PET detector based on differential time chain and continuous crystal

Abstract

The invention provides a PET detector based on a differential time chain and continuous crystals, which comprises a single crystal block, gamma rays enter the single crystal block, energy deposition is carried out in the single crystal block, photons are radiated, and SIPM on the surface of the single crystal block responds after absorbing the photons and outputs current pulses; the input end of the time reading circuit is connected with the output end of the SIPM, the time reading circuit is used for acquiring time information of current pulse output by the SIPM by using a differential time chain, the differential time chain comprises 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; and the input end of the positioning module is connected with the output end of the time reading circuit, and the positioning module is used for acquiring the positioning information of the photons in the single crystal block according to the time information. According to the invention, the PET detector based on the differential time chain and the continuous crystal is realized, and the spatial resolution of the PET detector is greatly improved.

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 resolution of the PET system for two adjacent locations in space. The hardware architecture of the PET system and the software algorithm for positioning will affect the quality of spatial resolution. The spatial resolution of the PET detector directly affects the quality of image reconstruction and the accuracy of lesion detection and positioning in the later stage. Therefore, improving the spatial resolution of the PET system has always been one of the hot topics of discussion.

[0003] In terms of hardware structure, a lot of research has been done and good spatial resolution has been achieved. Spatial resolution is mainly affected by two factors. One is the size of the crystal. A crystal as a pixel directly affects the size of the PET spatial resolution. Generally speaking, the smaller the crystal size, the higher the spatial resolution. The other is the deposition position of the photon. The deposition position of the photon in the crystal is random. There are errors in positioning the photon on the crystal surface, which causes image artifacts and reduces the spatial resolution. In this regard, in order to improve the spatial resolution of PET, the following solutions are proposed.

[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 defects. As the crystal size decreases, the probability of scattered emission will increase, the sensitivity of the PET detector will decrease, and the edge deterioration will be obvious. Higher requirements are placed on the crystal process, and at the same time, the resources, power consumption, and area consumed by the peripheral circuit 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, and the signal quality will decrease. These will reduce the spatial resolution of the PET detector. Therefore, the improvement in spatial resolution brought about by cutting the crystal is limited. At present, the spatial resolution that can be obtained using this strategy is 0.55mm.

[0005] The spatial resolution can be improved by reducing image artifacts through DOI (Depth of Interaction) information. This improvement method is also limited, and its spatial resolution is difficult to exceed 0.5mm. In order to obtain DOI information, additional resources are generally required. Whether it is double-end readout or long-edge coupling, the readout circuit resources will increase. If a double-end readout method is adopted, the long edge coupling of the crystal replaces the short edge coupling to obtain more DOI information. Its main disadvantage is that the complexity of the readout circuit will increase. In long-axis PET, the resource consumption brought by thousands of crystals is immeasurable, and it is difficult to apply it to the actual PET system at one time.

[0006] Compared with discrete single crystals, the single crystal block structure can effectively solve the problems brought by discrete crystals, has better sensitivity and detection efficiency, and has greater potential in achieving higher spatial resolution. By coupling multiple SIPM arrays on the surface of a single crystal block, each SIPM is equivalent to a pixel, which determines the size of the spatial resolution. Through multilateral coupling, more accurate DOI information can be obtained and image quality can be improved. However, there are some limitations in using single crystal blocks to obtain high spatial resolution. The increase in the number of SIPM couplings will lead to an increase in readout circuits, and each channel requires a separate signal readout circuit. In addition, when photons enter the single crystal block, due to light diffusion, the SIPM coupled on the surface of a single crystal block will respond, which puts higher requirements on the data bandwidth. Whether the data bandwidth is reduced by channel merging or reducing the number of channels, the spatial resolution will be deteriorated. Some scholars have proposed to only consider the SIPM response within the 15% energy range near 511kev, or adopt the idea of ​​channel merging to add and sum the SIPMs of rows and columns before outputting, thereby reducing the data bandwidth, but sacrificing some effective data details, and its spatial resolution is also reduced. In addition, the complexity of the readout circuit will increase, especially when multiple coupling is used, which limits the application of single crystal block architecture in PET detectors.

[0007] Although the methods currently used can improve the spatial resolution of the PET system 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, so as to solve the defect that the spatial resolution of the PET detector in the prior art is limited, 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 the input end of the time readout circuit is connected to the output end of the SIPM, and the time readout circuit is used to obtain the time information of the SIPM output current pulse using a differential time chain, wherein the differential time chain includes a time digital converter and a differential chain, and the output of the digital converter is used as a latch clock of the differential chain;

[0012] A positioning module, the input end of which 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 a PET detector based on a differential time chain and a continuous crystal provided by the present invention, the time digital converter uses a system clock as a latch clock of a D flip-flop of each tap in the time digital converter, and the clock used by the differential chain comes from the output of the kth axis head in the time digital converter, where k is determined according to 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, and the global clock buffer is used to reduce the delay from the kth axis head to each trigger.

[0015] According to a 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 through 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 a PET detector based on a differential time chain and a continuous crystal provided by the present invention, multiple surfaces of the single crystal block are coupled with SIPMs.

[0026] The input end of the positioning module is also connected to the output end of the energy 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 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 differential time chain and continuous crystal provided by the present invention greatly simplifies the complexity of the readout circuit and the data bandwidth by utilizing the differential time chain readout architecture in time acquisition, so that the complexity of the readout circuit and the data bandwidth are no longer limiting factors of the single crystal block; in order to obtain more accurate positioning information, a multi-edge coupling architecture is used, and SIPM is coupled to multiple surfaces of the single crystal block; in terms of 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 idea of ​​channel merging is adopted to acquire energy, so as to reduce the data bandwidth and the complexity of the readout circuit, 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, the following briefly introduces 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 creative work.

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

[0031] Figure 2 It is a schematic diagram of the differential time chain structure used by the PET detector based on the differential time chain and the 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 schematic diagram of merging and partitioning two adjacent channels for energy measurement in a PET detector based on a differential time chain and a continuous crystal provided by the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] PET is an imaging technology that can display the metabolic level of biological molecules in vivo. It plays an important role in the diagnosis of diseases such as cancer, tumors, and Alzheimer's disease. The resolution of PET detectors is a key factor in determining imaging quality. It determines whether the imaged object can be "clearly seen" and "accurately seen". Clearer imaging quality makes it possible to image fine structures inside the human body separately. At the same time, it can also promote early screening of diseases and reduce the probability of missed diagnosis. Although the spatial resolution of PET detectors has been greatly improved with the development of process technology, 0.5mm is still the threshold of PET detectors. Therefore, it is important to study how to improve the resolution of PET detectors.

[0036] Combine the following Figures 1 to 4The PET detector based on differential time chain and continuous crystal of the present invention is described, 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 the input end of the time readout circuit is connected to the output end of the SIPM, and the time readout circuit is used to obtain the time information of the SIPM output current pulse using a differential time chain, wherein the differential time chain includes a time digital converter and a differential chain, and the output of the digital converter is used as a latch clock of the differential chain;

[0039] A positioning module, the input end of which 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, through the research in recent years, crystal cutting can no longer meet higher spatial resolution. Therefore, this embodiment uses a more potential single crystal block structure.

[0042] Figure 1 The figure shows the single crystal block structure used in PET detectors. The white part of the structure is the single crystal block used, and the green part is the coupled SIPM. The single crystal block adopts single-sided or multi-sided coupling. Figure 1 (a) is the bottom coupling method, (b) is the double side coupling method, (c) is the bottom and top coupling method, and (d) is the full-cover coupling method. Figure 1 In addition to the coupling method shown, other coupling methods with more edges can also be used to obtain the best spatial resolution. It can be seen that 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 the single crystal block, which can be injected from above the single crystal block and deposited in the single crystal block, emitting visible light photons that are then received by the SiPM. SiPM is a solid-state high-gain radiation detector. The sensor based on the PN junction has single-photon sensitivity and generates a current pulse after absorbing a photon.

[0044] Secondly, in order 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 according to the SIPM response characteristics of a single crystal block. Under this architecture, only a global time digital converter and a multi-end differential chain are needed to complete the characterization of the photon arrival time of a single crystal block. Compared with traditional crystal time readout, its resources can be saved by up to 98%. The data format is output in a global clock plus absolute clock mode, and the number of data bits is greatly reduced compared to the traditional mode, and the data bandwidth is reduced. In addition, the time resolution of the differential time chain can reach 2ps. Accurate sampling of time is achieved. The differential-based time chain architecture successfully solves the problems of the complexity and bandwidth of the single crystal block readout circuit.

[0045] Figure 1 The deposition position and action depth of the photons are shown in red. The SIPM close to the deposition position of the photons will capture the visible light first and respond first. Conversely, the deposition position and action depth of the photons in the single crystal block 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 farthest SIPMs to the same γ photon will not exceed 100ps. This is the theoretical basis for the realization of differential time chain.

[0046] According to the response characteristics of the coupled SIPMs in a single crystal block to γ ​​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, which 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] Among them, the absolute time chain adopts the traditional long-chain TDC (Time Division Counter, time digital converter) architecture. According to the long-chain TDC structure conditions, the delay time of a carry chain CARRY8 is 30ps, and it has a total of 16 taps, and the average delay of each tap is about 2ps. Taking the system clock of 400MHZ as an example, 90 CARRY8s are required to form a long chain. In addition to using the long-chain TDC structure, other high-performance TDC architectures can also be selected to complete time 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], so the number of CARRY8 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 the time information in calibrating the deposition position, channel merging is used 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 position, the greater the photon energy captured. Conversely, the SIPM with greater energy is closer to the photon deposition position, thus locating the photon deposition position.

[0052] Finally, the obtained time information can be used for accurate positioning. On the one hand, the single crystal block architecture is adopted, and on the other hand, the circuit complexity is reduced through the differential time chain, and the time measurement resolution is improved. These improvements have greatly improved the spatial resolution of the PET detector.

[0053] This embodiment uses a single crystal block structure and a differential time chain readout architecture for time acquisition, which greatly simplifies the complexity of the readout circuit and the data bandwidth, so that the complexity of the readout circuit and the 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 embodiments, the time digital converter in this embodiment uses the system clock as the latch clock of the D flip-flop of each tap in the time digital converter, and the clock used by the differential chain comes from the output of the kth axis head in the time 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 kth 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] On the basis of the above-mentioned embodiment, in this embodiment, the length of the differential chain is determined according to the size of the single crystal block.

[0059] 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], so 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 of the signal reaching the differential chain will be greater than 30ps, even reaching the nanosecond level. In order to solve this problem, a fixed delay module IDELAY is embedded in the differential chain, and its maximum delay adjustment range can reach 2.1ns, which is used to compensate for the wiring delay of the signal reaching the differential chain and the latch clock delay of the D flip-flop of the differential chain, ensuring that the signal time difference between each differential chain and the absolute time chain is within the range of [-100ps, 100ps].

[0062] On the basis of 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. Input the same signal into the absolute delay chain and the differential chain. When the number of 1s in the differential chain D flip-flop latch output is equal to 8×k, where k is the number of differential chain CARRY8s. At this time, it is determined that the absolute clock signal and the differential chain signal arrive at the same time, and the delay time compensation value of IEDLEAY is the preset value. If the number of 1s in the differential chain latch output is less than 8×k, the delay time compensation value in IDELAY is reduced, otherwise the delay time compensation value of IDLEAY is increased.

[0068] Based on the above embodiment, in this embodiment, since IDELAY realizes the routing delay compensation of the signal line and the routing delay compensation of the clock signal, 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 on multiple surfaces of a single crystal block, and adopts a multi-edge coupling method to obtain accurate depth information of photons.

[0073] On the basis of the above embodiments, this embodiment further includes an energy readout circuit, 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 out 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, and 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 exact position, and energy information is only used for auxiliary calibration, the idea of ​​channel merging is adopted to acquire energy in order 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] The channel merging method includes 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, the gray part is a single crystal block, and the green part is a coupled SIPM. For example, if two adjacent SIPMs are grouped together and any one of the two SIPMs is selected as energy readout, the energy readout circuit resources will be reduced by 50%.

[0080] Using the high-resolution PET detector system based on differential time chain and continuous crystal proposed above, according to the spatial resolution calculation formula (speed of light / refractive index×time resolution), the detector resolution under this architecture can be obtained 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) The high-resolution PET detector architecture based on differential time chain and continuous crystal can achieve a spatial resolution of less than 0.5 mm. The single crystal block multi-sided coupling architecture replaces the traditional crystal cutting method, avoiding the problems of severe scattering and reduced sensitivity caused by edge deterioration.

[0084] (2) A new time readout circuit architecture is proposed, which is suitable for the simplified readout circuit of single crystal multi-sided coupling. The absolute time plus relative time method is adopted. When measuring, only one complete TDC long chain is needed to measure the absolute time, and the rest can use the short differential chain to obtain the relative time. The use of differential time chain greatly improves resource utilization. The length of the differential short chain is related to the size of the single crystal block. Taking 3mm as an example, a differential short chain only needs 6 CARRY8s, and the resource utilization is improved by 93%, and the time resolution can reach 2ps.

[0085] (3) Simplification of energy readout circuit. Adjacent SIPMs are partitioned or row and column outputs are combined to effectively reduce the number of energy measurement channels, simplify the energy readout circuit, and reduce resource utilization. This enables single crystal block multi-side coupling to be actually applied 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 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 the input end of the time readout circuit is connected to the output end of the SIPM, and the time readout circuit is used to obtain the time information of the SIPM output current pulse using a differential time chain, wherein the differential time chain includes a time digital converter and a differential chain, and the output of the digital converter is used as a latch clock of the differential chain; A positioning module, the input end of which 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.

2. The PET detector based on differential time chain and continuous crystal according to claim 1, characterized in that: The time-to-digital converter uses the system clock as the latch clock of the D flip-flop of each tap in the time-to-digital converter, and 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.

3. The PET detector based on differential time chain and continuous crystal according to claim 2, characterized in that: The time-to-digital converter further comprises a global clock buffer, wherein the global clock buffer is used to reduce the delay from the kth axis head to each trigger.

4. The PET detector based on differential time chain and continuous crystal according to claim 2, characterized in that: The length of the differential chain is determined according to the size of the single crystal block.

5. 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 arriving at the differential chain and a latch clock delay of a D flip-flop of the differential chain.

6. The PET detector based on differential time chain and continuous crystal according to claim 5, 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 latched outputs 1 of the D flip-flop 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.

7. The PET detector based on differential time chain and continuous crystal according to claim 6, characterized in that: The differential chain obtains the time information of the SIPM output current pulse through the following formula: t d =t s +(d-8×k)×t tdc 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.

8. The PET detector based on differential time chain and continuous crystal according to any one of claims 1 to 7, characterized in that: SIPMs are coupled to a plurality of surfaces of the single crystal block.

9. The PET detector based on differential time chain and continuous crystal according to any one of claims 1 to 7, characterized in that: It also includes an energy readout circuit, 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 out 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, and 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.

10. The PET detector based on differential time chain and continuous crystal according to claim 9, 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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