Mean random estimate from list mode data

Abstract

Systems and methods for estimating mean randomness include: acquiring list pattern data describing true and delayed coincidences detected by a positron emission tomography (PET) scanner during a scan of an object; determining, based on the list pattern data, delayed coincidences including said crystals for each crystal of the PET scanner and for each of a plurality of time periods of the scan; for each crystal, determining a single event rate associated with each time period based on the delayed coincidences determined for the crystal within the time period; for each time period, determining an estimated mean randomness for each of a plurality of crystal pairs based on the single event rate associated with the time period for each crystal of the crystal pair; and reconstructing an image of the object based on the estimated mean randomness for each time period and the detected true coincidences.

Description

Background Technology

[0001] In traditional positron emission tomography (PET) imaging, a radiopharmaceutical tracer is typically introduced into the patient via radial artery injection. The radioactive decay of the tracer produces positrons, which eventually encounter electrons and are annihilated. This annihilation produces two photons traveling in approximately opposite directions.

[0002] A detector ring surrounding the body detects emitted photons, identifies "coincidences," and reconstructs a PET image based on these coincidences. A coincidence is identified when two detectors positioned on opposite sides of the body detect the arrival of two photons within a specific coincidence time window. Because the two "coincidence" photons propagate in approximately opposite directions, the positions of the two detectors determine the response line (LOR), along which annihilation events may occur.

[0003] A "true" coincidence indicates the detection of two coincident photons arising from a single annihilation event located on the LOR between the two detectors. A "random" coincidence indicates two coincident photons not arising from the same annihilation event. A "scattered" coincidence is a type of true coincidence where the two coincident photons originate from the same annihilation event, but the annihilation event is not located on the LOR between the two detectors because one or both photons interact and scatter within the body or medium.

[0004] Conventional PET scanners detect all matches, regardless of whether they are true, random, or scattered matches. Since only true matches represent spatial information about the distribution of tracers within the body, random and scattered matches should be resolved before and / or during image reconstruction. Software- and / or hardware-based methods can be used to estimate random matches.

[0005] One current approach involves delaying the detection of an input channel for an event. For example, as described above, the main hardware logic detects all true coincidences along all LORs. An additional "delayed logic" receives the same input as the main logic, but delays one input channel by, for example, tens of nanoseconds (e.g., 5 times the coincidence window), before performing coincidence detection. As a result, the delayed logic does not detect any actual true coincidences.

[0006] The coincidences detected by delayed logic (i.e., delayed coincidences) are currently used to estimate the single-event rate for each crystal in the PET scanner. The single-event rate is the rate at which a crystal detects a valid (i.e., energy-qualified) photon during the scanning process. Next, for each LOR (i.e., each crystal pair (i, j)), a mean random smoothing model is constructed. : ,in and It is the single-event rate of crystals i and j, and It conforms to the time window. Some techniques further construct the mean random based on delayed conformity counts. Apply rescaling. In either case, during the reconstruction of the PET image from the detected true match, the mean obtained can be randomized, as is known in the art.

[0007] The aforementioned techniques fail to adequately account for variations in the single-event rate over time. These variations include, but are not limited to, continuous bed motion (CBM) scans and stationary scans in which tracer distribution changes rapidly. Furthermore, the aforementioned scaling performed plane-by-plane is noise-sensitive. This sensitivity is particularly problematic for tilt segments in CBM acquisition, which are noisy due to their low count counts and are especially prevalent in long-axis field-of-view PET scanners. Attached Figure Description

[0008] Figure 1A and 1B The detection of conformity according to some embodiments is shown.

[0009] Figure 2 A PET detector according to some embodiments is shown.

[0010] Figure 3 This is a block diagram of a conformity detection system according to some embodiments.

[0011] Figure 4 This is a block diagram of a system for reconstructing images from PET data according to some embodiments.

[0012] Figure 5 Includes flowcharts of a process for estimating the randomness of the mean, according to some embodiments.

[0013] Figure 6 The determination of the cone-sum of a crystal is shown in some embodiments.

