Our objective was to optimize a scanning protocol for CT colonography using a multidetector scanner with an adaptive-array matrix in a phantom study. A colonic phantom with 27 simulated lesions was examined using a multidetector CT scanner (Somatom Plus 4 Volume Zoom, Siemens, Erlangen, Germany). Three scanning protocols were tested: (a) collimation 1.0 mm, slice thickness 1.0 mm; (b) collimation 2.5 mm, slice thickness 3.0 mm; and (c) collimation 2.5 mm, slice thickness 5.0 mm. Image analysis was performed by two radiologists blinded to the construction of the phantom and to imaging parameters. Sensitivity for lesion detection and image quality was assessed. Computed tomographic colonography detected 27 of 27 lesions with protocol 1 (sensitivity 100%); 26 of 27 lesions with protocol 2 (sensitivity 96.3%); and 23 of 27 lesions with protocol 3 (sensitivity 85.2%). Image quality was graded as optimal for protocol 1, good for protocol 2, and poor for protocol 3. When multidetector CT equipment with an adaptive-array matrix is used, protocols with 1.0- or 3.0-mm slice thickness have a sensitivity for the detection of lesions higher than that obtained with a 5.0-mm slice thickness. Image quality decreases progressively when the slice thickness is increased; therefore, in a clinical setting, we recommend the use of a high-resolution protocol (1.0-mm effective slice thickness) with the highest pitch value in order to reduce scanning time. Dose exposure for such a protocol if combined with 80 mAs is within the range limits recommended by the ICRP for abdominal CT scanning.