Energy window optimization of PET detectors for SPECT imaging

We developed add-on SPECT imaging capability on an animal PET scanner to implement a cost effective dual modality imaging system. Using PET detectors, designed for 511 keV photons, to acquire relatively low energy gamma rays from SPECT radionuclides has some inherent downsides, including the sub-optimal energy resolution and background radioactivity from ¹⁷⁶Lu, a long-lived radioisotope of Lutetium that is present in most of the modern PET detector scintillators. To address these downsides, this study was carried out to optimize the detector energy window setting in SPECT imaging mode. A LabPET-8 scanner with a pinhole collimator insert was studied through a Monte Carlo simulation package. The energy spectra of detector crystal's intrinsic ¹⁷⁶Lu, and a cylindrical imaging phantom's ^99mTc emissions were simulated and assessed. The optimal energy window setting was derived by searching the lower and upper energy thresholds that maximize the noise-equivalent-count rate (NECR), an index adapted from its PET counterpart. The characteristics of ¹⁷⁶Lu energy spectrum obtained from simulation matches that reported in the literature and reproduces the features of ¹⁷⁶Lu energy spectra measured on the LabPET-8 scanner. The optimal energy window setting derived from the simulation was 86 to 182 keV. In a hot-rod and a uniform phantom imaging study, this energy window achieved better image quality than the other two energy window settings: open-energy window, and 120 to 160 keV. In the open-energy window setting, the lower energy threshold is set to a minimal value above detector electronic noise, and the upper threshold is open.