Patent Application
20180038966
Silicon photomultipliers (SiPMs), which are also commonly referred to as MicroPixel Photon Counters (MPPC) or MicroPixel Avalanche Photodiodes (MAPD) have become popular for use as photosensors in positron emission tomography (PET) scanners. SiPMs are tiled arrays of up to tens of thousands of avalanche photodiodes of typical size between 10 to 100 microns, connected in parallel on a common silicon substrate and working on common load. The output of an SiPM device is typically connected to a buffer amplifier, which can be implemented as a transimpedance amplifier. In PET detectors, several SiPM devices are grouped together and are optically coupled to scintillator crystals that convert the 511 kiloelectronvolt (keV) annihilation photons, forming a detector block.
In a clinical whole-body PET scanner, which consists of a large number of detector blocks arranged in a ring around the patient bore, each block conventionally shares timing, position and energy readout electronics to reduce the total number of readout channels for cost consideration. The coincidence resolving time (CRT) of each detector block is an important factor affecting the image quality of the reconstructed distribution of the positron-emitting radioisotope in patients.
Compared to conventional vacuum photomultiplier tube (PMT), SiPM has the advantages of compact size and allowing mass production with reduced variability and lower cost per unit of photosensitive area. Other superior characteristics are related to operation and performance, such as: higher photon detection efficiency (PDE), lower bias voltage, better timing resolution, and insensitivity to magnetic fields. However, SiPM has the disadvantages of higher dark count rate, slower fall time of output pulse relative to PMT, and signal-correlated spurious effects such as cross-talk and after-pulsing. These effects are cumulative with the number of SiPM devices connected into a PET detector block, and result in significant timing resolution degradation as the block size is increased.
Embodying devices and methods provide a universal readout circuit design for an SiPM-based PET detector at a high multiplexing level along with methods and procedures for improving coincidence resolving time (CRT) of PET systems. Embodying devices simplify the readout electronic, thus reducing detector cost while maintaining excellent timing performance.
An embodying pixel readout circuit can provide event timing information with minimized dark noise contribution while removing optical crosstalk from other channels. For light-sharing events (e.g., Compton scattering event between adjacent pixels), multiple timing signals can be utilized in the digital domain to derive better estimation of gamma ray (i.e., about a 511 keV annihilation photon) interaction time. For the same event, a block readout circuit can sum the output signal of all SiPMs in the analog domain to achieve minimal quantum noise.
Energy channel 210 includes summer/weighting circuit 212 that provides the control/processing logic circuit 240 with energy level and two-dimensional position (X and Y) of a radiation event on separate outputs. In some implementations, the circuit is configured to generate a weighted sum of the energy signals as well as the row and column coordinates from all SiPMs in the array.
Block timing channel 220 generates a timing signal HitB and a validation signal HitV that are provided to the control/processing logic circuit. In block timing channel 220 the output signals from all SiPMs in the array/block are summed together in summer 221. The summed signal can be used by both a time discriminating circuit for generating timing information and a validation circuit for validation of a true event.
The time discriminating circuit shapes the summed signal with shaping circuit 223 that high-pass filters the summed signal with a short time constant (e.g. 1-10 nanoseconds) to provide a high degree of timing accuracy. The shaped signal is then compared to the current threshold, which is set just above the dark current noise level. If the pulse is above the predetermined threshold of leading edge discriminator 225, the block timing trigger HitB provides timing information to the control/processing logic circuit 240.
The validation circuit of block timing channel 220 includes validity shaping circuit 224 with a relatively long time constant (e.g. 5-25 nanoseconds) for confirmation whether a valid radiation event has been detected by any of the SiPM in the block. The threshold level of validation channel discriminator 226 is set higher than the dark current noise level to generate a validation signal (HitV). This validation signal is also fed to control/processing logic circuit 240 for reduction of dead time. If a block timing trigger HitB is present but the validation signal (HitV) is not present, the pulse is known to be the result of a noise event and rejected for image reconstruction.
For each SiPM in the block, SiPM_i (i=1-N), an output signal from front end buffer 202, is provided to a corresponding pixel channel 230-i (e.g., 230-1, 230-2, . . . , 230-N). Each pixel channel includes a shaping circuit 232, a leading edge discriminator 234, and a delay circuit 236. A corresponding timing signal HitP_i (e.g., HitP1, HitP2, . . . , HitPN) is provided from each pixel channel to control/processing logic circuit 240. To obtain accurate time stamp, the pixel channels are configured with a short shaping time (1-10 ns) and a low threshold.
When the optical signal is of low amplitude with a large decay time constant, e.g. in bismuth germanium oxide (BGO) based detectors, the validation circuit (validity shaping circuit 224 and validation channel discriminator 226) may be designed with a longer shaping time (>10 ns) compared to the shaping time of the pixel channel shaping circuits 232 and block timing shaping circuit 223. Consequently, a higher threshold level could be implemented in the validation circuit, which reduces the count rate while effectively discriminating radiation event with low energy deposition.
