System and method for improving counting rate of TOF-PET system

Abstract

The invention discloses a system and a method for improving the counting rate of a TOF-PET (time of flight-positron emission tomography) system. The system comprises a data processing module and a plurality of acquisition units, wherein each acquisition unit comprises a detector module and an electronics module; the detector module is used for detecting gamma photon pairs generated by positron annihilation in a detected object, converting nuclear signals generated according to the detected gamma photon pairs into electric signals and inputting the electric signals into the electronics module; the electronics module screens the input electric signals to obtain real coincidence cases and collects the real coincidence cases; the data processing module is used for taking the received two real coincidence cases in the same coincidence time window as a pair of real coincidence cases, and calculating the time difference between the two real coincidence cases in each pair of real coincidence cases and the corresponding detector module; and determining the position of the detected object according to the statistical result of the time difference of each pair of real coincidence cases. The theoretical effective counting rate can be increased by several times or even one magnitude, and the sensitivity of the system is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of positron emission tomography (PET) imaging technology, and in particular to a system and method for improving the count rate of a TOF-PET system by utilizing electronic hardware-level pre-coincidence triggering. Background Art

[0002] Positron emission tomography (PET) is an advanced medical imaging technique that uses the physical properties of positron-emitting radioisotopes to achieve dynamic imaging of molecular interactions and metabolic activities in vivo. PET has been widely used in the diagnosis, monitoring and research of diseases, including cancer, cardiovascular disease and nervous system diseases. In addition to improving sensitivity, the introduction of time-of-flight (TOF) information can significantly improve the performance of PET systems. This is because the temporal distribution of annihilation photons is proportional to the spatial uncertainty of the annihilation position. Therefore, higher temporal resolution means more accurate annihilation point positioning and better image signal-to-noise ratio. The requirements of PET systems for high sensitivity and high temporal resolution place strict requirements on the number of channels, count rate and time accuracy of the readout electronics.

[0003] With the advantages of short dead time, low power consumption and high integration, the time measurement technology based on the timing discrimination circuit combined with high-precision TDC has become the first choice for the design of readout electronics in TOF-PET systems. This time measurement scheme converts the amplified nuclear signal into a fast-front pulse signal through a discriminator (comparator), etc., and then inputs the TDC to obtain the time information of the pulse arrival. However, the time performance of this technical solution is mainly limited by the discrimination module. The timing accuracy is related to the amplitude and time of the rising edge. There is a time walk effect. The timing accuracy is usually around 20ps, and the counting rate is in the order of tens of kcps to hundreds of kcps.

[0004] Another high-precision time measurement solution is based on waveform digitization technology. Compared with high-speed ADC (sampling rate>1GSPS), the waveform sampling chip based on switched capacitor array technology (SCA) combined with the waveform sampling solution of ADC, through the combination of high-speed analog sampling and slow readout, retains the physical information such as time and energy in the original nuclear signal while reducing power consumption, and the timing accuracy can be less than 10ps. However, the slow readout process brings a long dead time, and the counting rate is usually in the order of several kcps to tens of kcps, which limits its application in high counting rate scenarios such as TOF-PET. At present, there is relatively little research on improving the counting rate of this technical solution. Only some research teams have increased the counting rate to twice the original through simulation using the ping-pong architecture, but this also brings double the hardware cost. Summary of the invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to provide a system and method for improving the count rate of a TOF-PET system by using electronic pre-coincidence triggering. By adopting the electronic hardware-level pre-coincidence triggering design, the effective count rate of the SCA+ADC waveform sampling electronics is improved.

