PET detector based on differential time chain and continuous crystal

Abstract

The present invention provides a PET detector based on a differential time chain and a continuous crystal. The detector comprises: a single crystal block, into which gamma rays are incident, where energy is deposited within the block, emitting photons. A SIPM on the surface of the single crystal block absorbs the photons and responds by outputting current pulses. A time readout circuit, the input of which is connected to the output of the SIPM. The time readout circuit uses the differential time chain to obtain time information of the SIPM output current pulses. The differential time chain includes a time-to-digital converter and a differential chain, the output of which serves as a latch clock for the differential chain. A positioning module, the input of which is connected to the output of the time readout circuit. The positioning module uses the time information to obtain the location of the photons within the single crystal block. The present invention implements a PET detector based on the differential time chain and the continuous crystal, significantly improving the spatial resolution of the PET detector.

Description

Technical Field

[0001] The present invention relates to the technical field of PET (Positron Emission Tomography) detectors, and in particular to a PET detector based on a differential time chain and a continuous crystal. Background Art

[0002] PET spatial resolution refers to the minimum ability of a PET system to distinguish between two closely spaced locations. Both the PET system hardware architecture and the positioning software algorithms influence spatial resolution. The spatial resolution of the PET detector directly impacts image reconstruction quality and the accuracy of subsequent lesion detection and localization. Therefore, improving the spatial resolution of PET systems has long been a hotly debated topic.

[0003] Extensive research has been conducted on hardware architecture, resulting in impressive spatial resolution. Spatial resolution is primarily influenced by two factors: crystal size. A crystal, acting as a pixel, directly impacts PET spatial resolution. Generally speaking, smaller crystals yield higher spatial resolution. Another factor is the photon deposition location. Photons deposit randomly within a crystal, so uniformly targeting the crystal surface can introduce errors, resulting in image artifacts and reduced spatial resolution. Several approaches have been proposed to improve PET spatial resolution.

[0004] Reduce the crystal size and cut the crystal into smaller sizes to obtain higher spatial resolution. Although reducing the crystal size can effectively improve the spatial resolution of PET detectors, this method has its drawbacks. As the crystal size decreases, the probability of scattered emission increases, the sensitivity of the PET detector decreases, and the edge deteriorates significantly. Higher requirements are placed on the crystal process, and at the same time, the resources consumed by the peripheral circuit, power consumption, area, etc. will increase exponentially. In addition, too small a crystal size also makes it difficult to position the crystal. When the crystal size is smaller than the SIPM (Silicon Photomultiplier, semiconductor photodetector) size, one-to-one coupling will no longer be applicable, and a one-to-many coupling method must be used, which reduces the signal quality. These will all reduce the spatial resolution of the PET detector. Therefore, the improvement in spatial resolution brought about by cutting the crystal is limited. Currently, the spatial resolution that can be obtained using this strategy is 0.55mm.

[0005] While improving spatial resolution by reducing image artifacts through DOI (Depth of Interaction) information is limited, its spatial resolution rarely exceeds 0.5mm. Acquiring DOI information generally requires additional resources, increasing readout circuit resources for both dual-end readout and long-edge coupling. Using dual-end readout, the long edge of the crystal is coupled instead of the short edge, allowing for more DOI information. However, the main drawback is the increased complexity of the readout circuitry. In long-axis PET, the resource consumption associated with thousands of crystals is incalculable, making it difficult to implement in practical PET systems.

[0006] Compared to discrete single crystals, single-crystal block structures effectively address the challenges inherent in discrete crystals, offering improved sensitivity and detection efficiency and the potential for achieving higher spatial resolution. By coupling multiple SIPM arrays onto the surface of a single crystal block, each SIPM acts as a pixel, determining the spatial resolution. Multi-sided coupling enables more accurate DOI information and improves image quality. However, achieving high spatial resolution using single crystal blocks also has limitations. Increasing the number of coupled SIPMs leads to increased readout circuitry, requiring separate signal readout circuitry for each path. Furthermore, when a photon enters a single crystal block, light diffusion causes all SIPMs coupled to the surface of the block to respond, increasing data bandwidth requirements. Reducing data bandwidth through channel merging or reducing the number of channels will degrade spatial resolution. Some researchers have proposed considering only the SIPM response within the 15% energy range around 511 keV, or employing channel merging to sum the SIPMs in rows and columns before outputting the result. This reduces data bandwidth, but sacrifices some effective data detail, resulting in a decrease in spatial resolution. In addition, the complexity of the readout circuit will increase, especially when multi-side coupling is used, which limits the application of single crystal block architecture in PET detectors.

