Count-rate dependent resolution degradation from pulse pile-up on the HRRT

Pulse pile-up at high count rates is one factor that degrades spatial resolution of PET images. Most PET scanners with block detector designs suffer misidentification of event position, moving events from the edge to block center at high count rates. For the HRRT with LSO-LYSO phoswich detectors, there is an extra dimension of pile-up, i.e., inter-layer pile-up, which tends to push events to the LYSO layer at high count rates. A study was performed to characterize the resolution variation across the FOV through reconstructing simulated point sources at given levels of pile-up probability (PP). For 20% PP, in-plane FWHM was increased from 1.40 mm to 1.51 mm at the center of the FOV. At a radial distance of 72 mm (axially centered), the radial FWHM was asymmetric and tangential FWHM was 2.18 mm. At an axial distance of 72 mm (transaxially centered), inplane and axial FWHMs were 1.66 mm and 1.54 mm. Thus, higher pile-up probability resulted in greater resolution degradation as well as more inhomogeneous point spread functions. These results were supported by phantom scan results: radial resolution was degraded by 10% at the transaxial periphery, while the degradation of axial counterpart was less evident.