[0006] The technical solution of the present invention is:

[0007] A method for improving the count rate of a TOF-PET system, wherein the TOF-PET system includes a data processing module and a plurality of acquisition units, each of which includes a TOF-PET detector module and a TOF-PET electronics module; the steps include:

[0008] 1) Using the TOF-PET detector module to detect the gamma photon pairs generated by the annihilation of positrons in the detected object, and converting the nuclear signals generated by the detected gamma photon pairs into electrical signals and inputting them into the TOF-PET electronics module of the acquisition unit;

[0009] 2) Each of the TOF-PET electronic modules screens the input electrical signals, and when the energy of the nuclear signal meets the set energy screening condition, generates a trigger signal and sends it to each of the acquisition units; when the i-th acquisition unit receives the trigger signal generated by the TOF-PET electronic module in the i-th acquisition unit and the trigger signal generated by the TOF-PET electronic module in other acquisition units in the t-th coincidence time window, the TOF-PET electronic module in the i-th acquisition unit collects the nuclear signal in the t-th coincidence time window as a true coincidence case;

[0010] 3) The data processing module receives the real coincidence events collected by the TOF-PET electronics module, and regards two real coincidence events within the same coincidence time window as a pair of real coincidence events; extracts the time and energy information in each pair of real coincidence events, and calculates the time difference between the two real coincidence events in each pair of real coincidence events arriving at their corresponding TOF-PET detector modules, counts the time differences of each pair of real coincidence events, and determines the position of the detected object based on the statistical results.

[0011] Furthermore, each of the TOF-PET detector modules is connected in series, and the last-level TOF-PET detector module is connected to the data processing module; each of the TOF-PET electronics modules includes a comparator, a waveform sampling circuit and an FPGA; the comparator is used to perform energy screening on the input nuclear signal, and when the energy of the nuclear signal meets the set energy screening condition, a trigger signal is generated and sent to the FPGA in each acquisition unit; the waveform sampling circuit is used to acquire the input nuclear signal to obtain a coincidence signal and send it to the FPGA; when the FPGA receives the trigger signal generated by the comparator in its acquisition unit and the trigger signal generated by other acquisition units in the i-th coincidence time window, it retains the coincidence signal in the i-th coincidence time window and sends it as a true coincidence case to the next-level TOF-PET electronics module; the last-level TOF-PET detector module sends the received true coincidence case to the data processing module.

[0012] Furthermore, the waveform sampling circuit includes a DRS4 chip and an ADC; the DRS4 chip is used to perform high-speed analog sampling on the input nuclear signal and then input it into the ADC; the ADC is used to digitize the input analog signal to obtain a digitized waveform.

[0013] Furthermore, adjacent acquisition units are connected via high-bandwidth cables for transmitting the trigger signal; the FPGA determines whether the received trigger signal is within the same matching time window via an AND gate.

[0014] Furthermore, the data processing module calculates the position of positron annihilation according to the time t1 when one gamma photon arrives at the TOF-PET detector module and the time t2 when the other gamma photon arrives at the TOF-PET detector module in each pair of true coincidence cases. Where c is the speed of light.

[0015] A system for improving the count rate of a TOF-PET system, characterized in that it comprises a data processing module and a plurality of acquisition units, each of the acquisition units comprising a TOF-PET detector module and a TOF-PET electronics module;

[0016] The TOF-PET detector module is used to detect the gamma photon pairs generated by the annihilation of positrons in the detected object, and convert the nuclear signals generated by the detected gamma photon pairs into electrical signals and input them into the TOF-PET electronics module of the acquisition unit;

[0017] The TOF-PET electronics module is used to screen the input electrical signals, and when the energy of the nuclear signal meets the set energy screening conditions, a trigger signal is generated and sent to each of the acquisition units; wherein when the i-th acquisition unit receives the trigger signal generated by the TOF-PET electronics module in the i-th acquisition unit and the trigger signal generated by the TOF-PET electronics module in other acquisition units in the t-th coincidence time window, the TOF-PET electronics module of the i-th acquisition unit collects the nuclear signal in the t-th coincidence time window as a true coincidence case;

[0018] The data processing module is used to receive the real coincidence events collected by the TOF-PET electronics module, and regard two real coincidence events within the same coincidence time window as a pair of real coincidence events; extract the time and energy information in each pair of real coincidence events, and calculate the time difference between the two real coincidence events in each pair of real coincidence events when they reach their corresponding TOF-PET detector modules, count the time differences of each pair of real coincidence events, and determine the position of the detected object based on the statistical results.

