CN112932515B - Time correction method for TOF-PET - Google Patents

Time correction method for TOF-PET Download PDF

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CN112932515B
CN112932515B CN202110125737.6A CN202110125737A CN112932515B CN 112932515 B CN112932515 B CN 112932515B CN 202110125737 A CN202110125737 A CN 202110125737A CN 112932515 B CN112932515 B CN 112932515B
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赵晓坤
张勇
叶宏伟
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Minfound Medical Systems Co Ltd
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Abstract

The invention provides a time correction method for TOF-PET, which comprises the steps of firstly calculating time deviation among detector sub-modules by using coincidence data to obtain a time correction table of the time correction table, substituting original data to correct time information, and then calculating the time deviation among pixels on the basis of the time correction table to obtain a total time correction table. The shell source size corresponds to the FOV and can cover two or more sub-modules; the size of a large-size shell source is not needed, and the method is suitable for the condition that time deviation exists between detectors or electronics; and all LORs of the whole ring can be corrected, the calculation efficiency is higher, the iterative convergence is faster, and the obtained time precision is higher.

Description

Time correction method for TOF-PET
Technical Field
The invention relates to the technical field of positron emission computed tomography imaging, in particular to a time correction method for TOF-PET.
Background
Positron Emission Tomography (PET) imaging system is a nuclear medicine imaging device, and by injecting radioactive tracer drug into human body, the drug can produce a certain biological process with specific cell or tissue in human body, and by detecting a pair of gamma rays produced by radioactive nuclide decay, so as to obtain distribution diagram of radioactive tracer drug in human body and implement tomographic imaging.
The radioactive tracer injected into the human body decays to produce positrons, which annihilate with the majority of negative electrons present in the body, thereby producing a pair of gamma photons of approximately 180 degrees in opposite directions. The Line between the detectors that receive these two gamma photons is called the Line Of Response (LOR). If two gamma photons are detected that originate from the same electron annihilation event and neither photon is scattered from the medium, this event is referred to as a true coincidence event. If two gamma photons are detected that originate from the same electron annihilation event and at least one of the photons is scattered from the medium, this event is called a scatter coincidence. If the two photons detected originate from two different annihilation events, this event is referred to as random coincidence. The random coincidence count rate is proportional to the square of the system single count rate, while the true coincidence count rate is proportional to the first power of the system single count rate. So when the activity is high, or the axial field of view covers a long PET system, the stochastic coincidence events become a factor that severely affects the PET image quality. With the wide use Of fast scintillation crystals and the application Of Time Of Flight (TOF), the occurrence position Of each true coincidence event on the LOR can be estimated more accurately, and a new method is provided for removing useless random events to the maximum extent. The advantages brought by removing useless random events before image reconstruction are two-sided, (1) the counting of the useless events is reduced, the speed of data processing image reconstruction is improved, and (2) the background noise is reduced to the maximum extent, and the image quality is improved.
For PET with time-of-flight technology (TOF-PET), on a particular line of response (LOR), since the speed of light C is known, the position of the occurrence of a photon, i.e. the emission position of the positron, i.e. the position of the decay of the radiotracer (distance from the center of the LOR) x, can be determined by equation (1) from the difference in time t of arrival of two gamma photons at the detector:
Figure GDA0003632468850000021
the time resolution δ t of the TOF-PET system has a decisive influence on the location of the decay position of the radioactive tracer, as shown in formula (2), the higher the time resolution is, the higher the location accuracy is, the better the image quality is, the worse the time resolution is, the lower the location accuracy is, and the worse the image quality is.
Figure GDA0003632468850000022
In addition, the time resolution of the system also has a large influence on the counting rate of random events, and the random events can increase background noise in the image reconstruction process to influence the image quality. The random event count rate can be represented by equation (3):
NR＝2τN1N2 (3)
wherein N is1And N2Respectively, representing the count rate of gamma rays detected by each of a pair of detectors, and 2 tau representing the time window of the coincidence event, the size of the time window being dependent on the time resolution of the system. Therefore, the time resolution of the system is improved, the number of random events can be effectively reduced, and the image quality is further improved.
