CT colonography (protocol)

Last revised by Raymond Chieng on 24 Sep 2023

Computed tomographic (CT) colonography, also called CTC, virtual colonoscopy (VC) or CT pneumocolon, is a powerful minimally invasive technique for colorectal cancer screening. 

  • screening test for colorectal carcinoma

  • colon evaluation after incomplete or unsuccessful optical (conventional) colonoscopy

  • assessment of strictures

  • to better evaluate the colon proximal to obstructing neoplasms detected by conventional colonoscopy

  • patients with contraindications to or refusing optical colonoscopy

  • patient preparation

    • for optimal image quality, the colon should be clean and completely distended

    • residual stool and fluid may lead to a false negative or false positive diagnosis

    • residual stool may be "tagged" using oral contrast agents such as Gastrografin

  • bowel distension

    • optimal colonic distention is critical to technical success for proper intraluminal evaluation of the large bowel

    • distension can be achieved via a pressure-regulated device with carbon dioxide (preferred) or room air

  • intravenous contrast

    • not necessary for colonic interpretation although it is used in some centers for better assessment of the remaining abdominal organs

    • if used, the time difference between scanning in supine and prone positions means the first acquisition may be portal venous, but the second acquisition will be a more excretory (urographic) phase

  • antispasmodic agent

    • IV/IM hyoscine-N-butylbromide (Buscopan), an antimuscarinic drug reduces colonic motion, leading to higher quality images and reduced patient discomfort

    • IV glucagon is used in some countries/institutions as a first or second-line antiperistaltic agent: its efficacy is contentious

  • CT scanning is ideally performed on a multidetector computed tomography (MDCT) scanner in both supine and prone positions with a thin collimation

  • slice thickness of CT colonography may range from 1.25 mm to 5 mm 8.

  • CT colonography may detect polyps size from 2 mm or larger, however, the significance of detecting polyps less than 5 mm is of questionable clinical significance 8.

  • image review with the use of two-dimensional (2D) and three-dimensional (3D) displays is strongly advised for optimal evaluation

  • CTC more difficult to perform if a colostomy is present as there is no natural sphincter mechanism to retain the gas

  • general CT contraindications e.g. pregnancy, claustrophobia, etc.

  • history of severe adverse reaction/anaphylaxis to iodinated contrast media

  • patients at high risk for a gastrointestinal tumor (e.g. Lynch syndrome) may not be good candidates for CTC screening 7

Virtual colonoscopy has several advantages over optical colonoscopy:

  • less invasive procedure, therefore complication rate lower

  • takes less time

  • can visualize colon beyond the obstruction or narrowing

  • detects extracolonic pathology

  • residual fecal material can give rise to wrong interpretation

  • biopsy specimen cannot be taken at the time of the procedure

  • exposure to ionizing radiation

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