What is the preferred CT (Computed Tomography) chest protocol, with or without contrast, for diagnosing an aortic aneurysm?

CT Chest with Contrast is the Preferred Protocol for Diagnosing Aortic Aneurysm

CT angiography (CTA) with intravenous contrast is the recommended imaging modality for diagnosing thoracic aortic aneurysm, as it provides critical information about the aortic lumen, wall, and surrounding structures. 1

Rationale for CT with Contrast

• CTA is considered the reference standard for aortic aneurysm diagnosis and management decision-making, providing high-resolution images that allow for accurate measurement and characterization 1
• Contrast enhancement is essential as it allows visualization of the aortic lumen, which cannot be adequately assessed with non-contrast studies 1
• CTA provides a 3D dataset that can be reconstructed in multiple planes, allowing for precise measurement of the aneurysm perpendicular to the flow lumen 1
• The American College of Radiology explicitly recommends CTA chest for radiological diagnosis of suspected thoracic aortic aneurysm 1

Limitations of Non-Contrast CT

• Non-contrast CT as a standalone study can detect changes in aortic diameter and identify intramural hematoma, but has limited utility in comprehensive aneurysm evaluation 1
• The lack of IV contrast leads to underestimation of non-calcified atherosclerotic plaque and does not provide adequate assessment of the aortic lumen 1, 2
• While non-contrast CT can identify displaced calcified intimal flaps in some cases, it has lower sensitivity for detecting aortic pathology compared to contrast-enhanced studies 2

Technical Considerations

• Electrocardiogram (ECG) gating is particularly valuable for imaging the ascending aorta to minimize cardiac motion artifacts and ensure accurate measurements 1
• CTA protocols for aortic pathology typically include arterial and delayed contrast phases for comprehensive evaluation 1
• The scan range should include the entire thoracic aorta and may be extended to include the abdomen and pelvis if there is concern for extension of disease 1
• 3D rendering is a required element of CTA, allowing for detailed visualization of the aneurysm and its relationship to branch vessels 1

Special Circumstances

• In patients with renal insufficiency where contrast is contraindicated, MR angiography (MRA) is an alternative with similar sensitivity and specificity to CTA 1
• For patients with known intramural hematoma (IMH), a dual-phase protocol including both non-contrast and contrast-enhanced images may improve diagnostic accuracy 1
• Dual-energy CT allows for virtual non-contrast images that can help visualize intramural hematoma while reducing overall radiation exposure 1

Clinical Application

• CTA not only diagnoses the aneurysm but also provides critical information for treatment planning, including relationship to branch vessels and suitability for endovascular repair 1
• Measurements should be made perpendicular to the long axis of the aorta (outer-to-outer wall) for standardized evaluation 1, 3
• CTA allows for assessment of complications such as rupture, dissection, or thrombus formation that may affect management decisions 1

In summary, while non-contrast CT can identify some features of aortic aneurysms, CT angiography with intravenous contrast is the preferred protocol for comprehensive evaluation, accurate measurement, and treatment planning for thoracic aortic aneurysms 1.