The time resolution of the system is usually represented by the full width at half maximum of a time difference statistical map (time spectrum) of all coincidence events acquired by a radioactive source placed in the center of the FOV, and ideally, the statistical time spectrum should conform to a strict narrow gaussian distribution, and a gaussian peak is located where the time difference is equal to 0, but in practice, the statistical map has a certain shift and spread, which results in the reduction of the time resolution of the system.
The reasons for the deterioration of the time performance of TOF-PET systems can be summarized as follows:
(1) differences in physical properties between crystals. For a TOF-PET system, although the same crystal is used, due to the difference between the physical properties of the crystal itself, the time response of different crystals to incident gamma rays is different, such as scintillation of the crystal, propagation of photons within the crystal, etc.;
(2) differences in electronic systems. Due to differences in manufacturing processes and the like, signal transmission speed, amplification factor, response time, delay time and the like of an electronic system connected with different crystals are different, and aging degree, voltage, threshold value and the like of electronics also cause the difference in response time; in addition, there may be a large time deviation between the detectors due to cables (e.g., synchronous clock lines, etc.).
(3) External environmental factors. Variations in the external environment, such as temperature, humidity, etc., can also lead to different electronic delays.
For a TOF-PET system, the number of signal channels is as large as several tens of thousands, and in order to increase the time resolution of the overall system, it is required that each signal channel (pixel) be time-aligned. At present, a shell source placed in the center of the FOV or a line source rotating at a constant speed around the center of the FOV is mainly used to collect coincidence data, and the time deviation of each pixel is obtained by decoding the time spectrum of each pixel to other pixels in the coincidence data and calculating the mean value of the time spectrum. However, the method of rotating the source needs a motor to assist in rotation, the structure is complex, the time correction table obtained by the method of directly correcting the shell source can only accurately correct the LOR within the FOV covered by the radioactive source, and the LOR outside the FOV cannot have a good correction effect, so that the shell source is often large in size to cover more FOVs. Moreover, both methods are premised on that the time deviation of all pixels of the whole ring is random (uniformly distributed), and when the time deviation exists among modules of the detector, the time spectrum of a single pixel may become disordered, so that the time deviation value cannot be accurately obtained.
In summary, the prior art mainly has the following defects:
1. the line source needs a motor to assist in rotating, and the structure is complex;
2. the shell source has high requirements on size and can cover the clinical FOV;
3. the LOR correction effect of the radioactive source covering outside the FOV is not ideal;
4. when there is a large time deviation between detectors or electronics, the correct time deviation value for each pixel cannot be obtained.
Disclosure of Invention
In order to overcome the technical defects, the invention aims to provide a time correction method for TOF-PET, which does not need a shell source with a large size, is suitable for the condition that time deviation exists between detectors or electronics, and has the advantages of higher calculation efficiency, quicker iteration convergence and higher time precision.
The invention discloses a time correction method for TOF-PET, which comprises the following steps: placing a shell source in the center of a field of view (FOV) and setting measurement parameters; collecting data packets which accord with events in a set time period; obtaining the time difference T between the current sub-module and the sub-module opposite to the current sub-module from the data packet conforming to the event1Time difference T between the next circumferentially adjacent sub-module of the current sub-module and the sub-module directly opposite to the current sub-module2Time difference T between the next axially adjacent submodule of the current submodule and the directly opposite submodule of the current submodule3(ii) a By time difference T1Time difference T2Sum time difference T3Acquiring circumferential and axial time deviation of a current sub-module and an adjacent sub-module thereof, traversing all sub-modules of the whole PET system, and acquiring a first-stage time correction table; time correction using each submodule in the first-stage time correction tableCorrecting the time information of the corresponding coincident events in the data packets conforming to the events by the positive value to obtain updated data packets conforming to the events; acquiring time difference distribution of coincidence events corresponding to each pixel through the updated data packet of the coincidence events to acquire a time correction value of the pixel, wherein the time correction value of each pixel forms a pixel time correction table, and the pixel time correction table and the first-stage time correction table are integrated to acquire a total time correction table; correcting the corresponding time information of the coincident events in the data packets of the coincident events by using the total time correction table to obtain updated data packets of the coincident events; repeatedly acquiring the total time correction table through the updated data packet which accords with the event, and repeatedly acquiring k times until an iteration termination rule is met to acquire a final time correction table; and storing the final time correction table to a TOF-PET control system in a lookup table mode, and realizing real-time online inquiry and correction of the time output value of the corresponding pixel channel during measurement.
