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Abdomen and PelvisDemonstration data

CT Colonography

CT colonography (CTC, virtual colonoscopy) is a minimally invasive examination of the entire colon and rectum using thin-section CT of the cleansed, gas-distended colon. Combined 2D and 3D interpretation allows detection of polyps and masses with performance comparable to optical colonoscopy for clinically significant lesions.

Indications

  • Colorectal cancer screening in average-risk adults
  • Incomplete optical colonoscopy (e.g. redundant or obstructed colon)
  • Patients unable or unwilling to undergo optical colonoscopy or with high sedation risk
  • Evaluation of colon proximal to an obstructing carcinoma
  • Follow-up surveillance where endoscopy is contraindicated

Contraindications & Cautions

  • Acute diverticulitis, toxic megacolon or suspected colonic perforation
  • Symptomatic acute colitis
  • Recent colonic biopsy with deep resection or recent surgery (risk of perforation)
  • Inability to tolerate bowel preparation or colonic insufflation

Patient Preparation

  • Cathartic bowel preparation (e.g. polyethylene glycol or sodium phosphate regimen)
  • Oral faecal tagging with barium and/or iodinated contrast to label residual stool and fluid
  • Low-residue diet in the day(s) preceding the examination
  • Clear explanation of insufflation to improve tolerance and distension

Technique & Parameters

  • Rectal catheter with automated CO2 insufflation to achieve optimal colonic distension
  • Supine and prone (or decubitus) acquisitions to redistribute residual fluid and stool
  • Low-dose thin-section acquisition (≤1.25 mm collimation) reconstructed at 1 mm intervals
  • Intravenous contrast reserved for symptomatic/staging studies, not routine screening
  • Consider hyoscine/glucagon spasmolytic to improve distension where permitted

Systematic Review

  • Confirm adequate distension of all colonic segments on both positions
  • Perform primary 2D review with 3D endoluminal fly-through problem solving (or vice versa)
  • Differentiate polyps from tagged stool using mobility between positions and internal attenuation
  • Measure lesions in the 2D window on the long axis of the polyp head
  • Review extracolonic organs for incidental significant findings

Key Findings & Significance

  • Sessile or pedunculated soft-tissue polyps arising from the colonic wall
  • Annular or polypoid masses causing luminal narrowing (apple-core morphology)
  • Tagged mobile residual stool that changes position and often contains gas or high-density tag
  • Diverticulosis and haustral fold thickening
  • Extracolonic findings such as aortic aneurysm, renal or hepatic lesions

Differential Considerations

  • Residual tagged stool versus true polyp (mobility and attenuation are discriminators)
  • Prominent ileocaecal valve or lipoma versus neoplastic polyp
  • Diverticular stricture versus annular carcinoma
  • Inverted diverticulum mimicking a polyp
  • Extrinsic compression versus intrinsic mural mass

Pearls & Pitfalls

  • Report clinically significant polyps using CT Colonography Reporting and Data System (C-RADS)
  • Polyps ≥6 mm are generally reported; ≥10 mm warrant referral for polypectomy
  • Dual positioning is critical to distinguish mobile stool from fixed polyps and to complete distension
  • Suboptimal preparation or distension markedly reduces sensitivity and should be stated explicitly

Structured Report

  • Use C-RADS to categorise colonic (C0-C4) and extracolonic (E0-E4) findings
  • Report size, morphology and segmental location of each significant polyp or mass
  • Comment on adequacy of preparation, tagging and distension
  • Provide management recommendation (surveillance versus colonoscopy referral)
  • Document clinically important extracolonic findings and follow-up needs

References

  • ACR-SAR-SCBT-MR Practice Parameter for the Performance of CT Colonography in Adults
  • C-RADS: CT Colonography Reporting and Data System, American College of Radiology
  • Radiographics: Pickhardt PJ. CT Colonography: Pitfalls in Interpretation
  • ACR Appropriateness Criteria: Colorectal Cancer Screening

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.