[0007] Although the currently used methods can improve the spatial resolution of PET systems to a certain extent, there is no way to achieve a spatial resolution of 0.5 mm. Some methods are even not feasible in PET detectors due to certain reasons, such as circuit complexity. Summary of the Invention

[0008] The present invention provides a PET detector based on a differential time chain and a continuous crystal, which is used to solve the defect of limited improvement of the spatial resolution of PET detectors in the prior art, and realizes a PET detector based on a differential time chain and a continuous crystal, thereby greatly improving the spatial resolution of the PET detector.

[0009] The present invention provides a PET detector based on a differential time chain and a continuous crystal, comprising:

[0010] a single crystal block into which gamma rays are incident, and energy is deposited inside the single crystal block, emitting photons, and a SIPM on a surface of the single crystal block absorbs the photons and responds by outputting a current pulse;

[0011] a time readout circuit, wherein an input terminal of the time readout circuit is connected to an output terminal of the SIPM, and the time readout circuit is used to obtain time information of the SIPM output current pulse using a differential time chain, wherein the differential time chain includes a time-to-digital converter and a differential chain, and the output of the digital converter serves as a latch clock of the differential chain;

[0012] A positioning module, wherein the input end of the positioning module is connected to the output end of the time readout circuit, and the positioning module is used to obtain the positioning information of the photon in the single crystal block according to the time information.

[0013] According to the present invention, a PET detector based on a differential time chain and a continuous crystal is provided. The time-to-digital converter uses a system clock as the latch clock of the D-type flip-flops of each tap in the time-to-digital converter. The clock used by the differential chain comes from the output of the kth axis head in the time-to-digital converter, where k is determined by the length of the differential chain.

[0014] According to a PET detector based on a differential time chain and a continuous crystal provided by the present invention, the time-to-digital converter further includes a global clock buffer, which is used to reduce the delay from the k-th axis head to each trigger.

[0015] According to the PET detector based on a differential time chain and a continuous crystal provided by the present invention, the length of the differential chain is determined according to the size of the single crystal block.

[0016] According to a PET detector based on a differential time chain and a continuous crystal provided by the present invention, the differential chain also includes a fixed delay module, which is used to compensate for the wiring delay of the current pulse reaching the differential chain and the latch clock delay of the D trigger of the differential chain.

[0017] According to a PET detector based on a differential time chain and a continuous crystal provided by the present invention, the delay time compensation value of the fixed delay module is obtained by the following steps:

[0018] Inputting the same signal into the time-to-digital converter and the differential chain, and determining the product of the width and the number of carry chains in the time-to-digital converter;

[0019] If the number of D flip-flop latch outputs 1 of the differential chain is equal to the product, the delay time compensation value of the fixed delay module remains unchanged;

[0020] If the number of D flip-flop latch outputs 1 of the differential chain is less than the product, the delay time compensation value of the fixed delay module is reduced;

[0021] If the number of D flip-flop latch outputs 1 of the differential chain is greater than the product, the delay time compensation value of the fixed delay module increases.

[0022] According to a PET detector based on a differential time chain and a continuous crystal provided by the present invention, the differential chain obtains the time information of the SIPM output current pulse by the following formula:

[0023] t d =t s +(d-8×k)×t tdc

[0024] Among them, t s is the absolute time output by the time-to-digital converter, d is the number of 1s output by the D flip-flop latch of the differential chain, k is the length of the differential chain, and t tdc is the average delay time of each tap in the time-to-digital converter.

[0025] According to the PET detector based on differential time chain and continuous crystal provided by the present invention, multiple surfaces of the single crystal block are coupled with SIPM.

[0026] The input end of the positioning module is also connected to the output end of the energy readout circuit. The positioning module is used to obtain the positioning information of the photon in the single crystal block according to the time information and the energy information.

[0027] According to a PET detector based on a differential time chain and a continuous crystal provided by the present invention, the energy readout circuit is used to obtain the energy information by merging row and column data or merging adjacent channels.

[0028] The PET detector based on a differential time chain and a continuous crystal provided by the present invention greatly simplifies the complexity of the readout circuit and the data bandwidth by utilizing a differential time chain readout architecture for time acquisition, so that the readout circuit complexity and data bandwidth are no longer limiting factors of the single crystal block. To obtain more accurate positioning information, a multi-edge coupling architecture is used, coupling SIPM to multiple surfaces of the single crystal block. For energy information acquisition, since positioning can mainly rely on time information to obtain the exact position, and energy information is only used for auxiliary calibration, the concept of channel merging is adopted to obtain energy, thereby reducing the data bandwidth and readout circuit complexity, thereby greatly improving the spatial resolution of the PET detector. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction is given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 This is a schematic diagram of the single crystal block structure used in the PET detector based on the differential time chain and continuous crystal provided by the present invention;

[0031] Figure 2 Schematic diagram of the differential time chain structure used by the PET detector based on the differential time chain and continuous crystal provided by the present invention;

[0032] Figure 3 The present invention provides an IDELAY adaptive delay adjustment method in a PET detector based on a differential time chain and a continuous crystal;

[0033] Figure 4 It is a structural diagram of the combined partitioning of two adjacent energy measurement channels in a PET detector based on a differential time chain and a continuous crystal provided by the present invention. DETAILED DESCRIPTION

[0034] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0035] PET is an imaging technology that can visualize the metabolic levels of biomolecules in vivo. It plays a crucial role in the diagnosis of diseases such as cancer, tumors, and Alzheimer's disease. The resolution of a PET detector is a key factor in determining image quality. It determines how clearly and accurately the imaged object can be seen. Clearer imaging quality enables the isolated imaging of fine structures within the human body. It also promotes early screening for diseases and reduces the likelihood of missed diagnoses. Although the spatial resolution of PET detectors has greatly improved with technological advancements, 0.5mm remains the threshold for PET detectors. Therefore, research on improving the resolution of PET detectors is crucial.

[0036] The following combination Figures 1 to 4The present invention is described as follows: a PET detector based on a differential time chain and a continuous crystal, comprising:

[0037] a single crystal block into which gamma rays are incident, and energy is deposited inside the single crystal block, emitting photons, and a SIPM on a surface of the single crystal block absorbs the photons and responds by outputting a current pulse;

[0038] a time readout circuit, wherein an input terminal of the time readout circuit is connected to an output terminal of the SIPM, and the time readout circuit is used to obtain time information of the SIPM output current pulse using a differential time chain, wherein the differential time chain includes a time-to-digital converter and a differential chain, and the output of the digital converter serves as a latch clock of the differential chain;

[0039] A positioning module, wherein the input end of the positioning module is connected to the output end of the time readout circuit, and the positioning module is used to obtain the positioning information of the photon in the single crystal block according to the time information.

[0040] The positioning information of the photons in the single crystal block includes the deposition position and the action depth of the photons in the single crystal block.

[0041] First, recent research has shown that crystal cutting can no longer meet the requirements for higher spatial resolution. Therefore, this embodiment uses a single crystal bulk structure with greater potential.

[0042] Figure 1 The figure shows the single crystal block structure used in PET detectors. The white part in the structure is the single crystal block used, and the green part is the coupled SIPM. The single crystal block adopts single-side or multi-side coupling. Figure 1 (a) is the bottom side coupling method, (b) is the double side coupling method, (c) is the bottom side and top side coupling method, and (d) is the full-coverage coupling method. Figure 1 In addition to the coupling method shown, other multi-sided coupling methods can also be used to obtain the best spatial resolution. As can be seen, one SIPM acts as a pixel, and the number of coupled SIPMs directly affects the spatial resolution of the crystal and also determines the resource consumption.

[0043] Gamma rays are injected into a single crystal block, perhaps from above, where they are deposited, emitting visible light photons that are then received by the SiPM. The SiPM is a solid-state, high-gain radiation detector. Based on a PN junction, the sensor exhibits single-photon sensitivity and generates a current pulse upon photon absorption.

[0044] Secondly, to simplify the complexity of the time readout circuit and reduce the data bandwidth, a time measurement architecture based on a differential time chain is proposed based on the SIPM response characteristics of a single crystal block. In this architecture, only a global time-to-digital converter and multiple differential chains are needed to characterize the arrival time of photons in a single crystal block. Compared with traditional crystal time readout, it can save up to 98% of resources. The data format is output using a global clock plus an absolute clock, which significantly reduces the number of data bits and the data bandwidth compared to traditional methods. In addition, the time resolution of the differential time chain can reach 2ps, achieving precise sampling of time. The differential-based time chain architecture successfully solves the problems of single crystal block readout circuit complexity and bandwidth.

[0045] Figure 1 The deposition position and depth of photons are shown in red. The SIPM close to the deposition position will capture visible light first and respond first. Conversely, the deposition position and depth of photons in the single crystal can be determined based on the response time of each SIPM. The crystal size is generally within 3 cm. According to the propagation speed of light (3×10 8 m / s), the response time of the two most distant SIPMs to the same gamma photon will not exceed 100 ps. This is the theoretical basis for the implementation of the differential time chain.

[0046] Based on the response characteristics of the coupled SIPMs in a single crystal block to gamma photons, taking a 3mm single crystal block as an example, the maximum time difference of all couplings in a single crystal block will not exceed 100ps. This means that the resources required to solve the arrival time of each SIPM using relative time will be greatly reduced.

[0047] like Figure 2 The differential time chain architecture adopted in this embodiment includes an absolute time chain and multiple differential chains, and each differential clock chain corresponds to one SIPM.

[0048] The absolute time chain utilizes a traditional long-chain TDC (Time Division Counter) architecture. Based on the long-chain TDC structure, the delay of a carry chain (Carry8) is 30 ps. With 16 taps, the average delay per tap is approximately 2 ps. For example, with a 400 MHz system clock, 90 Carry8s are required to form a long chain. In addition to the long-chain TDC architecture, other high-performance TDC architectures are also available for time-to-digital conversion.

[0049] For the design of the differential chain, taking a 3mm single crystal block as an example, the time difference between the photons in the single crystal block reaching each SIPM is between [-100ps, 100ps]. Therefore, the number of CARRY8s in the differential chain is 6, which saves about 93% of resources compared to traditional time readout circuits.

[0050] In the energy information readout, since the main function of energy information is to assist time information in calibrating the deposition position, channel merging is adopted to obtain energy in order to reduce the number of output channels, reduce the data bandwidth, and simplify the complexity of the readout circuit.

[0051] Due to the conversion of light loss and light absorption, the closer the SIPM is to the photon deposition location, the greater the photon energy captured. Conversely, the SIPM with greater energy is closer to the photon deposition location, thus locating the photon deposition location.

[0052] Finally, the obtained time information can be used for accurate positioning. On the one hand, the use of a single crystal block architecture, and on the other hand, the use of a differential time chain to reduce circuit complexity and improve time measurement resolution, these improvements have greatly improved the spatial resolution of PET detectors.

[0053] This embodiment uses a single crystal block structure and a differential time chain readout architecture for time acquisition, thereby greatly simplifying the complexity of the readout circuit and the data bandwidth. As a result, the readout circuit complexity and data bandwidth are no longer limiting factors of the single crystal block, thereby greatly improving the spatial resolution of the PET detector.

[0054] Based on the above embodiment, the time-to-digital converter in this embodiment uses the system clock as the latch clock of the D flip-flop of each tap in the time-to-digital converter. The clock used by the differential chain comes from the output of the kth axis head in the time-to-digital converter, where k is determined according to the length of the differential chain.

[0055] When the number of CARRY8s in the differential chain is 6, k is 6.

[0056] On the basis of the above embodiment, the time-to-digital converter in this embodiment further includes a global clock buffer, and the global clock buffer (Global Buffer, BUFG) is used to reduce the delay from the k-th axis head to each trigger.

[0057] It should be noted that in order to minimize the delay from the kth tap of the absolute clock chain to each D flip-flop, a BUFG is needed and bound to the global clock tree to ensure that the latch clock delay of the differential chain is minimized.

[0058] Based on the above embodiment, in this embodiment, the length of the differential chain is determined according to the size of the single crystal block.

[0059] Taking a 3 mm single crystal block as an example, the time difference between photons in the single crystal block reaching each SIPM is between [-100 ps, ​​100 ps]. Therefore, the number of CARRY8s in the differential chain is 6, that is, the length of the differential chain is 6.

[0060] Based on the above embodiment, the differential chain in this embodiment further includes a fixed delay module, and the fixed delay module IDELAY is used to compensate for the wiring delay of the current pulse reaching the differential chain and the latch clock delay of the D flip-flop of the differential chain.

[0061] Due to the influence of internal layout and routing, the time difference between signals reaching the differential chain can be greater than 30ps, even reaching the nanosecond level. To address this issue, a fixed delay module, IDELAY, is embedded in the differential chain. Its maximum delay adjustment range is up to 2.1ns. This module compensates for the routing delay of signals reaching the differential chain and the latch clock delay of the differential chain's D flip-flops, ensuring that the signal time difference between each differential chain and the absolute time chain is within the range of [-100ps, 100ps].

[0062] Based on the above embodiment, the delay time compensation value of the fixed delay module in this embodiment is obtained by the following steps:

[0063] Inputting the same signal into the time-to-digital converter and the differential chain, and determining the product of the width and the number of carry chains in the time-to-digital converter;

[0064] If the number of D flip-flop latch outputs 1 of the differential chain is equal to the product, the delay time compensation value of the fixed delay module remains unchanged;

[0065] If the number of D flip-flop latch outputs 1 of the differential chain is less than the product, the delay time compensation value of the fixed delay module is reduced;

[0066] If the number of D flip-flop latch outputs 1 of the differential chain is greater than the product, the delay time compensation value of the fixed delay module increases.

[0067] This embodiment proposes an IDELAY automatic delay calibration method, such as Figure 3 As shown in the figure, the same signal is input to both the absolute delay chain and the differential chain. When the number of 1s in the differential chain D-type flip-flop latch outputs equals 8×k, where k is the number of differential chain CARRY8s, it is determined that the absolute clock signal and the differential chain signal arrive simultaneously, and the delay compensation value in IDLEAY is the preset value. If the number of 1s in the differential chain latch output is less than 8×k, the delay compensation value in IDLEAY is reduced; otherwise, the delay compensation value in IDLEAY is increased.

[0068] Based on the above embodiment, in this embodiment, IDELAY realizes the routing delay compensation of the signal line and the clock signal, so it can be ignored. The differential chain obtains the time information of the SIPM output current pulse through the following formula:

[0069] t d =t s +(d-8×k)×t tdc

[0070] Among them, t s is the absolute time output by the time-to-digital converter, d is the number of 1s output by the D flip-flop latch of the differential chain, k is the length of the differential chain, and t tdc is the average delay time of each tap in the time-to-digital converter.

[0071] On the basis of the above embodiments, multiple surfaces of the single crystal block in this embodiment are coupled with SIPMs.

[0072] In order to obtain more accurate positioning information, this embodiment uses a multi-edge coupling architecture to couple SIPMs to multiple surfaces of a single crystal block, using a multi-edge coupling approach to obtain accurate photon action depth information.

[0073] Based on the above embodiments, this embodiment further includes an energy readout circuit, the input end of which is connected to the output end of the SIPM, and the energy readout circuit is used to read the energy information of the SIPM output current pulse through channel merging;

[0074] The input end of the positioning module is also connected to the output end of the energy readout circuit. The positioning module is used to obtain the positioning information of the photon in the single crystal block according to the time information and the energy information.

[0075] This embodiment can not only obtain the scintillation pulse time information, but also obtain the pulse energy information.

[0076] In terms of energy information acquisition, since positioning can mainly rely on time information to obtain the accurate position, energy information is only used for auxiliary calibration. Therefore, the idea of ​​channel merging is adopted to obtain energy, so as to reduce the data bandwidth and readout circuit complexity, and further improve the spatial resolution of the PET detector.

[0077] On the basis of the above embodiments, the energy readout circuit in this embodiment is used to obtain the energy information by merging row and column data or merging adjacent channels.

[0078] Channel merging methods include merging and outputting the data of each row and each column, or merging and outputting adjacent channels. Other partitioning methods can also be used to reduce the number of output channels, reduce data bandwidth, and simplify the complexity of the readout circuit.

[0079] In order to simplify the energy readout circuit, taking the adjacent channel merge output as an example, the partition result is as follows Figure 4 As shown in the figure, the gray part is the single crystal block and the green part is the coupled SIPM. For example, if two adjacent SIPMs are grouped together and any one of them is selected as the energy readout, the energy readout circuit resources will be reduced by 50%.

[0080] Using the proposed high-resolution PET detector system based on differential time chains and continuous crystals, the detector resolution under this architecture can be calculated according to the spatial resolution calculation formula (speed of light / refractive index × time resolution) as follows.

[0081] 3×10 8 / 1.5×2×10 -12 =0.4mm

[0082] In summary, the present invention has the following advantages:

[0083] (1) A high-resolution PET detector architecture based on a differential time chain and continuous crystals can achieve a spatial resolution of less than 0.5 mm. The use of a single crystal block multi-edge coupling architecture replaces the traditional crystal cutting method, avoiding problems such as severe scattering and reduced sensitivity caused by edge deterioration.

