The effectiveness of conventional axial CT in evaluating the kidneys is well established; however, a few limitations adversely affect the ability of this technique to detect and characterize renal masses. Inconsistent patient breathing may result in misregistration artifacts, which can cause portions of the kidneys to be missed during a scanning sequence. Patient motion during scan acquisition (which usually takes approximately 2 sec) can cause significant artifact and degrade images. Even when patients are cooperative, if a small (diameter £ 2x image collimation) mass is not centered exactly within a CT image, partial volume artifacts can result in inaccurate image attenuation measurements. These artifacts can also hinder evaluation of subtle features within cystic lesions, such as septations, wall thickening, and nodularity.