The virtuosity of virtuality or how real is virtual colonography.

T he term ‘‘virtual colonoscopy’’ was initially coined by Vinig et al in 1994 who demonstrated the feasibility of creating three dimensional pictures of the colon using spiral computed tomography (CT) technology. Since the term ‘‘virtual colonoscopy’’ is misleading and implies a colonoscopic procedure rather than generation of images, this methodology should be referred to as virtual colonography (VC). VC can be performed using CT or magnetic resonance imaging (MRI). In this issue of Gut, Ajaj and colleagues present an evaluation of a new MR colonography technique employing a water enema for contrasting and extending the colon [see page 1738]. The so called ‘‘dark lumen’’ technique, which was first published in a small feasibility study and further described in a recently published review by the same group, offers the advantage that the lumen of the colon is pictured black whereas the colonic wall as well as polyps are brightly enhanced in T1 weighted sequences following the application of intravenous paramagnetic contrast. So what is the difference between this technique and that used in other published MR colonography studies? The common procedure is still administration of enemas mixed with paramagnetic contrast such as gadolinium, resulting in a ‘‘white’’ lumen in the T1 sequences. The high bright intraluminal signal contrasts with the black signal of the colonic wall; however, air bubbles and residual faeces also appear black with this technique, necessitating collecting data sets from both the prone and supine patient positions to differentiate real ‘‘sessile’’ polyps from ‘‘wandering’’ air bubbles or faeces. Hence the advantages of the ‘‘black lumen’’ are obvious. Because of the bright appearance of the bowel wall in contrast with the dark lumen, most pathological changes within the wall, such as inflammation or polyps, can be better depicted. Furthermore, examination time is shorter as only one dataset (prone position) is necessary. Does this new technique influence patient acceptance for VC compared with conventional colonoscopy? Several studies have so far addressed this issue and demonstrated that patients in general prefer VC. Probably the most embarrassing procedure during VC is colonic insufflation with air, carbon dioxide, or enemas containing contrast media to achieve maximal colonic distension, which also has to be performed in the case of the ‘‘dark lumen’’ technique. However, one has to bear in mind that patients judge bowel cleansing as the most negative experience which none the less has to be performed for VC, similar to colonoscopy. Therefore, the general acceptance for VC would probably markedly increase if it could be performed without bowel cleansing. Research in this area is ongoing and several feasibility studies have been published investigating the so called ‘‘faecal tagging’’ with contrast media, mainly barium, which was administered in the days prior to colonography together with regular meals.