Coincidence Time Correction (CTC) method for TOF-PET scanners with correction to account for misalignment of calibration phantom

The Coincidence Time Correction (CTC) calibration of PET scanners is required to detect coincident events and reduce random events. CTC is usually performed using a centrally located radioactive source. High timing resolution TOF PET scanners are sensitive to deviation of the reference source from the scanner´s central line. We have developed an iterative CTC algorithm that corrects on a Line of Response (LOR) basis for shift of the reference source. CTC values are calculated in an iterative process performed on a single data set (no need for multiple acquisitions). The CTC value for every crystal is gradually adjusted at every iteration “i” by a ΔCTC_i. The final CTC value is the sum of adjustments from all iterations. For faster convergence we have separated a low-frequency component of ΔCTC, due mostly to differences in cable lengths, and a high-frequency component, due to the crystal´s individual delays. This separation of components improved convergence by requiring one half of the iterations required without component separation. We showed that a shift of 20 mm of the reference source, representing up to a 130 ps timing shift, is properly corrected by the algorithm.