[0019] The advantages of the present invention are as follows:

[0020] Through the electronic hardware-level pre-coincidence trigger design and sampling control, efficient screening and readout of effective cases are achieved. The collected cases are all real cases that have already matched. This can avoid the increase of dead time in the readout and processing of invalid cases in the time measurement route based on SCA+ADC, and greatly reduce the pressure on the host computer to process data. Compared with traditional systems without integrated pre-coincidence trigger design, the theoretical effective count rate can be increased by several times or even an order of magnitude, greatly improving the sensitivity of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a system structure diagram of the present invention.

[0022] Figure 2 This is the structural schematic diagram of the TOF-PET electronics module. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention but not to limit the scope of the present invention.

[0024] The present invention relates to a method for improving the count rate and realizing high sensitivity of TOF-PET based on electronic pre-coincidence trigger design. The embodiments of the present invention are as follows:

[0025] 1) The entire TOF-PET system consists of three parts: TOF-PET detector module, TOF-PET electronics module and data processing module. Among them, the TOF-PET electronics module is a waveform sampling electronics board. Figure 2 The connection relationship between two detector modules and the data processing module of the electronics module is demonstrated. The TOF-PET detector module is used to detect gamma photons produced by positron annihilation and generate corresponding detection signals. Its specific structure and composition depend on the requirements of different PET systems.

[0026] 2) The waveform sampling electronics board is directly connected to the detector module to receive and extract the physical information in the nuclear signal generated by the gamma photon, including time, energy, etc. In addition, the electronics system is also responsible for judging the matching signal. Figure 2 As shown, the entire waveform sampling electronics board consists of three parts: a comparator, a waveform sampling circuit, and a data processing unit FPGA. The waveform sampling circuit is composed of a DRS4 chip based on SCA technology and an ADC, which can provide several to hundreds of analog sampling channels with a sampling rate of the order of GSPS. The signal output by the front-end detector is first amplified and shaped by the front-end electronics, and then enters the DRS4 chip through a differential amplifier circuit for high-speed analog sampling. After the sampling process is completed, the retained charge of the internal sampling unit is read out to the external ADC at a rate of 33MHz for digitization. The digitized waveform enters the FPGA to extract physical information such as time and energy, and finally the data is aggregated and transmitted to the next-level board through SerDes ( Figure 2 The waveform sampling board 1 transmits the data to the waveform sampling board 2), and the last board aggregates the data of all boards and transmits it to the data processing module via 10 Gigabit Ethernet. In addition, FPGA is also responsible for the logic control of DRS4 and ADC, and provides the clock required by DRS4 and ADC.

[0027] 3) Figure 2 The specific design and method of using electronic hardware-level pre-coincidence trigger to improve system sensitivity are as follows:

[0028] S1: The detector nuclear signal is sampled by the input waveform sampling electronics board and is also input into the threshold comparator. When the amplitude of the nuclear signal is higher than the comparison threshold set by the comparator, that is, when the energy screening condition is met, the generated over-threshold pulse signal will be input into the FPGA as a trigger signal. In this process, the comparator threshold can be adjusted online by the FPGA.

[0029] S2: If the detector module and the electronics module that match it also generate trigger signals and the two trigger signals are within the same matching time window, then the pair of matching signals is a true matching event. The time window is selected by the AND gate constructed by the FPGA. The trigger signal is transmitted through the cable between the waveform sampling electronics board.

