Representative study images demonstrating observer performance at varying dose levels. (A) a 6 mm metastasis in segment 3 of the liver was identified by 2, 3 and 1 abdominal radiologists at routine dose with filtered back projection (left, 200 quality reference mAs), 50% routine dose with IR (middle, 100 quality reference mAs), and 50% routine dose with FBP (right, 100 quality reference mAs), respectively, at contrast-enhanced abdominal CT. (B) a 5 mm solid nodule was identified by 2, 3 and 3 thoracic radiologists at routine dose with FBP (left, 70 quality reference mAs), 7% routine dose with FBP (middle, 5 quality reference mAs), and 4% routine dose with IR (2.5 quality reference mAs), respectively, at unenhanced chest CT. (C) an acute infarct of the posterior left insula and adjacent parietal operculum and temporal lobe was identified by 3, 3, 3 and 1 neuroradiologists using routine dose with IR (250 effective mAs), 40% routine dose with IR (100 effective mAs), 20% routine dose with IR (50 effective mAs), and 10% routine dose with IR (25 effective mAs), respectively, at unenhanced head CT.