Xishan Sun

Development of an Eight-Channel Time-Based Readout ASIC for PET Applications

An eight-channel readout ASIC has been developed for reading output signals from solid-state photomultipliers for positron emission tomography applications. This ASIC converts both the signal charge and occurring time to digital timing pulses so that only a time-to-digital converter is required for further signal processing. This provides the advantages of simplified circuit design, reduced power consumption, and suitability for applications that have a large number of readout channels. The ASIC uses a fully current mode preamplifier to achieve high bandwidth ( >; 100 MHz), high time resolution (better than ~ 1 ns FWHM), and low power consumption (a few mW/ch). The linear dynamic range of charge measurement is adjustable and can be extended up to 103 pC. The chip has been fabricated with 0.35 μm 2P4M CMOS technology. A test prototype board has been developed and used for ASIC functionality and performance evaluation. Our preliminary studies show that the targets have been successfully achieved.

Development of a prototype PET scanner with depth-of-interaction measurement using solid-state photomultiplier arrays and parallel readout electronics

In this study, we developed a prototype animal PET by applying several novel technologies to use solid-state photomultiplier (SSPM) arrays to measure the depth of interaction (DOI) and improve imaging performance. Each PET detector has an 8 × 8 array of about 1.9 × 1.9 × 30.0 mm(3) lutetium-yttrium-oxyorthosilicate scintillators, with each end optically connected to an SSPM array (16 channels in a 4 × 4 matrix) through a light guide to enable continuous DOI measurement. Each SSPM has an active area of about 3 × 3 mm(2), and its output is read by a custom-developed application-specific integrated circuit to directly convert analogue signals to digital timing pulses that encode the interaction information. These pulses are transferred to and are decoded by a field-programmable gate array-based time-to-digital convertor for coincident event selection and data acquisition. The independent readout of each SSPM and the parallel signal process can significantly improve the signal-to-noise ratio and enable the use of flexible algorithms for different data processes. The prototype PET consists of two rotating detector panels on a portable gantry with four detectors in each panel to provide 16 mm axial and variable transaxial field-of-view (FOV) sizes. List-mode ordered subset expectation maximization image reconstruction was implemented. The measured mean energy, coincidence timing and DOI resolution for a crystal were about 17.6%, 2.8 ns and 5.6 mm, respectively. The measured transaxial resolutions at the center of the FOV were 2.0 mm and 2.3 mm for images reconstructed with and without DOI, respectively. In addition, the resolutions across the FOV with DOI were substantially better than those without DOI. The quality of PET images of both a hot-rod phantom and mouse acquired with DOI was much higher than that of images obtained without DOI. This study demonstrates that SSPM arrays and advanced readout/processing electronics can be used to develop a practical DOI-measureable PET scanner.

TIMPIC-II: The second version time-based-readout ASIC for SSPM based PET applications

A second version ASIC for front-end detector readout, TIMPIC-II, has been developed for Solid-State Photomultiplier (SSPM) based PET applications. It uses the previously developed and evaluated time-based-readout (TBR) architecture. However, several major changes have been made to make TIMPIC-II more flexible and suited for PET applications, including adding a common energy trigger to select the true events and using a constant width integrator to improve linearity. A special logic unit is added to combine the energy and the timing pulses into one output signal that reduces half of the output pins. A 16channel chip has been designed and fabricated with 0.3S,.m 2P4M CMOS technology. The die area is 3mm × 3mm, and the chip is provided in a compact 14mm × 14mm BGA package. TIMPIC-II initial evaluated result shows that the trigger and TBR with control logic function works as designed. And the ASIC specifications including linearity, intrinsic noise and timing jitter, etc. are well achieved as the linear regression R > 0.999 in full dynamic range, intrinsic energy resolution is better than 0.1 % FWHM of SOOpC and the timing jitter standard deviation is 100-300ps for different input signal range. This ASIC is also tested and used for a PET front-end detector module with FPGAbased TDC and acquisition.

Energy and timing measurement of a PET detector with time-based readout electronics

New time-based readout (TBR) electronics has been developed and evaluated for its performance and application for PET detectors. It consists of a leading edge (LE) timing threshold for timing pickoff that provides a signal timing t1; a charge integration followed by a constant discharge between two specific timings t2 and t3 that are used to directly measure the signal energy. The timing-walk error caused by LE from signals of different amplitudes can be measured with t1 as a function of different signal amplitudes that is proportional to the time difference between t2 and t3. With this pre-calibration, timing-walk error can be accurately corrected. Therefore, both signal timing and energy can be accurately measured with digital timing signals without using CFD and ADC. These timing signals were controlled and in principle can be measured by an FPGA that can apply many signal logic and data correction algorithms. A single channel discrete component TBR circuit has been implemented in PC board, and 8-channel ASIC chips have been developed for feasibility evaluations and PET detector applications. Initial functionality and performance evaluations have been conducted. Signal amplitude measurement accuracy and linearity are very good; the measured timing accuracy from a pulse is the same as a standard CFD and reaches to ∼100 ps resolution with the test setup. Both suitable energy and coincidence timing resolutions (∼17% and ∼1.0–2.0 ns) were achieved with PMT or Solid-State PM (SSPM) array based PET detectors. Initial studies to acquire flood source crystal identification map has demonstrated the advantages of applying parallel readout with TBR electronics that read out and process signals from each pixel of SSPM array independently. With its relatively simple circuit and low cost, TBR electronics is expected to provide suitable front-end signal readout electronics for compact photon detectors such as SSPM array that require large number of output channels and demand high performance in energy and timing.

