problems in the imaging system adopted by the traditional coded aperture imaging technology: 1. The noise of the image is large, reducing the signal-to-noise ratio, spatial resolution, and imaging speed. 2. Increasing the thickness of the device will make the internal scattering more serious, making it difficult to improve the gamma collection efficiency or imaging speed by increasing the detector thickness. 3. It causes the relative weakening of the brightness of the pixels at the edge of the detector. If the information of these pixels is used in the image reconstruction process, serious artifacts will be caused. Compton imaging is only affected by 1 and 2 due to different imaging calculation principles.

[0006] Among them, the coded aperture imaging system consists of a coded plate and a position-sensitive probe, and the coded plate is arranged in front of the position-sensitive probe. The coded plate is a plate usually made of heavy metals with strong gamma absorption, drilled with holes in a specific arrangement. Gamma rays will pass through the holes and irradiate the position-sensitive probe, while it is difficult to penetrate the part blocked by the heavy metals. By performing decoding processing such as deconvolution or statistical iteration on the shadow image cast by the coded plate on the position-sensitive probe, the distribution image of the radiation source can be obtained.

[0007] The coded-aperture imaging system has a wide range of applications in low-energy gamma imaging. However, in the case of high-energy gamma rays, gamma photons will penetrate the coded aperture. At the same time, due to the increased internal scattering in the position-sensitive detector at high energies, the contrast between the shadow part and the bright spot part decreases, seriously affecting the spatial resolution and imaging speed of the system.

[0008] The Compton imaging system usually consists of multiple position-sensitive and energy-sensitive detectors arranged in front and back. Events are formed by coincidence in time through multiple detectors. The position and energy information of two deposition points are substituted into the Compton scattering formula to calculate the direction angle information of the source.

[0009] The Compton imaging system utilizes the scattering effect between multiple detectors. However, event screening is usually relatively strict, requiring exactly two scattering points located on different detectors. It is difficult to determine the data points to be used when there are many deposition points. Therefore, the effective counting rate of this system is very low, and the imaging time will be very long. In addition, this system requires both position and energy information at the same time, the system will be relatively complex and expensive. In addition, the randomness of this method is very large at low counts. Summary of the Invention

[0010] The purpose of the present invention is to provide a gamma imaging detector based on depth information to achieve high-speed and high-performance medium- and high-energy gamma imaging.

[0011] To achieve the above purpose, the present invention provides a gamma imaging noise reduction detector based on depth information, including a layered detector, a host computer, and an acquisition circuit connected between the two. The layered detector is a multi-layer structure formed by arranging multiple single-layer position-sensitive detectors in front and back. The thickness direction of the layered detector is the same as the front-back direction, and the side close to the gamma ray source is the front; the acquisition circuit is used to collect the time stamp, position information, and amplitude information of the electrical pulse signals output by the position-sensitive detectors; the host computer is responsible for collecting the numbers of the acquisition circuit and generating a detector image. It forms the same particle event with the electrical pulse signals having the same time stamp, determines the position information in the thickness direction of the activated pixel data corresponding to each electrical pulse signal through the number of the acquisition circuit, and only accumulates the activated pixel data of the position-sensitive detector at the frontmost position in the thickness direction into the detector image.

[0012] The single-layer position-sensitive detector includes a single-layer scintillator array and a photoelectric conversion array. The scintillator array is used to convert the incident gamma rays into scintillation photons that can be collected by the photoelectric conversion array, and the photoelectric conversion array is used to collect the scintillation photons and convert them into electrical signals.

[0013] The scintillator array and the photoelectric conversion array of a single position-sensitive detector are arranged in sequence along the propagation direction of the gamma rays, and the array pixels of the scintillator array and the photoelectric conversion array are aligned with each other.

[0014] The scintillator array is a GAGG:Ce crystal array, and the photoelectric conversion array is a SiPM array.

[0015] The number of layers of the layered probe is 3 to 5 layers.

[0016] The gamma imaging detector based on depth information further includes a coding plate disposed in front of the layered probe.

[0017] The layered probe further includes an adapter bracket for fixing the position-sensitive probes of multiple single layers. The adapter bracket includes multiple adapter plates for fixing the position-sensitive probes of single layers, and connecting rods for connecting and fixing adjacent adapter plates.

[0018] At least a part of the adapter plate is disposed on the periphery of the scintillator array and the photoelectric conversion array, and an adapter plug for connecting the acquisition circuit is provided on the periphery of the adapter plate. The photoelectric conversion array is electrically connected to the adapter plug of the adapter plate, and the adapter plug is electrically connected to the acquisition circuit via a flexible cable.

