Prospects for time-of-flight PET using LSO scintillator

We present measurements of the timing properties of lutetium orthosilicate (LSO) scintillator crystals coupled to a photomultiplier tube (PMT) and exc ited by 511 keV photons. These crystals have dimensions suitable for use in PET cameras (3x3x30 mm(3)). Coincidence timing resolution of 475 ps fwhm i s measured between detectors utilizing two such crystals, significantly wor se than the 300 ps fwhm predicted based on first principles for small cryst als and measured in 3 mm cubes. This degradation is found to be caused by t he scintillation light undergoing multiple reflections at quasirandom angle s within the scintillator crystal, which has two effects. First, it slows d own the effective information propagation speed within the crystal (to an e ffective (n) over cap=3.9-5.3). Since the incident annihilation photon trav els with n=1, information from interactions at different depths arrives at the PMT with different time delays. Second, the random nature of the reflec tion angles (and path lengths) introduce dispersion and so a 10%-90% rise t ime of 1 ns to the optical signal.