[0014] Figure 7 The following is an example of a random estimate of the mean based on different acquisition time periods, according to some embodiments.

[0015] Figure 8 This is a block diagram of a PET / CT imaging system according to some embodiments. Detailed Implementation

[0016] The following description is provided to enable anyone in the art to make and use the described embodiments. Various modifications will be apparent to those skilled in the art.

[0017] The inventors have discovered that, compared to existing models, the stochastic smoothing model... An improved estimate of the mean stochasticity is provided. By providing a better estimate of the mean stochasticity, the embodiment can provide improved PET images reconstructed based on it. This improvement is particularly significant when the single event rate varies greatly over time. A stochastic smoothing model can be used without the need for additional heuristics, the application of which can lead to data correction errors and thus degrade image quality.

[0018] The mean stochastic estimation based on the above model requires determining the single-event rate for each pair of crystals i,j as a function of time. From a processing perspective, this determination may be impractical because estimating single events from a sine plot is impractical, and direct access to the single events for each crystal is unavailable. The inventors have solved this problem by estimating the single-event rate over time based on the cone sum representing the delay coincidence over time.

[0019] Advantageously, the number of cone sums determined for a given time period (representing the delayed coincidences detected within that time period) is equal to the number of crystals in the PET scanner. Conversely, the delayed coincidence sine curve represents delayed coincidences using the number of LORs equal to the square of the number of crystals. Therefore, the determined cone sums include a highly compressed representation of delayed coincidences, which is easier to compute and process further. Furthermore, cone sums can be determined directly and efficiently from PET scan list pattern data.

[0020] Figure 1A and 1B The detection of conformity according to some embodiments is shown. Figure 1A This is an axial view of the aperture 105 of the detector ring 100 and the imaging object 110 disposed therein. The imaging object 110 may include a human body, a phantom, or any other suitable object. Figure 1B yes Figure 1A The image shows a radial view of detector ring 100 and object 110. In the example shown, detector ring 100 consists of any number (eight in this example) of adjacent and coaxial rings of detectors 150. Each detector 150 may include any number of scintillator crystals and electrical transducers.

[0021] Assume that annihilation events 120, 130, 140, and 142 occur at various locations within object 110. As described above, the injected tracer produces positrons, which are annihilated by electrons to produce two 511 keV gamma photons that travel in approximately opposite directions. Figure 1A and Figure 1A Each annihilation event, as represented in the diagram, results in a detected match. As mentioned above, true matches represent valid image data, while scattered and random matches represent noise.

[0022] A coincidence is detected when a pair of detectors receive two gamma photons within a coincidence time window, as determined based on the calculated arrival times of the two gamma photons at their respective detectors. Event 120 is associated with a true coincidence because event 120 results in the reception of two gamma photons within the coincidence time window, and because the location of annihilation event 120 lies on LOR 125, which connects the locations of the detectors that received the two gamma photons.

[0023] Event 130 is associated with scattering coincidence because even if the two gamma photons generated by event 130 are detected within the coincidence time window, the location of annihilation event 130 is not located on LOR 135 connecting the locations of the two photons. This could be due to a change in orientation of at least one of the two gamma photons within object 110 caused by Compton (i.e., inelastic) or coherent (i.e., elastic) scattering.

[0024] Events 140 and 142 are two separate annihilation events that result in the detection of random coincidences. (As...) Figure 1B As shown, one of the photons generated by event 140 is absorbed in object 110, and one of the photons generated by event 142 escapes detection by any detector 150 of detector ring 100. The remaining photons happen to be detected within the coincidence time window, even though no annihilation event occurs on LOR 145, which connects to the location where the coincidence photons were received.

[0025] Since only true unscattered coincidences indicate the location of annihilation events, random coincidences and scattered coincidences are often subtracted from the acquired PET data or otherwise used to correct the acquired PET data during PET image reconstruction.

[0026] Typically, a PET detector includes one or more scintillation elements and one or more electrical transducers. The scintillation element generates photons with energies of several electron volts (eV) in response to receiving 511 keV photons generated by an annihilation event. The electrical transducer converts the low-energy photons generated by the scintillation element into electrical signals. According to some embodiments, the electrical transducer may include, for example, a SiPM, PMT, or semiconductor-based detector.

