Is a plain abdominal computed tomography (CT) scan sufficient for diagnosing an abdominal aortic aneurysm?

Plain Abdominal CT for Abdominal Aortic Aneurysm

A plain (non-contrast) abdominal CT scan can diagnose an abdominal aortic aneurysm and is sufficient for screening purposes, but contrast-enhanced CTA remains the reference standard for definitive diagnosis and treatment planning. 1

Diagnostic Performance of Non-Contrast CT

Non-contrast CT demonstrates superior sensitivity (83-89%) compared to ultrasound (57-70%) for detecting AAA, with equally high specificity (>98%) for both modalities. 1

Key Advantages of Non-Contrast CT:

• Provides accurate aortic diameter measurements when the abdominal aortic contour is well visualized and can be distinguished from adjacent structures 1
• Detects aortic wall calcifications, which ultrasound cannot assess 1
• Evaluates thoracic and iliac aortic abnormalities that may be missed by ultrasound 1
• Particularly beneficial in patients with obesity or poor sonographic windows where ultrasound fails 1
• Low-dose non-contrast CT exhibits similar accuracy and reproducibility of measurements compared to CTA 1

Critical Measurement Technique

You must measure the outer-to-outer (OTO) aortic diameter perpendicular to the long axis of the aorta—this is obligatory for non-contrast CT images where the aortic wall and lumen cannot be distinguished. 1

• Use multiplanar reformatted images for tortuous aneurysms to avoid artifactual accentuation of a single dimension 1, 2
• The Society for Vascular Surgery now recommends outer wall to outer wall measurements (not inner wall to inner wall) 1

When Non-Contrast CT is Insufficient

Proceed to Contrast-Enhanced CTA When:

• Pre-operative planning is needed—CTA provides near 100% sensitivity and specificity and is the reference standard for management decisions 1, 3
• Evaluating symptomatic patients with acute abdominal or back pain—CTA can detect rupture, dissection, and thrombus characteristics 1, 3
• Assessing branch vessel involvement, which is critical for surgical or endovascular planning 1, 3
• Determining extent of iliac artery involvement (present in approximately 5% of AAAs) 3, 2

Clinical Algorithm

For Asymptomatic Screening:

• Non-contrast CT is acceptable as a screening examination, especially when ultrasound is inadequate 1
• AAAs are frequently diagnosed incidentally on non-contrast CT performed for other indications 1

For Suspected AAA Requiring Definitive Diagnosis:

• If non-contrast CT confirms AAA presence, proceed to CTA for complete characterization before any intervention 1, 3
• CTA must include iliofemoral arteries ("run-off") to evaluate access vessels 3, 2

Important Limitations to Recognize

Non-contrast CT cannot assess:

• Intraluminal thrombus characteristics or extent 1
• Dissection flaps or signs of imminent rupture 3
• Branch vessel involvement or stenosis 1, 3
• Inflammatory changes in the aortic wall 4

These limitations mean non-contrast CT should not be used as the sole imaging modality for treatment planning or symptomatic patients. 1, 3

Common Pitfalls to Avoid

• Do not rely on non-contrast CT alone for pre-operative assessment—contrast-enhanced CTA is mandatory for surgical or endovascular planning 1, 3
• Do not use non-contrast CT as initial imaging in symptomatic patients with suspected rupture—these patients require immediate CTA 1, 3
• Ensure proper perpendicular measurement technique to avoid overestimation from oblique measurements 1, 2