Bias Voltage Calibrations for a 1-millimeter Resolution Clinical PET System

We are developing a 1-millimeter resolution clinical PET system dedicated to loco-regional (e.g., head/neck, breast, etc.) cancer imaging. The prototype of the system consists of 1,024 detector modules, each of which employs two position-sensitive avalanche photodiodes, coupled to 8 × 8 lutetium-yttrium oxyorthosilicate crystal elements. The analog signals from the detector modules are processed by ASICs, and the output signals of the ASICs are digitized by analog-to-digital converters. It is critical to properly bias position-sensitive avalanche photodiodes to maximize the performance of the detector modules. In this work, we have calibrated the detector module bias voltages so that the digitized values of 511 keV photopeak signals from all PSAPDs align to 1,500 ADC index units. Seven measurements of 511 keV photopeak ADC index units were taken for different bias voltages in 5 V steps. We fit exponentials to the voltage versus photopeak ADC index unit plots to extract calibration bias voltages. The average of the resulting bias voltages across the total of 2,048 PSAPDs in the system was 1726.7 V. Average 511 keV energy resolutions of PSAPDs, which showed more than or equal to 20 V of differences between coarse-calibrated bias voltages and fine-calibrated bias voltage, improved from 18.5 % to 14.5 %.