Development of an 8-channel time based readout ASIC for PET applications

An eight-channel readout ASIC has been developed for reading out signals from Silicon Photomultiplier (SiPM) for PET applications. It converts both signal charge and occurring time to digital timing pulses and only needs TDC. It has the advantages of simplified circuit design, reduced power consumption and suitable for large number of readout channels. The ASIC uses a fully current mode preamplifier to obtain high bandwidth (>100MHz) with a few mW/ch power consumption. The dynamic range for charge measurement is adjustable and can be extended up to 103 pC. The chip has been fabricated in 0.35μm 2P4M CMOS technology. A test prototype board for ASIC evaluation has been developed and the preliminary tests show the time jitter is 150ps (rms) by electronics only with injected 30 ns rise time signals, and the coincidence timing is about 1.7 ns (FWHM) for 511 keV photo peak events with LYSO/SiPM detectors. The charge dynamic range is measured to be ∼10bit for electronics only, and better than 1% for detector signals. A simple theoretic analysis shows that the timing and charge resolutions are limited dominately by the detector dark noise in our test and will be studied further. Preliminary evaluations show that the ASICs functionalities and performance targets have been successfully achieved.

Capacitor based multiplexing circuit for silicon photomultiplier array readout

Several different multiplexing readout methods have been investigated for reading out silicon photomultiplier (SiPM) arrays. However, it is still challenging by using these methods to maintain signal integrity for overall good signal and imaging performance while reducing the number of readout and processing channels. One common issue to resistor based multiplexing method is the position-dependent timing shift among different channels, which can in principle be calibrated and corrected but add complexity to the detector calibration and operation process, and can be very difficult to apply for a practical PET system for routine imaging applications. To solve such and other problems, we explored a capacitor-based multiplexing method for our PET detector to read SiPM with a common cathode which has not been addressed previously. To achieve good detector performance, we required output signal without undershot/overshot suited for excellent charge integration, and without timing shift among different channels. The design applies a capacitor network to divide the charge of signals from a SiPM into two branches, with the division of charge based on the position of the SiPM in the network. Only one capacitor value is needed. The number of readout channels can be reduced from N×N to 2N. Evaluation circuit was tested with pulsed signals and a practical PET detector which consisted of an 8×8 SiPM array and LYSO scintillator array. The results showed that signal rise and fall times from different channels were the similar, no output signal undershot, and no timing shift among different channels. The resistive and capacitive multiplexing methods were compared for their noise level, energy resolution, rise time, and timing resolution as function of channel numbers. Capacitive multiplexing method shows better noise and timing performance with much better timing and energy consistent from all detector area. A PCB circuit board with capacitor multiplexing has been developed for PET detector applications.

SU-C-207A-06: On-Line Beam Range Verification with Multiple Scanning Particle Beams: Initial Feasibility Study with Simulations

PURPOSE To investigate the feasibility and requirement for intra-fraction on-line multiple scanning particle beam range verifications (BRVs) with in-situ PET imaging, which is beyond the current single-beam BRV with extra factors that will affect the BR measurement accuracy, such as beam diameter, separation between beams, and different image counts at different BRV positions. METHODS We simulated a 110-MeV proton beam with 5-mm diameter irradiating a uniform PMMA phantom by GATE simulation, which generated nuclear interaction-induced positrons. In this preliminary study, we simply duplicated these positrons and placed them next to the initial protons to approximately mimic the two spatially separated positron distributions produced by two beams parallel to each other but with different beam ranges. These positrons were then imaged by a PET (∼2-mm resolution, 10% sensitivity, 320×320×128 mm^3 FOV) with different acquisition times. We calculated the positron activity ranges (ARs) from reconstructed PET images and compared them with the corresponding ARs of original positron distributions. RESULTS Without further image data processing and correction, the preliminary study show the errors between the measured and original ARs varied from 0.2 mm to 2.3 mm as center-to-center separations and range differences were in the range of 8-12 mm and 2-8 mm respectively, indicating the accuracy of AR measurement strongly depends on the beam separations and range differences. In addition, it is feasible to achieve ≤ 1.0-mm accuracy for both beams with 1-min PET acquisition and 12 mm beam separation. CONCLUSION This study shows that the overlap between the positron distributions from multiple scanning beams can significantly impact the accuracy of BRVs of distributed particle beams and need to be further addressed beyond the established method of single-beam BRV, but it also indicates the feasibility to achieve accurate on-line multi-beam BRV with further improved method.

