SPECT imaging of high energy isotopes and iso- topes with high energy contaminants with rotating slat collimators Roel Van Holen *1 , Steven Staelens 1 and Stefaan Vandenberghe 1 1 ELIS Department, MEDISIP, Ghent University, IBBT, IBiTech, De Pintelaan 185 block B, B-9000 Ghent, Belgium Email: Roel Van Holen * - Roel.VanHolen@UGent.be; Steven Staelens - Steven.Staelens@UGent.be; Stefaan Vandenberge - Stefaan.Vandenberghe@UGent.be; * Corresponding author Abstract Background: Quantitative SPECT imaging is limited by many factors, including penetration of high-energy photons through the collimator. Even small peaks of high energy give large contamination in the photopeak energy window. Rotating slat collimators typically offer a 40- to 50-fold increase in geometric detection efficiency compared to parallel hole collimators while the amount of high-energy contamination is expected to remain similar. The aim of this study is to show that SPECT for isotopes with high energy peaks benefits from the use of a rotating slat collimator by significantly reducing the effect of high-energy contamination. Results: A simulation study was performed for I-123 which has high-energy emissions besides the main energy peak at 159 keV. Results show that the rotating slat collimator is able to reduce the contribution of high energy photons from about 60% to about 8% of all detected photons in the photopeak energy window. This results in a recovered contrast gain of about 25%. For I-131, there was a reduction in contamination from 65% to 49% when using a rotating slat instead of a parallel hole collimator. This resulted in an average hot contrast improvement of 12% and average cold contrast improvement of 8% for tomographic images. Conclusions: Rotating slat collimators have an additional advantage regarding image quality for isotopes with high energy emissions, especially if the contamination arises from photons with energies higher than the main emission energy, due to the relative lower number of measured collimator-penetrating photons with respect to geometrically collimated photons 1