What is the recommended imaging modality for diagnosing an infrarenal (below the kidney) aneurysm?

Imaging for Infrarenal Abdominal Aortic Aneurysm

Recommended Imaging Modality

CT angiography (CTA) is the gold standard and primary imaging modality for diagnosing infrarenal abdominal aortic aneurysms, providing comprehensive evaluation of aneurysm size, morphology, and involvement of branch vessels including the iliac arteries. 1, 2

Initial Diagnostic Approach

For Asymptomatic Patients or Screening

• Ultrasound (US) is the first-line imaging modality for initial detection and surveillance of infrarenal AAA, with sensitivity and specificity approaching 100% 1, 2, 3
• US should be a dedicated examination measuring the leading-edge to leading-edge anteroposterior (AP) diameter in the proximal, mid, and distal infrarenal aorta 1
• Measurements should use the outer-to-outer (OTO) technique perpendicular to the long axis of the aorta 2
• If US is inadequate or technically limited, proceed directly to non-contrast CT 2

For Symptomatic Patients

• CTA is the preferred initial imaging modality for patients presenting with acute abdominal or back pain, pulsatile mass, or suspected rupture 1, 2, 4
• CTA provides submillimeter, isotropic 3-D datasets allowing accurate assessment of aneurysm extent and complications 1, 2
• The scan range must include the iliofemoral arteries to evaluate access vessels for potential intervention 2

Pre-Intervention Planning

CTA is mandatory before any surgical or endovascular repair, as it provides critical information about: 1

• Precise aneurysm dimensions and morphology
• Proximal neck characteristics (length, diameter, angulation, calcification)
• Involvement of renal and visceral arteries
• Distal landing zones in the iliac arteries
• Presence of mural thrombus

Key Technical Considerations

• Use multiplanar reformatted images for tortuous aneurysms to avoid artifactual measurement errors 1, 2
• Automated centerline 3-D software should be employed for accurate diameter measurements in curved vessels 1
• Approximately 5% of AAAs are juxtarenal or suprarenal, which may not be adequately visualized by US alone, making CTA essential 1, 5

Alternative Imaging Options

MR Angiography (MRA)

• MRA with IV contrast is an acceptable alternative when CTA is contraindicated (e.g., severe iodinated contrast allergy) 1, 2
• Non-contrast MRA techniques are available for patients with severe renal insufficiency, though they have longer acquisition times and increased motion artifacts 1, 2
• MRA is suitable for patients with contraindications to both iodinated contrast and radiation exposure 1

Catheter Arteriography

• Has very limited utility and should only be considered when both CTA and MRA are contraindicated (e.g., severe renal dysfunction with cardiac pacemaker) 1
• The combination of intra-arterial injection with lower contrast load may decrease nephropathy risk in select cases 1

Common Pitfalls to Avoid

• Do not rely solely on ultrasound for pre-operative planning, as it underestimates aneurysm size by approximately 4 mm compared to CTA and cannot adequately visualize juxtarenal extension or iliac involvement 1, 2
• Avoid standard CT without contrast, as it cannot adequately define vascular anatomy or detect dissection flaps 1
• Do not use abdominal radiographs, intravenous urography, or blood pool radionuclide imaging, as these are not recommended for AAA diagnosis or surveillance 1
• Ensure lateral aortography views when angiography is performed, as AP projections frequently suggest renal artery involvement due to infrarenal aortic elongation and buckling 5
• Always measure perpendicular to the vessel axis using multiplanar reformations to avoid overestimation in tortuous vessels 1, 2