A time-based single transmission-line readout with position multiplexing

We developed a time-based single-transmission-line readout method for time-of-flight positron emission tomography (PET) detectors. The 2D position of a silicon photomultiplier (SiPM) array was encoded in the upper and lower widths of a specially prepared L-shaped tag pulse followed by the original scintillation signal. A PET detector setup was configured using a 4 × 4 array of LSO crystals optically coupled one-to-one to a 4 × 4 SiPM array. Two pulse width modulator circuits were employed per SiPM anode signal channel and a total of 32 width-modulated digital pulses were summed and merged with a delayed common-cathode signal. The final output was analyzed using timestamps crossing two-level threshold voltages. All 16 crystals were clearly separated on a positioning map. The average energy and coincidence time resolutions were 15.0 ± 1.1% and 288.7 ± 29.3 ps after proper correction process, respectively. A 3D position decoding capability was also shown by the remarkable discrimination performance in a phoswich PET detector setup (LSO and LGSO), resulting from well-preserved scintillation signals. The proposed method enables a time-based single-channel readout with 3D gamma ray interaction position decoding capability without compromising on detector performance. This method provides gamma ray energy and arrival time information as well as 2D and depthwise interaction positions of the phoswich detectors through one channel readout. Thus, channels can be reduced by at least 4-5 times compared to typically employed charge-sharing-based position multiplexing method; this significantly reduces the burden of data acquisition on the PET system.

Optical Simulation and Experimental Assessment with Time-Walk Correction of TOF-PET Detectors with Multi-Ended Readouts

As a commonly used solution, the multi-ended readout can measure the depth-of-interaction (DOI) for positron emission tomography (PET) detectors. In the present study, the effects of the multi-ended readout design were investigated using the leading-edge discriminator (LED) triggers on the timing performance of time-of-flight (TOF) PET detectors. At the very first, the photon transmission model of the four detectors, namely, single-ended readout, dual-ended readout, side dual-ended readout, and triple-ended readout, was established in Tracepro. The optical simulation revealed that the light output of the multi-ended readout was higher. Meanwhile, the readout circuit could be triggered earlier. Especially, in the triple-ended readout, the light output at 0.5 ns was observed to be nearly twice that of the single-ended readout after the first scintillating photon was generated. Subsequently, a reference detector was applied to test the multi-ended readout detectors that were constructed from a 6 × 6 × 25 mm³ LYSO crystal. Each module is composed of a crystal coupled with multiple SiPMs. Accordingly, its timing performance was improved by approximately 10% after the compensation of fourth-order polynomial fitting. Finally, the compensated full-width-at-half-maximum (FWHM) coincidence timing resolutions (CTR) of the dual-ended readout, side dual-ended readout, and triple-ended readout were 216.9 ps, 231.0 ps, and 203.6 ps, respectively.

DOI estimation through signal arrival time distribution: a theoretical description including proof of concept measurements

The challenge to reach 10 ps coincidence time resolution (CTR) in time-of-flight positron emission tomography (TOF-PET) is triggering major efforts worldwide, but timing improvements of scintillation detectors will remain elusive without depth-of-interaction (DOI) correction in long crystals. Nonetheless, this momentum opportunely brings up the prospect of a fully time-based DOI estimation since fast timing signals intrinsically carry DOI information, even with a traditional single-ended readout. Consequently, extracting features of the detected signal time distribution could uncover the spatial origin of the interaction and in return, provide enhancement on the timing precision of detectors. We demonstrate the validity of a time-based DOI estimation concept in two steps. First, experimental measurements were carried out with current LSO:Ce:Ca crystals coupled to FBK NUV-HD SiPMs read out by fast high-frequency electronics to provide new evidence of a distinct DOI effect on CTR not observable before with slower electronics. Using this detector, a DOI discrimination using a double-threshold scheme on the analog timing signal together with the signal intensity information was also developed without any complex readout or detector modification. As a second step, we explored by simulation the anticipated performance requirements of future detectors to efficiently estimate the DOI and we proposed four estimators that exploit either more generic or more precise features of the DOI-dependent timestamp distribution. A simple estimator using the time difference between two timestamps provided enhanced CTR. Additional improvements were achieved with estimators using multiple timestamps (e.g. kernel density estimation and neural network) converging to the Cramér-Rao lower bound developed in this work for a time-based DOI estimation. This two-step study provides insights on current and future possibilities in exploiting the timing signal features for DOI estimation aiming at ultra-fast CTR while maintaining detection efficiency for TOF PET.