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

The light distribution along one edge of a PET (positron emission tomography) scintillation crystal was investigated by Monte Carlo simulation. This position-dependent light distribution can be used to measure the position of the 500-keV photon interaction, in the crystal on an event-by-event basis, thus reducing radial elongation. The predicted full width at half maximum (FWHM) of the light distribution on the 3×30 mm surface of a 3×10×30 mm bismuth germanate (BGO) crystal surrounded by diffuse reflector is 3.0 mm. This light distribution is constant for widths (originally 3 mm) between 1 and 6 mm, but decreases from 3.0 to 1.8 mm FWHM as the height is reduced from 10 to 3 mm. Other geometrical modifications were simulated, including corner reflectors on the opposing 3×30 mm surface. A promising geometry is a 2.2×5×30 mm BGO crystal, for which a 2.2 mm FWHM light distribution is predicted, which should yield a PET detector module with a depth of interaction measurement resolution of 3.6 mm FWHM