Preferably, the time difference T between the current sub-module and the sub-module opposite to the current sub-module is obtained from the event-compliant data packet1Time difference T between the next circumferentially adjacent sub-module of the current sub-module and the sub-module directly opposite to the current sub-module2Time difference T between the next axially adjacent submodule of the current submodule and the directly opposite submodule of the current submodule3(ii) a By time difference T1Time difference T2Sum time difference T3Obtaining the time deviation of the current sub-module and the adjacent sub-module in the circumferential direction and the axial direction, traversing all the sub-modules of the whole PET system, and obtaining a first-stage time correction table comprises the following steps: the TOF-PET system comprises n x m detector sub-modules, wherein m represents m detector rings distributed axially, and n represents n detector sub-modules on each detector ring; obtaining sub-module S from the data packet conforming to the eventα，βWith sub-modules directly opposite thereto
Figure GDA0003632468850000041
Time difference T of1Next circumferentially adjacent sub-module Sα+1，βAnd sub-module
Figure GDA0003632468850000042
Time difference T of2Next axially adjacent submodule Sα，β+1And sub-module
Figure GDA0003632468850000043
Time difference T of3(ii) a By time difference T1Sum time difference T2Obtaining C of any sub-module on the same ringα，0Time correction value C ofα，0(ii) a By time difference T1Sum time difference T3Obtaining C of any submodule in axial directionα，βTime correction value C ofα，β，Cα，0And Cα，βA first level time correction table is constructed.
Preferably, the sub-module S is obtained from the event-compliant data packetα，βWith sub-modules directly opposite thereto
Figure GDA0003632468850000044
Time difference T of1The method comprises the following steps: obtaining sub-module S in the data packet conforming to the eventα，βWith sub-modules directly opposite thereto
Figure GDA0003632468850000045
And obtaining the mean value of the time differences of all the coincidence events, and recording the mean value as a first mean value, wherein the first mean value is the time difference T1(ii) a The next annularly adjacent sub-module S is obtained from the data packet conforming to the eventα+1，βAnd sub-module
Figure GDA0003632468850000046
Time difference T of2The method comprises the following steps: obtaining the next circumferentially adjacent sub-module S in the data packet conforming to the eventα+1，βAnd submodule
Figure GDA0003632468850000047
And obtaining the time difference distribution of all coincidence eventsThe mean value of the time differences of all the coincidence events is recorded as a second mean value, and the second mean value is the time difference T2(ii) a The next axially adjacent sub-module S is obtained from the event-compliant data packetα，β+1And sub-module
Figure GDA0003632468850000048
Time difference T of3The method comprises the following steps: obtaining the next axially adjacent sub-module S in the event-compliant data packetα，β+1And sub-module
Figure GDA0003632468850000049
And obtaining the mean value of the time differences of all the coincidence events, and recording as a third mean value, wherein the third mean value is the time difference T3。
Preferably, the time correction value
Figure GDA00036324688500000410
Time correction value
Figure GDA0003632468850000051
Preferably, the obtaining, by the updated coincidence event data packet, a time difference distribution of the coincidence events corresponding to each pixel to obtain a time correction value of the pixel, where the time correction value of each pixel forms a pixel time correction table, and the obtaining a total time correction table by integrating the pixel time correction table and the first-stage time correction table includes: acquiring time difference distribution of coincidence events corresponding to each pixel through the updated data packet of the coincidence events to acquire a time correction value of the pixel, wherein the time correction value of each pixel forms a pixel time correction table, namely a second-stage time correction table; and synthesizing the first-stage time correction table and the second-stage time correction table at all the time to obtain a total time correction table.
