• Which of four combinations of low residue diet, reduced laxative dose and oral contrast agent is best tolerated by patients and optimally prepares the colon prior ro CT colonography
  

• to compare the diagnostic performance of CT colonography after reduced bowel preparation and faecal tagging with conventional colonoscopy in patients undergoing both procedures
  

CT colonography has been shown to be accurate in detection of colorectal neoplasia in enriched patient populations and is generally preferred by patients to conventional endoscopy and barium enema. It is advocated as an acceptable alternative for colorectal cancer screening and is increasingly used in the investigation of symptomatic patients. Ideally, like conventional colonoscopy, CT colonography requires a clean colon, free of residual stool and fluid. There is however good evidence that the arduous bowel purgation regimes required may produce adverse effects such as electrolyte imbalance, and are more detrimental to overall patient experience and compliance than any subsequent investigation. An ability to perform CT colonography with reduced bowel purgation, whilst maintaining acceptable diagnostic accuracy, would undoubtedly make the technique a more attractive proposition both for population screening and investigation of symptomatic patients.

Interest has been generated in the use of orally ingested dilute barium or ionated contrast medium prior ro CT colonography to "label" or "tag" residual fluid or faecal matter reduced laxative. Such oral contrast agents are highly attenuating to X-rays such that labeled residual bowel contents appear white on CT scanning and are readily distinguished from true colonic pathology.

Despite the early promise of a few limited studies, there is no consensus as to the optimum oral contrast type, dose and concentration. Anecdotally barium tends to best label solid residue whereas iodinated contrast best labels fluid, but this assumption has not been proven. Furthermore it is known that iodinated contrast tends to draw fluid into the bowel, producing a wet colon, which may not be suited to CT colonography. However, this indrawing of fluid by iodinated contrast such as gastrograffin produces a mild laxative effect, which may obviate the need for further formal purgation.

The study aims to establish the optimum reduced preparation-tagging regimen, establish patient experience and document diagnostic performance compared to conventional colonoscopy.