Time resolution for scattered and unscattered coincidences in a TOF PET scanner

Time resolution is a key parameter in the performance of a TOF PET scanner, and it is usually measured in the system with unscattered data and used as the width of the TOF kernel in reconstruction. In fact, the time distribution of detected coincidence events is a function of the energy of the detected photons, and trues and scatter events have different time distributions. While the reconstruction TOF kernel should match the true, unscattered data, the scatter simulation used for scatter correction should match the time distribution of the scatter data. This work, using experimental measurements and Monte Carlo simulation, confirmed that the time resolution in a PET scanner is dependent on the energy of the detected photon pair. Moreover, it was observed that trues, single and multiple scatter in general have different time distributions. The FWHM of the time distribution of trues and scatter in different configurations of phantoms and energy threshold were evaluated. The width of the time distribution for scatter was observed to be much larger than for trues, particularly because of the effect of multiple scatter, if low energy thresholds are used. On the other hand, the use of a high energy threshold (for example, 435 keV) reduces the timing FWHM of scatter and the weight of multiple scatter, and allows the use of single scatter simulation as a good approximation of the data time distribution in a realistic PET scan.