Preferably, the acquiring a time difference distribution of coincidence events corresponding to each pixel to acquire a temporal correction value of the pixel includes: and acquiring the mean value of the time difference distribution of the coincidence events corresponding to each pixel, thereby acquiring the time correction value of the pixel.
Preferably, the time correction value of the total time correction table is:
Figure GDA0003632468850000052
wherein A is any pixel point, i is i pixel points which are consistent with the pixel point A in the view field, i is more than or equal to 0 and less than or equal to N, and deltaAThe time offset of pixel a.
Preferably, the time correction value of the pixel point a of the final time correction table is:
Figure GDA0003632468850000053
preferably, the shell source is a positive electron source, including Na-22, Ge-68, FDG-18; the size of the shell source corresponds to a field of view that minimally covers only the two opposite modules.
Preferably, the measurement parameters include a field of view and a time window.
After the technical scheme is adopted, compared with the prior art, the method has the following beneficial effects:
1. the shell source of the positive electron source such as Na-22, Ge-68, FDG-18 and the like used in the invention has the advantages that the size of the shell source corresponds to the FOV and can cover two or more sub-modules;
2. the method is characterized in that the size of a shell source with large size is not needed, and the method is suitable for the situation that time deviation exists between detectors or electronics, the time deviation between detector sub-modules is calculated by using coincidence data to obtain a time correction table of the time correction table, the coincidence data is substituted into original data to correct time information, and on the basis, the time deviation between pixels is calculated to obtain a total time correction table;
3. all LORs of the whole ring can be corrected, the calculation efficiency is higher, the iteration convergence is faster, and the obtained time precision is higher.
Drawings
FIG. 1 is a flow chart of a method of time correction for TOF-PET provided by the present invention;
FIG. 2 is a schematic diagram of a circular sub-module level time correction of the time correction method for TOF-PET according to the present invention;
FIG. 3 is a schematic axial sub-module level time correction of the time correction method for TOF-PET according to the present invention;
FIG. 4 is a typical coincidence event time difference plot of the time correction method for TOF-PET provided by the present invention;
FIG. 5 is a schematic diagram of pixel-level time correction of the time correction method for TOF-PET according to the present invention.
Reference numerals: 1-shell source, 2-daughter detector, 3-coincidence event.
Detailed Description
The advantages of the invention are further illustrated in the following description of specific embodiments in conjunction with the accompanying drawings.
Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, like numbers in different drawings represent the same or similar elements unless otherwise indicated. The implementations described in the exemplary embodiments below are not intended to represent all implementations consistent with the present disclosure. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the present disclosure, as detailed in the appended claims.
The terminology used in the present disclosure is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used in this disclosure and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should also be understood that the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.
It is to be understood that although the terms first, second, third, etc. may be used herein to describe various information, such information should not be limited to these terms. These terms are only used to distinguish one type of information from another. For example, first information may also be referred to as second information, and similarly, second information may also be referred to as first information, without departing from the scope of the present disclosure. The word "if," as used herein, may be interpreted as "at … …" or "when … …" or "in response to a determination," depending on the context.
In the description of the present invention, it is to be understood that the terms "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. indicate orientations or positional relationships based on those shown in the drawings, and are merely for convenience of description and simplicity of description, but do not indicate or imply that the device or element referred to must have a particular orientation, be constructed in a particular orientation, and be operated, and thus, are not to be construed as limiting the present invention.
