Randoms estimation for list-mode reconstruction for the ECAT HRRT

A randoms correction method has been implemented for list-mode image reconstruction of data from the ECAT HRRT (High Resolution Research Tomograph). The random coincidence rate for each list-mode event is estimated from the relevant block singles rates, the estimated coincidence timing window (τ), and the fractional efficiency of the crystal within the block. From a decaying phantom study, τ ranged from 2.80-2.97 nsec, consistent with the expected value of 3 nsec, having only a slight variation with count rate. Because the fractional crystal efficiencies were also found to be count rate dependent with higher LVSO/LSO relative efficiency at higher count rates, we have determined that a count rate dependent correction is necessary. No photon energy/object size dependence was found for the efficiencies.