[0019] The host is also configured to configure and send the parameters of each acquisition circuit to control the acquisition circuit and process the read data.

[0020] A main control program module and a post-processing module are installed on the host. The main control program module on the host simultaneously controls and reads multiple acquisition circuits, and outputs a raw data file with time stamps, amplitude information, the numbers of the acquisition circuits, and the activated pixel numbers. The post-processing module is configured to: compare the time stamps to form the same particle event for the electrical pulse signals with the same time stamp; for the same particle event, determine the position information in the thickness direction of the activated pixel data corresponding to each electrical pulse signal according to the number of the acquisition circuit, discard the activated pixel data corresponding to the position-sensitive probe at the rear in the thickness direction, and only retain the activated pixel data corresponding to the position-sensitive probe at the front in the thickness direction to obtain the processed particle event; then, perform image integration summation on the processed particle event to obtain the processed image.

[0021] A GUI program module is also installed on the host to facilitate the interaction between the personnel and the main control program module, send instructions and monitor the operating status through the main control program module.

[0022] The gamma imaging detector based on depth information of the present invention divides a relatively thick detector into a layered probe to obtain the depth information of particle energy deposition, and then screens the activated pixels through the depth information to reduce the noise influence caused by internal scattering in the detector crystal in medium and high energy gamma ray imaging. By inferring the first scattering point from the reaction depth information obtained by the multi-layer detector, the secondary scattering noise of medium and high energy gamma rays can be excluded, and the imaging quality after saturation can be maintained or even improved while increasing the imaging speed, thus solving the contradiction between imaging speed and imaging quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic structural diagram of a gamma imaging detector based on depth information of the present invention.

[0024] Figure 2A and Figure 2B is a schematic structural diagram of a layered probe of a gamma imaging detector based on depth information of the present invention, where Figure 2A is a front view, Figure 2B is a side view.

[0025] Figure 3 is a working flowchart of the mainframe of a gamma imaging detector based on depth information of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] As Figure 1 shown is a schematic structural diagram of a gamma imaging detector based on depth information according to an embodiment of the present invention. As Figure 1 shown, the gamma imaging detector based on depth information includes a layered probe 100, a mainframe 200, and an acquisition circuit 300 connected between the two. The acquisition circuit 300 is connected to the mainframe 200 via a network cable.

[0027] The layered probe 100 is a multi-layer structure formed by arranging a plurality of single-layer position-sensitive probes 10 in a front-to-back manner. Thus, the collection ability for medium and high energy gamma rays is improved through the layered probe 100. That is to say, each single-layer position-sensitive probe 10 is a single-layer structure, corresponding to one layer in the layered probe 100. The thickness direction of the layered probe 100 is the same as the front-to-back direction. Relative to the gamma ray radiation source, the side closer to the gamma ray radiation source is the front, and the side farther from the radiation source is the back.

[0028] The structures of different single-layer position-sensitive probes 10 are the same and independent of each other. Among them, the single-layer position-sensitive probe 10 is in the form of an array, including a single-layer scintillator array 11 and a photoelectric conversion array 12. The scintillator array 11 is used to convert the incident gamma rays into scintillation photons that can be collected by the photoelectric conversion array 12, and the photoelectric conversion array 12 is used to collect the scintillation photons and convert them into electrical signals.

[0029] The scintillator array 11 and the photoelectric conversion array 12 of the single position-sensitive probe 10 are arranged in sequence along the propagation direction of the gamma rays (i.e., the scintillator array 11 is in the front and the photoelectric conversion array 12 is in the rear), and the array pixels of the scintillator array 11 and the photoelectric conversion array 12 are aligned with each other. In this embodiment, the array forms of the scintillator array 11 and the photoelectric conversion array 12 are similar to those of a conventional position-sensitive probe. In this example, a square array such as 12×12 is adopted, each pixel is also square, the pixel size is such as 3×3 mm, and an isolation and reflection layer is provided between the pixels.

[0030] In this embodiment, the scintillator array 11 is a GAGG:Ce crystal array, and the photoelectric conversion array 12 is a SiPM array. GAGG:Ce is a scintillation crystal with a relatively large density, a relatively large light yield, and a fast decay time. Other crystals are also acceptable. SIPM is a semiconductor photomultiplier device used to achieve photoelectric conversion. It should be noted that GAGG:Ce crystals and SIPM are only one implementation scheme. In other embodiments, other crystals can also be adopted for the scintillator array 11, and other materials can also be adopted for the photoelectric conversion array 12.

[0031] In this embodiment, the number of layers of the layered probe 100 is optimized and determined according to the thickness of each single-layer position-sensitive probe 10 and the gamma-ray energy range in the application scenario, usually 3 to 5 layers.

