3D nonlinear least squares position estimation in a monolithic scintillator block

The 3D interaction position of a 511 kev gamma ray in a monolithic scintillator can be determined by nonlinear least squares fitting of the measured light distribution at the exit surface to an underlying model. This model is based on solid angles of direct detected light and reflected light, combined with a background due to diffuse reflection. The advantage is that this method does not rely on training data or detector parameter calibration and it estimates x, y and DOI simultaneously. Simulating a 20 × 20 × 10 mm block of LSO mounted onto two adjacent S8550 Hamamatsu APD matrix, we achieved an intrinsic resolution over whole block of 0.81 mm, 0.98 mm and 1.30 mm FWHM in x, y and DOI direction.