Count-Rate Dependent Component-Based Normalization for the HRRT

Component-based normalization is an important technique for PET scanners with a high number of lines of response (LOR), e.g., 4.5 times 10⁹ for the HRRT. It reduces the problem of measuring the sensitivity of each LOR to that of estimating the individual crystal efficiencies(epsiv), e.g., 119808 for the HRRT. We propose a component-based method to compute epsiv for the HRRT. In addition, the block design of the HRRT produces pulse pile-up which causes apparent changes in epsiv with count rate. These effects occur within the block and between the front (LSO) and back (LYSO) crystal layers. We use a rotating source to measure the values and a decaying uniform phantom to account for variations with count rate. The computation of efficiencies is achieved with ~1% statistical noise with an acquisition of ~1 h. Count rate dependency of epsiv is implemented as a linear model in terms of block singles rate. Four approaches to modify epsiv with count rate were compared. Among them, an independent parameter for each crystal produced the best results, both visually and quantitatively. Failure to account for the count rate dependency in epsiv leads to high resolution artifacts in the reconstructed images, most visible in the transverse plane, in the center of the field-of-view.