[0032] The layered probe 100 further includes an adapter bracket for fixing a plurality of single-layer position-sensitive probes 10. As Figure 2A and Figure 2B shown, in this embodiment, the adapter bracket includes a plurality of adapter plates 31 for fixing the single-layer position-sensitive probes 10, and connecting rods 32 for connecting and fixing between adjacent adapter plates 31. Among them, the single-layer position-sensitive probe 10 is fixed on the adapter plate 31 through a plastic shell 33, long studs 34, and nuts 35. The single-layer position-sensitive probe 10 is accommodated in the space jointly defined by the adapter plate 31 and the plastic shell 33. The long studs 34 and nuts 35 are fixed at the reserved holes of the adapter plate 31.

[0033] Preferably, at least a part of the adapter plate 31 is arranged on the periphery of the scintillator array 11 and the photoelectric conversion array 12, and an adapter plug 311 for connecting the acquisition circuit 300 is provided on the periphery of the adapter plate 31. Each adapter plate 31 has a lead for connecting the photoelectric conversion array 12 to the adapter plug. Therefore, the photoelectric conversion array 12 is electrically connected to the adapter plug 311 of the adapter plate 31, and the adapter plug 311 is electrically connected to the acquisition circuit 300 via a flexible cable. Thus, the electrical signal of the photoelectric conversion array 12 is transmitted to the acquisition circuit 300.

[0034] The layered probe 100 of the present invention will not cause mutual blockage between the front and rear multi-layer position-sensitive probes 10, resulting in inability to measure. First, the adapter board 31, the adapter plug, and the flexible cable are arranged on the periphery of the scintillator array 11 and the photoelectric conversion array 12. Secondly, medium and high-energy gamma rays have strong penetrability, the density of the PCB and the flexible cable is low, and there is a very high probability that no reaction and energy deposition will occur. The multi-layer position-sensitive probes 10 are arranged front and back to increase the probability of reaction and energy deposition, and at the same time, to distinguish the first deposition and the secondary scattering deposition as much as possible.

[0035] Please refer to again Figure 1 The acquisition circuit 300 is used to acquire the timestamp, position information, and amplitude information of the electrical pulse signals output by the photoelectric conversion array 12.

[0036] In this embodiment, the acquisition circuit 300 is an A5202 acquisition board, which provides the bias voltage required for the normal operation of the photoelectric conversion array 12, and reads the electrical pulse signals generated by the photoelectric conversion array 12. Each A5202 acquisition board has 64 independent readout channels and can control and read 64 pixels. Each readout channel has a preamplifier, fast shaping filter, slow shaping filter, peak detection circuit, and discriminator. The electrical pulse signals of the photoelectric conversion array 12 are input to the fast shaping filter and the slow shaping filter after passing through the preamplifier. The fast shaping filter is connected to the discriminator to generate a self-trigger signal. The slow shaping filter is connected to the peak detection circuit, and its peak value is used for subsequent analog-to-digital conversion. The self-trigger signals of all 64 channels are logically ORed to generate a global trigger signal, which is used to determine the timestamp and trigger each independent channel to perform analog-to-digital conversion. After analog-to-digital conversion, the timestamp, channel information (i.e., position information), and amplitude information of the electrical pulse signals can be obtained, and this data packet will be sent to the memory on this A5202 acquisition board waiting for the host computer to read.

[0037] The number of A5202 acquisition boards can be set to one or more according to the number of pixels of the single-layer position-sensitive probe 10, but the A5202 acquisition boards do not need to be arranged in a layered manner and are not a multi-layer structure.

[0038] The host 200 is set to regard the electrical pulse signals from different position-sensitive probes 10 with the same timestamp as the information of the same particle event; for the same particle event, according to the number of the acquisition circuit, determine the position information in the thickness direction of the activated pixel data corresponding to each electrical pulse signal, and only accumulate the activated pixel data of the frontmost position-sensitive probe 10 in the thickness direction into the detector image.

