What is the recommended CT (Computed Tomography) angiography for patients presenting with bright red rectal bleeding?

CT Angiography of Abdomen and Pelvis for Bright Red Rectal Bleeding

For patients presenting with bright red rectal bleeding, CT angiography (CTA) of the abdomen and pelvis is the recommended imaging modality, particularly when the patient is hemodynamically unstable or has suspected active bleeding. 1

Indications for CTA Abdomen and Pelvis

• CTA should be the first-line investigation in patients with active lower gastrointestinal bleeding (LGIB) with hemodynamic instability (shock index ≥1) 1
• CTA is preferred over colonoscopy in unstable patients as it:
  • Can rapidly localize bleeding sources in the upper GI tract, small bowel, or lower GI tract 1
  • Does not require bowel preparation 1
  • Is widely available and can be rapidly accessed 1
• CTA has superior sensitivity compared to conventional angiography, detecting bleeding rates as low as 0.3 mL/min (versus 0.5-1.0 mL/min for conventional angiography) 1

Technical Considerations

• CTA requires proper timing to coincide with peak arterial enhancement 1
• Essential elements of CTA include:
  • Appropriate timing of contrast administration
  • Multiplanar reconstructions/reformats
  • 3D renderings 1
• CTA should be performed in preference to a standard "general contrast CT" done in the delayed/portal-venous phase 1
• Dual-energy CT techniques can improve detection of active bleeding through:
  • Low-keV virtual monoenergetic images
  • Iodine-only images 1, 2

Diagnostic Performance

• CTA has reported sensitivity of 79-95% and specificity of 95-100% in retrospective clinical studies of LGIB 1
• In evaluation of active bleeding, CTA has demonstrated active extravasation of contrast in 78.9% to 84.2% of cases 1
• The diagnostic yield decreases if bleeding is intermittent 1, 3

Clinical Algorithm for Bright Red Rectal Bleeding

1. For hemodynamically unstable patients (shock index >1):

   • Perform CTA abdomen and pelvis as first-line investigation 1, 4
   • If CTA is negative but bleeding continues, consider immediate upper endoscopy to exclude an upper GI source 1, 4

2. For hemodynamically stable patients:

   • Begin with direct anorectal examination (anoscopy/proctoscopy) 1, 4
   • If anorectal examination is negative, proceed with colonoscopy 1
   • Consider CTA if colonoscopy is inconclusive or patient's condition deteriorates 5, 6

Common Pitfalls to Avoid

• Delaying CTA in hemodynamically unstable patients by attempting colonoscopy first 1, 4
• Using standard contrast-enhanced CT instead of properly timed CTA for suspected active bleeding 1
• Failing to consider that bright red rectal bleeding may represent an upper GI source (up to 15% of cases) 1, 4
• Relying solely on portal-venous phase imaging; arterial phase is critical for detecting active extravasation 1, 6

Additional Benefits of CTA

• Provides valuable preoperative information for potential endovascular or surgical intervention 1, 5
• Can be beneficial for patients who have undergone intervention where bleeding was not localized or controlled 1
• Allows for detection of vascular abnormalities such as angiodysplasia, varices, or aberrant vessels 3, 7