Improvement in spatial resolution of dual-ended readout of 100 mm long LYSO Crystals through use of systematic crystal surface roughing

We are investigating using dual-ended readout of axially-oriented long thin scintillator crystals in detectors for a compact geometry preclinical PET system. In this work we explore the effects of using regular patterns of surface roughing to modulate the light transport in 100 mm long LYSO crystals with the goal of improving the spatial resolution of this dual-ended readout design. The long surfaces of 3 × 2 × 100 mm
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LYSO crystals, initially polished on all sides, were systematically roughed in a band pattern with 0.3 mm bands of roughed surface spaced at 5 mm increments over the central 8 cm of the crystal length. Cases of 1 to 4 surfaces of the crystal roughed with this pattern were explored. Each crystal was wrapped in Teflon and read out at either end using a position sensitive photomultiplier tube (PSPMT). An electronically collimated beam of 511 keV photons was aimed perpendicularly to the long direction of the crystal and data acquired at nine locations from 1 to 9 cm of the crystal length in steps of 1 cm. The ratio of two PMT signals was used to find the axial position of interaction and spatial resolution in light sharing direction while their sum was used to determine the light output and energy resolution. The spatial resolution improved from an average of 9.1 mm with no surface treatment to 5.6, 4.7 or 4.2 mm for banding patterns on 1, 2 or 4 surfaces, respectively. As expected, the total light collected decreased with increasing number of roughed surfaces, resulting in a decrease in the energy resolution from 11.1% FWHM for the untreated crystal to 12.0%, 13.5%, and 13.6% for the patterns on 1, 2, and 4 surfaces, respectively. When the spatial resolution is corrected for the beam width, we estimate the resolution for the case of 4 banded surfaces to be 3.4 mm FWHM. This promising improvement in spatial resolution fulfils the design criteria of our proposed compact preclinical PET system based on axially-oriented dual-ended readout detectors.