Low Prevalence Population

9.6.2.1 3D Methods

Pickhardt et al. (2003) studied the accuracy of CT colonography in an asymptomatic population of 1233 patients of average risk for colorectal cancer. A primary 3D endoluminal bidirectional fly-through was used for detection of polyps, after electronic cleansing. Non-visualized areas were presented to the observers after the fly-through (Fig. 9.3). Pickhardt reported a sensitivity and specificity for large adenomatous polyps of respectively 92 and 96%. For adenomatous polyps larger than 6 mm the results were respectively 86 and 80%. The authors concluded that CT virtual colonoscopy with the use of a 3D approach is an accurate screening method for the detection of colorectal neoplasia. It even compared favorably with optical colonoscopy.

van Gelder et al. (2004b) evaluated CT-colonog-raphy with a primary 3D approach, using an unfolded cube display method (Fig. 9.4). The study included 249 surveillance patients, 20 patients having a history of mild symptoms. CT colonography reported a sensitivity and specificity of 76 and 92% for large polyps and of both 70% for medium polyps (between 6 and 9 mm). Van Gelder et al. concluded that CT colonography and colonoscopy have a similar ability to identify individuals with large polyps in patients at increased risk for colorectal cancer.

2D Methods

Less favorable results were reported in studies performed by Johnson et al. and Cotton et al.

Johnson et al. (2003a) studied the accuracy of CT colonography in a population of 703 asymptomatic patients. CT colonography was reviewed, in contrary to the latter two studies, with a primary axial 2D method combined with 2D MPR and 3D for problem solving. This was done by three reviewers. The sensitivity reported for detection of large colorectal polyps was between 32 and 73%, depending on the reader. The specificity ranged from 97 to 98%. For medium sized polyps the sensitivity ranged from 29 to 57%, the specificity from 88 to 95%. The author concluded that in this low prevalence population, the detection rates of CT colonography were inferior to colonoscopy.

Cotton et al. (2004) reported a sensitivity of 52% for large polyps and 32% for medium-sized polyps in a population of 615 patients. The specificity was 96 and 93% respectively. As in the study performed by Johnson, the detection rates of CT colonography were inferior to colonoscopy.

2D vs 3D Methods

Macari et al. (2000) compared the findings of two review methods and conventional colonos-copy in detecting colorectal polyps in 42 patients. In method 1, axial 2D datasets were examined in a cine mode using 3D review for problem solving. In method 2, datasets were examined exactly as in method 1, and subsequent to that review, datasets were examined with simultaneous 3D fly-through and MPR images. Using method 1, three of five medium-sized polyps and one large polyp were detected. With method 2, the same polyps were seen as with method 1. No additional polyps were detected. Macari concluded that primary axial 2D CT colonography with 3D problem solving was comparable to complete 2D and 3D CT colonography in detecting colorectal polyps.

van Gelder et al. (2004c) compared primary 2D evaluation with a primary 3D evaluation method (unfolded cube projection) in a series of 77 patients. Mean sensitivity for large polyps for the primary 3D and 2D review methods were 83 and 72%, respectively. The specificity was 92 and 94% respectively. Fewer perceptive errors, although not statistically significant (p=0.06), were made with the primary 3D method than with the primary 2D method although at expense of a slight increase of the number of false positives.

In contrast to Macari, Van Gelder at al. concluded that 3D review of CT colonography seems to improve polyp detection as fewer perceptive errors are made.

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