Crystal-level timing calibration using cascaded photons of 60Co point source for long axial animal PET system

Abstract Objective. Timing calibration is essential for positron emission tomography (PET) system as it enhances timing resolution to improve image quality. Traditionally, positron sources are employed for timing calibration. However, the photons emitted by these sources travel in opposite directions, necessitating that positrons annihilate at multiple locations to collect coincidence data across a greater number of lines of response (LORs). To overcome this limitation, this study proposes a timing calibration method utilising a 60Co point source. Approach. The 60Co source emits cascaded photons without angular correlation, allowing the collection of coincidence events throughout the field of view (FOV) with a single 60Co point source positioned at the centre of the FOV to determine the timing offsets of the pixels. Leveraging the properties of 60Co, we propose a calibration method and implement it on a long axial animal PET system. Initially, we calibrated the timing offsets of the pixels within two blocks to establish reference detectors, and subsequently employed a 60Co point source to determine the timing offsets of all the pixels in the system relative to these reference detectors. In addition, we evaluated the system's timing resolution before and after the calibration to validate the efficacy of the proposed method. Main results. We measured the timing offsets of the pixels across the entire system, ranging from -5.0 to 2.0 ns. After implementing the timing offset lookup table, the system timing resolution was improved from 6.30 ns before calibration to 1.04 ns. Significance. In this study, the 60Co source is employed for timing calibration, offering the advantages of operational simplicity, broad applicability, and potential application in time-of-flight (TOF) PET.