Fecal tagging: MR colonography without colonic cleansing

Colorectal cancer, mostly arising (>90%) from preexisting adenomatous polyps, continues to be the second leading cause of cancer death. Magnetic resonance colonography (MRC) permits accurate detection of colonic polyps with a diameter larger than 10 mm. Because residual colonic stool cannot be differentiated from polyps, MRC requires a clean colon. However, the rigors associated with colonic cleansing considerably reduce patient acceptance. The need for colonic cleansing could be eliminated, if stool were to acquire a signal intensity different from polyps and identical to the enema used to fill and distend the colon. In principle, there are two approaches to this concept of fecal tagging: dark polyps surrounded by bright stool and a bright enema, and bright polyps surrounded by dark stool and a dark enema. The first approach has been evaluated with some success. Gadolinium (Gd)-DOTA was administered as an oral contrast agent with meals preceding MRC based on the administration of a Gd-based enema. The high cost of Gd-based contrast has limited the clinical utility of this technique. In the second approach patients are provided with barium as an oral fecal tagging agent to render stool dark, and barium for the enema is used to distend the colon during MRC. The colonic wall and polyps arising from it can be made visible after intravenous administration of Gd-based extracellular contrast. This method provides sufficient contrast between the darkened colonic lumen and the brightly enhanced colonic wall to permit virtual endoscopic rendering. Preliminary results showed an exact correlation with findings of conventional endoscopy and surgery. Fecal tagging obviates bowel cleansing and therefore should enhance patient acceptance for MR colonoscopy. Barium as the tagging agent is promising because it is inexpensive, commercially available, and characterized by an excellent safety profile.