[0030] S3: True coincidence events that meet both the time and energy screening requirements will generate a trigger signal. The trigger signal is transmitted on the cable between the boards, and the electronics boards corresponding to the coincidence detectors use FPGA to control the DRS4 chip to collect the pair of coincidence events. In this process, scattered events, non-conforming events, etc. are eliminated, and the electronics board collects only confirmed events. The increase in count rate mainly lies in the increase in effective coincidence events:

[0031] Assuming that the detection efficiencies of a pair of coincidence detector modules are DE1 and DE2 respectively, when the electronic dead time is fixed and the network bandwidth for data transmission reaches its peak value, the number of events transmitted to the data processing module per unit time is SE. In the absence of electronic pre-trigger design,

[0032] SE=SA×(DE1+DE2) (1)

[0033] Where SA is the SA pair of gamma rays generated by the annihilation of the radiation source within the solid angle of a pair of coincidence detection modules. At this time, the theoretical coincidence count rate

[0034] CR1=SA×DE1×DE2 (2)

[0035] From formulas (1) and (2), we can see that:

[0036]

[0037] After adding the pre-coincidence trigger design, since the pair of coincidence detection modules collect only coincidence events, the theoretical coincidence event rate CR2 is:

[0038]

[0039] Compared with the design without pre-coincidence trigger, the coincidence count rate of the system with pre-coincidence trigger design is theoretically improved by:

[0040]

[0041] The detection efficiencies DE1 and DE2 are mainly related to the type and length of the scintillation crystals in the detector module, and their values ​​are less than 1.

[0042] S4: The collected digital coincidence signals are aggregated and packaged by the Gigabit Ethernet module of the waveform sampling electronics board and then transmitted to the data processing module. The data processing module performs statistical calculations on the time difference information of the coincidence cases and finally obtains the location of the object to be measured. The increase in the counting rate ensures the rapid realization of statistics and improves the sensitivity of the system.

[0043] Although the specific embodiments of the present invention are disclosed for the purpose of illustration, the purpose is to help understand the content of the present invention and implement it accordingly, those skilled in the art will understand that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the best embodiment, and the scope of the present invention is subject to the scope defined in the claims.

Claims

1. A method for improving the count rate of a TOF-PET system, wherein the TOF-PET system comprises a data processing module and a plurality of acquisition units, each of the acquisition units comprises a TOF-PET detector module and a TOF-PET electronics module; the steps comprising: 1) Using the TOF-PET detector module to detect the gamma photon pairs generated by the annihilation of positrons in the detected object, and converting the nuclear signals generated by the detected gamma photon pairs into electrical signals and inputting them into the TOF-PET electronics module of the acquisition unit; 2) Each of the TOF-PET electronic modules screens the input electrical signals, and when the energy of the nuclear signal meets the set energy screening condition, generates a trigger signal and sends it to each of the acquisition units; when the i-th acquisition unit receives the trigger signal generated by the TOF-PET electronic module in the i-th acquisition unit and the trigger signal generated by the TOF-PET electronic module in other acquisition units within the compliance time window, the TOF-PET electronic module of the i-th acquisition unit collects the nuclear signal within the t-th compliance time window as a true compliance case; 3) The data processing module receives the real coincidence events collected by the TOF-PET electronics module, and regards two real coincidence events within the same coincidence time window as a pair of real coincidence events; extracts the time and energy information in each pair of real coincidence events, and calculates the time difference between the two real coincidence events in each pair of real coincidence events arriving at their corresponding TOF-PET detector modules, counts the time differences of each pair of real coincidence events, and determines the position of the detected object based on the statistical results.

2. The method according to claim 1, characterized in that The TOF-PET detector modules are connected in series, and the last-stage TOF-PET detector module is connected to the data processing module; each TOF-PET electronics module includes a comparator, a waveform sampling circuit and an FPGA; the comparator is used to perform energy screening on the input nuclear signal, and when the energy of the nuclear signal meets the set energy screening condition, a trigger signal is generated and sent to the FPGA in each acquisition unit; the waveform sampling circuit is used to collect the input nuclear signal to obtain a coincidence signal and send it to the FPGA; when the FPGA receives the trigger signal generated by the comparator in the acquisition unit where it is located and the trigger signal generated by other acquisition units in the i-th coincidence time window, it retains the coincidence signal in the i-th coincidence time window and sends it as a true coincidence case to the next-stage TOF-PET electronics module; the last-stage TOF-PET detector module sends the received true coincidence case to the data processing module.

