What is the next best imaging modality for a suspected aortic arch aneurysm (AAA) with a left suprahilar convexity on chest X-ray (CXR)?

Next Best Imaging for Suspected Aortic Arch Aneurysm

CT angiography (CTA) of the chest is the recommended next imaging modality for a suspected aortic arch aneurysm identified as a left suprahilar convexity on chest X-ray. 1

Rationale for CTA as First-Line Imaging

• CTA provides comprehensive evaluation with high sensitivity and specificity for thoracic aortic aneurysms, allowing for detailed assessment of aneurysm location, size, and relationship to branch vessels 1
• CTA uses thin-section acquisition timed to coincide with peak arterial enhancement, producing a volumetric dataset that can be interpreted using transverse reconstructions, multiplanar reformations, and 3D renderings 1
• CTA offers advantages over other modalities including wide availability, speed of acquisition, and ability to detect complications such as thrombus and dissection 1, 2
• ECG-gated CTA acquisition is recommended to minimize cardiac motion artifact and allow for accurate orthogonal measurement of the thoracic aorta 1, 2

Limitations of Initial Chest X-ray

• While chest X-ray can identify abnormalities such as widened mediastinum or aortic tortuosity that suggest aortic aneurysm, it lacks sensitivity and specificity for definitive diagnosis 1, 3
• Chest X-ray findings such as left suprahilar convexity require confirmation with cross-sectional imaging for accurate diagnosis and characterization 1, 4
• Morphometric parameters on chest X-ray can help estimate aortic diameter but are insufficient for definitive diagnosis of thoracic aortic aneurysm 3

Alternative Imaging Options

MR Angiography (MRA)

• MRA chest is an acceptable alternative when CTA is contraindicated (e.g., severe contrast allergy, renal insufficiency) 1
• MRA provides similar sensitivity and specificity to CTA while allowing for post-processing capabilities 1
• Limitations include longer acquisition times, limited ability to characterize aortic wall calcifications, and potential contraindications (pacemakers, claustrophobia) 1, 2

Transesophageal Echocardiography (TEE)

• TEE can be considered in unstable patients who cannot be transported for CTA or when rapid bedside assessment is needed 1, 2
• TEE is useful for evaluating the aortic root and associated valvular abnormalities but has limited visualization of the anterior aortic arch due to interposition of the trachea and left main bronchus 1, 2

Important Considerations for CTA Protocol

• For thoracic aortic aneurysm evaluation, the scan should include the entire aorta from the aortic sinus through the iliac bifurcation 1
• Measurement of the maximal aortic diameter should be performed perpendicular to the long axis of the aorta (outer-to-outer wall diameter) 1, 5
• For patients who cannot tolerate iodinated contrast, non-contrast CT can still provide valuable information about aortic size and wall calcifications, though with reduced diagnostic accuracy 1

Pitfalls to Avoid

• Relying solely on chest X-ray findings without confirmatory cross-sectional imaging can lead to missed diagnoses 1, 6
• Non-ECG-gated studies may have variable image quality through the aortic root and coronary vessels due to cardiac motion 1
• Streak artifacts from implanted devices can limit CTA evaluation 1
• Oblique measurements can artificially accentuate aneurysm dimensions; always measure perpendicular to the vessel axis 5

CTA has become the standard non-invasive imaging modality for thoracic aortic disease due to its superior diagnostic accuracy, speed, and comprehensive evaluation capabilities 7. When a thoracic aortic aneurysm is suspected based on chest X-ray findings, prompt CTA evaluation is essential for accurate diagnosis and management planning.