CN112932517B - Background radioactivity realization time self-correction method and device - Google Patents
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
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Abstract
The application provides a background radioactivity realizing time self-correction method and a device, which relate to the technical field of positron emission computed tomography imaging, and acquire a coincidence count to a triggering stop condition by initializing a time self-correction acquisition condition of a PET system; selecting coincidence events with proper response line length, compensating response line flight time, performing time migration correction on time scaling migration phenomenon caused by energy difference of the coincidence events, correcting a large number of coincidence time differences of coincidence events, iteratively counting system response flight time spectrums of all detectors, evaluating optimal time deviation values of all detectors, and finally triggering iteration convergence and termination conditions to complete self-correction. According to the application, an external radioactive source and a complex operation flow are not introduced, so that the time correction task of the PET system based on the scintillation crystal containing lutetium element is completed; and when the time is self-corrected, the coverage of the detector with the ultra-large FOV range of the system is realized, and the statistical counting rate in unit time is improved.
Description
Technical Field
The application relates to a background radioactivity realizing time self-correcting method and device, belonging to the technical field of positron emission computed tomography imaging.
Background
The time-of-flight positron emission computed tomography (TOF-PET) is a relatively high-end nuclear medicine imaging technology and is widely applied to clinical tumor examination, localization and clinical pathology research. The principle is that a specific life nuclide with positron decay and biological metabolin are taken as carriers (such as glucose, nucleic acid, protein and the like) to form a specific tracer agent, the specific tracer agent is loaded into a human body to participate in the metabolism process, and a PET imaging system is utilized to image the concentration distribution diagram of the tracer agent in the human body; because the concentration distribution of the tracer is related to the metabolism intensity, the metabolism intensity distribution condition of the human body is indirectly reflected, and medical auxiliary diagnosis information is provided for doctors.
The TOF-PET system with high precision time resolution can acquire the detection position, energy and accurate flight time difference of the coincidence event, and compared with the traditional PET system, the TOF-PET system has more information of the coincidence flight time difference, so that the quality of a reconstructed image is improved.
Detectors for Positron Emission Tomography (PET) include scintillation detectors such as sodium iodide (NaI), bismuth Germanate (BGO), lutetium Silicate (LSO), lutetium Yttrium Silicate (LYSO). The lutetium-based scintillation crystal detector has better energy resolution, shorter decay time, higher light yield and becomes the preferred detector for PET systems. The nuclide lutetium 176 in the lutetium-based scintillator is not a stable nuclide, and emits a plurality of gamma rays with relatively large emission branches of 88, 203, 307keV and the like when beta-decay occurs.
The TOF-PET detection system is provided with tens of thousands of detector units, the detectors of different modules use different timing systems, and electric signals cause delay in the process of processing and transmitting in electronic elements, so that in order to obtain accurate flight time differences of coincidence signals, time correction is needed for all the detector units of the PET system.
Moreover, when the circuit of the PET system is aged and replaced, the corresponding time correction parameters are not applicable any more, so that the time performance is reduced, and the time correction must be executed again.
Currently, PET systems basically use geometrically regular radiation sources such as rod sources, column sources, shell sources, etc. to complete the time scale. These sources have a certain service life, require special storage devices or spaces, are inconvenient to operate, have a risk of radiation injury when operated for a long time and a short distance, and increase the use cost. But also increases the probability of scattering events, and the size of the external radiation source limits the detector coverage.
In summary, current positron emission computed tomography (PET) systems require the introduction of regularly shaped external positron sources, and also require predetermined positional information of the sources, to complete the temporal calibration of the detection system. This would increase the complexity and cost of operation of the time-stamping operation procedure while increasing the radiation dose experienced by the personnel involved.
Based on this, the present application has been made.
Disclosure of Invention
In order to solve the defects in the prior art, the application provides a method for realizing time self-correction of background radioactivity and a TOF-PET device, which do not need to use an external radioactive source and utilize the radioactive background of a PET system detector to realize time self-correction of a system.
The method provided by the application is applicable to all PET systems with intrinsic radioactivity detectors, such as PET systems based on lutetium-series scintillator detectors, and is characterized by collecting coincidence events triggered by intrinsic background radioactivity of lutetium-series scintillation crystals.
The beta-decay of one Lu-176 in the lutetium scintillation crystal can record the electronic signal emitted by the lutetium scintillation crystal by the detector of the lutetium scintillation crystal, and the corresponding gamma photons can be recorded by the lutetium scintillation crystal or other detectors to form a pair of effective coincidence events, wherein the corresponding flight time difference is the flight time length of the gamma photons between the two detectors, and a response line is formed on the corresponding two detection spaces.