3. The method according to claim 2, characterized in that The waveform sampling circuit includes a DRS4 chip and an ADC; the DRS4 chip is used to perform high-speed analog sampling on the input nuclear signal and then input it into the ADC; the ADC is used to digitize the input analog signal to obtain a digitized waveform.

4. The method according to claim 2, characterized in that: The adjacent acquisition units are connected via high-bandwidth cables for transmitting the trigger signal; the FPGA determines whether the received trigger signal is within the same matching time window via an AND gate.

5. The method according to claim 1, characterized in that The data processing module calculates the position of positron annihilation based on the time t1 when one gamma photon arrives at the TOF-PET detector module and the time t2 when the other gamma photon arrives at the TOF-PET detector module in each pair of true coincidence cases. Where c is the speed of light.

6. A system for improving the count rate of a TOF-PET system, characterized in that: It includes a data processing module and a plurality of acquisition units, each of which includes a TOF-PET detector module and a TOF-PET electronics module; The TOF-PET detector module is used to detect the gamma photon pairs generated by the annihilation of positrons in the detected object, and convert the nuclear signals generated by the detected gamma photon pairs into electrical signals and input them into the TOF-PET electronics module of the acquisition unit; The TOF-PET electronics module is used to screen the input electrical signals, and when the energy of the nuclear signal meets the set energy screening conditions, a trigger signal is generated and sent to each of the acquisition units; wherein when the i-th acquisition unit receives the trigger signal generated by the TOF-PET electronics module in the i-th acquisition unit and the trigger signal generated by the TOF-PET electronics module in other acquisition units in the t-th coincidence time window, the TOF-PET electronics module of the i-th acquisition unit collects the nuclear signal in the t-th coincidence time window as a true coincidence case; The data processing module is used to receive the real coincidence events collected by the TOF-PET electronics module, and regard two real coincidence events within the same coincidence time window as a pair of real coincidence events; extract the time and energy information in each pair of real coincidence events, and calculate the time difference between the two real coincidence events in each pair of real coincidence events when they reach their corresponding TOF-PET detector modules, count the time differences of each pair of real coincidence events, and determine the position of the detected object based on the statistical results.

7. The system according to claim 6, characterized in that The TOF-PET detector modules are connected in series, and the last-stage TOF-PET detector module is connected to the data processing module; each TOF-PET electronics module includes a comparator, a waveform sampling circuit and an FPGA; the comparator is used to perform energy screening on the input nuclear signal, and when the energy of the nuclear signal meets the set energy screening condition, a trigger signal is generated and sent to the FPGA in each acquisition unit; the waveform sampling circuit is used to collect the input nuclear signal to obtain a coincidence signal and send it to the FPGA; when the FPGA receives the trigger signal generated by the comparator in the acquisition unit where it is located and the trigger signal generated by other acquisition units in the i-th coincidence time window, it retains the coincidence signal in the i-th coincidence time window and sends it as a true coincidence case to the next-stage TOF-PET electronics module; the last-stage TOF-PET detector module sends the received true coincidence case to the data processing module.

8. The system according to claim 7, characterized in that The waveform sampling circuit includes a DRS4 chip and an ADC; the DRS4 chip is used to perform high-speed analog sampling on the input nuclear signal and then input it into the ADC; the ADC is used to digitize the input analog signal to obtain a digitized waveform.

9. The system according to claim 7, characterized in that The adjacent acquisition units are connected via high-bandwidth cables for transmitting the trigger signal; the FPGA determines whether the received trigger signal is within the same matching time window via an AND gate.

10. The system according to claim 6, characterized in that The data processing module calculates the position of positron annihilation based on the time t1 when one gamma photon arrives at the TOF-PET detector module and the time t2 when the other gamma photon arrives at the TOF-PET detector module in each pair of true coincidence cases. Where c is the speed of light.

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