Cold lesions: enhanced contrast using asymmetric photopeak windows.

Cold-lesion contrast resolution was measured as a function of photopeak window position, photon energy, and scatter configuration using a clinical scintillation camera capable of asymmetric photopeak imaging. Two radionuclide solutions, technetium-99m and thallium-201, were studied at five photopeak window positions: the symmetric setting and window settings that were 5%, 10%, 20%, and 30% asymmetrically high (asymmetry was defined here by the intrinsic percentage of count loss relative to the symmetric window setting). Low contrast was examined with small cold-sphere phantoms and large cold-cube phantoms to produce high- and low-scatter conditions, respectively. High contrast was examined with a Hine cylindrical step phantom. Lesion contrast improved markedly with small degrees of window asymmetry but showed less dramatic improvement when positions at the higher degrees of asymmetry were studied. For the high-scatter and low-contrast situations, the maximum improvement in lesion contrast tended to be inversely proportional to lesion size. The maximum improvement in contrast resolution ranged from 10% to 45%, depending on lesion size, scatter configuration, and the radionuclide solution used.