Pneumocolography and virtual colonoscopy: its clinical place and development

Abstract

Given the increasing strain on colonoscopy services, newer techniques designed to accurately diagnose colorectal cancer or significant colorectal adenomatous polyps should impact substantially on health care costs. These relatively noninvasive techniques, notably magnetic resonance imaging (MRI) and computed tomography (CT) colonography, are likely to be more accurate in displaying the entire colon given the variable rates of total colonoscopy achieved outside of major endoscopic centers. Moreover, although these newer diagnostic strategies still require good quality bowel preparation and smooth muscle relaxation, they are likely to be better tolerated by elderly or frail patients who are unable either to retain barium or undergo colonoscopic examination [1].There are a range of CT-based colonic imaging techniques available. Spiral (helical) CT pneumocolography was introduced by Amin et al.[2] who employed dynamic scanning with intravenous contrast-enhanced thin helical sectioning of the colon without any off-line image post-processing. The Mayo Clinic group led by Hara used CT colography with multiformatted two-dimensional CT images combined with depth-shaded volume rendering for an improved assessment of colorectal polyps [3,4]. Virtual colonoscopy (VC) was a term first used by Vining et al. [5] in 1994, whereby helical CT scanning was combined with advanced post-procedural image manipulation using perspective volume rendering algorithms to create a reconstructed view for hollow viscus tracking of the colon. More recently, endoluminal contrast-enhanced MR reconstructed colonography has been developed for similar purposes [6,7].The advantages of such technology lie in the assessment of the local pathology encountered, defining the presence of significant pericolic lymphadenopathy and in the demonstration of extra-colonic pathology, most notably hepatic metastatic disease. Spiral CT techniques have less motion artifacts, provide better tumor registration, optimal vascular opacification and hepatic parenchymal enhancement. These techniques have been validated in in vitro studies and in cases with proven colonic cancer and polyp disease (providAbstract Colonoscopy is an invasive technique, frequently incomplete and often poorly tolerated in elderly patients. New, less invasive modalities are being developed to diagnose moderate-sized adenomatous polyps and colorectal cancers which involve pneumo-or hydrocolography with high-resolution surface imaging, either by CT or MR scanning. This approach has recently been supplemented by complex image post-processing to provide a form of virtual colonoscopy which takes account of projected colonic direction and which can map for mucosal anomalies. It remains to be seen how sensitive these newer techniques are and whether defined algorithms for colonic representation can be agreed upon which will prove both accurate and costeffective both in symptomatic and screening populations.