Disclosure of Invention
In view of the above, the present invention provides a method and an apparatus for processing stacking events to solve the above technical problems.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
in a first aspect of the embodiments of the present invention, a method for processing stacking events is provided, which is used for a PET system, and includes the following steps:
acquiring an electric signal output by a PET detector;
carrying out digital integral real-time sampling on the electric signal, wherein a real-time sampling value is taken as an energy value of a first event at each moment;
monitoring a real-time sampling value of a rising edge of the electric signal;
when the continuous jump of the electric signal is monitored, judging that a stacking event occurs;
and after the accumulation event occurs, continuing to perform digital integral real-time sampling, and performing function fitting based on the real-time sampling value at each moment before the accumulation event occurs, wherein each moment after the accumulation event occurs takes the function fitting value as the energy value of the first event at the moment, and the difference value between the real-time sampling value and the function fitting value as the energy value of the second event at the moment.
Preferably, after the accumulation event occurs, the digital integral real-time sampling is continued, the function fitting is performed based on the real-time sampling value at each time before the accumulation event occurs, the function fitting value is used as the energy value of the first event at each time from the previous time when the first jump occurs, and the difference value between the real-time sampling value and the function fitting value is used as the energy value of the second event at the time.
Preferably, when the function fitting is performed, the function used is an exponential function.
Preferably, after acquiring the electrical signals output by the PET detector, the method further comprises the following steps:
differentiating the electrical signal output by the PET detector;
when a stacking event occurs, acquiring two narrow pulses;
screening the two narrow pulses to obtain two continuous time pulses;
and time calibration is carried out on the two continuous time pulses to obtain the time information of the first event and the second event.
According to a second aspect of embodiments of the present invention, there is provided an accumulation event processing apparatus for a PET system, comprising:
the electric signal acquisition module is used for acquiring an electric signal output by the PET detector;
the integral sampling module is used for carrying out digital integral real-time sampling on the electric signal, and the real-time sampling value is taken as the energy value of the first event at each moment;
the monitoring module monitors a real-time sampling value of the rising edge of the electric signal;
the judging module is used for judging that a stacking event occurs when the electric signal is monitored to continuously jump;
the function fitting module is used for performing function fitting based on real-time sampling values at all moments before the accumulation event occurs after the accumulation event occurs, and the function fitting values at all moments after the accumulation event occurs are used as energy values of a first event at the moment;
and the difference value calculation module performs digital integral sampling after the accumulation event occurs, and takes the difference value between the real-time sampling value and the function calculation value at each moment as the energy value of the second event at the moment.
Preferably, the function fitting module is configured to perform function fitting when a stacking event occurs, and start from a previous time point when a first jump occurs, and use a function calculation value as an energy value at the time point.
Preferably, the difference calculation module is configured to perform digital integral sampling after a stacking event occurs, and use a difference between a real-time sampling value and a function calculation value at each time as an energy value of a second event at the time.
Preferably, when the function fitting module performs function fitting, the function is an exponential function.
Preferably, the method further comprises the following steps:
the differential sampling module is used for carrying out digital differential real-time sampling on the electric signal;
the narrow pulse acquisition module acquires two narrow pulses after an accumulation event occurs;
the discrimination module discriminates the two narrow pulses to obtain two continuous time pulses;
and the calibration module is used for carrying out time calibration on the two continuous time pulses to obtain the time information of the first event and the second event.
Compared with the prior art, the method can effectively and accurately separate and process the stacking events, and obtain the energy information of the first event and the second event, so that the crystal identification accuracy, the energy correction accuracy and the like can be improved.
Detailed Description
The present invention will be described in detail below with reference to specific embodiments shown in the drawings. These embodiments are not intended to limit the present invention, and structural, methodological, or functional changes made by those skilled in the art according to these embodiments are included in the scope of the present invention.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in this specification 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.
As shown in fig. 1, a stacking event processing method for a PET system includes the following steps.
