Achieving sub-100 ps time-of-flight resolution in thick LSO positron emission tomography while reducing system cost: a Monte Carlo study

Purpose. Evaluating the time-of-flight (TOF) resolution improvement that could be obtained using an easy crystal block modification which enables depth of interaction (DOI) assessment and simplifies the detector assembling process. Method. A fast optical Monte Carlo (MC) code was developed. The code was evaluated versus measurements of the energy resolution, number of detected scintillation photons and TOF resolution (TOFr) reported for different crystal photodetector setups. Then, MC simulations were performed for a modified crystal block section of 8 × 8 mm2 in which two partial saw cuts allow light sharing between four detector pixels with a strong dependence on the DOI. Results. Relative differences between MC simulations and reported measurements were always below 10% for any quantities. The simulations showed that the best TOFr was obtained by leaving the partial saw cuts empty. This feature results from the fact that for a slant angle lower than 56 degrees, the scintillation photons undergo a lossless total reflection at the L[Y]SO → air boundary, which is hardly achievable using a reflector material. According to the simulations, this approach allows a TOFr improvement from 163 ps to 90 ps full width at half-maximum using a 22 mm thick LSO 0.2%Ca:Ce crystal coupled to a FBK-NUV-HD silicon photomultiplier. Conclusion. Sub-100 ps TOFr using thick LSO crystal appears achievable using this simple crystal block modification. The method reduces by a factor of 4 the number of crystal pixels to be covered by a reflective material and afterwards joined together. As clinical positron emission tomography contains about 60 000 crystal pixels, this benefit would reduce the assembling cost.