[0027] Figure 2A detector 200 according to some embodiments is shown. The detector 200 consists of eight microblocks, two of which are in the axial direction and four in the meridional direction. In one example, the microblocks comprise a grid of 5×5 lutetium silicate (LSO) scintillation crystals with dimensions of 3.2 mm × 3.2 mm × 20 mm. The microblocks can be coupled to a 4×4 array of SiPMs for receiving photons from them and generating electrical signals based thereon. Thus, the detector 200 comprises 200 crystals in rows of 10 crystals in the axial direction and 20 crystals in the meridional direction. Embodiments are not limited to the above description of the detector 200.

[0028] According to some embodiments, the detector ring 100 includes eight detectors in the axial direction and 38 detectors in the meridional direction. Thus, the detector ring 100 includes 60,800 detector crystals, with rows of 80 detector crystals in the axial direction and rows of 760 detector crystals in the meridional direction. Embodiments are not limited to these specifications.

[0029] Figure 3 This is a block diagram of a coincidence detection system 300 according to some embodiments. System 300 includes scintillation units 310, 320, and 330, corresponding electrical transducer units 312, 322, and 332, and corresponding signal processing components 314, 324, and 334. A coincidence detection unit 340 receives signals from each of the signal processing components 314, 324, and 334.

[0030] Each scintillation unit 310, 320, and 330 may include one or more scintillation crystals. For example, each of scintillation units 310, 320, and 330 may include a microblock of 5×5 crystal elements, a macroblock of 2×2 microblocks, or a detector consisting of two macroblocks. Embodiments are not limited to any particular configuration or construction of scintillation units 310, 320, and 330.

[0031] Each electrical transducer unit 312, 322, and 332 may include one or more PMTs, SiPMs, etc. The number of electrical transducers in each unit 312, 322, and 332 may be less than, equal to, or greater than the number of crystal elements in each scintillation unit 310, 320, and 330. According to some embodiments, the electrical transducer unit includes a 4×4 array of SiPMs for each microblock of 5×5 crystal elements in its corresponding scintillation unit.

[0032] Signal processing components 314, 324, and 334 receive electrical signals from corresponding electrical transducer units 312, 322, and 332, and perform signal processing to, for example, determine whether the signal represents a photon detection event, perform signal unpileing through pile-up rejection and / or correction methods, and associate the photon detection event with a specific detector crystal of the scintillation units 310, 320, and 330. Signal processing components 314, 324, and 334 can perform any suitable function and exhibit any suitable implementation.

[0033] The coincidence detection unit 340 receives all photon detection events that have passed energy qualification, referred to as single events, and identifies such event pairs that occur within a coincidence time window. As described above, the coincidence detection unit 340 also includes delay logic that delays the apparent arrival time of one event in each comparison, thereby identifying the aforementioned delayed coincidences. The coincidence detection unit 340 therefore outputs data specifying each identified pair and labeling each pair as a true coincidence or a delayed coincidence. For either type of coincidence, the output data also specifies the two detector crystals that received the photon detection event containing the coincidence.

[0034] Data output by the coincidence detection unit 340 and associated with coincidences detected over a period of time can be stored in a "list pattern" file. The list pattern file includes list pattern data that specifies for each coincidence the type (i.e., true or delayed), the two crystals receiving the photons of the coincidence, the energy level of each event, the time the coincidence was detected, and, in the case of time-of-flight (TOF) PET imaging, the difference between the arrival times of the two photons that caused the detected coincidence. This difference can be used to more accurately estimate the specific location of the annihilation event corresponding to the occurrence along the LOR. The list pattern data is not limited to the above.

[0035] Detected coincidences over a period of time can also be stored in a sinogram. A sinogram is a data array of angles relative to displacement for each LOR (Longest Range) of each detected coincidence. Delayed sinograms can store data associated with detected delayed coincidences, while true sinograms can store data associated with detected true coincidences. A sinogram includes a row containing the LORs at a specific azimuth angle φ. Each of these rows corresponds to a one-dimensional parallel projection of the tracer distribution at different coordinates. The sinogram stores the location of the LOR for each coincidence, such that all LORs passing through a single point in the volume depict a sine curve in the sinogram. Each coincidence can be represented by its LOR, energy level, time of coincidence occurrence, TOF data, and other information.

