What are the diagnostic approaches for Abdominal Aortic Aneurysm (AAA)?

Diagnostic Approaches for Abdominal Aortic Aneurysm (AAA)

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

Initial Diagnostic Evaluation

Ultrasound (US)

• Transabdominal US is the mainstay for AAA screening and surveillance with sensitivity and specificity approaching 100% 1
• Advantages include:
  • Non-invasive, portable, and can be performed in various settings including emergency departments 1
  • No radiation exposure or contrast requirements 3
  • Cost-effective for initial evaluation and follow-up 2
• Measurement technique:
  • The American Institute of Ultrasound in Medicine recommends measuring the greatest diameter from outer-to-outer (OTO) edges of the aortic wall 1
  • The UK National Health Services AAA Screening Programme uses maximal anterior-posterior inner-to-inner (ITI) diameter 1
• Limitations:
  • Underestimates AAA diameter by 1-3 mm compared to CT 1
  • In 1-2% of cases, adequate evaluation is impossible due to patient body habitus or excessive bowel gas 1
  • Pre-evaluation overnight fasting is recommended to reduce bowel gas 1
  • May not accurately delineate juxtarenal or suprarenal aneurysms (approximately 5% of AAAs) 1

CT and CT Angiography (CTA)

• CTA is considered the reference standard for AAA diagnosis and management decision-making 1, 2
• Indications for CTA:
  • Symptomatic patients with acute onset abdominal or back pain 1
  • Pre-operative assessment before endovascular or open surgical repair 1
  • When US evaluation is inadequate 1
  • When aneurysm reaches size threshold for repair (5.5 cm) 1
• Measurement technique:
  • Outer-to-outer (OTO) aortic diameter perpendicular to the long axis of the aorta is recommended 1
  • For tortuous aneurysms, multiplanar reformatted images with angle correction or curved planar reformatted images with automated centerline 3-D software should be used 1
• Advantages:
  • Provides detailed information about aneurysm morphology, branch vessel involvement, and access vessels 1
  • Can identify other pathologies that may affect management 1
  • Higher sensitivity than US in identifying AAAs 1
• Non-contrast CT:
  • Can be used when contrast is contraindicated 1
  • More sensitive than US in identifying AAAs 1
  • Particularly useful for suspected contained rupture to identify the crescent sign (dissecting hematoma) 1

Advanced Imaging Modalities

MR Angiography (MRA)

• Alternative to CTA for diagnosis and pre-intervention evaluation 1
• Indications:
  • Patients with contraindications to CTA (e.g., iodinated contrast allergy) 1
  • Can be performed without gadolinium-based contrast agents using techniques like time-of-flight, balanced steady-state free precession, phase-contrast, and quiescent-interval single-shot imaging 1
• Limitations:
  • Longer imaging acquisition times 1
  • Limited ability to characterize aortic wall calcifications 1
  • Contraindicated in patients with certain implants 1

Other Imaging Modalities

• Conventional aortography: Not recommended for initial diagnosis due to invasiveness and low sensitivity 1
• Abdominal radiography: Not recommended due to low sensitivity, though AAA may be incidentally discovered if aortic wall calcifications are visible 1
• FDG-PET/CT: Not recommended for initial diagnosis but may play a role in diagnosing inflammatory and mycotic aortic aneurysms 1

Diagnostic Algorithm

1. For asymptomatic patients with risk factors (age >65, male, smoking history, family history of AAA):

   • Start with transabdominal US 1, 2
   • If US is inadequate due to body habitus or bowel gas, proceed to non-contrast CT 1

2. For symptomatic patients (abdominal/back pain, pulsatile mass):

   • CTA is the preferred initial imaging modality 1
   • If contrast is contraindicated, consider non-contrast CT or MRA 1

3. For pre-operative assessment:

   • CTA is the gold standard 1, 2
   • Include iliofemoral arteries to evaluate access vessels 1
   • Include chest imaging in patients with thoracoabdominal AAA 1

Surveillance Recommendations

• For AAAs 4.5-5.4 cm: imaging every 6 months 1
• For AAAs 3.5-4.4 cm: imaging every 12 months 1
• For AAAs 3.0-3.4 cm: imaging every 3 years 1
• For AAAs 2.6-2.9 cm: imaging every 5 years 1

Common Pitfalls and Caveats

• US measurements typically underestimate AAA diameter compared to CT; account for this 1-3 mm difference when making management decisions 1
• Ensure proper measurement technique (perpendicular to the aortic long axis) to avoid overestimation due to oblique measurements 1
• Be aware that 5% of AAAs are juxtarenal or suprarenal, which may not be adequately visualized by US 1
• For tortuous aneurysms, a single dimension may be artificially accentuated by the curvature of the aorta; use multiplanar reformatted images 1
• Recognize that AAA can be incidentally discovered on imaging studies performed for other reasons 1