The worst thing about a colon cancer screening examination from a patient’s perspective is preparing for it. This is true for both optical colonoscopy as well as CT colonography (CTC). For the latter, tagging of residual fluid and stool with positive contrast material is essential. Oral tagging is the conventional method for CT colonoscopy, but radiologists at the University of Pisa in Italy believe that rectal iodine tagging can be an effective alternative with better patient acceptance. They report an evaluation of its use in the September issue of the European Journal of Radiology.
To maximize quality of a CTC, adequate bowel cleansing and tagging with barium and/or iodine contrast agents are necessary. The presence of fluid and or fecal residues within the colonic lumen may affect the evaluation of colonic lesions as it may hide or mimic polyps. Fecal tagging enables radiologists to distinguish polypoid lesions from tagged luminal residues. After oral administration of either iodinated contrast media or barium or both, only residual stool and fluid will be marked.
However, the taste of the ionic iodinated contrast agent sodium and meglumine amidotrizoate can be unpleasant for patients, and act like a laxative. With the oral administration of hyperosmolar iodine-based regimens, some patients may experience adverse reactions that include diarrhea, nausea and vomiting to severe reactions including aspiration, severe respiratory distress, and even congestive heart failure. It may also cause disturbances of water and electrolyte balance due to its hyperosmolarity. For this reason, oral administration is not recommended outside a hospital environment.
A more “popular” same-day preparation oral tagging regimen requires eating a low-residue diet and taking a mild laxative at each of three main meals starting two days prior to the exam. On the day of the CTC, patients drink a solution of 50 mL of sodium diatrizoate and meglumine diatrizoate diluted in water at the facility where the exam is to be performed three hours in advance. This can be inconveniencing and a discomfort to patients, whose three hour wait in the radiology department is interspersed with bouts of diarrhea.
Rectal tagging has the advantage of eliminating the three hour “same day preparation” advance wait time. And it enables a colonoscopy to be completed for patients when a conventional optical colonoscopy has been interrupted.
Emanual Neri, M.D., of the Diagnostic and Interventional Radiology – Department of Translational Research and New Technologies in Medicine and Surgery, and colleagues, enrolled 600 consecutive patients scheduled for CTC in a study to evaluate feasibility, homogeneity of fluid tagging, and diagnostic accuracy of CTC performed through rectal administration of a solution of iodinated contrast. Ninety-three patients were randomly selected to receive oral tagging; the remainder had rectal tagging. A total of 70 patients had both types of tagging when two exams for colon cancer screening were performed over a five year time period.
Patients who had positive CDC findings subsequently underwent optical colonoscopy. A total of 15 patients who had oral tagging had a total of 20 colonic lesions. Fifty-five patients in the rectal tagging group had a total of 77 lesions.
Image quality with both types of tagging was comparable. The researchers determined that mean per-polyp sensitivity of CTC with rectal tagging was 96.1% and 89.4% with oral tagging. Specificity was 95.3% and 95.8% respectively. Positive predictive values were 96.0% and 85.0% and negative predictive values were 98.8% and 97.2%.The overall exam time for patients having rectal tagging was about 18 minutes compared to about 3.5 hours for the patients who had oral tagging with the requisite wait time.
The disadvantage of rectal tagging was the need for patients to repeatedly turn themselves while lying on the CT table to ensure homogeneous intraluminal distribution and mixing of contrast medium. This proved to be difficult for elderly patients. Also, intraluminal enhancement with rectal tagging was not homogeneous through the entire colon. But, the authors pointed out, it did not result in lower diagnostic accuracy or image quality. They recommend its use as an alterative and effective method of fecal tagging for CTC exams.
Rectal tagging: a potential alternative to oral tagging for a CT colonography exam. Appl Radiol.