Development of compact DOI-measurable PET detectors for simultaneous PET/MR Imaging

It is critically needed yet challenging to develop compact PET detectors with high sensitivity and uniform, high imaging resolution for improving the performance of simultaneous PET/MR imaging, particularly for an integrated/inserted small-bore system. Using the latest “edge-less” SiPM arrays for DOI measurement using the design of dual-ended-scintillator readout, we developed several compact PET detectors suited for PET/MR imaging. Each detector consists of one LYSO array with each end coupled to a SiPM array. Multiple detectors can be seamlessly tiled together along all sides to form a large detector panel. Detectors with 1.5x1.5 and 2.0x2.0 mm crystals at 20 or 30 mm lengths were studied. Readout of individual SiPM or capacitor-based signal multiplexing was used to transfer 3D interaction position-coded analog signals through flexible-print-circuit cables to dedicated ASIC frontend electronics to output digital timing pulses that encode interaction information. These digital pulses can be transferred to, through standard LVDS cables, and decoded by a FPGA-based data acquisition positioned outside the MRI scanner for coincidence event selection. Initial detector performance measurement shows excellent crystal identification even with 30 mm long crystals, ~18% and 2.8 ns energy and timing resolutions, and around 2-3 mm DOI resolution. A large size detector panel can be scaled up with these modular detectors and different PET systems can be flexibly configured with the scalable readout electronics and data acquisition, providing an important design advantage for different system and application requirements. It is expected that standard shielding of detectors, electronics and signal transfer lines can be applied for simultaneous PET/MR imaging applications, with desired DOI measurement capability to enhance the PET performance and image quality.

Monte Carlo simulation based scatter correction in 3D list-mode image reconstruction

We developed a Monte Carlo simulation based scatter correction method for 3D list-mode image reconstruction, and tested the method with Monte Carlo simulations. First, an emission image without scatter correction was reconstructed using MLEM. A transmission image was generated with the CT image. Then, based on the emission and transmission images, GATE was used to simulate coincidence events with their line-of responses (LORs) grouped according to their spatial positions (e.g. interaction positions or detector modules). The scatter ratio (scatter vs total coincidences) in each LOR group was calculated and stored in a scatter table. Finally, the scatter table was applied in a new image reconstruction to correct the scatter on the basis of each LOR group. The method was implemented in a simulated brain-size PET, with 300×300×100 mm3 FOV, 2×2×30 mm3 LYSO crystals, and 5 mm depth-of-interaction (DOI) resolution. Images of a 150×150×80 mm3 PMMA phantom inserted with three different radioisotope distributions were studied, including a point source array, a hot rod matrix, and a uniform source. We used detector module as the criteria to group LORs. With scatter correction, image resolution was almost the same as measured by point sources at different FOV positions; hot-rod sources showed visually improved image quality with reduced background noise; image SNR of the uniform source was not impacted. This method has been successfully implemented in the brain-size PET with improved image quality. It can be potentially applied to other list-mode 3D PET systems, with considering the accuracy and variation of scatter ratio in LOR grouping.

Design and development of novel and practical PET detectors for advanced imaging applications

New DOI-measurable PET detectors have been designed, developed and evaluated with advanced silicon photomultiplier (SiPM) and readout technologies. The detector consists of an 8×8 array of 1.5×1.5×20 or 1.5×1.5×30 mm3 LYSO scintillators which is optically coupled to a 4×4 array of 3×3 mm2 SiPM array at each crystal array end through a 2 mm thick optical plate. Scintillator surfaces, reflectors and coupling were designed and fabricated to reserve the air-gap to achieve high depth-of-interaction (DOI) resolution and other detection performance. The insensitive edges around each detector is less than 0.2 mm, making it practical to seamlessly tile detectors together to assemble a large size detector panel for developing a PET system. A 16-ch ASIC based PCB readout electronics was developed to solve the challenging SiPM array readout problem. Each compact PCB contains 4 ASIC chips and one detector-level FPGA, with analog signal being inputted from each SiPM array through a flexible printed circuit cable, converted to digital timing pulses, processed online by FPGA to record interaction information (energy, timing, and position), and transferred through a fast LVDS connection to system FPGA for event selection and data acquisition. Initial measurements showed excellent crystal identification with all crystals were clearly separated from each other in a flood source image, with resolutions of energy, timing and DOI were around 17%, 2.7 ns and 2.0 mm (mean value), respectively. Overall, comparing to the previous prototype DOI detectors we developed, the new detector is simplified in design without using complicated light guide yet with significantly improved DOI resolution (from ~5 to ~2 mm), more compact packaging for making a large size flat-panel detector, and more integrated and fast readout electronics. The new detector and readout is expected leading to an advanced PET with leapfrog imaging performance improvement.