CT Colonography Route and Contrast Protocol
CT colonography requires rectal insufflation with air or carbon dioxide via a small-caliber rectal catheter, oral contrast agents (barium and/or iodine) for fecal and fluid tagging, and typically does NOT use intravenous contrast for screening purposes. 1
Route of Administration
Rectal Route for Colonic Distension
- A small-caliber rectal catheter is inserted into the rectum, followed by automated or manual insufflation of room air or carbon dioxide 1
- Automated insufflation is strongly preferred over manual insufflation because it results in improved colonic distention, has preset ramped flow rates, and includes automatic venting at predetermined intracolonic pressures for enhanced safety 1
- Carbon dioxide may reduce postprocedure cramping compared to room air 1
- A CT scout image should be performed prior to scanning to confirm adequate insufflation 1
Oral Route for Bowel Preparation
- Patients undergo full cathartic preparation with a clear liquid diet the day before the study, similar to colonoscopy requirements 1
- Cathartic agents include polyethylene glycol, oral sodium phosphate, or magnesium citrate 1
- Polyethylene glycol is safe and results in minimal fluid shifting, though it increases colonic fluid compared to sodium phosphate 1
- Oral sodium phosphate agents require smaller volumes but can cause electrolyte shifts when doses exceed 45 mL daily 1
Contrast Protocol
Oral Contrast for Fecal and Fluid Tagging
- Tagging of residual solid stool and fluid with barium and/or iodine oral contrast agents is preferred but not mandatory 1
- Stool tagging is generally achieved with ingestion of a barium suspension 1
- Fluid tagging is performed using an iodinated oral contrast agent 1
- Fecal and fluid tagging permits identification of submerged polyps and reduces false-positive examinations due to residual stool 1
- Use of tagging agents is impractical when CT colonography is performed following incomplete endoscopy 1
Intravenous Contrast
- Intravenous contrast is generally NOT given to patients undergoing screening 1
- IV contrast can be helpful in patients with more advanced symptoms or when characterization of solid organs is necessary (e.g., to evaluate potentially significant extracolonic findings or to stage an obstructing colon cancer) 1
- When IV contrast is needed, routine dose settings (100 mAs) should be used rather than low-dose technique 1
CT Acquisition Parameters
Scanning Positions
- Supine and prone CT acquisitions are mandatory to permit redistribution of colonic fluid and air, which improves polyp detection compared to a single position 1
- Decubitus position may be used as an alternative for patients unable to lie prone 2
Technical Specifications
- CT colonography should be performed using multidetector CT with a minimum of 4 detectors 1
- Protocols should use high spatial resolution with 3-mm slice thickness or less 1
- Low-dose technique (100 mAs) should be used for routine screening examinations 1
- The recommended kVp is 120 kV 1
- Automatic exposure control should be utilized when available, as it varies x-ray tube current and results in significant dose savings for average-sized patients 1
Important Clinical Caveats
Patient Selection Considerations
- Avoid oral phosphate-containing preparations in patients with GFR less than 60 mL/min/1.73 m² due to risk of phosphate nephropathy 3
- The entire procedure on the CT table takes approximately 10 minutes, with no sedation or recovery time needed 1
Quality Assurance
- Trained personnel should review 2D images of the colorectum prior to patient dismissal to ensure adequate visualization of all colonic segments 1
- Repeat scanning after re-inflation, changes in patient position, or intravenous contrast may be required if colonic segments are inadequately visualized due to collapse or excess fluid 1