Computed tomographic (CT) colonography, also called CTC, virtual colonoscopy (VC) or CT pneumocolon, is a powerful minimally invasive technique for colorectal cancer screening.
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Indications
- 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 conventional colonoscopy
Technique
- 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
Data acquisition and analysis
- CT scanning is ideally performed on a multidetector computed tomography (MDCT) scanner in both supine and prone positions with a thin collimation
- image review with the use of two-dimensional (2D) and three-dimensional (3D) displays is strongly advised for optimal evaluation
Findings
- colonic diverticulosis
- colon polyps
- colorectal carcinoma
- extrinsic lesions impressing on the colon
- extracolonic pathology
Contraindications
Absolute
- acute inflammatory conditions such as acute diverticulitis, active stage of ulcerative colitis or Crohn disease
- recent abdominal or pelvic surgery
Relative
- 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 allergy/anaphylaxis to iodinated contrast media
- protocols using barium sulfate contrast media may be used as an alternative
- patients at high risk for a gastrointestinal tumor (e.g. Lynch syndrome) may not be good candidates for CTC screening 7
Advantages
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
Disadvantages
- residual fecal material can give rise to wrong interpretation
- biopsy specimen cannot be taken at the time of the procedure
- exposure to ionizing radiation
