Virtual Colonoscopy Using Omnipaque as a Contrast Agent

Overview

CT colonography (CTC) is a validated screening exam for colorectal cancer. The diagnostic accuracy of CTC depends on the quality of the bowel cleansing and contrast tagging of residual stool and fluid. New bowel preparation media for CTC should be assessed for their efficacy and for patient satisfaction. Iohexol is currently approved by the FDA for oral use for imaging of the gastrointestinal tract. A potential advantage of using iohexol for CTC is that low-and iso-osmolar oral contrast agents have fewer risks than hyperosmolar contrast agents, which make them ideal for use in all patients, especially those who are frail and/or have multiple co-morbidities. For example, hyperosmolar contrast agents that are accidentally aspirated into the lungs during ingestion may result in life-threatening acute pulmonary edema and severe chemical pneumonitis, which is not the case for low- and iso-osmolar contrast agents. It may even be safer to use low- or iso-osmolar contrast agents for patients undergoing CTC on the same day as their incomplete colonoscopy, since they are at risk for aspiration from being sedated for their colonoscopy. Additionally, unlike hyperosmolar contrast agents, low- and iso-osmolar contrast agents do not cause sudden and massive fluid shifts, thus eliminating the risk of dangerous electrolyte imbalances.

Colorectal cancer is the third most common cancer and the second leading cause of cancer-related mortality in the United States when both sexes are combined. Given that the time interval between the appearance of a pre-malignant adenoma and its progression to invasive carcinoma is approximately 5.5 years for polyps \>10 mm and 10-15 years for smaller polyps, colorectal cancer is an ideal disease for screening because there is sufficient time to detect and remove pre-malignant adenomas. However, even though screening has been shown to decrease colorectal cancer mortality, only about 60% of people over the age of 50 have had the recommended screening tests. First introduced in 1994, computed tomography colonography (CTC) is a validated colorectal cancer screening exam by the American Cancer Society. It is a low-radiation dose CT scan of the colon that is performed after bowel preparation and gas insufflation of the colon. CTC has several advantages over optical colonoscopy including less invasiveness, no need for sedation, improved patient comfort, and the ability to identify extracolonic findings. The performance of CTC for polyp detection compares favorably to that of optical colonoscopy and CTC is rapidly gaining acceptance as a screening and diagnostic technique. For accurate diagnosis in CTC, the colon must be adequately cleansed and any residual stool and fluid must be tagged so that they do not obscure polyps or masses. Patients often find bowel preparation to be the most onerous part of the screening process because it is inconvenient and uncomfortable. Many bowel preparations for CTC and optical colonoscopy require ingestion of large volumes of laxative solutions that can lead to nausea, abdominal pain and diarrhea. Additionally, for CTC, currently the most widely used tagging agents are hyperosmolar iodinated and barium based agents such as diatrizoate meglumine and diatrizoate sodium solution (Gastrografin; Bracco Diagnostics, Monroe Township, NJ), a hyperosmolar ionic iodinated contrast agent. These hyperosmolar agents can also lead to nausea, abdominal pain and diarrhea, and can even result in dangerous electrolyte balances from rapid fluid shifts. Changing the current bowel preparation regimen to improve patient comfort, convenience, and safety would most likely increase patient compliance with CTC. Iohexol (Omnipaque; GE Healthcare, Milwaukee, WI) is a low-osmolar non-ionic iodinated contrast agent that is approved by the FDA for oral use. It is routinely used orally for CT scans of the abdomen and pelvis, and has been used experimentally in CTC. Iohexol is better tolerated by patients and has a better safety profile than hyperosmolar iodinated and barium based contrast agents. Study Hypotheses 1. Oral iohexol in conjunction with a minimally cathartic agent results in adequate bowel preparation for CTC. 2. Oral iohexol provides effective tagging of residual stool and fluid in standard screening and diagnostic CTC and same-day CTC following incomplete optical colonoscopy. 3. Oral iohexol is well tolerated by patients in terms of taste, ease of preparation, and side effects.