Next Step: Proceed Directly to CT Angiography
When ultrasound fails to adequately visualize or measure the abdominal aorta in a patient with suspected AAA, CT angiography (CTA) of the abdomen and pelvis with IV contrast is the definitive next step. 1, 2
Why CTA is the Appropriate Next Step
CTA is the reference standard for AAA diagnosis and management decision-making, providing submillimeter, isotropic 3-D datasets with high spatial resolution that allow accurate measurement of the outer-to-outer (OTO) aortic diameter perpendicular to the long axis of the aorta. 1, 2
Key Advantages of CTA Over Repeat Ultrasound
CTA overcomes ultrasound limitations that likely caused the initial inadequate visualization, including obesity, bowel gas, and inability to distinguish the aortic wall from adjacent structures. 1, 3
CTA provides comprehensive anatomic assessment including the full extent of any aneurysm (infrarenal, juxtarenal, or suprarenal), involvement of branch vessels, presence of mural thrombus, and evaluation of the iliofemoral arteries as potential access vessels. 1, 2
Measurements are more reliable on CT than ultrasound, with ultrasound typically underestimating aortic diameter by 1-3 mm compared to CT. 2, 4
Specific Imaging Protocol
The CTA should include:
Scan range from the diaphragm through the iliofemoral arteries to evaluate the full extent of aortic pathology and assess access vessels for potential endovascular repair. 1, 2
Thin-section acquisition timed to peak arterial enhancement with multiplanar reformations and 3D renderings, which are required elements of CTA. 5
OTO diameter measurements perpendicular to the long axis using angle-corrected multiplanar reformatted images, especially important if the aorta is tortuous. 1, 5
Common Pitfalls to Avoid
Do not order repeat ultrasound when the initial study was inadequate—the same technical limitations will persist. 3
Do not order non-contrast CT as it cannot assess for dissection, thrombus characteristics, or branch vessel involvement that are critical for management decisions. 5
Do not delay imaging to obtain additional non-invasive vascular studies first—definitive anatomic imaging is needed to establish the diagnosis and size. 5
Alternative if CTA is Contraindicated
If the patient has contraindications to iodinated contrast (severe renal insufficiency or contrast allergy):
MR angiography (MRA) with IV gadolinium contrast is an acceptable alternative for AAA diagnosis and pre-intervention evaluation. 1
Non-contrast MRA techniques are available for patients with severe 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. 1, 2
Management After CTA Confirms AAA Size
Once CTA establishes the aortic diameter:
AAA ≥5.5 cm: Urgent vascular surgery consultation for elective repair consideration. 1, 6
AAA 3.0-3.4 cm: Surveillance imaging every 3 years. 1
Aortic ectasia 2.6-2.9 cm: Surveillance imaging every 5 years. 1, 4
Future surveillance can typically be performed with ultrasound once the diagnosis and size are established by CT, reserving repeat CT for pre-operative planning if the aneurysm reaches intervention threshold or if symptoms develop. 4