[0036] Figure 4An imaging system 400 according to some embodiments is illustrated. Each component of system 400 can be implemented by any suitable combination of hardware and software. In some embodiments, one or more components can be implemented by a single software application.

[0037] System 400 includes a detector 410 as part of a detector ring and a corresponding scintillator 420. The scintillator 420 can be composed of... Figure 2 The description describes the composition of a single crystal. The embodiments are not limited to scintillator-based detectors. Direct conversion detectors (e.g., CZT and TIBr) may also be used in some embodiments.

[0038] Detector 410 detects gamma photons 435 emitted from volume 430. Systems for facilitating the emission of gamma photons from a volume are known in the art, particularly with respect to PET imaging described herein. As described above, the crystal of scintillator 420 receives gamma photons 435 and emits photons in response. Detector 410 receives photons, and each detector 410 generates an electrical signal based on the energy of the received photons and its own characteristic photoelectric response distribution.

[0039] The detector signal processing unit 440 receives electrical signals generated by each detector 410 and performs signal processing to, for example, determine whether the signal represents a photon detection event, perform signal destacking through stacking rejection, determine the event energy, and determine the event time. The detector signal processing unit 440 can perform any suitable function and exhibit any suitable implementation.

[0040] Within a given time period, the coincidence detection unit 445 receives all photon detection events identified by energy (e.g., between 435 and 585 keV) from all detectors 410 of the detector ring. Based on the reception time of each photon detection event, unit 445 identifies pairs of photon detection events received within the coincidence time window and determines that each such pair corresponds to a true coincidence with associated LOR and energy. The coincidence detection unit 445 can also determine a TOF value representing the difference in reception time of the photon detection events for each pair. The coincidence detection unit 445 also uses delay logic to identify delayed coincidences as described above. Data representing each detected coincidence (i.e., true and delayed) is stored in file 450.

[0041] The random correction unit 455 can estimate the mean randomness of each crystal pair based on file 450, as described herein. This estimation is based on data from file 450 associated with detected delay coincidences. The random correction unit 455 can correct the true coincidence data in file 450 to generate randomly corrected (i.e., net true) data. According to some embodiments, the scattering estimation unit 460 can then estimate the scattering coincidences based on the net true data. As is known in the art, this estimation can be based in part on modeling data 465.

[0042] The estimated scattering coincidences can be filtered 470. The reconstruction unit 475 executes a reconstruction algorithm to reconstruct the image based on the estimated scattering coincidences thus filtered and the randomly corrected coincidence data output from the random correction unit 455.

[0043] Figure 5 This includes a flowchart of a process 500 for estimating mean randomness from list pattern data, according to some embodiments. Mean randomness can be used to correct PET frames prior to image reconstruction.

[0044] Process 500 and the other processes described herein can be executed using any suitable combination of hardware and software. The software program code embodying these processes can be stored by any non-transitory tangible medium, including hard disks, volatile or non-volatile random access memory, DVDs, flash drives, and magnetic tapes, and executed by any suitable processing unit, including but not limited to one or more microprocessors, microcontrollers, processing cores, and processor threads. Embodiments are not limited to the examples described below.

[0045] Initially, at S510, the object is scanned using a PET scanner known in the art. According to some embodiments, the object includes phantoms, such as, for example, a uniformly filled water cylinder. A radionuclide tracer is injected into the object prior to scanning. The radionuclide tracer may include any suitable tracer, such as, but not limited to, fluorodeoxyglucose (FDG). The scan may include a conventional static PET scan or a CBM scan, and generates list pattern data describing delayed and true coincidences detected by the PET scanner during scanning as described above.

