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A recent trend in PET instrumentation is the use of silicon photomultipliers (SiPMs) for high resolution and time-of-flight (TOF) detection. Due to its small size, a PET system can use a large number of SiPMs and hence effective and scalable multiplexing readout methods become important. Unfortunately, multiplexing readout generally degrades the fast timing properties necessary for TOF, especially at high channel reduction. Previously, we developed a stripline (SL)-based readout method for PET that uses a time-based multiplexing mechanism. This method maintains fast timing by design and has been successfully used for TOF PET detectors. In this article, we present a more systematic study in which we examine how two important design parameters of the readout—the number of inputs on an SL ( $n_{\mathrm {SL}}$ ) and the pathlength between adjacent input positions ( $\Delta \ell $ )—affect its detection performance properties for PET. Our result shows that up to $n_{\mathrm {SL}}=32$ the readout can achieve accurate pixel discrimination and causes little degradation in the energy resolution. The TOF resolution is compromised mildly and a coincidence resolving time on the order of 300-ps FWHM can be achieved for lutetium–yttrium oxyorthosilicate- and SiPM-based detectors. We also discuss strategies in using the readout to further reduce the number of electronic channels that a PET system would otherwise need.

We report the development of a compact, highly modular SiPM-based PET detector that outputs digital data via a standard serial network interface and the use of such detectors in conjunction with a software platform to obtain a scalable system architecture for PET. The detector, approximately 2.5 cm wide and 10 cm long, is made of 4 LYSO/SiPM arrays arranged in a row. Each LYSO/SiPM array is consisted of a 13x13 LYSO array optically coupled to a 6x6 array of SensL MicroFC-30035 SiPMs. For the LYSO array, the crystal size is 1.89x1.89x13 mm3 and the pitch is 2.0 mm. For the SiPM array, the pixel size and pitch are 3.2 mm and 4.2 mm, respectively. The intrinsic resolution, energy resolution and coincidence time resolution of the detector are measured to be about 1.91 mm, 15% and 1.4 ns, respectively. Crystals in a detector exhibit rather uniform performance characteristics. Two demonstration systems are developed using the developed detectors and proposed architecture, including a single-ring system (~10.4 cm diameter, 10 cm length) using 12 detectors and a two-ring system (~19 cm diameter, 20 cm length) using 48 detectors. Imaging data are successfully obtained by using these systems and the results indicate that approximately 1 mm image resolution is achieved.

Recently, we have developed a stripline (SL) readout method and amulti-voltage threshold (MVT) digitizer for PET. The SL readout is amultiplexing method for reading multiple input signals that is easy toimplement and can support timeof-flight (TOF) detection. The MVT methodsdigitally samples a PET signal pulse with respect to a number of voltagethresholds and can be implemented by using the field programmable gate array(FPGA). The use of these two methods can greatly simplify the electroniccomplexity of a TOF PET detector. In this paper, we investigate theperformance properties of the SL readout when applied to different numbersof silicon photomultipliers (SiPM) from SensL. Outputs of the SL readout areacquired by using a 72-ch FPGA MVT board. Our results show that up to 36pixels per SL can be discriminated, a pixel energy resolution of 11-21%can be obtained, and a coincidence resolving time (CRT) of 520 ps can beachieved. Previously, we obtained 320ps CRT when applied the MVT to a singlepixel made of LYSO and SiPM. Therefore, we believe the CRT can be furtherimproved after optimization.

We are developing a MR insert PET detector using SiPM for small animal imaging. Our detector design is based on the strip-line signal readout that we have developed to efficiently handle large amount signals for SiPM-based TOF PET. The stripline readout has an advantageous feature suitable for building PET integrated within MR. In strip-line readout, the signal digitization is accomplished remotely from the detection module, thereby the front-end electronics of the detector within MR is minimal; the interference between two systems, e.g., magnetic field distortion due to PET, can be minimized accordingly. The feasibility of the strip-line readout for PET/MR has been demonstrated in our previous study. The current prototype MR insert PET is comprised of 8 detector modules, which are installed in a plastic cylindrical supporting structure. Each detector module consists of 8x4 LYSO arrays (3x3x10 mm3each) and two Hamamatsu S13361–3050NE-04 MPPC arrays (4x4, 3.2 mm pitch); 32 SiPM signals are routed in 4 strip-lines so that 8 LYSO/SiPM signals in a row are connected to each strip-line. The signals from strip-lines are fed to a 72-ch multi-voltage threshold (MVT) digitizer, which digitizes the input by TDCs implemented in FPGA. Experiments have been carried out to evaluate the detector performance, and image reconstruction also has been performed for a simple shape object. In the paper, the detail on the development of the MR insert PET are explained, and preliminary test results are presented.

Signal multiplexing for SiPM-based TOF PET to achieve high multiplexing ratio and keep the fast timing performance is a challenging issue. We have developed a multiplexing method using a strip-line; multiple SiPMs are connected to a single micro-strip transmission line, and the position of hit SiPM is determined by the signal arrival time difference on the strip-line. The strip-line method is scalable; high multiplexing ratio can be easily achieved by increasing the number of SiPMs connected on a strip-line. To test the scalable feature of the strip-line readout systematically, we have implemented several strip-lines with different trace lengths and varied the number of SiPMs (Hamamatsu S13361-3050NE) on the strip-lines up to 32. Experimental tests have been conducted to assess the influences of the strip-lines on detector performance: position, energy, and coincidence time resolution. Preliminary results from the study are presented, and our ongoing efforts on strip-line readout for TOF PET are discussed.