In order to achieve the above purpose, the technical scheme adopted by the application is as follows:
the correction method comprises the following steps:
(1) Initializing, namely initializing a time self-correcting acquisition condition of the PET system, and acquiring a condition from the coincidence counting to the triggering stopping condition;
(2) Selecting coincidence events, removing useless random coincidence events, and selecting coincidence events with proper response line length;
(3) Compensating the flight time of the response line, compensating the flight time required by the corresponding response line light to a reference response line according to the position information of the coincidence event, and newly calculating a corresponding time difference;
(4) Time migration correction, namely performing time migration correction on a time scaling migration phenomenon caused by the energy difference of the coincidence event;
(5) Correcting the time of the system detector, namely correcting the time difference of all selected coincidence events subjected to response line flight time compensation and time migration correction;
(6) Counting, namely counting the effective time-of-flight spectra of all detectors after time difference correction;
(7) Time deviation evaluation, namely calculating time deviation correction tables of all detector units after one statistical iteration according to the flight time spectrums of all detector units, and accumulating the time deviation correction tables into a system time deviation correction table;
(8) Iterating, repeating the time correction step, the statistics step and the time deviation evaluation step of the system detector until the iteration convergence and interruption conditions are triggered, completing the time deviation tables of all the detectors, and ending the self-correction of the background time.
The corresponding self-correcting device comprises a PET system with background radioactivity, and can form at least one pair of effective coincidence events in an acquisition module, a detection module corresponding to one response line, a coincidence event selection module (adopting a multi-energy window event discriminator and a response line length selector), a response line flight time compensation module, a time migration correction module, a system detector time correction module, a statistics module, a time deviation evaluation module and an iteration module.
The application provides a multi-energy window time discriminator, which is used for extracting effective coincidence events formed by triggering beta particles and gamma rays from coincidence events formed by triggering signals with different energies such as background beta particles, gamma rays, beta+gamma superposition, beta+beta superposition, gamma+gamma superposition signals and the like. The gamma photons with different energies in the radioactive background have energy windows of one type of corresponding multi-energy section, and the energy windows of beta particles are energy windows of another type of multi-energy section formed by subtracting the energy sections of gamma rays, beta+gamma superposition, beta+beta superposition and gamma+gamma superposition signals; the energy of both signals of a pair of coincidence events are identified as valid coincidence events in both energy windows, respectively.
A large number of effective coincidence events are acquired, and the application provides a response line selector which screens coincidence events with a length suitable for time correction from thousands of response lines of a PET system.
The application provides a response line flight time compensation module, which is used for acquiring a beta signal emission peak and gamma photon receiving peak shape of a narrower iterative correction time spectrum. The system has the functions that a pair of coincidence events can be formed by every two independent signal output subsystems of the PET system, the corresponding two detectors form a response line, and photon flight time differences corresponding to the response lines at different positions compensate the time differences of the coincidence events to the corresponding flight time differences of the reference response lines by designating the reference response lines.
The time scaling is caused to have the wandering phenomenon by the event-triggered signals with different energies, the time wandering relative difference value triggered by the different energies is calculated by referring to the energy signals, and the time difference conforming to the event is further corrected.
The multi-energy window time discriminator, the response line selector, the response line compensation module and the time walk correction module are used for processing and correcting a large number of coincidence time differences which accord with the cases, the system response flight time spectrum of each detector is counted in an iteration mode, the optimal time deviation value of each detector is estimated, and finally iteration convergence and termination conditions are triggered.
The principle and the beneficial technical effects of the application are as follows:
(1) The application does not need to use an external radioactive source: the time correction task of the PET system based on the scintillation crystal containing lutetium element is completed without introducing an external radioactive source and a complex operation flow; lutetium-based scintillation crystals contain the radionuclide Lu-176, which beta-decays with gamma photons of emission energy 202, 307 keV. The coincidence events formed on the detector by the beta particles and the emitted gamma photons are used for time correction of the detector, so that a traditional time correction method by an external radioactive source is omitted.
(2) Compared with the traditional external radioactive source, the size of the external radioactive source limits the coverage of the detector, and the large radial visual field acquires coincidence data during time self-correction, so that the coverage of the detector with the ultra-large FOV range of the TOF-PET system can be realized, and the statistical counting rate in unit time is improved;
(3) The application can simply, quickly and randomly check the time offset condition of a daily PET system, and can collect data at any time due to the stable background radioactivity of the PET system based on the lutetium scintillation crystal due to the ultra-long half-life (3.7E+10y) of the Lu-176, thereby completing the energy scale of the detector of the PET system and the time performance detection of the daily detector system.
