To determine whether colonic bubbles associated with CT colonography performed with iohexol for fecal/fluid tagging could be reduced by adding simethicone to the standard cathartic preparation.

CT colonography (CTC) is a recently developed radiological examination to find colorectal neoplasia. Fecal/fluid tagging using oral administration of contrast is an essential procedure for CTC. Iohexol, which has recently started being used as an agent for fecal/fluid tagging, has great advantages as it has much more tolerable taste and much lower rates of adverse effects such as clamping or diarrhea compared with traditionally used Gastrografin/Gastroview. However, iohexol is frequently associated with an occurrence of a lot of bubbles in the colon, which makes CTC interpretation more time-consuming and laborious.

Past experience in colonoscopy field suggests that simethicone, a safety-proven highly inexpensive over-the-counter medicine, might resolve this issue. Given the fact that patient convenience is an important factor for a successful CTC and the fact that time-intensive nature of CTC interpretation is one of the major deterrents to wide spread adoption of CTC while CTC also needs to be as time efficient as possible in order to effectively serve the role of population screening for colorectal cancer, investigating the effect of simethicone to prevent the colonic bubbles on the time efficiency of CTC interpretation would be important. If simethicone can resolve the colonic bubble problem, CTC can be performed more conveniently for the patients as well as for the interpreting radiologists.

This study is to determine whether colonic bubbles associated with CT colonography performed with iohexol for fecal/fluid tagging could be reduced by adding simethicone to the standard cathartic preparation.

Inclusion Criteria:

• Adult patients who are schedule to undergo colonoscopy for a suspicion of colonic neoplasia at the investigators' institution
• Those who agree to participate in the study

Exclusion Criteria:

• Colonoscopy for reasons other than detecting colonic neoplasia, e.g. evaluation of inflammatory bowel disease
• Contraindications to iodinated contrast including renal insufficiency, hypersensitivity, and hyperthyroidism
• Acute severe colonic obstruction which is likely preclude safe and successful performance of CTC
• Patient who is suspicious for colonic perforation
• Pregnancy
• Phenylketonuria (contraindication to simethicone)