CALIBRATION OF A PET DETECTOR MODULE THAT MEASURES DEPTH OF INTERACTION

We investigate two in situ calibration techniques (using gamma rays in cident from the patient side) for PET detector modules that measure in teraction depth via an analog ratio. For each crystal element in the c amera, we need to determine two unknowns: the gain ratio K of the two detector (PMT and PD) signals and the depth dependence alpha of the si gnals (defined as the ratio of the signal observed when the crystal is excited at the end closest to and furthest from the detector). The li ght collection is assumed to be linearly dependent on distance from th e detector end, in agreement with experimental results. The first meth od extracts K and alpha using (a) the ratio of the PMT and PD signals for interactions that occur in the detector end closest to the patient (the most probable depth) and (b) the shape of the PMT pulse height d istribution which reflects the exponential attenuation length in the d etector. The second method utilizes the fact that E=PD+PMT (the total energy estimator) is independent of Gamma=PD/(PD+PMT) (the depth estim ator) when K is correct, with a distribution position that is alpha de pendent. On simulated data, both the gain ratio and depth dependence a re determined with errors of 3% rms and 2% rms respectively, resulting in minimal degradation of energy and depth resolution.