What imaging modality is recommended for diagnosing and monitoring Abdominal Aortic Aneurysm (AAA)?

Imaging for Abdominal Aortic Aneurysm (AAA)

Initial Diagnosis

For asymptomatic patients with suspected AAA, ultrasound is the first-line imaging modality, while CT angiography (CTA) is the gold standard for symptomatic patients and pre-operative assessment. 1, 2

Asymptomatic Patients

• Transabdominal ultrasound is the mainstay for AAA screening and surveillance with sensitivity and specificity approaching 100% 2
• Ultrasound accurately measures aortic size, detects wall lesions such as mural thrombus or plaques, and is widely available, painless, and low cost 1
• The greatest diameter should be measured from outer-to-outer (OTO) edges of the aortic wall 2
• Ultrasound measurements typically underestimate AAA diameter compared to CT by 1-3 mm 2
• If ultrasound evaluation is inadequate (occurs in approximately 7% of cases), proceed to non-contrast CT 2

Symptomatic Patients

• CTA is the preferred initial imaging modality for patients presenting with acute onset abdominal or back pain, particularly with a pulsatile abdominal mass or significant AAA risk factors 1, 2
• CTA provides submillimeter, isotropic, 3-D datasets with high spatial resolution and is considered the reference standard for AAA diagnosis and management decision-making 1, 2
• Measurement of maximal aortic diameter based on OTO wall diameter perpendicular to the long axis of the aorta on CTA is the gold standard 1
• The scan range should include the iliofemoral arteries to evaluate access vessels, especially important for endovascular repair planning 1, 2

Pre-Operative Assessment

CTA is the optimal choice for pre-intervention studies before endovascular or open surgical repair. 1, 3

• CTA allows comprehensive evaluation of the aneurysm anatomy, branch vessels, and access vessels needed for surgical planning 1, 2
• Multiplanar reformatted images and 3-D renderings are essential components of pre-operative CTA 1
• For tortuous aneurysms, use multiplanar reformatted images to avoid artifactual accentuation of a single dimension 2

Alternative to CTA

• MR angiography (MRA) with IV contrast is an acceptable alternative when CTA is contraindicated (e.g., contrast allergy, renal insufficiency) 1, 2, 3
• Non-contrast MRA techniques are available for patients with renal insufficiency, though they have longer acquisition times and increased motion artifacts 2
• Catheter arteriography has very limited utility and should only be considered when both CTA and MRA are contraindicated 2, 3

Surveillance Imaging

Ultrasound is the preferred modality for AAA surveillance in stable patients. 2, 4

Surveillance intervals based on AAA size 2:

• Every 6 months for AAAs 4.5-5.4 cm
• Every 12 months for AAAs 3.5-4.4 cm
• Every 3 years for AAAs 3.0-3.4 cm
• Every 5 years for AAAs 2.6-2.9 cm

Post-EVAR Surveillance

Both ultrasound and CTA are acceptable for post-endovascular repair surveillance, though the optimal modality remains debated. 5, 6, 7

• High-quality duplex ultrasound is comparable to CTA for assessment of aneurysm size and graft patency after endovascular repair 5
• Contrast-enhanced ultrasound is useful in detecting, localizing, and quantifying endoleaks when following patients after EVAR 1, 6
• However, ultrasound has lower sensitivity (42.9%) for endoleak detection compared to CT in some studies, particularly in busy hospital vascular laboratories 7
• CTA remains more reliable for detecting clinically significant endoleaks, especially those near attachment sites or associated with aneurysm expansion 5, 7

Critical Measurement Technique

• Maximum aortic diameter must be measured perpendicular to the centerline of the aorta using 3-D and multiplanar reformatted images 1, 2
• Measurements in the axial plane relative to the patient's body may overestimate aortic diameter in tortuous aortas 1
• OTO wall measurements are recommended and can be 3-6 mm larger than inner-to-inner measurements 1

Common Pitfalls to Avoid

• Do not rely solely on ultrasound for pre-operative planning, as it may underestimate aneurysm size and inadequately visualize iliac arteries 2
• Approximately 5% of AAAs are juxtarenal or suprarenal, which may not be adequately visualized by ultrasound 2
• Failing to include the full iliofemoral system in imaging studies can miss critical information about access vessels for potential endovascular repair 2
• Catheter-based aortography may underestimate true aortic diameter if significant mural thrombus obscures the luminal contour 1