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CT Colonography with Computer-aided Polyp Detection: Volume and Attenuation Thresholds to Reduce False-Positive Findings Owing to the Ileocecal Valve¹

¹ From the Department of Radiology, National Institutes of Health, 10 Center Dr, Bldg 10, Rm 1C351, MSC 1182, Bethesda, MD 20892-1182 (S.D.O., R.M.S., J.Y.); National Naval Medical Center, Bethesda, Md (P.J.P.); Department of Radiology, University of Wisconsin Medical School, Madison, Wis (P.J.P.); Uniformed Services University of the Health Sciences, Bethesda, Md (P.J.P., J.R.C.); and Walter Reed Army Medical Center, Washington, DC (P.J.P., J.R.C.). Received July 22, 2005; revision requested September 23; revision received November 16; accepted December 20; final version accepted February 17, 2006. Supported by the Clinical Research Training Program, a public-private partnership supported jointly by the National Institutes of Health (NIH), and a grant to the Foundation for the NIH from Pfizer Pharmaceuticals Group; and in part by the Intramural Research Program of the NIH, Warren G. Magnuson Clinical Center. Address correspondence to R.M.S. (e-mail: rms{at}nih.gov).

Purpose: To retrospectively identify volume and average attenuation thresholds for differentiating between ileocecal valve (ICV) and polyp at computed tomographic (CT) colonography with computer-aided detection (CAD).

Materials and Methods: Informed consent (with consent for future retrospective research) and institutional review board (IRB) approval were obtained for the original prospective study. This retrospective study had IRB approval, as well, and was HIPAA-compliant. A total of 496 patients were selected from a larger screening population. CT colonographic images from 394 patients (227 men, 167 women; mean age, 58.0 years; range, 40–79 years) were used as a training set, and images from 102 patients (76 men, 26 women; mean age, 59.8 years; range, 46–79 years) were used as a test set. A series of 2742 volume and attenuation thresholds, for which segmented findings both larger in volume and lower in average attenuation were labeled as ICVs and remaining findings were labeled polyps, were applied to the training set to determine settings with 100% sensitivity for polyp detection and the highest specificity for ICV detection. The optimal settings were then applied to the test set. Significance was assessed with the Fisher exact test, and 95% confidence intervals (CIs) were computed for sensitivity and specificity.

Results: A total of 386 ICVs and 67 adenomatous polyps from the training set and 102 ICVs and 138 adenomatous polyps from the test set could be segmented with a three-dimensional segmentation algorithm. When supine and prone images were counted individually, 746 nonunique ICVs from the training set and 191 from the test set were segmentable. In the training set, a volume of 600 mm³ and an attenuation of 36 HU provided 100% sensitivity (67 polyps; 95% CI: 93%, 100%) and the optimal 83% specificity (618 of 746 ICVs; 95% CI: 80%, 85%). When applied to the test set, this combination provided 97% sensitivity (134 of 138 polyps; 95% CI: 92%, 99%) and 84% specificity (160 of 191 ICVs; 95% CI: 78%, 89%). Differences in sensitivity and specificity in the detection of polyps between the sets were not significant.

Conclusion: Volume and average CT attenuation thresholds can help differentiate most ICVs from true polyps.

© RSNA, 2006

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