The importance of achieving optimal colonic distension prior to CT colonography cannot be overstated, and the editors have justifiably devoted an entire chapter to the subject. Optimal luminal distension enables the reader to rapidly and confidently assess the colon, and undoubtedly improves diagnostic accuracy (Chen et al. 1999; Fletcher et al. 2000; Yee et al. 2003). Conversely inadequate distension may obliterate the colonic lumen and results in wall/haustral fold thickening (Fig. 5.1) thereby variously hiding or mimicking colorectal neoplasia (Fig. 5.2a,b) (Fletcher et al. 1999; Macari et al. 2001; Fenlon 2002). Interpretation times are increased when the colon is poorly distended but of greater importance is the potential to miss significant colonic pathology (Fig. 5.3a-c), occasionally rendering the examination non-diagnostic (i.e. necessitating requiring repeat examination or endo-scopic referral) and sometimes frankly misleading. For example a review of missed significant lesions

D. Burling, MD; S. Taylor, MD; S. Halligan, MD Intestinal Imaging Unit, Level 4V, St. Mark's Hospital, Watford Road, Harrow, HA1 3UJ, Middlesex, UK

from a recent large prospective multicentre trial of CT colonography (Rockey et al. 2005; Paulson et al. 2004) found suboptimal distension, along with poor preparation, was implicated as a contributing factor in 16 of 28 (57%) false negative examinations.

Several strategies have been shown to improve distension, the most notable being dual patient positioning (i.e. prone and supine scanning). Use of faster multi-detector row scanners and administration of intravenous spasmolytics (see section below) may also help. However, despite these strategies, suboptimal distension is unfortunately frequently encountered in day-to-day clinical practice.

Unlike bowel purgation, which is largely determined by intrinsic patient-related factors such as compliance and bowel transit time, colonic distension is greatly influenced by the colonographic practitioner present at the time of examination. Distending the colon may at first appear a relatively simple and trivial procedure and, intuitively, skills acquired for barium enema should be easily transferable to CT colonography. However, the colonographer is dis-advantaged. Unlike barium enema, where real time fluoroscopic screening is utilized to ensure satisfactory segmental distension, colonic insufflation prior to CT colonography is not performed under direct visualisation: A scout view must suffice. As a result, inadequate distension may only be fully appreciated once full data acquisition is complete. Furthermore, the aim for CT colonography is not simply adequate inflation but optimal distension, preferably with 'pencil-thin' wall and haustral folds. Colonic neopla-sia is generally better seen when the colon is well distended, a statement that holds true for both primary 2D and 3D analysis (Pickhardt 2004). Indeed, sessile and flat lesions with minimal protrusion into the lumen or causing subtle focal wall thickening may be invisible without optimal distension.

The purpose of this chapter is to provide an evidence-based review of strategies and techniques intended to safely optimise colonic distension, and to draw readers' attention to current areas of controversy.

Fig. 5.1. A poorly distended sigmoid colon (arrows) resulting in bulbous haustral folds on this supine scan demonstrates how inadequate distension can thwart confident and time efficient interpretation. Subsequent optical colonoscopy was unremarkable
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