Optimization of scanning parameters for CT colonography

To determine the optimal collimation, pitch and reconstruction interval for CT colonography, 10 spherical polyps between 1 mm and 10 mm diameter and made of tissue equivalent material with a CT number of 40 Hounsfield units (HU) were placed in the colon of an anthropomorphic phantom. The phantom was scanned at slice thicknesses of 3 mm, 5 mm and 7 mm and pitches of 1.0, 1.3, 1.5, 1.7 and 2.0 on an IGE Hispeed advantage system. Images were reconstructed for each scanning parameter at the minimum intervals allowed along the z-axis. The optimum scanning protocol was assessed by measuring maximum contrast between the polyp and air, sensitivity for detection of each polyp along the z-axis, and relative radiation dose. In addition, images were reviewed separately by two radiologists who graded polyp conspicuity as: 0, not seen; 1, faintly seen; 2, well seen. It was found that varying the scanning parameters caused a marked alteration in the maximum contrast between each polyp and air. For example, for the 5 mm polyp, the range of contrasts from best to worst case was 910-490 HU. It was noted that with contrasts of less than 500 HU, polyps were only faintly seen. A slice thickness of 3 mm with a pitch of 2 offers optimal polyp conspicuity with a relatively low radiation dose, we conclude that scanning parameters can be optimized for threshold contrast, radiation dose and subjective conspicuity. We propose an optimal parameter of 3 mm slice thickness and pitch 2.