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

The light distribution along one edge of a 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 expected full width at half maximum (FWHM) of the light distribution on the 3 x 30 mm(exp 2) surface of a 3 x 10 x 30 mm(exp 3) 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 x 30 mm(exp 2) 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 x 10 x 30 mm(exp 3) BGO simulation crystal results in an expected depth of interaction resolution of 7.5 mm FWHM.