Random coincidence estimation from single event rates on the Discovery ST PET/CT scanner

Random coincidence estimation from single crystal event rates has been implemented for the Discovery ST PET/CT scanner. The ST histograms every qualified photon (i.e., those meeting the energy threshold requirements) for each crystal in the detector ring, providing the capability to calculate random coincidence rates for each line of response in the scanner. The estimate is corrected for random coincidences lost at either end of the major system clock cycle (often referred to as the "picket fence" effect), and, at extremely high count rates, for coincidence events lost to capacity limitations of the coincidence electronics. The randoms estimate is not corrected for changes in the activity distribution due to decay during the course of the acquisition; this effect is small (<1% for an acquisition of one-half of a half-life), and the correction is only accurate if physical decay is the only process by which activity is leaving the LOR. Software to implement the randoms from singles estimate has been incorporated into both the 2D and 3D reconstruction processing paths for Discovery ST. The accuracy of the correction has been validated by comparing the estimate to a delayed event channel randoms estimate.