We report the development of a modularized compact positron emission tomography (PET) detector that outputs serial streams of digital samples of PET event pulses via an Ethernet interface using the UDP/IP protocol to enable rapid configuration of a PET system by connecting multiple such detectors via a network switch to a computer. Presently, the detector is [Formula: see text] in extent (excluding I/O connectors) and contains an [Formula: see text] array of [Formula: see text] one-to-one coupled lutetium-yttrium oxyorthosilicate/silicon photomultiplier pixels. It employs cross-wire and stripline readouts to merge the outputs of the 216 detector pixels to 24 channels. Signals at these channels are sampled using a built-in 24-ch, 4-level field programmable gate arrays-only multivoltage threshold digitizer. In the computer, software programs are implemented to analyze the digital samples to extract event information and to perform energy qualification and coincidence filtering. We have developed two such detectors. We show that all their pixels can be accurately discriminated and measure a crystal-level energy resolution of 14.4% to 19.4% and a detector-level coincidence time resolution of 1.67 ns FWHM. Preliminary imaging results suggests that a PET system based on the detectors can achieve an image resolution of [Formula: see text].

We have developed a signal multiplexing method for a SiPM-based time-of-flight (TOF) positron emission tomography (PET). In the method, multiple SiPM signals are connected to a single strip-line, and the position of hit SiPM is decoded from the arrival time difference measured at the two ends of a strip-line. Since the position resolution on strip-line is directly proportional to the accuracy of the measured time, a high speed (≥ 1 GSPS) waveform sampling electronics, e.g., Domino Ring Sampler (DRS4), has been used to determine the time stamp of the strip-line signal as precise as possible: e.g., the electronic time resolution of the DRS4 is measured to be 5 ps. Although the DRS4 waveform sampling provides a good timing accuracy, its large latency time required for digitizing samples could be a limiting factor for high count rate operation. Therefore, in an effort to increase the counting performance, we have come up with a new design of the strip-line board that can work with commonly used time-to-digital converters (TDC) with 50-100 ps electronic time resolution, which provide high counting capability. Hamamatsu MPPC 4×4 arrays (S12642-0404PA-50, 3×3 mm2 area for each) are employed in the new strip-line board; 32 MPPC signals are routed to 4 strip-lines on the board. For verification purpose, we have conducted experimental tests to readout the current strip-line board by using the OpenPET electronics, which is a general purpose data acquisition developed for nuclear medicine research community. In the test, LYSO scintillator arrays (3×3×20 mm3 each) are coupled to MPPCs on the strip-line board, and the detector is irradiated by a 22Na source with 10 μCi activity. The initial results obtained by using OpenPET are very promising: each MPPC on the strip-line is well identified with 1.2 mm FWHM resolution, and 13.0% FWHM of energy resolution for 511 keV gamma. The results are comparable to the ones by using DRS4 waveform sampler running at 2.5 GSPS.

We are developing a time-of-flight (TOF) positron emission tomography (PET) scanner for brain imaging using SiPMs. In our development, we have adopted a strip-line based signal multiplexing method for efficient channel reduction for SiPMs; the method is using a single strip-line on which multiple SiPMs are connected for sharing readout, and the position of hit SiPM is decoded from the arrival time difference measured at the two ends of the strip-line. We have conducted experimental tests for evaluating the performance of the detectors based on the strip-line signal readout. Hamamatsu MPPC array (S12642-0404PA-50, 3.2 mm pitch) is employed in the detector module; 32 MPPC signals from two MPPCs arrays are routed to 4 strip-lines on the current strip-line board. For the tests, two detectors are used in a coincidence setup, and an array of 4×4 LYSO scintillator (3×3×20 mm3, 3.2 mm pitch) is coupled to the MPPC array in each detector. A 22Na source with 3 μCi activity is used, and the waveform of the detectors signals are collected by using a Domino Ring Sampler (DRS4) evaluation board running at 2.5 giga sampling rate. The preliminary results show that the position on the strip-line is determined with ∼1.0 mm precision from the measured differential time on the strip-line. The average energy and coincidence time resolution are measured to be ∼13% FWHM and ∼726 ps FWHM, respectively, at 511 keV photo-peak. A systematic investigation of the effects of strip-line on detector performance is carried out in parallel by using a specially designed strip-line board. The test results is presented in the paper.

Based on the multi-voltage threshold (MVT) method, we have proposed the FPGA-only implementation for positron emission tomography (PET) detectors. It realizes required comparators and time-to-digital convertors (TDCs) on FPGAs. Thus, its performance can be easily enhanced by optimizing the inside algorithm without hardware modifications or improved with the development of FPGA technologies. In this paper, we leverage the advantages of the FPGA-only implementation to develop a new digitizer board that can provide 72 digitizing channels. This digitizer board significantly increases its readout density than before. And the power consumption is also reduced from 330 mW per channel to 150 mW per channel. By connecting the board to a pair of 6 × 6 LYSO/SiPM arrays, a micro-system, useful for coincidence detection and imaging small objects, is built. We also introduce a TDC multiplexing method when implementing MVT digitizers which apparently reduces the required FPGA resources by 70%. These freed resources can be reused to further improve the integration of readout system. With the micro-system, our preliminary results indicate a coincidence timing resolution of ∼530 ps (FWHM) and an energy resolution of ∼15% (at 511 keV) when the signal rate is 0.9 Mcps, regardless whether TDC multiplexing method is used.