Extension of the SSS PET scatter correction algorithm to include double scatter

PET scatter correction typically employs a single Compton scatter simulation which is scaled to the emission data to account for multiple scatter. However, Monte Carlo simulations suggest that for modern scanners with good energy resolution and a narrow photopeak energy window, only double scatter makes a significant contribution to this multiple scatter. Thus a single plus double scatter simulation could be a very good approximation to the total scatter. Consequently, we have extended our single scatter simulation (SSS) algorithm to include an estimate of double scatter contributions. These are efficiently computed by considering a subset of pairs of the single scatter points. By fully accounting for the physics, an absolute scaling is achieved, and no sinogram tail fitting is required. The double scatter simulation (DSS) results agree well with independent Monte Carlo simulations. Computation time for SSS+DSS increases by a small multiple of the time required for SSS only, but remains clinically viable. Results for simulated and measured PET studies are shown, and the absolutely scaled SSS+DSS is compared to the standard relative scaling of SSS only.