Downscatter contamination from high-energy photons of 124iodine in 2D and 3D PET

Iodine is an attractive isotope for medical imaging; ¹²⁴I makes available to PET a large set of compounds and protocols traditionally used in X-ray and SPECT imaging and offers additional clinical opportunities in molecular and genomic imaging. However, ¹²⁴I has a very complex decay scheme in which higher energy contaminants that compete for abundance with the positron decay. This paper analysis, via Monte Carlo studies, the impact of contaminates from ¹²⁴I on 2D and 3D PET imaging systems. Various geometries and detector materials have been simulated. Results show two trends. The presence of a significant amount of lead or tungsten in the field-of-view in the 2D scanner makes it more sensitive to this contamination by causing downscatter into the PET energy window; improved energy resolution decreases the sensitivity to the contamination. Overall, the contamination in the primary energy window is in the range of 20% higher in a 2D imager then in a comparable 3D geometry, an effect that increases with improving energy resolution. A significant proportion of the gamma events are emitted in coincidence with positrons and present an additional and challenge to imaging, but the physical advantage of 3D remains, such that the noise in the coincidence data relative to 2D is reduced from the ¹⁸F or pure-beta emitter imaging case. Varying with energy resolution, the relative improvement in SNR over the 2D geometry ranges from 11%-61%.