The effect of preparation is important, not only in terms of cleansing of the colon of residual stool, but also of residual fluid. Most CTC reports have been done on consenting research patients who are already scheduled to undergo a non-research conventional colonoscopy. The preparation is often chosen by the gastroenterologist. A colonic lavage with 4 L of polyethylene glycol is know to leave significant residual fluid which can hide polyps unless a unique strategy is used to reveal polyps submerged in fluid. Granted, in a well-distended colon, the use of both supine and prone views will theoretically move the fluid and reveal the polyp on at least one view, reader confidence is affected by not seeing the lesion on both views and sometimes the segment is collapsed on one view, thus precluding the possibility of correct diagnosis. Therefore in CTC screening, when the preparation is chosen by the radiologist a relatively "dry" preparation is preferred, using phosphosoda or magnesium citrate. Bisacodyl tablets can also be used to reduce residual rectal fluid. Polyps submerged in fluid can be revealed by use of additional decubitus views, use of intravenous contrast or use of fluid tagging with barium and/ or water soluble orally administered contrast. Thus differences in these techniques can affect the diagnostic quality of the CTC and affect the validity of combining data from different investigators.
Recent evidence points to the efficacy of stool and residual fluid tagging, independent of the use of electronic subtraction (or "cleansing") by specialized software programs (Iannaconne et al. 2004; McFarland and Zalis 2004). Data should be analyzed separately for patients undergoing tagging regimens and those that do not. Details of tagging options are discussed elsewhere in this book.
Controversy also remains regarding the use of spasmolytics: glucagon (primarily used in the United States) and Buscopan (used in Europe, but not approved for use in the United States). These drugs may affect patient comfort (and thus the reporting of patient satisfaction) and colonic distention. Spasmolytics may theoretically improve or hinder colonic distention. On the one hand, colonic spasm is minimized. However the ileocecal valve may become incompetent allowing reflux of gas into the small bowel. The effect of ileocecal valve reflux will depend on the method of distension. If a mechanical pump is used which keeps the colon distended at a set pressure, then the reflux may not hinder colonic distension. If manual distension is used, additional gas insufflation just prior to scanning will be needed to compensate for the gas refluxed into the small bowel. For these reasons, reporting of medication used, its route and dosage and timing of administration, is also important.
The CT parameters will also impact the quality of the 2D, multiplanar and 3D reconstructed images. The details of scanner type, collimation, pitch, detector array for volume scanner, gantry rotation time, kVp, mA, mAs, radiation dose modulation, reconstruction kernel, reconstruction slice thickness and interval, pixel size, will all be important. Because of the complexity of these parameters, often data combined from different institutions is combined based on broad groups focusing on only key parameters such as: single vs multidetector scan; collimation over 3 mm vs 3 mm and less; high dose vs low radiation dose (a very subjective term since a wide range of dosages have been used). Other seemingly less important factors, many nevertheless, are significant, such as the method of breath hold. Older papers used multiple breath holds or a breath hold followed by quiet breathing. Volume CT scanners such as 16, 32, 40 and 64 slice scanners are capable of shorter breath holds (depending on the detector array and collimation used) thus minimizing artifacts due to respiratory motion.
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