Despite unanimous consensus in favour of dual position scanning amongst 27 international experts in 2003 (Barish et al. 2005), a minority still promulgate single position scanning.
However, the evidence in support of dual position scanning is strong (Yee et al. 2003; Fletcher et al. 2000; Chen et al. 1999; Morrin et al. 2002). These studies have found that colonic segments obscured by either faecal residue/fluid or poor distension will be revealed by redistribution in the complementary position; for example, the rectum is usually optimally distended with the patient lying prone while the transverse colon is usually best distended on the supine acquisition since it is least dependent in this position. Unsurprisingly, improved segmental visualisation significantly increases polyp detection. An early study (Chen et al. 1999) using manual insufflation to distend the colon with room air, showed the majority of colonic segments (59%) were inadequately distended if only one acquisition (prone or supine) was assessed. However when data from both acquisitions were combined, a large majority of segments (87%) were adequately distended and polyp detection rates were increased. Later studies (Yee et al. 2003; Fletcher et al. 2000) confirmed these findings and demonstrated significantly improved polyp detection with dual position scanning owing to improved distension and therefore segmental visualisation.
Advocates of single position scanning stress the additional radiation burden of scanning patients twice routinely, and choose to perform an additional scan only if visualization is deemed inadequate on the initial study. By necessity, this approach requires constant supervision. Moreover, studies of luminal visualisation (Yee et al. 2003; Chen et al. 1999; Morrin et al. 2002) suggest that a second scan will be required frequently. It is possible that in the future scanning may be performed routinely in a single position but this will require improved automated insufflation methods and possibly electronic subtraction of tagged faeces and fluid. However, at the time of writing most authorities would strongly recommend routine dual position data acquisition.
Almost all published descriptions of CT colonog-raphy techniques recommend acquiring the supine dataset first, followed by the prone scan (Ristvedt et al. 2003; Yee et al. 2003; Gluecker et al. 2003; Svensson et al. 2002). This anecdotal recommendation is likely a result of insufflation being performed in the supine position, which subsequently dictates the initial scan acquisition. Also, if intravenous contrast is utilised, this is frequently administered with the first scan and usually the supine position is chosen for convenience despite some evidence suggesting distension may be better overall on the prone scan (Morrin et al. 2002). However, in the authors experience, the grade of distension is independent of the initial scan position (Burling et al. 2005). Left lateral decubitus positioning is an effective alternative to the prone position for the second CT acquisition (following the supine scan), particularly for immobile and elderly patients, or patients with respiratory disease (Gryspeerdt et al. 2004).
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