Double scatter simulation for more accurate image reconstruction in positron emission tomography

Quantitative reconstruction algorithms for positron emission tomography (PET) require estimating the scattered annihilation radiation contributions to the measured data. This is commonly done by simulating only the single scatter contribution, then scaling this component to the data to account for multiple scatter. This scaling step is sometimes problematic due to inconsistencies and statistical noise in these data. Monte Carlo (MC) simulations suggest that for modern scanners with good energy resolution and a narrow photopeak energy window, multiple scatter is dominated by double scatter, so that a single plus double scatter simulation could account for all but a few percent of the scatter arising from within the field of view (FOV) of the simulation. Consequently, we have extended our single scatter simulation (SSS) algorithm to include double scatter contributions. These are efficiently computed by considering a subset of pairs of the single scatter points. This simulation discriminates the time-of-flight offsets of the scattered radiation as well. By fully accounting for the physics, an absolute scaling is achieved such that no scaling relative to measured data is required to model scatter from within the FOV. The double scatter simulation (DSS) results agree well with independent MC 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 phantom and human PET studies are presented.