Development and Performance Optimization of a Multiplexed DOI-Encoding PET Detector Using the TOFPET2 ASIC

Depth-of-interaction (DOI) positron emission tomography (PET) detectors are essential in high-resolution small animal PET. The dual-ended readout detector configuration provides high DOI resolution; however, signal multiplexing schemes are often needed to reduce manufacturing costs. TOFPET2 is a low-cost, multichannel application-specific integrated circuit (ASIC) designed for individual silicon photomultiplier (SiPM) readout, with performance in multiplexed configurations not well known. This study evaluates a highly multiplexed, high-resolution, dual-ended readout PET detector using TOFPET2. The PET detector was constructed with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$23\times 23$ </tex-math></inline-formula> array of LYSO crystals (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.785\times 0.785\times $ </tex-math></inline-formula> 20 mm3) coupled to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6\times 6$ </tex-math></inline-formula> SiPM arrays (pitch <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${=}3$ </tex-math></inline-formula>.3 mm) via 1.2 mm PMMA sheets. Custom boards reduced 72 SiPM signals to eight (four per end) before feeding into TOFPET2 channels. The detector was biased at 27, 27.5, and 28 V, with ASIC discriminator thresholds optimized based on flood map quality. Optimal flood maps were obtained at 27.5 V. The mean energy and DOI resolution were <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10.5~\pm ~1.6$ </tex-math></inline-formula>% and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.3~\pm ~0$ </tex-math></inline-formula>.3 mm, respectively, with a coincidence time resolution (CTR) of 1.3 ns between two detectors. This work demonstrates the feasibility of employing TOFPET2 to achieve high spatial, energy, and DOI resolution in a 9:1 multiplexed system.