In the description of the present invention, unless otherwise specified and limited, it is to be noted that the terms "mounted," "connected," and "connected" are to be interpreted broadly, and may be, for example, a mechanical connection or an electrical connection, a communication between two elements, a direct connection, or an indirect connection via an intermediate medium, and specific meanings of the terms may be understood by those skilled in the art according to specific situations.
In the following description, suffixes such as "module", "component", or "unit" used to denote elements are used only for facilitating the explanation of the present invention, and have no specific meaning in themselves. Thus, "module" and "component" may be used in a mixture.
Referring to fig. 1-5, the present invention discloses a time correction method for TOF-PET, comprising the steps of:
s1, placing the shell source 1 in the center of the visual field, and setting measurement parameters; collecting a data packet which accords with an event 3 in a set time period;
s2, obtaining the time difference T between the current sub-module and the sub-module opposite to the current sub-module from the data packet conforming to the event 31Time difference T between the next circumferentially adjacent sub-module of the current sub-module and the sub-module directly opposite to the current sub-module2Time difference T between the next axially adjacent submodule of the current submodule and the directly opposite submodule of the current submodule3(ii) a Tong (Chinese character of 'tong')Time difference of passage T1Time difference T2Sum time difference T3Acquiring circumferential and axial time deviation of a current sub-module and an adjacent sub-module thereof, traversing all sub-modules of the whole PET system, and acquiring a first-stage time correction table;
s3, correcting the corresponding time information conforming to the event 3 in the data packet conforming to the event 3 by using the time correction value of each sub-module in the first-stage time correction table to obtain an updated data packet conforming to the event 3;
s4, acquiring time difference distribution of coincidence events corresponding to each pixel through the updated coincidence event data packet to acquire a time correction value of the pixel, wherein the time correction value of each pixel forms a pixel time correction table, and the pixel time correction table and the first-stage time correction table are integrated to acquire a total time correction table;
s5, correcting the corresponding time information conforming to the event 3 in the data packet conforming to the event 3 by using the total time correction table to obtain an updated data packet conforming to the event 3; repeatedly acquiring a total time correction table through the updated data packet which accords with the event 3, and repeatedly acquiring k times until an iteration termination rule is met to acquire a final time correction table;
and S6, storing the final time correction table into a TOF-PET control system in a lookup table mode, and realizing real-time online query and correction of the time output value of the corresponding pixel channel during measurement.
In this embodiment, the control system in step S6 is an FPGA module of the front-end electronic system, and in other embodiments, the control system may also be other processing modules, which is not limited herein.
In step S1, the time for collecting the data packet corresponding to event 3 in the set time period needs to be long enough to ensure that the time spectrum of each pixel of the detector 2 array has enough statistical counts. Over a period of time of the data acquisition process, data packets of coincidence events 3 between all detectors 2 of the entire TOF-PET system can be acquired.
In step S1, a shell source 1 is a positive electron source and comprises Na-22, Ge-68 and FDG-18; the size of the shell source 1 corresponds to the field of view (FOV) that can minimally cover only the two opposite modules. The measurement parameters include the FOV and time window.
Preferably, referring to fig. 2-3, in step S2, assuming that the detector array includes n × m sub-detectors 2, the data packet corresponding to event 3 includes n × m detection sub-modules, where m denotes m detector rings distributed axially, and n denotes n detector sub-modules on each detector ring.
Analyzing the data packet conforming to the event 3, taking the minimum time reading unit as a unit, recording the data packet as a SubModule (S), and acquiring any SubModule S from the data packet conforming to the event 3α，βWith sub-modules directly opposite thereto
Figure GDA0003632468850000081
Time difference T of1Next circumferentially adjacent sub-module Sα+1，βAnd sub-module
Figure GDA0003632468850000082
Time difference T of2Next axially adjacent submodule Sa，β+1And submodule
Figure GDA0003632468850000083
Time difference T of3. By time difference T1Sum time difference T2Obtaining S of any sub-module on the same ringα，0Time correction value C ofα，0(ii) a Transit time difference T1Sum time difference T3Obtaining S of any submodule in axial directionα，βTime correction value C ofα，β，Cα，0And Cα，βA first level time correction table is constructed.
