What is the recommended imaging protocol for diagnosing and monitoring an aneurysm using a CT (Computed Tomography) scan?

CT Scan Protocol for Aneurysm Diagnosis and Monitoring

CT angiography (CTA) is the optimal imaging protocol for diagnosing and monitoring aneurysms, providing high-resolution, three-dimensional visualization with accurate measurements that correlate well with operative findings. 1

Initial Diagnosis Protocol

For Abdominal Aortic Aneurysms (AAA):

• First-line screening: Ultrasound (US) for initial detection in asymptomatic patients

  • Cost-effective, non-invasive, and widely available
  • No radiation exposure

• Definitive diagnosis and pre-intervention assessment: CTA abdomen and pelvis with IV contrast

  • Technical specifications:
    • Multidetector CT with submillimeter, isotropic acquisition
    • Contrast concentration: 270-320 mg Iodine/mL 2
    • Scan range: Include iliofemoral arteries for access vessel evaluation
    • Include chest imaging for thoracoabdominal aneurysms 1

• Measurement technique:

  • Outer wall to outer wall (OTO) diameter perpendicular to the long axis of the aorta
  • This is considered the reference standard for AAA diagnosis and management 1
  • Note: US tends to underestimate aneurysm size by approximately 4mm compared to CTA 1, 3

For Cerebral Aneurysms:

• Gold standard: Cerebral arteriography (invasive angiography) 1
• Preferred non-invasive option: CTA head with IV contrast (rated 8/9 in appropriateness) 1
  • Particularly superior for evaluation of clipped aneurysms
  • Can detect aneurysms ≥5mm with good accuracy 4

Monitoring Protocol

For AAA Surveillance:

• Surveillance intervals based on aneurysm size: 3

  • 25-29 mm: Every 4 years
  • 30-39 mm: Every 3 years
  • 40-49 mm: Annually
  • 50-55 mm (men) or 45-50 mm (women): Every 6 months
  • ≥55 mm (men) or ≥50 mm (women): Consider intervention rather than surveillance

• Imaging modality for routine surveillance:

  • Ultrasound for stable, uncomplicated AAAs
  • Switch to CTA when:
    • US does not allow adequate measurement
    • AAA reaches size threshold for intervention (≥5.5 cm)
    • Pre-operative planning is needed 3
    • Rapid expansion is detected (≥10 mm/year or ≥5 mm/6 months)

For Cerebral Aneurysm Follow-up:

• For untreated cerebral aneurysms: 1

  • Best followed using the same imaging modality on which the aneurysm was initially found
  • CTA head with IV contrast (rated 8/9 in appropriateness)
  • Consider switching to MRA if stable over time to reduce radiation exposure

• For previously treated cerebral aneurysms: 1

  • CTA head with IV contrast (rated 8/9 in appropriateness)
  • CTA is superior for evaluation of clipped aneurysms
  • MRA is superior for evaluation of coiled aneurysms

Special Considerations

• Dual-energy CTA can be used to characterize AAAs with reduced IV contrast dose without compromising image quality 1

• CT without contrast may be useful when:

  • Evaluating for suspected contained rupture (to better diagnose dissecting hematoma) 1
  • IV contrast is contraindicated
  • Note: Noncontrast CT has been found to be more sensitive than US in identifying AAAs 1

• Women have similar growth rates as men but a four-fold higher rupture risk at the same diameter, requiring lower diameter thresholds for intervention 3

• Saccular morphology of smaller aneurysms may be associated with increased rupture risk below standard size thresholds 1, 3

• 3D reconstruction yields valuable topographic information for planning endovascular or surgical intervention 5

By following these protocols, clinicians can accurately diagnose aneurysms, monitor their growth, and determine the optimal timing for intervention, ultimately reducing morbidity and mortality associated with aneurysm rupture.