Control/processing logic circuit 240 registers the position of pixel channels having hits (i.e., radiation events). The timing, energy, and two-dimensional position information of a radiation event are sent to an external processor (e.g., time-to-digital converter (TDC), field programmable gate array, microcontroller, microprocessor, etc.) for further processing. For single pixel events, logic circuit 240 is configured to send the timing signal of triggered pixel channel HitP to the external processor. Since the dark noise is low and there is no crosstalk, this configuration yields the best timing performance.
For multiple pixel events (e.g., Compton scattering events), logic circuit 240 is configured to record the first two pixel-channel signals (HitP) to cover most of the energy deposition. Using the first two signals from pixel channels, an optimal time stamp may be estimated when combining with the time signal HitB.
Time stamp estimation can be performed by methods in accordance with embodying methods. The time stamp estimation obtained by the embodying methods can be applied to obtain improved CRT, which results in an improvement in signal-to-noise ratio and image quality over conventional methods.
In accordance with embodying methods, for a single pixel event (e.g.,
One method can obtain time stamp directly using the block channel timing signal (HitB). In a second embodying method, timing determination can be made based on the measurement of the different energy deposition in multiple pixels. The timing signal of the pixel channel may be used when it is triggered by an energy deposition close to about 511 keV. Or the timing stamp could utilize the block timing signal when energy deposition is more uniformly distributed on multiple pixels. This timing determination can be based on look-up table data, and/or curve-fitting equation(s). In accordance with a third embodying method, all three timing signals e.g., pixel A, pixel B, and block channel data may be considered together.
In accordance with embodiments, timing information can be recovered by using a look-up table (with interpolation) and/or curve fitting equations applied to energy and timing data for pixel A, pixel B, and block signal depending on different energy range. For example, a multi-pixel event timing estimation can be obtained using data from either pixel A and pixel B, which collects most of optical photons (e.g., about >80%); otherwise the hybrid timing estimation can be obtained using both pixel data and block data.
Major sources of noise in timing determination include quantum noise, dark current/electronic noise, optical photon propagation inside crystal, and timing jitter of SiPM response. Because the block channel readout has dark current noise and electronic noise contribution from a number (N) of pixels and corresponding buffer amplifiers, the variance of block timing signal σBlock2 typically is far greater than the variance of pixel timing signal σPixel2. For multiple pixel events, the variance of pixel timing signal is dominated by the optical photon collection (energy deposition) in individual SiPM device.
The decision process 300 determines first (step 305), whether the radiation event is a single pixel event (see
If the event is a multiple pixel event (i.e., two or more pixels), a determination is made on whether there are more than two pixels (step 315). If the event involves more than two pixels, the time stamp is obtained from the block timing channel (step 320).
If there are only two pixels involved in the event, their respective pixel-channel signals (e.g., time trigger signal TA and time trigger signal TB), along with block signal Tblock are obtained (step 325). The energy depositions in each of the two triggered SiPMs are estimated, step 330, based on the weighted SiPM signal from energy channel 210. The time signals TA, TB from each pixel channel represent an independent measurement of the arrival time of the 511 keV annihilation photon. The estimated energy depositions (e.g. EA and EB) are used to calculate, step 335, an estimated time stamp (Tstamp) as an average of the measured time signals weighted by the inverse of their respective variances.
A weighted mean of the time stamp Tstamp from the same event can be expressed as shown in equation 1:
From these independent measurements (pixel channels and/or block channel) of the same event, the estimated Tstamp presents a reduction in the variance and thus improves the CRT.
Embodying systems and methods can process noisy and slow signals generated by SiPM, to offer improved timing performance with a reduction in the number of electronic channels. This reduction in readout channels, and the ability to accurately process noisy and slow signals can be used to produce low cost non-TOF and TOF-PET scanners while offering good timing capabilities.
In accordance with some embodiments, a computer program application stored in non-volatile memory or computer-readable medium (e.g., register memory, processor cache, RAM, ROM, hard drive, flash memory, CD ROM, magnetic media, etc.) may include code or executable instructions that when executed may instruct and/or cause a controller or processor to perform methods discussed herein such as time stamp determination in PET scanners using individual pixel-based channel and block channel data from a SiPM array, as described above.
The computer-readable medium may be a non-transitory computer-readable media including all forms and types of memory and all computer-readable media except for a transitory, propagating signal. In one implementation, the non-volatile memory or the computer-readable medium may be external memory.
Although specific hardware and methods have been described herein, note that any number of other configurations may be provided in accordance with embodiments of the invention. Thus, while there have been shown, described, and pointed out fundamental novel features of the invention, it will be understood that various omissions, substitutions, and changes in the form and details of the illustrated embodiments, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Substitutions of elements from one embodiment to another are also fully intended and contemplated. The invention is defined solely with regard to the claims appended hereto, and equivalents of the recitations therein.
