What is the recommended imaging protocol for diagnosing and managing an aortic (abdominal aorta) aneurysm using Computed Tomography (CT) scan?

CT Protocol for Abdominal Aortic Aneurysm Diagnosis and Management

CT angiography (CTA) with IV contrast is the gold standard imaging protocol for diagnosing and managing abdominal aortic aneurysms, providing superior diagnostic accuracy and essential information for treatment planning. 1

Initial Imaging Protocol for Suspected AAA

CTA Protocol Components

• Scan Type: CT angiography (CTA) of abdomen and pelvis with IV contrast
• Required Elements:
  1. Precise timing to coincide with peak arterial enhancement
  2. Thin-section CT acquisition with multiplanar reformations
  3. 3D renderings (essential component that distinguishes CTA from standard CT) 1
• Measurement Technique: Outer-to-outer (OTO) wall diameter perpendicular to the long axis of the aorta 1
• Scan Range: Should include iliofemoral arteries for access vessel evaluation and chest in patients with thoracoabdominal AAA 1

Alternative Protocols When CTA Is Not Possible

1. Standard CT with IV contrast: Can accurately assess aortic diameters if contour is well visualized, but lacks 3D rendering capabilities 1
2. Non-contrast CT: Useful when contrast is contraindicated; more sensitive than ultrasound but cannot distinguish aortic wall from lumen 1
3. MRA with IV contrast: Alternative when radiation exposure is a concern or for long-term surveillance; limitations include longer acquisition times and limited ability to characterize wall calcifications 1, 2

Advanced Imaging Features

Measurement Techniques

• Use multiplanar reformatted images angle-corrected for aortic curvature or curved planar reformats with automated centerline 3D software 1
• Measure maximal aortic diameter based on outer-to-outer wall diameter perpendicular to the long axis 1

Critical Features to Assess

1. Aneurysm characteristics:

   • Location, size, and extent
   • Presence and distribution of thrombus (larger and eccentric thrombus associated with rapid enlargement) 1, 3
   • Wall calcification (increases wall stress) 1
   • Saccular morphology (higher rupture risk) 2

2. Anatomical relationships:

   • Proximity to renal arteries
   • Involvement of iliac arteries
   • Branch vessel involvement 1

3. Complications:

   • Signs of rupture or impending rupture (crescent sign, draped aorta, adjacent vertebral erosion) 1
   • Inflammatory changes (perianeurysmal fibrosis) 4

Special Considerations

Dual-Energy CTA

• Allows characterization of AAAs with reduced IV contrast dose without compromising image quality 1
• Particularly useful in patients with renal impairment

Pre-operative Planning

• CTA is the imaging procedure of choice before endovascular or open surgical repair 1, 5, 6
• Complete assessment should include:
  • Precise aneurysm measurements
  • Evaluation of proximal and distal landing zones
  • Assessment of access vessels (iliofemoral arteries) 1, 6

Surveillance Protocol

Based on aneurysm size:

• 30-39 mm: CT every 3 years
• 40-44 mm: CT annually
• 45-54 mm (men) or 45-50 mm (women): CT every 6 months
• ≥55 mm (men) or ≥50 mm (women): Consider repair rather than continued surveillance 2

Pitfalls to Avoid

1. Measurement errors:

   • Failing to measure perpendicular to the vessel axis
   • Not using outer-to-outer wall measurements
   • Not accounting for vessel tortuosity

2. Inadequate scan range:

   • Missing thoracic extension in thoracoabdominal aneurysms
   • Inadequate coverage of iliac arteries for endovascular planning

3. Misinterpretation:

   • Overlooking small contained ruptures
   • Missing inflammatory components of aneurysms
   • Underestimating thrombus burden, which correlates with expansion rate 3

By following this protocol, clinicians can accurately diagnose AAAs, assess rupture risk, plan appropriate interventions, and monitor disease progression to reduce morbidity and mortality.