Effects of asymmetric photopeak windows on flood field uniformity and spatial resolution for scintillation cameras

The use of asymmetric high windows is known to reduce the amount of scatter that is present in an image. In this study intrinsic and extrinsic flood field uniformity and line spread functions were used to quantitate the effects of window asymmetry, using energy and renormalization maps based on Tc-99m. Five different windows with settings of intrinsic symmetric, 5, 10, 20, and 30% asymmetric (defined as count loss relative to the symmetric window) were used. Extrinsic measurements were obtained with several combinations of forward and backscatter. Uniformity was expressed in terms of the Uniformity Index and NEMA parameters. Spatial resolution was evaluated from FWHM, FWTM, and MTF values. For Tc and Tl-201, flood field uniformity showed a generally linear decrease as a function of asymmetry. Although intrinsic field uniformity for Tc was better than for Tl at each setting, in the presence of scatter Tc values deteriorated more rapidly with increasing window asymmetry and in some conditions were inferior to those of Tl. I-131 uniformity indices were poorer than those of Tc and Tl for all settings. FWHMs and FWTMs showed improvement with window asymmetry only in the presence of scatter, FWTMs for Tc decreased by 15% at 10% asymmetry but only reached 20% at 30% asymmetry. By comparison, Tl values decreased by 17% at 10% asymmetry and exhibited no further change up to 30%. Cold sphere lesion contrast followed a similar pattern for Tc and Tl. These results demonstrate that improvement in spatial resolution and contrast can be achieved with the use of asymmetrical windows, but potential benefit in terms of lesion detectability must be considered from the perspective of decreased field uniformity