Calibrations of the Integrated Circuit Readout for a High Resolution Preclinical CZT PET Imaging System

This work focuses on the development of a software control scheme which is capable of (A) automated ASIC calibration, (B) per channel analog readout electronics programming, and (C) executing user imaging protocols for a preclinical sub 1-mm spatial resolution CZT PET system. The system is built from 24 dual-CZT detector modules; each CZT crystal is 40 x 40 x 5 mm3 and the electrodes deposited on each crystal in the dual module are arranged back-to-back in an anode-cathode-cathode-anode configuration. Each CZT crystal has 39 anode strips, and 38 electrodes. Anode strips are 100 μm width with a 1 mm pitch and steering electrode has 400 μm width. The cathode side of our detector module is composed of 8 strips which are 4.9 mm width with a pitch of 5 mm. Crystals are configured in an "edge-on" orientation in which photons enter the 5x40 mm2 and pass through a much thicker CZT volume resulting in a detection efficiency equivalent to that of 2 cm of LSO scintillator. The cathode and anode strips are supported by a custom front end electronics board designed around the RENA-3 (Readout Electronics for Nuclear Applications) ASIC. A sub-assembly of 2 x 18 RENA boards has been constructed and currently contains a total of 2592 channels. The full system will have 4x48 RENA boards which is supported by 13,824 channels. Given the need to set preamplifier, amplifier, and triggering parameters per channel, a robust operations infrastructure is necessary to program, and operate each RENA 3 board. We have developed an iterative, multi-channel programming and DAC threshold calibration scheme and performed preliminary testing on our CZT PET sub-assembly. Using a subset of detectors, a total of 432 channels were programmed and calibrated. Programming and calibration required a mean time of 5.17 minutes per ASIC with a standard deviation of 0.39 minutes. Once the code was validated, a portion of the system was DAC calibrated and activated to acquire anode energy spectra using Ge-68 and Cs-137.