And calculating the average value of the distribution of each time difference, obtaining the time deviation of two adjacent submodules according to the average value difference of the two distributions, searching the whole system, and obtaining the time deviation between the submodules. Namely: sub-module S in data packet for obtaining coincidence event 3α，βWith sub-modules directly opposite thereto
Figure GDA0003632468850000084
Is consistent with the time difference distribution of event 3, andobtaining a first average value of the time differences of all the coincidence events 3, wherein the first average value is the time difference T1(ii) a Obtaining next annularly adjacent sub-module S in data packet conforming to event 3α+1，βAnd submodule
Figure GDA0003632468850000085
And obtaining a second average value of the time differences of all the coincidence events 3, wherein the second average value is the time difference T2(ii) a Obtaining the next axially adjacent sub-module S in the data packet conforming to event 3αβ+1And submodule
Figure GDA0003632468850000086
And obtaining a third mean value of the time differences of all the coincidence events 3, wherein the third mean value is the time difference T3。
Specifically, S is first counted on the first ring (m ═ 0)0，0With its face-to-face module
Figure GDA0003632468850000087
The distribution of time differences between all coincidence events 3, as shown in FIG. 5, and fitting the time spectrum using a double Gaussian function to a bimodal, bimodal mean (parameter P in FIG. 5)1And P4) The average of the time spectra is the mean of the time spectra, i.e. the time offset of the two sub-blocks,
Figure GDA0003632468850000088
same method statistics and S0，0Adjacent next submodule S1，0And
Figure GDA0003632468850000089
the distribution of time differences between all coincident events 3, the time offset between the two sub-modules is obtained
Figure GDA00036324688500000810
The submodule S can be obtained through the time deviation of every two submodules1，0And S0，0In betweenThe time offset is:
Figure GDA0003632468850000091
S0，0defining the time deviation as 0 for the start submodule, then S1，0Time correction value C of submodule1，0＝-T(1，0)，(0，0)。
In the same way, by the module S1，0And
Figure GDA0003632468850000092
time deviation therebetween
Figure GDA0003632468850000093
Module S2，0And
Figure GDA0003632468850000094
time deviation therebetween
Figure GDA0003632468850000095
A sub-module S can be obtained2，0And S1，0Time deviation T between(2，0)，(1，0)Then submodule S2，0Time correction value C of2，0＝C1，0-T(2，0)(1，0)。
By analogy, any sub-module S on the ring can be obtainedα，0The time correction value of (a) is:
Figure GDA0003632468850000096
in the same way, as shown in fig. 2, each sub-module on the 0 th ring is used as a starting module, all the sub-modules in the axial direction are traversed, and any sub-module S in the axial direction can be obtainedα，βThe time correction value of (a) is:
Figure GDA0003632468850000097
therefore, the time correction values of all the sub-modules on the detector array, namely the first-stage time correction table, are obtained.
Preferably, in step S4, the updated event 3 compliant packet is used to obtain the time difference distribution corresponding to each pixel and obtain the mean value of the event 3 compliant time difference distribution corresponding to each pixel to obtain the time correction value of each pixel, where the time correction value of each pixel constitutes the pixel time correction table, i.e. the second-stage time correction table.