[0039] Such as Figure 3As shown, the host 200 is also configured to configure and send the parameters of each acquisition circuit to control the acquisition circuit 300 and process the read-out data. The parameters that the host 200 configures and sends to each acquisition circuit 300 mainly include: SiPM bias voltage, maximum current protection threshold, acquisition board working mode, global trigger signal source, data packet save trigger signal source, acquisition board output port signal source, preamplifier gain, shaping time, discriminator trigger threshold, zero suppression threshold. The main purpose of the host 200 is to: adjust the bias voltage and maximum current protection threshold of the photoelectric conversion array 12 to enable the normal operation of the photoelectric conversion array 12 and protect the integrated chip, and adopt different preamplifier gains, shaping times, discriminator trigger thresholds, and zero suppression thresholds for different types and energies of radiation sources so that the signals processed by the read-out channel meet the analog-to-digital conversion range limit and can normally output the energy value. The acquisition circuit 300 starts to work only after receiving the acquisition circuit parameters and acquisition commands, enabling the host 200 to collect data from multiple acquisition boards and perform file writing sorting. File writing sorting means sorting the particle events of all probes at the same timestamp according to the depth information, and only retaining the probe information closest to the radiation source, so as to infer the position of the first energy deposition.

[0040] The host 200 is equipped with a main control program module 201, a GUI (Graphical User Interface) program module 202, and a post-processing module 203.

[0041] The main control program module 201 on the host 200 can simultaneously control and read multiple acquisition circuits 300, and save or transfer the timestamp and amplitude information of the electrical pulse signals from different acquisition circuits 300 in a self-defined format to output a raw data file with timestamp, amplitude information, the number of the acquisition circuit, and the activated pixel number (i.e., the particle position). The number of the acquisition circuit corresponds to the depth information. Among them, the activated pixel number represents the coordinate position of the particle on the same position-sensitive probe 10, and the acquisition board number corresponds to which position-sensitive probe 10 the particle is specifically deposited on.

[0042] The post-processing module 203 is configured as follows: event combination, that is, comparing timestamps to form electrical pulse signals with the same timestamp from different position-sensitive probes 10 into the same particle event; depth sorting, that is, for the same particle event, determining the position information in the thickness direction of the activated pixel data corresponding to each electrical pulse signal according to the number of the acquisition circuit, discarding the activated pixel data corresponding to the position-sensitive probe 10 at the rear in the thickness direction, and only retaining the activated pixel data corresponding to the position-sensitive probe 10 at the forefront in the thickness direction to obtain the processed particle event; thereby, realizing the speculation of the first energy deposition position and excluding redundant scattering information; subsequently, performing image integration summation on the processed particle event to obtain the processed image. Among them, image integration summation refers to statistically summing the number of particle events occurring at each pixel point of the position-sensitive probe 10 within a period of time, and using the total number of particle events at each pixel point as the brightness of that pixel point.

[0043] In this embodiment, if particle events from different position-sensitive probes 10 have the same timestamp, they are combined into the same particle event, and thus the same timestamp is used as a prerequisite for determining that particle events from different position-sensitive probes 10 are the same particle event.

[0044] For convenience of operation, the main control program module 201 can communicate with the GUI program module 202 through the socket protocol, and the GUI program module 202 is for the user to send instructions and monitor the running status.

[0045] In addition, the gamma imaging detector based on depth information of the present invention may further include a coding plate provided in front of the layered probe 100, enabling the detector to image. The coding plate is an existing one, made of heavy metal with strong gamma absorption and drilled with holes in a specific arrangement. Thus, the gamma imaging detector based on depth information of the present invention adopts coded aperture imaging technology, greatly increasing the light flux, so it has the advantages of high resolution of pinhole imaging and high imaging rate of coded imaging.

[0046] In the prior art, to improve the detection efficiency and imaging speed, for example, a traditional single-layer detector is adopted and a relatively thick detector crystal is used to improve the detection efficiency. However, due to more serious scattering in the thick crystal, the imaging performance of the relatively thick detector crystal is usually lower than that of the thin detector after counting saturation, and the first deposition and secondary scattering deposition will be mixed in the same particle event, making it difficult to improve the gamma collection efficiency or imaging speed by increasing the detector thickness.

[0047] The gamma imaging detector based on depth information of the present invention divides a relatively thick detector into a layered probe to obtain the depth information of particle energy deposition, and then screens the activated pixels through the depth information to reduce the noise caused by internal scattering in the detector crystal in medium- and high-energy gamma-ray imaging. By inferring the first scattering point through the reaction depth information obtained by the multi-layer detector, the secondary scattering noise of medium- and high-energy gamma rays can be excluded, and the imaging quality (such as contrast-to-noise ratio, resolution) after saturation can be maintained or even improved while increasing the imaging speed, thus solving the contradiction between imaging speed and imaging quality.

[0048] Compared with traditional coded-aperture imaging, this technology has a faster imaging speed and higher quality. Compared with the Compton imaging system with multi-layer probes, this technology has relatively loose event coincidence conditions, which can effectively improve the counting rate and acquisition speed.

