CT Colonography (Virtual Colonoscopy)

Indications

• Colorectal cancer screening (average risk)
• Incomplete optical colonoscopy or patients unable/unwilling to undergo it
• Evaluation of obstructing colonic lesion (proximal colon mapping)

Patient prep

• Cathartic bowel prep (PEG or sodium phosphate-based) the day before
• Fecal/fluid tagging: oral barium and/or iodinated oral contrast (e.g., diatrizoate/Gastrografin) with meals the day before to tag residual stool/fluid
• Low-residue diet day prior; NPO morning of exam
• Colonic insufflation with CO2 (preferred, automated) or room air at time of exam via thin rectal catheter

Contrast

Technique

• Insufflate colon with CO2 to adequate distention; verify on scout
• Scan supine AND prone (or decubitus) to redistribute gas/fluid and mobilize residual stool
• Low-dose technique (low mAs, dose modulation); 120 kV
• Thin collimation, 0.625-1.25 mm recon for 2D/3D fly-through
• Interpret with primary 2D and 3D endoluminal navigation; CAD optional

Series / Sequences

Key points

• Both supine and prone (or decubitus) acquisitions required to distinguish polyps from residual stool and to overcome poor distention
• Stool/fluid tagging is essential to differentiate tagged residue from true polyps
• Adequate distention (CO2 preferred) is the most important quality factor
• Report with C-RADS; polyps >=6 mm clinically relevant; >=10 mm warrant colonoscopy
• Low radiation dose technique; screening is typically non-IV-contrast

References