What imaging modality is best for detecting abdominal aortic aneurysms (AAAs) in asymptomatic patients with risk factors such as smoking, hypertension, or a family history of aneurysms?

Imaging for Abdominal Aortic Aneurysms

Ultrasound is the primary imaging modality for detecting abdominal aortic aneurysms in asymptomatic patients, offering 95% sensitivity and near 100% specificity without radiation exposure. 1, 2

First-Line Imaging: Ultrasound

Transabdominal ultrasound of the abdominal aorta is recommended as the gold standard for AAA screening and surveillance. 1, 2 This modality:

• Reliably detects AAA presence in nearly all patients with sensitivity and specificity approaching 100% 1
• Provides accurate measurements of aortic diameter comparable to CT when using proper technique 1, 3
• Can be performed in multiple settings including emergency departments due to machine portability 1
• Poses negligible risk to patients with no radiation exposure 1, 2

Technical Considerations for Ultrasound

The American Institute of Ultrasound in Medicine recommends measuring the greatest diameter from outer-to-outer (OTO) edges of the aortic wall, though the UK National Health Service uses inner-to-inner (ITI) measurements. 1 Pre-evaluation overnight fasting reduces bowel gas interference. 1

Ultrasound underestimates AAA diameters by 1-3 mm compared to CT, but this does not affect screening accuracy. 1 In 1-2% of cases, adequate visualization is impossible due to large body habitus or excessive bowel gas. 1

Advanced Imaging: CT Angiography

When AAA reaches the size threshold for repair (≥5.5 cm in men, ≥5.0 cm in women) or becomes symptomatic, CT angiography is required for preoperative planning. 1

CTA provides:

• Precise anatomic detail for surgical or endovascular approach planning 1
• Assessment of abdominal branch vessel involvement 1
• Evaluation of access vessels for endovascular repair 1
• Characterization of aneurysm morphology including saccular features that increase rupture risk below 5.5 cm 1

For ruptured AAA, CT angiography with non-contrast phase followed by contrast injection is the gold standard for confirming rupture and surgical planning. 4

CT Limitations

Contrast-enhanced CT is not generally accepted as a first-line screening tool due to radiation exposure and cost. 1 However, many AAAs are incidentally discovered on abdominal CT scans performed for other indications. 1

Alternative Advanced Imaging: MR Angiography

MRA may be substituted when CT cannot be performed, such as in patients with iodinated contrast allergy. 1

MRA offers:

• Accurate and reproducible aortic diameter measurements comparable to CTA using black-blood sequences 1
• No radiation exposure 3
• Functional and hemodynamic data including aortic wall stiffness and blood flow quantification 1

MRA is contraindicated in patients with severe renal insufficiency requiring gadolinium contrast, though non-contrast MRA techniques are now robustly applicable for diameter determination. 1, 3

Imaging NOT Recommended

Conventional Angiography

Aortography is invasive, time-consuming, and poses risks of embolization, perforation, and bleeding, making it unsuitable for AAA screening or routine evaluation. 1 Its sole utility is in patients with significant contraindications to both CTA and MRA. 1

Plain Radiography

Abdominal radiographs have low sensitivity for AAA detection and cannot accurately evaluate AAA morphology or extent, though calcified aneurysms may be incidentally visible. 1

Screening Protocol by Risk Group

Men aged 65-75 years who have ever smoked (≥100 cigarettes lifetime) should receive one-time ultrasound screening. 1, 2 This reduces AAA-related mortality by 42-43%. 1, 2

First-degree relatives of AAA patients aged ≥50 years should receive ultrasound screening unless an acquired cause is clearly identified. 1

Men aged ≥75 years (regardless of smoking) or women aged ≥75 years who are current smokers or hypertensive may be considered for screening. 1

Important Caveat

A single negative ultrasound examination around age 65 virtually excludes future AAA rupture risk, as the 10-year incidence of new AAAs is only 0-4%, with none exceeding 4.0 cm diameter. 1, 2 Rescreening after normal initial ultrasound provides negligible benefit. 2

Surveillance Imaging

For small AAAs (3.0-5.4 cm), ultrasound surveillance intervals are: 1

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

For aneurysms 4.0-5.5 cm with concerning morphology (saccular shape), CTA should be performed before continuing ultrasound surveillance to better characterize rupture risk. 1