Step 100, acquiring an electric signal output by a PET detector. And acquiring the electric signal output by the PET detector through an amplifying circuit, and amplifying the electric signal.
Step 101, performing digital integral real-time sampling on the electric signal, wherein the real-time sampling value at each moment is taken as the energy value of the first event at the moment.
After a single gamma ray hits the crystal, the digital integral real-time sampling is carried out on the electric signal output by the PET detector. Because the stacking event is a probability event, the occurrence probability is in direct proportion to the activity of the detected target, and when the occurrence probability is unknown, the electric signal output by the PET detector is sampled in real time through an AD sampling circuit according to a conventional digital integration method.
Step 102, monitoring a real-time sampling value of a rising edge of the electric signal.
And in the digital integral sampling process, the change condition of the real-time sampling value at each moment is monitored in real time.
And 103, judging that a stacking event occurs when the electric signal is monitored to continuously jump.
Due to the existence of noise, the sample value jumping is normal, but the sample value jumping at two consecutive time instants is abnormal, i.e. the consecutive jumping occurs, which is a typical sign of the occurrence of a pile-up event. As shown in FIG. 2, ti-2To tiAt the moment, the sampling value generates continuous jump, and the original waveform which tends to be attenuated has large amplitude change. And if continuous jumping does not exist all the time, continuing to carry out digital integral sampling to obtain the energy value of the first event.
And step 104, after the accumulation event occurs, continuing to perform digital integral sampling, and performing function fitting based on the real-time sampling value at each moment before the accumulation event occurs, wherein each moment after the accumulation event occurs takes the function fitting value as the energy value of the first event at the moment, and the difference value between the real-time sampling value and the function fitting value as the energy value of the second event at the moment.
After a stacking event occurs, continued digital integration sampling and function fitting may be performed simultaneously. When a pile-up event occurs, the digital integration actually samples the energy value, which is not the energy value of a true single gamma ray hitting the crystal, but the energy values resulting from two or more gamma ray hits the crystal are superimposed. At the moment, if the gravity center method is used for sampling, the crystal calculation error is directly caused, and the image quality is influenced; with one-to-one sampling, the acquisition results are also not amenable to efficient energy correction due to energy contamination. Therefore, the invention adopts real-time sampling for obtaining the energy value before the occurrence of the accumulation event and adopts function fitting for obtaining the energy value after the occurrence of the accumulation event through the processing chips such as the FPGA and the like, thereby effectively stripping the polluted energy and maximally restoring the energy value after the hit of the real single event. The function is fitted until complete decay of the first event is complete, obtaining energy values at each time of the first event after a pile-up event has occurred.
And 105, obtaining energy information of the first event and the second event.
After the accumulation event occurs, the subsequent energy of the first event is obtained by adopting function fitting, so that the polluted energy can be effectively stripped, and the energy value after the hit of the real single event is reduced to the maximum extent; and the energy of the second event is acquired, and the function fitting value of the first event is subtracted from the real-time sampling value, so that the accuracy of the energy of the second event is ensured. Therefore, the energy information of each moment before and after the accumulation event occurs is summarized, and the respective complete energy information of the first event and the second event can be obtained. Therefore, when the accumulation event occurs, the energy information of the first event and the second event can still be effectively reduced, so that the operations such as crystal position calculation, energy correction and the like can be carried out, and the crystal identification accuracy, the energy correction accuracy, the system sensitivity, the noise equivalent count value and the like can be improved.
Furthermore, in order to make the calculation result more accurate, after the accumulation event occurs, function fitting and digital integral sampling are simultaneously performed, starting from the previous moment when the first jump occurs, the function fitting value is used as the energy value of the first event at each moment, and the difference value between the real-time sampling value and the function fitting value is used as the energy value of the second event at the moment. Since two consecutive transitions need to be identified to determine whether a pile-up event occurs, the actual occurrence time of the pile-up event should be the previous time of the first transition.