Specifically, after time offsets between detector sub-modules on the TOF-PET system are sequentially calculated, time references of all the detector sub-modules are aligned to 0, and at this time, the time offsets between pixels can be considered to be random, that is, uniformly distributed. On the basis, the time spectrum of all the coincidence events 3 corresponding to each pixel is counted. Referring to fig. 4, for a particular pixel a (which may be located anywhere on the system, T)A＝tA+δA) In the FOV, there are a large number of pixels Bi(0≤i≤N,TB＝tB+δB) These coincidental LORs can be counted, and the mean of the temporal distribution of pixel a is:
Figure GDA0003632468850000098
wherein, tA、
Figure GDA0003632468850000099
Is the time, delta, of the true arrival of a photon at the sub-detector 2A、
Figure GDA00036324688500000910
Is the time offset of the pixel. Since the radioactive source is placed in the center of the FOV and the shell source 1 itself is of an axisymmetric structure, the source is placed in the center of the FOV
Figure GDA00036324688500000911
The mean value is 0, which is a symmetrical distribution centered at 0. For a large number of pixels Bi(0≤i≤N)，The time deviation of these pixels can be considered statistically as a random uniform distribution, i.e. the average value is 0, i.e. can be considered as
Figure GDA0003632468850000101
Is 0. Therefore, the deviation of the mean value of the time spectrum of the specific pixel A from 0 is the time deviation delta of the pixelA。
And traversing the whole ring to obtain the time deviation of all the pixels, namely a second-stage time correction table.
After the first-stage time correction table and the second-stage time correction table of all the time are integrated, a total time correction table can be obtained, and the total time correction table belongs to any submodule Sa，βThe total temporal correction value of any pixel a of (a) is:
Figure GDA0003632468850000102
preferably, in step S5, after k iterative corrections, the time offset of each pixel is calculated, so that the time offset of each pixel on the ring tends to be more and more ideal, and the obtained time correction table is more accurate, and the time correction value of the pixel point a of the time correction table is:
Figure GDA0003632468850000103
in step S6, the final time correction table is stored in the memory and is sent to an electronic system (FPGA) at the front end of the TOF-PET in the form of a look-up table (LUT), so as to realize real-time online query and correction of the time output value of the corresponding pixel channel during measurement.
Compared with the prior art, the shell source 1 has the size corresponding to the FOV and can cover two or more sub-modules; the size of the shell source 1 is not required to be large, and the method can be simultaneously suitable for the condition that time deviation exists between the sub-detectors 2 or electronics; and all LORs of the whole ring can be corrected, the calculation efficiency is higher, the iterative convergence is faster, and the obtained time precision is higher.
It should be noted that the embodiments of the present invention have been described in terms of preferred embodiments, and not by way of limitation, and that those skilled in the art can make modifications and variations of the embodiments described above without departing from the spirit of the invention.

Claims (7)

1. A time correction method for TOF-PET, comprising the steps of:
placing a shell source in the center of a view field, and setting measurement parameters; collecting data packets which accord with events in a set time period;
obtaining the time difference T between the current sub-module and the sub-module opposite to the current sub-module from the data packet conforming to the event1Time difference T between the next circumferentially adjacent sub-module of the current sub-module and the sub-module directly opposite to the current sub-module2Time difference T between the next axially adjacent submodule of the current submodule and the directly opposite submodule of the current submodule3(ii) a Transit time difference T1Time difference T2Sum time difference T3Obtaining the time deviation of the current sub-module and the adjacent sub-module in the circumferential direction and the axial direction, traversing all the sub-modules of the whole PET system, and obtaining a first-stage time correction table comprises:
the control system of the TOF-PET comprises n x m detector sub-modules, wherein m represents m detector rings distributed axially, and n represents n detector sub-modules on each detector ring; obtaining sub-module S from the data packet conforming to the eventα，βWith sub-modules directly opposite thereto
Figure FDA0003632468840000011
Time difference T of1Next circumferentially adjacent sub-module Sα+1，βAnd sub-module
Figure FDA0003632468840000012
Time difference T of2Next axially adjacent submodule Sα，β+1And sub-module
Figure FDA0003632468840000013
Time difference T of3(ii) a By time difference T1Sum time difference T2S for obtaining any sub-module on the same ringα，0Time correction value C ofα，0(ii) a Transit time difference T1Sum time difference T3Obtaining S of any submodule in axial directionα，βTime correction value C ofα，β，Cα，0And Cα，βForming a first-stage time correction table;
correcting the corresponding time information of the coincident events in the data packets of the coincident events by using the time correction values of the sub-modules in the first-stage time correction table to obtain updated data packets of the coincident events;
acquiring time difference distribution of coincidence events corresponding to each pixel through the updated data packet of the coincidence events to acquire a time correction value of the pixel, wherein the time correction value of each pixel forms a pixel time correction table, and the pixel time correction table and the first-stage time correction table are integrated to acquire a total time correction table
Figure FDA0003632468840000014
Wherein A is any pixel point, i is i pixel points which are consistent with the pixel point A in the view field, i is more than or equal to 0 and less than or equal to N, and deltaAThe time deviation of the pixel point A is obtained;
correcting the time information of the corresponding coincidence events in the data packets conforming to the events by using the total time correction table to obtain updated data packets conforming to the events; and repeatedly acquiring the total time correction table through the updated data packet conforming to the event for k times until an iteration termination rule is met to obtain a final time correction table
Figure FDA0003632468840000021
And storing the final time correction table to a TOF-PET control system in a lookup table mode, and realizing real-time online inquiry and correction of the time output value of the corresponding pixel channel during measurement.