[0046] For each crystal in the PET scanner, in S520, the number of delay coincidences that include that crystal and occur within a given time period is determined from the list pattern data. S520 includes determining the aforementioned sum of cones for each crystal in the PET scanner within the given time period. As described above, the list pattern data will determine each determined delay coincidence... Two crystals i and j are associated with the PET scanner. Therefore, the number of determined delay coincidences associated with each crystal i of the PET scanner can be determined as follows: .

[0047] This time period can include any sub-time period of the total scan time. In some embodiments using CBM scanning, the duration of the time period is equal to the time required for the bed to move ~5 cm. Therefore, this duration depends on the speed of bed movement. For the purposes of this description, it will be assumed that the duration of the time period is 1 second, and that the given time period in the first iteration of S520 is the first second of the PET scan. Therefore, in this example, S520 includes determining from the list pattern data all detected delay coincidences occurring during the first 1 second of the scan.

[0048] Figure 6 Each dashed line represents a LOR (Late Optical Response) that occurs during the first 1 second of the scan and is associated with another crystal in crystal 600 and detector ring 100. It should be understood that many more delay coincidences may be detected during the first 1 second of a PET scan, and Figure 6 Only those delay coincidences associated with crystal 600 are shown. Therefore, seven delay coincidences associated with crystal 600 are determined in S520. This determination is repeated for each crystal of the PET scanner.

[0049] In S530, it is determined whether the list pattern data includes an additional time period. If so, the process returns to S520 to determine the delay coincidence for each crystal within the next time period. Continuing the example above, the next time period could include the next (i.e., the second) second of the PET scan. The process continues in this manner to calculate the cone sum for each crystal of the PET scanner within each specified time period of the PET scan.

[0050] In S540, and for each time period, the single event rate for each crystal is determined based on the cone sum determined for each crystal within that time period. In one example, S540 includes using a cone sum specific to each time period determined for each crystal. For each crystal i, the sequential monotonic coordinate ascending algorithm is solved iteratively. Alternatively, the simultaneous update equation can be solved iteratively, again using a cone and a time-specific equation determined for each crystal. To calculate the single event rate for each crystal i during the time period, and where =Number of cones and contributing LORs:

[0051] in .

[0052] Next, and for each time period, in S550, the mean is estimated randomly for each crystal pair based on the time-specific single-event rate for each crystal. For example, the single-event rate determined in S540 for crystal i,j for time period T is used. The mean of the crystal pair (i, j) over time T is random. identified as .

[0053] Figure 7 The diagram shows the S550 for each time period. Determine the mean random Single event rate Indicates the time period to be acquired The single-event rate for each crystal in the PET scanner during the period. Using these single-event rates, the mean random estimate for each crystal pair (i, j) can be determined as follows: During the time period The mean of all crystal pairs in the PET scanner during the period is expressed as a random estimate. Similar determinations occur in each time period. To determine the corresponding random estimate of the mean .

[0054] In S560, an image of the object is reconstructed based on the estimated mean random and true correspondence between the list pattern data and the data for each crystal pair. According to some embodiments, and also as... Figure 7 As shown, some embodiments of S560 include random estimation of the mean for a time period. Summation is performed to generate a stochastic estimate of the composite mean. For example, for each time period Inner crystal stochastic estimation of the mean Summation is performed to generate a stochastic estimate of the composite mean. And perform this summation for each crystal pair to generate a random estimate of the composite mean. This summation can include the above-mentioned stochastic smoothing model. (i.e., as shown) The implementation of ).

[0055] For reference Figure 4 As described, S560 may include, in any known or becoming known manner, randomizing the mean of the detected true coincidence, scattering estimate, and composite estimate. This method reconstructs PET images, as described in this paper. Compared to images reconstructed from PET data using existing techniques, these PET images exhibit less noise and a higher signal-to-noise ratio.

[0056] Figure 8 A PET / CT system 800 is shown for performing one or more of the procedures described herein. Embodiments are not limited to system 800.

[0057] System 800 includes a stand 810 defining an aperture 812. As is known in the art, stand 810 houses a PET imaging assembly for acquiring PET image data and a CT imaging assembly for acquiring CT image data. As is known in the art, the CT imaging assembly may include one or more X-ray tubes and one or more corresponding X-ray detectors.