[0049] For the gamma imaging noise reduction detector of the present invention, through simulation experiments, for gamma photons with an energy of 1 MeV, the contrast-to-noise ratio of the image processed by the gamma imaging noise reduction detector of the present invention is increased by 67% compared with the coded-aperture gamma imaging detector without additional processing. The detector crystal of the gamma imaging noise reduction detector of the present invention uses a five-layer layered probe. Compared with the design of a traditional thin detector without additional processing, the contrast-to-noise ratio can be increased by 25%, and the imaging speed can be increased by 33%, effectively alleviating the contradiction between the particle collection efficiency of the detector and the imaging quality. In addition, the gamma imaging noise reduction detector of the present invention can also effectively eliminate the phenomenon of lower brightness of the edge pixels of the detector, avoiding the introduction of additional artifact errors during image reconstruction.

Claims

1. A gamma imaging noise reduction detector based on depth information, characterized in that: It includes a layered probe, a host and a collection circuit connected therebetween. The layered probe is a multi-layer structure formed by arranging a plurality of single-layer position-sensitive probes in a front-to-back manner. The thickness direction of the layered probe is consistent with the front-to-back direction, and the side closer to the gamma ray radiation source is the front. The acquisition circuit is used to acquire the timestamp, position information and amplitude information of the electric pulse signal output by the position sensitive probe; The host is responsible for collecting the numbers of the acquisition circuits and generating detector images. It combines electrical pulse signals with the same timestamp into the same particle event, and determines the position information in the thickness direction of the activated pixel data corresponding to each electrical pulse signal through the numbers of the acquisition circuits, and only accumulates the activated pixel data of the frontmost position-sensitive probe in the thickness direction into the detector image.

2. The gamma imaging noise reduction detector based on depth information according to claim 1, characterized in that: The single-layer position-sensitive probe comprises a single-layer scintillator array and a photoelectric conversion array. The scintillator array is used to convert incident gamma rays into scintillation photons that can be collected by the photoelectric conversion array. The photoelectric conversion array is used to collect the scintillation photons and convert them into electrical signals.

3. The gamma imaging noise reduction detector based on depth information according to claim 2, characterized in that: The scintillator array and the photoelectric conversion array of a single position sensitive probe are arranged in sequence along the propagation direction of the gamma rays, and the array pixels of the scintillator array and the photoelectric conversion array are aligned with each other; and / or The scintillator array is a GAGG:Ce crystal array, and the photoelectric conversion array is a SiPM array.

4. The gamma imaging noise reduction detector based on depth information according to claim 1, characterized in that: The layered probe has 3 to 5 layers.

5. The gamma imaging noise reduction detector based on depth information according to claim 1, characterized in that: It also includes a coding plate arranged in front of the layered probe.

6. The gamma imaging noise reduction detector based on depth information according to claim 1, characterized in that: The layered probe also includes an adapter bracket for fixing a plurality of single-layer position-sensitive probes. The adapter bracket includes a plurality of adapter plates for fixing a single-layer position-sensitive probes, and a connecting rod for connecting and fixing adjacent adapter plates.

7. The gamma imaging noise reduction detector based on depth information according to claim 6, characterized in that: At least a portion of the adapter board is arranged on the periphery of the scintillator array and the photoelectric conversion array, and an adapter plug for connecting to the acquisition circuit is provided on the periphery of the adapter board. The photoelectric conversion array is electrically connected to the adapter plug of the adapter board, and the adapter plug is then electrically connected to the acquisition circuit via a flexible flat cable.

8. The gamma imaging noise reduction detector based on depth information according to claim 1, characterized in that: The host is also configured to configure and send parameters of each acquisition circuit to control the acquisition circuit and process the read data.

9. The gamma imaging noise reduction detector based on depth information according to claim 1, characterized in that: The host is installed with a main control program module and a post-processing module; The main control program module on the host computer controls and reads multiple acquisition circuits at the same time, and outputs a raw data file with a timestamp, amplitude information, the number of the acquisition circuit, and the number of activated pixels; The post-processing module is set to: compare timestamps so that the electrical pulse signals with the same timestamp are grouped into the same particle event; For the same particle event, the position information of the activation pixel data corresponding to each electric pulse signal in the thickness direction is determined according to the number of the acquisition circuit, the activation pixel data corresponding to the position sensitive probe at the rear in the thickness direction is discarded, and only the activation pixel data corresponding to the position sensitive probe at the front in the thickness direction is retained to obtain the processed particle event; Subsequently, image integration and summation are performed on the processed particle events to obtain a processed image.

10. The gamma imaging noise reduction detector based on depth information according to claim 9, characterized in that: The host is also installed with a GUI program module to facilitate personnel to interact with the main control program module, send instructions and monitor the operating status through the main control program module.

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