In 2006, over 55,000 Americans are expected to die of colorectal cancer (CRC) (1). It is estimated that there is over a 5% chance that an American will develop CRC in their lifetime and over a 2% chance that an American will die from CRC (2). Fortunately, in most cases, controlling CRC is possible with the proper screening methods and the subsequent removal of suspicious polyps, adenomas, and carcinomas. If CRC is detected at an early stage, the 5-year survival rate is 90%. Unfortunately, less than 40% of colorectal cancers are detected at an early stage. Once the cancer has developed distant metastases, the 5-year survival rate is less than 10% (2).From 1998 to 2002, the incidence rate of CRC decreased by 1.8% per year, which is believed to have occurred due to increased awareness of the importance of CRC screening and consequent polyp removal (2). However, CRC is still predicted to cause about 10% of all cancer-related deaths in the United States in 2006. Relatively accurate screening methods are available to patients, but patient participation in CRC screening continues to be low. Computed tomography colonography (CTC) is a relatively new screening technology that aims to achieve a high patient acceptance, diagnostic accuracy, and screening effectiveness thereby decreasing mortality rates due to CRC.

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Background and Aims The factors that influence the conspicuity of polyps on CT colonography (CTC) are poorly understood. The aim of this study is to compare radiologists' visual assessment of polyp conspicuity to quantitative image features and show the relationship between visual conspicuity and the detection of colonic polyps by computer-aided detection (CAD) on CT colonography. Methods One polyp (size range 6 – 10 mm) was selected from the CTC examination of each of 29 patients from a larger cohort. All patients underwent oral contrast-enhanced CTC with same-day optical colonoscopy with segmental unblinding. The polyps were analyzed by a previously validated CAD system and placed into one of two groups (detected [n=12] or not detected [n=17] by CAD). The study population was intentionally enriched with polyps that were not detected by the CAD system. Four board-certified radiologists, blinded to the CAD results, reviewed 2D and 3D CTC images of the polyps and scored the conspicuity of the polyps using a 4-point scale (0=least conspicicuous, 3=most conspicuous). Polyp height and width were measured by a trained observer. A t-test (two-tailed, unpaired equal variance) was done to determine statistical significance. Intra- and inter-observer variability of the conspicuity scores was assessed using the weighted kappa test. Regression analysis was used to investigate the relationship of conspicuity to polyp height and width. Results A statistically significant difference was found between the average conspicuity scores for polyps that were detected by CAD compared to those that were not (2.3±0.6 vs. 1.4±0.8) (p = .004). There was moderate intra-observer agreement of the conspicuity scores (weighted kappa 0.57±0.09). Inter-observer agreement was fair (average weighted kappa for 6 pair-wise comparisons, 0.38±0.15). Conspicuity was correlated with manual measurement of polyp height (r2 = 0.38 – 0.56, p < .001). Conclusions CAD tends to detect polyps that are more visually conspicuous. Polyp height is a major determinant of visual conspicuity. Therefore, CAD developers may need to specifically target flatter and less conspicuous polyps in order for CAD to better assist the radiologist.