Performance of Positron Imaging Systems as a Function of Energy Threshold and Shielding Depth

The effect of energy discrimination and shielding on positron imaging data quality was investigated using a detector pair to simulate a ring positron emission tomograph. Formulas are presented relating the sensitivity, random fraction, and scatter fraction for a detector pair to the same parameters for a ring system. Data were fitted to detector pair expressions for the variation of the above parameters with shielding depth in order to obtain information on the effect of energy threshold level. These fitted curves were used to determine the sensitivity, random fraction, and scatter fraction, as well as an overall data quality factor as a function of energy threshold level and shielding depth. Data were obtained for both NaI(T1) and BGO detector types using activity levels in the range of 1.5 μCi/cm3. Results show that for NaI(T1) detectors, the lowest possible energy threshold level is optimal, with the corresponding optimal shielding depth determined by the level of activity to be imaged. For BGO detectors, a tradeoff exists between energy thresholds of 100-400 keV and shielding depths of 15-30 cm with smaller shielding depths requiring higher energy thresholds.