CT Colonography Protocols

Specific CT colonography protocols should be established at a local level and should be based on the currently available published evidence. Protocols should address the method of bowel preparation (clean colon vs fluid or faecal tagging), use or not of intravenous contrast, use or not of spasmolytics, method of colon distension, scanning parameters, and methods of interpretation. The specifics of many of these options are discussed in subsequent chapters.

The basic equipment required for the CT colonog-raphy examination is little more than a red rubber catheter with a hand held insufflation bulb similar to that used for barium enema examinations. There are a variety of rectal catheters available of varying size, typically 5-15 mm in diameter. Although we routinely use a balloon-tipped enema catheter, many researchers now avoid balloon insufflation. Traditionally room air has been the gas of choice for colonic insufflation at CT colonography due to its availability and lack of additional expense. However, there is a growing body of evidence advocating the use of carbon dioxide (CO2) which is associated with less abdominal cramps and is more rapidly reabsorbed (Yee and Galindo 2002). CO2 is supplied from a refillable cylinder via a disposable administration set which allows constant gas pressure influx with the facility to record both gas pressures and the volume of CO2 administered.

In our practice, the radiologist is responsible for the practicalities of rectal tube insertion and subse quent colonic insufflation. Depending on departmental time constraints, radiology staffing and volume of CT colonography examinations, consideration may be given to training a dedicated CT colonography technician or nurse and this has been successfully established in some institutions. Furthermore, some centres allow patients to 'self-inflate' in order to improve patient acceptance of the technique.

Much research has been published from both in vivo and phantom studies on the effect of different scan parameters (particularly slice collimation, pitch and mAs) on the quality of CT colonography studies, the associated artefacts and patient radiation doses. As with any radiologic study there will be a trade off between image quality (the diagnostic value of the study) and radiation dose. Typical parameters for CT colonography will be specifically discussed in a later chapter. In establishing a CT colonography service it is important to agree on a standard scan protocol so that patients are imaged and the data set acquired in a consistent and reproducible manner. This creates a uniformity among studies, which facilitates interpretation and comparison with previous studies. Published literature advocates that all patients be scanned in both supine and prone positions as dual positioning allows redistribution of air, stool and fluid and is associated with an increased sensitivity for polyp detection. In one particular study, the reported sensitivity of CT colonography for detection of polyps greater than 10 mm in size was 92.7% for dual positioning compared with 58.5 and 51% for supine and prone scanning alone (YeE et al. 2003).

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