Drawings
FIG. 1 is a schematic diagram of a typical background coincidence event for a PET system composed of lutetium-based scintillation detectors;
FIG. 2 is a flow chart of the background time self-correction in the present embodiment;
fig. 3 is a graph showing the comparison of the time-of-flight spectra before (up) and after (down) the background time self-correction in this example.
Detailed Description
In order to make the technical means of the present application and the technical effects achieved thereby clearer and more complete disclosure, an embodiment is provided, and the following detailed description is given with reference to the accompanying drawings:
as shown in fig. 2, the system and method for implementing time self-correction of background radioactivity of the present embodiment first needs to determine a PET system with background radioactivity in a detection module, and can form a pair of effective coincidence events in an acquisition module. Such as lutetium-based scintillation crystal based PET systems; as shown in fig. 1, in a given coincidence time window, assuming that the Lu-176 nuclear species in detector 101 releases one β -particle and multiple γ -photons in one β -decay, the β -particle triggers detector 101 to record one electronic signal, while the emitted γ -photon is recorded as another electronic signal at detector 103, these two signals forming a pair of valid coincidence events, corresponding to one response line 102 (as shown in fig. 1); during the coincidence time window, the signals recorded first in the coincidence event are called first signals, and the signals recorded later are called second signals; each pair of coincidence events comprises position and energy information of the corresponding two signals, time sequence and actually measured flight time difference information.
The specific self-correction method comprises the following steps:
step 201, initializing acquisition conditions according to time self-correction conditions, and acquiring statistics of the time of flight spectrum from the background coincidence data to each detector suitable for the PET system. The acquisition conditions are as follows: the coincidence event of the radial visual field of the PET system is not limited, the energy window is fully opened, a proper coincidence time window is preset according to the electronic condition of the actual PET system, and the time offset (generally preset to be zero) of each corresponding detector is initialized;
step 202, calling a coincidence event selection function module, calling a multi-energy window event discriminator to discriminate the effectiveness of a first signal and a second signal of each coincidence event, judging whether the coincidence event determines coincidence triggered by Beta particles and gamma photons emitted by a Beta-decay, eliminating useless random coincidence events, and improving the true coincidence event occupation ratio of a coincidence event set; invoking a response line length selector to select a coincidence event of an appropriate response line length (the appropriate response line length is typically greater than the inscribed radius of the PET system);
step 203, then, using a response line compensation module to designate the photon flight time of a specific response line length as a reference flight time (generally using the radial inscribed diameter of the PET system as a reference), calculating the difference between the real flight time and the reference flight time required by each pair of detector pairs conforming to the event selected in the photon traversing step 202, and compensating the difference to the actually measured flight time difference of each pair of conforming events; wherein the time difference of each step is the flight time difference corrected by the previous step;
step 204, correcting the time difference of flight compensated in step 203 by using a time walk correction module to correct the time walk deviation caused by the energy difference of the coincidence event;
step 205, then, a system detector time correction module is called to correct the system time deviation of each coincidence event flight time difference corrected in step 204;
step 206, a statistics function module is called, and the coincidence events processed in step 205 are counted to generate a flight time spectrum corresponding to each detector;
step 207, finally, invoking a time deviation evaluation module, calculating the time spectrum of flight positioning emission response peak and the peak position of receiving response peak of each detector unit, calculating the time deviation correction table of all the detector units after one statistics iteration according to the average value of the two peak positions, namely one iteration time correction, and accumulating the obtained time deviation correction table of each detector unit to a system time deviation correction table;
step 208, entering an iteration module, and repeating steps 205, 206, 207 and 208 until the iteration convergence and interruption conditions are triggered, so as to complete the time deviation table of all the detectors, wherein the iteration convergence and interruption conditions can be specified maximum iteration times or deviation correction values after each iteration reach specified deviation thresholds and the like.
Step 209, the self-correction of the background time is finished.
In summary, the coincidence time-of-flight spectra of each detector after correction exhibits significant emission peaks and receptions compared to the time-of-flight spectra before (up) and after (down) the time correction (see fig. 3).
The foregoing is a further detailed description of the provided technical solution in connection with the preferred embodiments of the present application, and it should not be construed that the specific implementation of the present application is limited to the above description, and it should be understood that several simple deductions or substitutions may be made by those skilled in the art without departing from the spirit of the present application, and all the embodiments should be considered as falling within the scope of the present application.
