virtual colonography

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

Indications

  • screening test for colorectal carcinoma
  • colon evaluation after incomplete or unsuccessful conventional colonoscopy
  • assessment of strictures
  • to better evaluate the colon proximal to obstructing neoplasms detected by conventional colonoscopy
  • patients with contraindications 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 although it is used in some centers
  • 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 multi-detector 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

Contraindications

  • acute inflammatory conditions such as acute diverticulitis, active stage of ulcerative colitis or Crohn disease
  • recent abdominal or pelvic surgery
  • CTC cannot be performed if a colostomy is present as there is no natural sphincter mechanism to retain the tube
  • general CT contraindications e.g. pregnancy, claustrophobia, etc.
  • history of severe allergy / 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

Advantages

Virtual colonoscopy has several advantages over conventional 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

See also

Siehe auch: