Lu-176 based time skew correction method for TOF-PET detectors

Objective. Time calibration is an important procedure for correcting time shifts inherently included in TOF-PET. In this work, we developed a time skew (TS) correction method that uses the intrinsic radioactivity originating from the natural decay of 176 Lu, a constituent element of the scintillator. Approach. The TOF-PET detector has a one-to-one optical coupling between the scintillators and photosensors, enabling independent readout of each channel. For electrical TS correction caused by the electronic circuitry, the electrical TS is estimated from the time-stamp difference between beta particles from 176 Lu, used as a trigger, and the simultaneously emitted prompt gamma rays. The correction is performed in two stages, first within each detector and then between detectors. Although crystal TS originating from the scintillator can also be corrected using 176 Lu, obtaining sufficient statistics requires a long acquisition time. Therefore, we focused on the linear correlation between the light output of each crystal and the crystal TS, and implemented a simple crystal TS correction based on a formulation derived from the existing timing correction method. Main results. When the proposed method was applied to coincidence measurements using a pair of TOF-PET detectors, a coincidence resolving time of 232 ps was achieved using only 176 Lu background data, which was comparable to the 230 ps obtained with the existing method using an external source. Significance. The developed method is expected not only to simplify timing correction but also to reduce downtime during detector replacement.