Claims (5)
1. A method for realizing time self-correction of background radioactivity, comprising the following steps:
(1) Initializing a time self-correcting acquisition condition of the PET system, and acquiring a condition from coincidence counting to triggering stopping;
(2) Selecting coincidence events, removing useless random coincidence events, and selecting coincidence events with proper response line length; the method comprises the steps of carrying out validity identification on first signals and second signals of all coincidence events by a multipotent window event discriminator, judging whether the coincidence events determine coincidence triggered by Beta particles and gamma photons emitted by one Beta-decay through a multipotent window, eliminating useless random coincidence events, and improving the true coincidence event duty ratio of an event set; selecting a coincidence event with a proper response line length by adopting a response line length selector; a pair of signals recorded first in a coincidence event is called a first signal, and signals recorded later are called a second signal;
(3) Compensating the flight time of the response line, compensating the photon flight time of the corresponding response line length to a reference response line according to the position information of the coincidence event, and recalculating the corresponding time difference;
(4) Time migration correction, namely performing time migration correction on a time scaling migration phenomenon caused by the energy difference of the coincidence event;
(5) Correcting the time of the system detector, namely correcting the time difference of all selected coincidence events subjected to response line flight time compensation and time migration correction;
(6) Counting, namely counting the effective time-of-flight spectra of all detector units after time difference correction;
(7) Time deviation evaluation, namely calculating time deviation correction tables of all detector units after one statistical iteration according to the flight time spectrums of all detector units, and accumulating the time deviation correction tables into a system time deviation correction table;
(8) Iterating, repeating the system detector time correction step, the statistics step and the time deviation evaluation step until the iteration convergence and interruption conditions are triggered, completing the time deviation correction tables of all detector units, and ending the self-correction of the background time.
2. The method for implementing time self-correction of background radioactivity according to claim 1, wherein: in the step (1), the acquisition condition includes a coincidence event which does not limit the radial view of the PET system, a full-open energy window, a proper coincidence time window preset according to the electronic condition of the actual PET system, and a zero time offset preset for each corresponding detector unit.
3. The method for implementing time self-correction of background radioactivity according to claim 1, wherein: in the step (8), the time-of-flight spectrum positioning emission response peak and the peak position of the receiving response peak of each detector unit are calculated, and the time offset correction table of all the detector units after one statistical iteration is calculated according to the average value of the two peak positions, so that one iteration time correction is performed.
4. The device comprises a detection module, a PET system with background radioactivity, a detection module and a detection module, wherein at least one pair of effective coincidence events can be formed in the acquisition module, and each pair of effective coincidence events corresponds to one response line; the method is characterized in that: and also comprises
The initialization module is used for initializing the acquisition conditions of the time self-correction of the PET system, and acquiring the acquisition coincidence counting to the triggering stop conditions;
the coincidence event selecting module is used for removing useless random coincidence events and selecting coincidence events with proper response line length; the method comprises the steps of carrying out validity identification on first signals and second signals of all coincidence events by a multipotent window event discriminator, judging whether the coincidence events determine coincidence triggered by Beta particles and gamma photons emitted by one Beta-decay through a multipotent window, eliminating useless random coincidence events, and improving the true coincidence event duty ratio of an event set; selecting a coincidence event with a proper response line length by adopting a response line length selector; a pair of signals recorded first in a coincidence event is called a first signal, and signals recorded later are called a second signal;
the response line flight time compensation module is used for compensating the photon flight time of the corresponding response line length to the reference response line according to the position information of the coincidence event, and recalculating the corresponding time difference;
the time migration correction module is used for performing time migration correction on a time calibration migration phenomenon caused by the energy difference of the coincidence event;
the system detector time correction module is used for correcting time difference of all selected coincidence events subjected to response line flight time compensation and time migration correction;
a statistics module for counting the effective time-of-flight spectra of all detector units corrected for time differences;
the time deviation evaluation module is used for calculating time deviation correction tables of all detector units after one statistical iteration according to the flight time spectrums of all detector units and accumulating the time deviation correction tables into a system time deviation correction table;
and the iteration module is used for repeatedly using the system detector time correction module, the statistics module and the time deviation evaluation module until triggering iteration convergence and interruption conditions to finish the time deviation correction tables of all detector units and finishing the self-correction of the background time.
5. The background radioactivity realizing time self-correcting device according to claim 4, wherein: the coincidence event selection module employs a multi-energy window event discriminator and a response line length selector.
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| CN111568452A (en) * | 2020-05-25 | 2020-08-25 | 上海联影医疗科技有限公司 | PET system state detection method, PET system state detection device, computer equipment and storage medium |
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