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.