Effect of asymmetric photopeak windows on lesion detectability

Recent advances in scintillation camera technology allow for the clinical application of asymmetric high photopeak windows which may improve spatial resolution, enhance contrast and increase lesion detectability. This research evaluated the response of a scintillation camera operated using symmetric and asymmetric photopeak windows for two radionuclides, Thallium-201 and Technetium-99m; and, using Technetium-99m, a receiver operating characteristic (ROC) study was performed to evaluate cold lesion detectability as a function of photopeak window location. Intrinsic and extrinsic uniformity and spatial resolution measurements were performed using five photopeak window locations corresponding to intrinsic symmetric and 5, 10, 20 and 30% asymmetric windows. Cold lesion contrast was evaluated using three different phantoms and was specified by a ratio of lesion-to-background count densities. The results demonstrated a progressive loss of flood field uniformity for increased window asymmetry. Spatial resolution measured in the presence of scattering exhibited statistically significant improvement with increased window asymmetry. Lesion contrast improved markedly with small degrees of window asymmetry but showed less dramatic changes for the higher degrees of asymmetry. Eight radiologists and three physicists participated in the ROC study. The readers evaluated six image sets representing two lesion diameters, 2.5 and 2.2 cm, imaged using three photopeak windows: symmetric, and 10 and 20% asymmetric