Monte Carlo optimization of depth-of-interaction resolution in PET crystals

The light distribution along one edge of a positron emission tomography (PET) scintillation crystal was investigated with a Monte Carlo simulation. This position-dependent light can be used to measure the 511-keV photon interaction position in the crystal on an event-by-event basis, thus reducing radial elongation. The expectant full width at half maximum (FWHM) of the light distribution on the 3-mm*30-mm surface of a 3-mm*10-mm*30-mm bismuth germanate (BGO) crystal surrounded by a diffuse reflector was determined to be 3.0 mm. This light distribution does not change as the width (originally 3 mm) is varied from 1 to 6 mm, but decreases monotonically from 3.0 to 1.8 mm FWHM as the height (originally 10 mm) is reduced to 3 mm. Other geometrical modifications were simulated, including numerous corner reflectors on the opposing 3-mm *30-mm surface, which reduced the FWHM to 2.4 mm. The response of a dual-wedge photodiode combined with the predicted light distribution for the 3-mm*10-mm*30-mm BGO scintillation crystal results in an expected depth of interaction resolution of 7.5 mm FWHM.<>