Recommended Imaging for Suspected Abdominal Aortic Aneurysm
For symptomatic patients with suspected AAA presenting with acute abdominal or back pain, CT angiography (CTA) of the abdomen and pelvis with IV contrast is the recommended initial imaging modality, while ultrasound is preferred for asymptomatic screening. 1
Clinical Context Determines Imaging Choice
The imaging approach differs fundamentally based on whether the patient is symptomatic or asymptomatic:
Symptomatic Patients (Acute Pain, Pulsatile Mass)
- CTA abdomen and pelvis with IV contrast is the reference standard and should be ordered immediately 1
- CTA provides submillimeter, isotropic 3-D datasets with high spatial resolution that allow accurate measurement of maximum aortic diameter using outer-to-outer (OTO) wall measurements perpendicular to the long axis of the aorta 1
- The scan range must include the iliofemoral arteries to evaluate access vessels for potential endovascular repair, as approximately 5% of AAAs involve the iliac arteries 1, 2, 3
- CTA can detect critical complications including thrombus, dissection flap, and signs of imminent or contained rupture with near 100% sensitivity and specificity 3, 4
Asymptomatic Patients (Screening/Surveillance)
- Transabdominal ultrasound is the first-line imaging modality with sensitivity and specificity approaching 100% 2
- Ultrasound measurements typically underestimate AAA diameter by 1-3 mm compared to CT 2
- If ultrasound is inadequate or inconclusive, proceed to non-contrast CT or CTA 2, 3
Technical Requirements for CTA
When ordering CTA, ensure the following elements are included:
- Timing: Thin-section CT acquisition timed to coincide with peak arterial enhancement 1
- Reconstructions: Multiplanar reformations are mandatory, especially for tortuous aortas to avoid overestimation of diameter 1, 3
- 3-D renderings: This is a required element that distinguishes CTA from standard contrast-enhanced CT 1
- Measurement technique: OTO aortic diameter perpendicular to the long axis using angle-corrected multiplanar reformatted images 1, 2
Alternative Imaging Options
When Contrast is Contraindicated
- MR angiography (MRA) with IV contrast is an acceptable alternative to CTA for diagnosis and pre-intervention evaluation 1, 2
- Non-contrast MRA techniques (time-of-flight, balanced steady-state free precession) are available but have longer acquisition times and increased motion artifacts 1, 5
- Non-contrast CT is more sensitive than ultrasound for identifying AAAs but cannot assess for dissection, thrombus characteristics, or branch vessel involvement 1, 3
Imaging Modalities to Avoid
- Catheter-based aortography is invasive with low sensitivity and may underestimate true aortic diameter if significant mural thrombus is present 1
- Aortography should only be considered as part of endovascular AAA interventions, particularly in emergent cases of ruptured AAA 1
Critical Pitfalls to Avoid
- Do not start with ultrasound in symptomatic patients – while excellent for screening, ultrasound underestimates aneurysm size by approximately 4 mm compared to CTA and cannot adequately assess for rupture or iliac involvement 2, 3
- Do not order CT without contrast for symptomatic patients – non-contrast CT cannot assess for dissection, thrombus characteristics, or branch vessel involvement critical for management decisions 3
- Do not measure aortic diameter in the axial plane alone – this may overestimate diameter in tortuous aortas; always use multiplanar reformatted images perpendicular to the centerline 1, 2
- Do not forget to include iliofemoral vessels in the scan range – this information is essential for surgical or endovascular planning 1, 2, 3
Regarding Chest CT
Chest CT should be included in the scan range only if there is concern for thoracoabdominal AAA (aneurysm extending above the diaphragm) 1, 6. For isolated infrarenal AAA, chest imaging is not routinely necessary unless clinical suspicion exists for thoracic involvement.