Influence of Tagged Fecal Material on Detectability of Colorectal Polyps at CT: Phantom Study

OBJECTIVE. The purpose of this study was to determine the influence of tagged material on the minimal radiation dose needed to detect colorectal polyps at CT. MATERIALS AND METHODS. The study was conducted in two phases. In the first, three experienced observers determined the visibility of sessile polyps (6 mm) at five contrast levels (300, 490, 790, and 1,040 HU and air) and five tube charge levels (10, 14, 20, 28, and 40 mAs) in an anthropomorphic phantom. Each polyp was present in one of eight possible locations. The mean tube charge threshold for 90% correct responses was determined for each contrast level. Blinded observers performed independent 2D readings. In the second phase of the study, three 150-cm virtual colons were evaluated at two contrast levels (300 and 480 HU) and at five tube charge levels between 20 and 80 mAs. The three colons contained 18 randomly located polyps. The mean tube charge threshold for 90% sensitivity was determined for each contrast level. RESULTS. In the first phase of the study, the estimated tube charge thresholds for 300, 480, and 790 HU were 24.0, 16.3, and 6.2 mAs. At 1,040 HU and in air, all polyps were detected at the lowest tube charge setting (10 mAs). In the second phase, the tube charge thresholds for 90% sensitivity at 300 and 480 HU were 70 and 35 mAs, respectively. CONCLUSION. If polyps are covered by fecal material, a considerably higher tube charge setting is needed for adequate visualization than is needed for polyps in a completely cleansed colon, especially when the density of the tagged residue is low.