Virtual Colonoscopy Using Omnipaque as a Contrast Agent
Virtual Colonoscopy Using Omnipaque as a Contrast Agent
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.
Inclusion Criteria:

 -  age ≥18 years

 -  able to provide written informed consent

 Exclusion Criteria:

 -  hypersensitivity to iodine-containing compounds,

 -  pregnant or lactating,

 -  diuretic therapy, and

 -  enrollment in other clinical research trials involving investigational drugs