Specifically, since the rising edge of the visible light signal generated by the gamma ray hitting the crystal after being converted into the electrical signal is a typical exponential curve, the fitting function used in the function fitting may be an exponential function.
In an alternative embodiment of the present invention, the exponential function is formulated as
Wherein t is time, y (t) is energy value at t moment, K
1Is the proportionality coefficient of the exponential curve, K
2Is the decay exponent of an exponential curve, the proportionality coefficient K
1Attenuation index K
2Are all constants. Here, the time t is in units of sampling periods of the AD conversion chips, and if the sampling frequency of the AD conversion chips is 200M, the sampling period is 5ns, and t of the time axis is incremented in units of 5 ns. As shown in FIG. 2, t
iFor the moment when the pile-up event is identified, t
i-2For the moment when the pile-up event actually occurs, t
i+nThe time at which the first event completely decays to completion after the heap event occurs.
In the function formula, constant K1、K2The real-time sampling values of any two moments before the accumulation event occurs are substituted into a function formula for calculation, and the two selected moments can be the 1/3 th position and the 2/3 th position of the acquired moment respectively; constant K1、K2And an approximate value can be obtained through calculation of waveform data at the acquired moment.
In an alternative embodiment of the present invention, as shown in fig. 3, after acquiring the electrical signals output by the PET detector, the method further comprises the following steps:
step 201, differentiating the electrical signal output by the PET detector.
Step 201 and step 101 can be performed synchronously, the electric signal output by the PET detector is amplified by an amplifier and divided into two parts, one part enters an AD sampling circuit for digital integral real-time sampling, and the other part enters a differentiating circuit for differentiation. The purpose of differentiation is to efficiently acquire two narrow pulses when a pile-up event occurs.
In step 202, when a pile-up event occurs, two narrow pulses are acquired.
And step 203, screening the two narrow pulses to obtain two continuous time pulses.
Two narrow pulses enter a discriminator to obtain two continuous time pulses, and the discriminator can be a CFD discriminator or a D L ED discriminator.
Step 204, time calibration is performed on the two continuous time pulses to obtain time information of the first event and the second event.
The two continuous time pulses, after entering the TDC circuit, can be time-scaled to obtain the time information of the first event and the second event. The TDC circuit in the invention can be a circuit with an external TDC chip as a core, and can also be a TDC module in a processing chip such as an FPGA.
The energy information and the time information of the two events before and after are respectively combined together through the processing chips such as the FPGA and the like, when the accumulation events occur, the accumulation events are effectively separated, and the energy information and the time information of the two events are accurately acquired.
The front-end processing circuit used in the specific step may include, as shown in fig. 6, an amplifying circuit 610, an AD sampling circuit 620, and a control circuit 630, where an output end of the amplifying circuit 610 is electrically connected to an input end of the AD sampling circuit 620, and an output end of the AD sampling circuit 620 is electrically connected to an input end of the control circuit 630. The amplifying circuit 610 is used for receiving and amplifying the electric signal output by the PET detector, the AD sampling circuit 620 is used for performing digital integration on the amplified electric signal, and the control circuit 630 is used for acquiring an energy value acquired by the digital integration, and simultaneously monitoring and processing Pileup in real time. The control circuit 630 may be an FPGA chip.
The front-end processing circuit further comprises a differential circuit 640, a discriminator 650 and a TDC circuit 660, wherein the input end of the differential circuit 640 is electrically connected with the output end of the amplifying circuit 610, the output end of the differential circuit 640 is electrically connected with the input end of the discriminator 650, the output end of the discriminator 650 is electrically connected with the input end of the TDC circuit 660, the differential circuit 640 is used for carrying out digital differential real-time sampling on an electric signal output by a PET detector, the discriminator 650 is used for discriminating a narrow pulse signal, and the TDC circuit 560 is used for carrying out time calibration on a time pulse.