[0084] (2) A new time readout circuit architecture is proposed, which is suitable for a simplified readout circuit for single crystal multi-sided coupling. Using the absolute time plus relative time method, only one complete TDC long chain is needed to measure the absolute time during measurement, and the rest can be used to obtain the relative time using a short differential chain. The use of a differential time chain greatly improves resource utilization. The length of the differential short chain is related to the size of the single crystal block. For example, for a 3mm differential short chain, only 6 CARRY8s are needed, which increases resource utilization by 93% and enables a time resolution of 2ps.

[0085] (3) Simplification of the energy readout circuit. Adjacent SIPMs can be partitioned or row-column outputs can be combined to effectively reduce the number of energy measurement channels, simplify the energy readout circuit, and reduce resource utilization. This enables the practical application of single-crystal multi-sided coupling in PET detector systems.

[0086] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A PET detector based on differential time chain and continuous crystal, characterized in that: include: a single crystal block into which gamma rays are incident, and energy is deposited inside the single crystal block, emitting photons, and a SIPM on a surface of the single crystal block absorbs the photons and responds by outputting a current pulse; a time readout circuit, wherein an input terminal of the time readout circuit is connected to an output terminal of the SIPM, and the time readout circuit is configured to obtain time information of the SIPM output current pulse using a differential time chain, wherein the differential time chain includes a time-to-digital converter and a differential chain, and the output of the time-to-digital converter serves as a latch clock of the differential chain; a positioning module, wherein an input end of the positioning module is connected to an output end of the time readout circuit, and the positioning module is used to obtain positioning information of the photon in the single crystal block according to the time information; The time-to-digital converter uses a system clock as a latch clock for a D flip-flop of each tap in the time-to-digital converter. The clock used by the differential chain comes from the output of the kth tap in the time-to-digital converter, where k is determined according to the length of the differential chain. The length of the differential chain is determined according to the size of the single crystal block.

2. The PET detector based on differential time chain and continuous crystal according to claim 1, characterized in that: The time-to-digital converter further includes a global clock buffer configured to reduce a delay from the kth tap to each flip-flop.

3. The PET detector based on differential time chain and continuous crystal according to claim 1, characterized in that: The differential chain further includes a fixed delay module, which is used to compensate for a wiring delay of the current pulse reaching the differential chain and a latch clock delay of a D flip-flop of the differential chain.

4. The PET detector based on differential time chain and continuous crystal according to claim 3, characterized in that: The delay time compensation value of the fixed delay module is obtained by the following steps: Inputting the same signal into the time-to-digital converter and the differential chain, and determining the product of the width and the number of carry chains in the time-to-digital converter; If the number of D flip-flop latch outputs 1 of the differential chain is equal to the product, the delay time compensation value of the fixed delay module remains unchanged; If the number of D flip-flop latch outputs 1 of the differential chain is less than the product, the delay time compensation value of the fixed delay module is reduced; If the number of D flip-flop latch outputs 1 of the differential chain is greater than the product, the delay time compensation value of the fixed delay module increases.

5. The PET detector based on differential time chain and continuous crystal according to claim 4, characterized in that: The differential chain obtains the time information of the SIPM output current pulse through the following formula: ； in, is the absolute time output by the time-to-digital converter, d is the number of 1s output by the D flip-flop latch of the differential chain, k is the length of the differential chain, is the average delay time of each tap in the time-to-digital converter.

6. The PET detector based on differential time chain and continuous crystal according to any one of claims 1 to 5, characterized in that: SIPMs are coupled to a plurality of surfaces of the single crystal block.

7. The PET detector based on differential time chain and continuous crystal according to any one of claims 1 to 5, characterized in that: It also includes an energy readout circuit, wherein the input end of the energy readout circuit is connected to the output end of the SIPM, and the energy readout circuit is used to read the energy information of the SIPM output current pulse through channel merging; The input end of the positioning module is also connected to the output end of the energy readout circuit. The positioning module is used to obtain the positioning information of the photon in the single crystal block according to the time information and the energy information.

8. The PET detector based on differential time chain and continuous crystal according to claim 7, characterized in that: The energy readout circuit is used to obtain the energy information by combining row and column data or combining adjacent channels.

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