Recommended Technique

It is clear form the above discussion that the practitioner has many options available when attempting to optimise colonic distension prior to scan acquisition. While some techniques have an established evidence-base, others are largely a matter of personal preference. Whatever regime is chosen, it is clear that good distension is absolutely pivotal to the success of any CTC examination. The follow section will provide the reader with details of the authors' preferred methods.

Written patient information is provided and posted to the patient along with the bowel preparation approximately two weeks prior to examination. On the day of the examination, the radiologist or radiology resident greets the patient, checks they have understood what the procedure involves, they have no contraindications to hyoscine butylbromide or intravenous contrast and are happy to proceed. They are asked to evacuate the rectum just prior to entering the scanner room. In an attempt to improve compliance, patients are routinely warned that they will experience abdominal bloating and mild discomfort and the importance of good colonic distension for the accurate interpretation of their scan is stressed.

The authors favour carbon dioxide as the distension agent and in the past have slowly administered this via gentle compression of a filled enema bag as described in the section above. However, we now utilize an automated insufflator delivering carbon dioxide via a narrow calibre catheter, reserving a balloon catheter for the very occasional patient with anal incontinence. The patient is asked to lie supine initially so that an intravenous catheter can be sited if intravenous contrast is to be used, and for administration of Buscopan if not contraindicated. The patient is then asked to lie in the left lateral decubi-tus position and a lubricated rectal catheter, already attached to the insufflation device, is inserted. For all patients the maximum pressure shutdown dial is set initially at 25 mm Hg. Insufflation is commenced and after approximately 1.5 L have been introduced, the patient is turned into the supine position. Distension is then continued, titrated to patient tolerance, and sustained if rectal pressure remains low (i.e. below 15 mm Hg), providing the patient does not complain of undue abdominal discomfort. Once either the patient is mildly uncomfortable or intraluminal pressure consistently remains above 25 mm Hg (such that further insufflation is automatically prevented, which usually occurs following administration of 24 L of gas), a first CT scout image is acquired. If distension is deemed optimal by the supervising radiologist, then the full supine scan is acquired in a single breathhold. As long as the patient is comfortable the authors prefer to leave the insufflator device switched on during scanning, but turn down the pressure limit to 15 mm Hg so that this minimal rectal pressure is maintained. If the patient is uncomfortable, the device is paused to ensure that no further gas is insufflated until such time as the patient is happy for it to be recommenced. If distension is suboptimal despite the device recording rectal pressures exceeding 25 mm Hg, the catheter is checked and repositioned because it may be that its tip is occluded against the rectal wall. If unsuccessful, we will either then reposition the patient (e.g. prone) or gently manually palpate the abdomen to encourage redistribution of gas.

Once the supine study has been acquired, the rectal catheter is left in situ and the patient asked to turn prone. A second scout is performed and if distension is deemed suboptimal, the pressure limit will be increased to 25 mm Hg to encourage further gas insufflation. A further scout is performed and when this demonstrates optimal insufflation, the second study is acquired. The examination is then complete and the rectal catheter removed. The patient is reassured that much of the insufflated gas will be absorbed (rather than expelled), and that any abdominal cramping should ease within a few minutes.

5.11

Conclusion

There are several strategies available to the practitioner for optimising colonic distension and, if used appropriately, the time and effort invested will be rewarded by easier and more accurate interpretation. The authors recommend ongoing quality assurance measures are adopted by all departments performing CT colonography in order to minimise failure rates due to inadequate distension. Finally, safety concerns about CT colonography will likely diminish with more judicious use of rectal balloon catheters.

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