Effect of lower energy threshold on single and multiple scatter distributions in positron volume imaging

Simulations of the scatter distributions in the Advance tomograph have been performed using the SimSET software package. The Zubal phantom was used, centered at the heart with activity only in the heart or in all soft tissues except the lungs. Data were binned axially for: (1) -7.6 to 7.6 cm (the full axial field of view); (2) -0.884 to 0.844 cm; and (3) 2.53 to 4.22 cm. Simulations were performed with both a 45.6 cm long (full object) and a 15.2 cm long (short object) section of the phantom with 250 million decays. The total scatter to trues ratio for the full object varied from 1.20 (300 keV, all soft tissues) to 0.35 (425 keV, heart only). The ratio of single scatters from the short object simulation compared to the total scatters from the full object varied from 40% at 300 keV to 60% at 425 keV. While the multiple scatters were well estimated with a Gaussian convolution of the single scatter events (for the full object), significant errors may occur if the attenuation and activity outside the FOV is not included in the estimation of the single scatters.