[0058] PET imaging assemblies can include any number or type of detectors in any configuration known in the art. Typically, a detector includes one or more scintillation elements and one or more electrical transducers. The scintillation element generates photons with energies of several electron volts in response to receiving 511 keV photons generated by an annihilation event. LSO and yttrium lutetium silicate (LYSO) scintillators exhibit suitable stopping capabilities and rapid scintillation decay, and can be used in high count rate scenarios.

[0059] An electrical transducer converts low-energy photons generated by a scintillation element into an electrical signal. According to some embodiments, the electrical transducer may include a silicon-based photomultiplier (SiPM) or photomultiplier tube (PMT). Some embodiments employ a block detector that includes more scintillation elements than electrical transducers. In a block detector, multiple electrical transducers receive scattered low-energy photons resulting from the absorption of one of the photons annihilated at 511 keV. The relative outputs of the transducers are compared to determine the absorption location, which in turn identifies the scintillation element or crystal determined to have received the annihilated photon.

[0060] The injection system 818 can be operated to deliver a calibrated injection of FDG, iodine, or other radiopharmaceuticals to a patient before and / or during a PET scan. In some embodiments, the injection system 818 is integrated into a bench 810. The injection system 818 may support a wired or wireless communication link with the control system 820 for receiving information on specified dose, injection protocol, and scan delay.

[0061] Bed 815 and base 816 are operable for moving a patient lying on bed 815 into and out of port 812 before, during and after imaging. In some embodiments, bed 815 is configured to translate over base 816, and in other embodiments, base 816 may move with bed 815 or alternatively move from bed 815.

[0062] The movement of the patient into and out of the port 812 allows for scanning of the patient using CT and PET imaging elements of the gantry 810. This scanning can be performed based on scanning parameters such as scan range and corresponding scan speed. According to some embodiments, during such scanning, the bed 815 and base 816 can provide continuous bed movement and / or step-and-shoot motion.

[0063] The control system 820 may include any general-purpose or special-purpose computing system. Therefore, the control system 820 includes one or more processing units 822 and a storage device 830 for storing program code. The processing unit 822 is configured to execute processor-executable program code to cause the system 820 to operate as described herein. The storage device 830 may include one or more fixed disks, solid-state random access memory, and / or removable media (e.g., thumb drives) mounted in a corresponding interface (e.g., a USB port).

[0064] Storage device 830 stores the program code of control program 831. One or more processing units 822 can execute control program 831 to control hardware components to inject radiopharmaceuticals into the patient, move the patient through orifice 812 past the PET detector on bench 810, and detect coincidence events occurring within the patient, in conjunction with PET system interface 823, bed interface 825, and injection interface 827. Detected events can be stored in memory 830 as list-mode data 834.

[0065] One or more processing units 822 may also execute control program 831 to, in conjunction with CT system interface 824, cause radiation sources within gantry 810 to emit radiation into the body within aperture 812 from different projection angles, and control corresponding detectors to acquire two-dimensional CT data. As described above, CT data can be acquired substantially simultaneously with PET data, and can be used for attenuation correction of simultaneously acquired list pattern data 834 known in the art. In this regard, control program 831 may also be executed to reconstruct PET images 836 from list pattern data 834 of PET scans using any known or becoming known reconstruction algorithm.

[0066] According to some embodiments, the storage device 830 also includes a random estimation procedure 832 for estimating the mean randomness used in the reconstruction. As described above and in detail below, this estimation utilizes the single-event rate of different scan time periods, which are then determined based on delay coincidence time periods and crystal-specific cone sums.

[0067] PET image 836 can be transmitted to terminal 840 for display via terminal interface 826. Terminal 840 may include a display device and an input device coupled to system 820. Terminal 840 may receive user input for controlling the display of data, operation of system 800, and / or the processing described herein. In some embodiments, terminal 840 is a separate computing device, such as, but not limited to, a desktop computer, laptop computer, tablet computer, and smartphone.

