Evaluation of imaging parameters for scintillation camera imaging of positron-emitting radionuclides in the Compton region

The interest to use scintillation camera systems for imaging 511 keV photons from positron emitting radionuclides has increased due to the greater availability of ¹⁸FDG and it´s increased utility in nuclear medicine. A prerequisite is a collimator designed for such high photon energies. However, even for relatively modern cameras, hardware limitations can make such high photon energy imaging difficult. This work has evaluated the use of an high energy window in the Compton region for imaging. The optimal energy window was determined by a Monte Carlo simulation study using sixty-four sets of energy windows (15% and 45%) where the centre of the window ranged from 511 keV to 196 keV in steps of 5 keV. For each window, projections of a line source in a homogeneous cylinder phantom of Lucite were acquired and the FWHM, FWTM, and the system sensitivity were calculated. Also, events from scattered photons were separated allowing the calculation of the scatter fraction. The simulations were validated by measurement of ¹⁸F for some energy windows. Simulation studies show that the system spatial resolution FWHM for windows in the Compton region is comparable to photopeak acquisitions. An optimal window was around 320 keV. The authors are basing these results on WB simulation studies with a clinical ¹⁸FDG distribution and a nonhomogeneous anthropomorphic computer phantom