TOFPET detectors for MR inserts based on strip-line readout and waveform sampling

WE have previously demonstrated a stripline readout for TOFPET detectors that can achieve significant reduction in the number of readout channels while maintaining good detection performance [1-3]. The principle of this time-based multiplexing readout is depicted in Figs. 1(a) and 1(b). As shown, a stripline receives the outputs of several photodetectors (PD). When a PD produces a signal at time t e , two identical pulses are created at the position of the PD but travel in opposite directions along the stripline, arriving the ends at time, say, t 1 and t 2 . Evidently, t 1 = t e + l 1 /v and t 2 = t e + l 2 /v = t 2 + (L − l 1 )/v, where l 1 and l 2 are the path lengths from the PD to the ends, L is the total length of the stripline, and v is the pulse propagation speed on the stripline. It follows that dt = t 2 − t 1 = L/v − 2(l 1 /v) and t = (t 1 + t 2 )/2 = t e + L/2v. As L and v are fixed numbers, the differential time (DT), dt, thus encodes the PD's position on the stripline and the average time, t, the event time. The pulses are highly correlated; therefore, the size of the pulse, which is related to the event energy in PET, can be measured off either one of the pulses by using the peak or integrated amplitude. Now, the path-length between neighboring PDs on the strpiline can be small, for example, 3–6 mm. The propagation speed of the pulse, on the other hand, is approximately half the speed of light. Thus, the difference in the DT reading of two adjacent PDs can be on the order of 20–40 picoseconds (ps) and the DT measurement needs to be highly precise. To achieve this, we proposed to digitally sample the pulses at 2–5 GHz by employing an inexpensive, low power-comsumption waveform-sampling technology such as the DRS4 from the Paul Sherrer Institut (PSI) (see Fig. 1(c)) and apply digital-signal processing to extract accurate time information. Subject to the count-rate requirement, multiple striplines can be simply connected to increase the number of SiPMs per line and therefore the readout is scalable. Also, with the stripline readout the data-acquisition (DAQ) electronics can be detached from the detector (the use of up to 10-meter long cables to connect the DAQ electronics and detector have been successfully tested); this feature can be useful for developing systems that are subject to tight space constraints.