[0068] Each component of system 800 may include other elements necessary for its operation, as well as additional elements for providing functionality beyond that described herein. Each functional component described herein may be implemented in computer hardware, program code, and / or one or more computing systems that execute such program code known in the art. Such computing systems may include one or more processing units that execute processor-executable program code stored in a memory system.

[0069] Those skilled in the art will understand that various adaptations and modifications can be configured for the above embodiments without departing from the claims. Therefore, it should be understood that the claims may be implemented in ways different from those specifically described herein.

Claims

1. A system comprising: A positron emission tomography (PET) scanner comprising multiple crystals, the PET scanner performing a scan of an object and generating list pattern data describing true and delayed coincidences detected by the PET scanner during the scan; Processing unit, used for: For each crystal, the number of delay coincidences including that crystal is determined from the list pattern data for each of the multiple time periods of the scan; For each crystal, a single event rate associated with each time period is determined based on the number of delay coincidences determined for all crystals in the plurality of crystals within the time period; For each time period, based on the single event rate associated with each crystal of the crystal pair for that time period, an estimated mean random is determined for each of the plurality of crystal pairs, wherein the estimated mean random determined for the crystal pair for the first time period is different from the estimated mean random determined for the crystal pair for the second time period. For each of the multiple crystal pairs, a composite mean random is determined based on the estimated mean random determined for each time period of the crystal pair; as well as An image of the object is reconstructed based on the composite estimated mean of each of the plurality of crystal pairs and the detected true coincidence; as well as A bed, used to support the object and move during scanning. The duration of each of the plurality of time periods depends on the speed at which the bed moves during the scan.

2. The system according to claim 1, wherein, Determining the single event rate associated with a time period of a crystal involves determining, for each crystal i, the event rate. ,in It is equal to the delay coincidence determined for crystal i within the time period.

3. The system according to claim 1, wherein, Determining the single event rate associated with a time period of a crystal involves determining, for each crystal i, the event rate. ,in ,and It is equal to the delay coincidence determined for crystal i within the time period, and wherein =To The number of LORs that have made contributions.

4. The system according to claim 1, The positron emission tomography (PET) scanner performs a second scan of the second object and generates second list pattern data, which describes the second true coincidence and the second delayed coincidence detected by the PET scanner during the second scan, and Processing unit, used for: For each crystal, a second number of second delay coincidences including that crystal are determined from the second list pattern data for each of the multiple time periods of the second scan; For each crystal, a second single-event rate associated with each second time period is determined based on the second number of second delay coincidences determined for all crystals in the plurality of crystals within the second time period; For each second time period, a second estimated mean random is determined for each of the plurality of crystal pairs based on the second single event rate associated with the second time period of each crystal of the crystal pair, wherein the second estimated mean random determined for the crystal pair in the second time period is different from the second estimated mean random determined for the crystal pair in another second time period. For each of the multiple crystal pairs, a second composite estimate random is determined based on the second estimate random determined for each crystal pair in the second time period; as well as An image of the second object is reconstructed based on the second composite estimate mean of each of the plurality of crystal pairs, randomly and the detected second true coincidence.

5. A method comprising: Obtain a list of pattern data describing true and delayed coincidences detected by a positron emission tomography scanner during the scanning of an object; The bed supporting the object is moved during the scan; as well as For each crystal of the positron emission tomography scanner, the number of delay coincidences including the crystal is determined from the list pattern data for each of the multiple time periods of the scan; For each crystal, the single event rate associated with each time period is determined based on the number of delay coincidences determined for all of the plurality of crystals for the time period; For each time period, based on the single event rate associated with each crystal of the crystal pair for that time period, an estimated mean random is determined for each of the plurality of crystal pairs, wherein the estimated mean random determined for the crystal pair for the first time period is different from the estimated mean random determined for the crystal pair for the second time period. For each of the multiple crystal pairs, a composite mean random is determined based on the estimated mean random determined for each time period of the crystal pair; as well as An image of the object is reconstructed based on the composite estimated mean of each of the plurality of crystal pairs, randomized, and the detected true coincidence. The duration of each of the plurality of time periods depends on the speed at which the bed moves during the scan.

6. The method according to claim 5, wherein, Determining the single event rate associated with a time period of a crystal involves determining, for each crystal i, the event rate. ,in It is equal to the delay coincidence determined for crystal i within the time period.

7. The method according to claim 5, wherein, Determining the single event rate associated with a time period of a crystal involves determining, for each crystal i, the event rate. ,in ,and It is equal to the delay coincidence determined for crystal i within the time period, and wherein =To The number of LORs that have made contributions.

8. The method according to claim 5, further comprising: Acquire second list pattern data describing the second true coincidence and the second delayed coincidence detected by a positron emission tomography scanner during the second scan of the second object; For each crystal, for each of the second plurality of time periods of the second scan, a second number of second delay coincidences including that crystal is determined from the second list pattern data; For each crystal, a second single-event rate associated with each second time period is determined based on the second number of second delay coincidences determined for all crystals in the plurality of crystals within the second time period; For each second time period, a second estimated mean random is determined for each of the plurality of crystal pairs based on the second single event rate associated with the second time period of each crystal of the crystal pair, wherein the second estimated mean random determined for the crystal pair in the second time period is different from the second estimated mean random determined for the crystal pair in another second time period. For each of the multiple crystal pairs, a second composite estimate random is determined based on the second estimate random determined for each crystal pair in the second time period; as well as An image of the second object is reconstructed based on the second composite estimate mean of each of the plurality of crystal pairs, randomly and the detected second true coincidence.

9. A non-transitory computer-readable medium storing processor-executable process steps, which, when executed by a processing unit of a computing system, cause the computing system to: Obtain a list of pattern data describing true and delayed coincidences detected by a positron emission tomography scanner during the scanning of an object; The bed supporting the object is moved during the scan; as well as For each crystal of the positron emission tomography scanner, the number of delay coincidences including the crystal is determined from the list pattern data for each of the multiple time periods of the scan; For each crystal, the single event rate associated with each time period is determined based on the number of delay coincidences determined for all of the plurality of crystals for the time period; For each time period, based on the single event rate associated with each crystal of the crystal pair for that time period, an estimated mean random is determined for each of the plurality of crystal pairs, wherein the estimated mean random determined for the crystal pair for the first time period is different from the estimated mean random determined for the crystal pair for the second time period. For each of the multiple crystal pairs, a composite mean random is determined based on the estimated mean random determined for each time period of the crystal pair; as well as An image of the object is reconstructed based on the composite estimated mean of each of the plurality of crystal pairs, randomized, and the detected true coincidence. The duration of each of the plurality of time periods depends on the speed at which the bed moves during the scan.

10. The medium according to claim 9, wherein, Determining the single event rate associated with a time period of a crystal involves determining, for each crystal i... ,in It is equal to the delay coincidence determined for crystal i within the time period.

11. The medium according to claim 9, wherein, Determining the single event rate associated with a time period of a crystal involves determining, for each crystal i... ,in ,and It is equal to the delay coincidence determined for crystal i within the time period, and wherein =To The number of LORs that have made contributions.

12. The medium according to claim 9, when executed by a processing unit of a computing system, the processor-executable process steps further cause the computing system to: Acquire second list pattern data describing the second true coincidence and the second delayed coincidence detected by a positron emission tomography scanner during the second scan of the second object; For each crystal, for each of the second plurality of time periods of the second scan, a second number of second delay coincidences including that crystal is determined from the second list pattern data; For each crystal, a second single-event rate associated with each second time period is determined based on the second number of second delay coincidences determined for all crystals in the plurality of crystals within the second time period; For each second time period, a second estimated mean random is determined for each of the plurality of crystal pairs based on the second single event rate associated with the second time period of each crystal of the crystal pair, wherein the second estimated mean random determined for the crystal pair in the second time period is different from the second estimated mean random determined for the crystal pair in another second time period. For each of the multiple crystal pairs, a second composite estimate random is determined based on the second estimate random determined for each crystal pair in the second time period; as well as An image of the second object is reconstructed based on the second composite estimate mean of each of the plurality of crystal pairs, randomly and the detected second true coincidence.

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