Computed tomography colonography (virtual colonoscopy): update on technique, applications, and future developments.

Computed topography colonography (CTC) was first described in 1994 as a rapid, non-invasive imaging method to investigate the colon and rectum. Since the advent of CTC, it has been regarded as a potential alternative technique to conventional colonoscopy for detection of colorectal polyps and cancers. Patients undergo standard bowel preparation 24 to 48 hours before the procedure, using either a standard barium enema preparation or balanced polyethylene glycol (PEG) solution. This rapid examination, without the use of sedation or intervention, is well-tolerated by patients. The potential for limited bowel preparation can reduce discomfort associated with traditional purging techniques significantly, and result in an improved perception of the screening study. CTC is performed using a single or multislice CT scanner, with acquisition of volumetric data from the entire colon. Multislice technology enables fast scanning with high resolution. To minimize the radiation dose, efforts have been made to adapt the tube current to the minimum accepted dose while not diminishing study performance. Acquired CT data are transferred onto a dedicated workstation equipped with navigator software, which permits the radiologist to obtain multiplanar reformations as well as construct an endoluminal model of the air-distended colon. Currently, the most widely accepted application for CTC is following incomplete colonoscopy. Other indications that await further clinical validation include colorectal screening. The collective experience of published studies shows CTC to be an accurate tool for detection of clinically significant colorectal polyps. Specificity and sensitivity of CTC are excellent for polyps larger than 10 mm.