Effects of Positron Range and Annihilation Photon Acolinearity on Image Resolution of a Compton PET

In order to assess the effects of positron range and acolinearity on image resolution of a Compton PET scanner designed for sub-millimeter resolution in mice, Monte Carlo simulations were performed using EGS4 code. The PET device consists of a BGO ring (17.6 cm dia., 16 cm length, and 2 cm thickness segmented into 3 mm times 3 mm times20 mm crystals) surrounding a concentric position-sensitive silicon ring (4 cm dia. times4 cm long times1.6 cm thick segmented into 16 layers of 0.3 mm times0.3 mm times1 mm elements) for the scatter detector. For each detected event, interaction position was obtained from the silicon pad in which a single Compton scatter occurs and the BGO crystal which has maximum energy deposited. Acolinearity was considered in sampling the direction vector of annihilation photon pairs. The cusp-like distribution of positron range in water was added in sinogram. An intrinsic detector resolution of 230 mum FWHM is obtained. By considering positron range and acolinearity, the overall image resolutions of ¹⁸F, ¹¹C, ¹³N, and ¹⁵O point sources in water are 360, 440, 490, and 550 mum FWHM, respectively. Image quality was evaluated with 2-D images of cylindrical source for each positron emitter in a water cylinder reconstructed with filtered back projection (FBP). Monte Carlo simulation indicates the blurring effect from positron range of various isotopes dominates the image resolution of the Compton PET instrument. A negligibly small effect on resolution was introduced from acolinearity due to the small diameter of the silicon detector.