The development of computed tomography (CT) independently by both Godfrey N Hounsfield and Allan M Cormack in 1972 has forever changed the practice of medicine in the detection, surveillance and treatment of disease. In the past three decades, we have seen an explosion in technological innovation, particularly in the field of CT. As CT has become more sophisticated, so has the radiologist in the detection and diagnosis of disease.

In 1994, Vining and Gelfand introduced computed tomographic colonography (CTC), also referred to as virtual colonoscopy (VC), as a tool to evaluate the

Resident in Radiology, Department of Radiology, Brigham and Women's Hospital, 75 Francis Street Boston, Massachusetts 02115 USA K. J. Mortele, MD

Associate Professor of Radiology, Harvard Medical School; Director, Abdominal and Pelvic MRI; Associate Director, Division of Abdominal Imaging and Intervention, Department of Radiology, Brigham and Women's Hospital, 75 Francis Street Boston, Massachusetts 02115 USA

insufflated colon (Vining et al. 1994). Early work in CT colonography involved patient populations with an increased risk of colon cancer with the goal of detecting colorectal cancer. Also, early studies were performed with single detector CT scanners with thick collimated slices and were primarily read in the 2D axial plane. We have advanced significantly from the days of single detector scanners with 4, 8, 16, and 64 multidetector-row scanners now available. Total volume imaging in a single breath-hold, as a result of multidetector-row scanning, has been shown to improve accuracy of polyp detection by decreasing breathing artifacts (Gryspeerdt et al. 2004). Also, due to significant software improvements, post-processing reformations are currently reconstructed in any plane in a manner of seconds (Bruzzi et al. 2001) and continued advancement in 3-D software development has made virtual endo-scopic flythrough of the colon feasible (Siemens 2002).

The practical execution of CT colonography is still somewhat variable: patient bowel preparation, method of insufflating the colon, scanning acquisition parameters and post-processing software vary. Methods of interpretation also vary with some proponents advocating a primary 3D read with 2D images for problem solving versus a primary 2D read with 3D flythrough for problem solving (Dachman et al. 1998). No technique has yet been proven to be superior to any other consistently and differences are seen regionally. Overall, however, the CT colo-nography literature has shown consistent improvement in the sensitivity and specificity of polyp and colorectal cancer detection as the technology has improved.

CT colonography as a screening tool for colon cancer continues to improve and a credible alternate and non-invasive tool to evaluate the colon now exists. Initial studies in the mid- to late 1990s demonstrated sensitivities of polyp detection ranging from 50 to 90% for polyps larger than 1 cm with specificities ranging from 70 to 90% (Van Dam et al. 2004). As the technology and application of

CTC developed over time, the detection of colon cancer and polyps, even those smaller than 10 mm, improved (Yee et al. 2001). In certain patient populations, CTC may in fact be the examination of choice for evaluating the colon, compared to available current alternatives, such as double-contrast barium enema (DCBE), flexible sigmoidoscopy and conventional colonoscopy. Studies have proven that CTC is better at detecting colon cancers and polyps compared to DCBE, and arguably is as good as conventional colonoscopy for the same purposes. CT colonography has yet to be adopted and integrated into the screening algorithm.

This chapter explores the current indications and contraindications of CTC, and provides recommendations regarding which patients are eligible to undergo CTC. Current reimbursable indications in the US by major third party payers are briefly described. Lastly, the current technologies under development with possible future indications are discussed.

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