Visualization modes for CT colonography using cylindrical and planar map projections

Purpose: The purpose of this study was to demonstrate the limitations to th e effectiveness of CT colonography, colloquially called virtual colonoscopy (VC), for detecting polyps in the colon and to describe a new technique, m ap projection CT colonography using Mercator projection and stereographic p rojection, that overcomes these limitations. Method: In one experiment, data sets from nine patients undergoing CT colon ography were analyzed to determine the percentage of the mucosal surface vi sible in various visualization modes as a function of field of view (FOV). In another experiment, 40 digitally synthesized polyps of various sizes (10 , 7, 5, and 3.5 mm) were randomly inserted into four copies of one patient data set. Both Mercator and stereographic projections were used to visualiz e the surface of the colon of each data set. The sensitivity and positive p redictive value (PPV) were calculated and compared with the results of an e arlier study of visualization modes using the same CT colonography data. Results: The percentage of mucosal surface visualized by VC increases with greater FOV but only approaches that of map projection VC (98.8%) at a dist orting, very high FOV. For both readers and polyp sizes of greater than or equal to 7 mm, sensitivity for Mercator projection (87.5%) and stereographi c projection (82.5%) was significantly greater (p < 0.05) than for viewing axial slices (62.5%), and Mercator projection was significantly more sensit ive than VC (67.5%). Mercator and stereographic projection had PPVs of 75.4 and 78.9%, respectively. Conclusion: The sensitivity of conventional CT colonography is limited by t he percentage of the mucosal surface seen. Map projection CT colonography o vercomes this problem and provides a more sensitive method with a high PPV for detecting polyps than other methods currently being investigated.