Gastrointestinal contrast medium is routinely administered for abdominal CT imaging to distend the bowel and distinguish it from abnormal structures. Oral contrast medium may be negative, e.g. water or methylcellulose preparation, or positive, e.g. barium sulphate suspension or iodinated solution.

Water is particularly useful if examining the stomach, duodenum and pancreatic regions, with methylcellulose better at distending the small bowel (e.g. for enteroclysis/enterography). Mannitol is a negative agent that is less readily resorbed than water and leads to better small bowel distension.

Positive agents have wide applications within abdominal imaging, but barium suspensions should be avoided where there is a possibility of perforation and iodinated solutions should be avoided in hyperthyroidism. Positive contrast media also increase X-ray attenuation and will lead to higher dose settings when using automated exposure control.

Rectal contrast agents may also be used; in particular, the combination of positive oral agent(s) to 'tag' faeces together with insufflation of the colon with air or CO2 is routinely practised in colonography. In certain circumstances, contrast medium may be introduced into other body cavities, including the peritoneum, bladder or a fistula. These situations typically require the use of iodinated contrast medium.

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