The front-end processing circuits are all conventional configurations used in practice, and no additional circuit is required to be added.
Corresponding to the foregoing embodiments of the heap event processing method, the present disclosure also provides embodiments of a heap event processing apparatus.
Referring to fig. 4, a block diagram of an embodiment of an accumulation event processing apparatus according to the present invention, which can be applied to a PET system, comprises:
an electric signal acquisition module 400 for acquiring an electric signal output by the PET detector;
the integral sampling module 410 is used for carrying out digital integral real-time sampling on the electric signal, and the real-time sampling value at each moment is taken as the energy value of the first event at the moment;
a monitoring module 420 for monitoring a real-time sampling value of a rising edge of the electrical signal;
the judging module 430, when the continuous jump of the electric signal is monitored, judging that a stacking event occurs;
the function fitting module 440 is used for performing function fitting based on real-time sampling values at all times before the occurrence of the accumulation event after the occurrence of the accumulation event, and taking the function fitting values at all times after the occurrence of the accumulation event as energy values of a first event at the moment;
and the difference value calculating module 450 continues to perform digital integral sampling after the accumulation event occurs, and the difference value between the real-time sampling value and the function fitting value is used as the energy value of the second event at each moment after the accumulation event occurs. Wherein the function fitting value is obtained by the function fitting of the function fitting module 440.
Through the function fitting module 440 and the difference value calculating module 450, the energy values of the first event and the second event after the accumulation event occurs are obtained, and then complete energy information of the first event and the second event is obtained.
Further, in order to make the calculation result more accurate, the function fitting module 440 is configured to perform function fitting based on real-time sampling values at times before the occurrence of the accumulation event after the occurrence of the accumulation event, and start from a time before the occurrence of the first jump, where each time uses a function fitting value as an energy value of a first event at the time; the difference calculating module 450 is configured to continue to perform digital integral sampling after the occurrence of the accumulation event, and start at a previous time when the first transition occurs, and each time takes a difference between the real-time sampling value and the function fitting value as an energy value of a second event at the time.
Specifically, since the rising edge of the visible light signal generated by the gamma ray hitting the crystal after being converted into the electrical signal is a typical exponential curve, the fitting function used in the function fitting may be an exponential function.
In an alternative embodiment of the present invention, the exponential function is formulated as
Wherein t is time, y (t) is energy value at t moment, K
1Is the proportionality coefficient of the exponential curve, K
2Is the decay exponent of an exponential curve, the proportionality coefficient K
1Attenuation index K
2Are all constants.
In the function formula, constant K1、K2Any two real-time sampling values at the moment before the accumulation event occurs are substituted into a function formula for calculation, and the two selected moments can be the 1/3 th position and the 2/3 th position of the acquired moment respectively; constant K1、K2And an approximate value can be obtained through calculation of waveform data at the acquired moment.
In an alternative embodiment of the present invention, as shown in fig. 5, the method further includes:
a differential sampling module 510 for performing digital differential real-time sampling on the electrical signal;
a narrow pulse acquisition module 520 for acquiring two narrow pulses after the occurrence of the accumulation event;
a discrimination module 530 for discriminating the two narrow pulses to obtain two continuous time pulses;
the calibration module 540 performs time calibration on the two continuous time pulses to obtain time information of the first event and the second event.
With regard to the apparatus in the above-described embodiment, the specific manner in which each module performs the operation has been described in detail in the embodiment related to the method, and will not be elaborated here.
For the device embodiments, since they substantially correspond to the method embodiments, reference may be made to the partial description of the method embodiments for relevant points. The above-described embodiments of the apparatus are merely illustrative, and some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the present disclosure. One of ordinary skill in the art can understand and implement it without inventive effort.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
It will be understood that the invention is not limited to the precise arrangements described above and shown in the drawings and that various modifications and changes may be made without departing from the scope thereof. The scope of the invention is limited only by the appended claims.