2. The time correction method according to claim 1, wherein said obtaining a sub-module S from said event-compliant data packetα，βWith sub-modules directly opposite thereto
Figure FDA0003632468840000022
Time difference T of1The method comprises the following steps: obtaining sub-module S in the data packet conforming to the eventα，βWith sub-modules directly opposite thereto
Figure FDA0003632468840000023
And obtaining the mean value of the time differences of all the coincidence events, and recording the mean value as a first mean value, wherein the first mean value is the time difference T1；
The next annularly adjacent sub-module S is obtained from the data packet conforming to the eventα+1，βAnd sub-module
Figure FDA0003632468840000024
Time difference T of2The method comprises the following steps: obtaining the next circumferentially adjacent sub-module S in the data packet conforming to the eventα+1，βAnd sub-module
Figure FDA0003632468840000025
And obtaining the mean value of the time differences of all the coincidence events, and recording the mean value as a second mean value, wherein the second mean value is the time difference T2；
The next axially adjacent sub-module S is obtained from the event-compliant data packetα，β+1And sub-module
Figure FDA0003632468840000026
Time difference T of3The method comprises the following steps: obtaining the next axial phase in the data packet conforming to the eventNeighbor module Sα，β+1And sub-module
Figure FDA0003632468840000027
And obtaining the mean value of the time differences of all the coincidence events, and recording as a third mean value, wherein the third mean value is the time difference T3。
3. The time correction method according to claim 1, characterized in that the time correction value
Figure FDA0003632468840000028
Time correction value
Figure FDA0003632468840000029
4. The time correction method according to claim 1, wherein the obtaining, through the updated coincident event data packet, a time difference distribution of coincident events corresponding to each pixel to obtain a time correction value of the pixel, the time correction value of each pixel constituting a pixel time correction table, and the obtaining a total time correction table by integrating the pixel time correction table and the first-stage time correction table comprises:
acquiring time difference distribution of coincidence events corresponding to each pixel through the updated data packet of the coincidence events to acquire a time correction value of the pixel, wherein the time correction value of each pixel forms a pixel time correction table, namely a second-stage time correction table;
and integrating the first-stage time correction table and the second-stage time correction table at all the time to obtain a total time correction table.
5. The time correction method according to claim 1, wherein the obtaining a time difference distribution of coincidence events corresponding to each pixel to obtain a time correction value of the pixel comprises:
and acquiring the mean value of the time difference distribution of the coincidence events corresponding to each pixel, thereby acquiring the time correction value of the pixel.
6. The time correction method of claim 1, wherein the shell source is a positive electron source comprising Na-22, Ge-68, FDG-18;
the size of the shell source corresponds to a field of view that minimally covers only the opposite two sub-modules.
7. The time correction method of claim 1, wherein the measurement parameters include a field of view and a time window.
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