What type of MRI (Magnetic Resonance Imaging) should be ordered for a patient with a left aortic arch and an aberrant right subclavian artery?

MRI for Left Aortic Arch with Aberrant Right Subclavian Artery

For a patient with left aortic arch and aberrant right subclavian artery, a dedicated cardiovascular magnetic resonance (CMR) study with 3D imaging of the thoracic aorta and its branches should be ordered to evaluate for associated thoracic aortic aneurysm and to assess for compression of adjacent structures. 1, 2

Rationale for CMR Selection

CMR provides several advantages for evaluating this aortic arch anomaly:

1. Complete visualization of vascular anatomy:

   • Allows detailed assessment of the aortic arch, its orientation, and the origin and course of arch vessels 1
   • Enables visualization of potential Kommerell's diverticulum, which may be present at the origin of the aberrant vessel 1, 2

2. Assessment of associated complications:

   • Evaluates for thoracic aortic aneurysm (TAA), which is significantly associated with left aortic arch with aberrant right subclavian artery 1
   • According to AHA/ACC guidelines, 2-8% of patients with this anomaly have associated thoracic aortic dissection 1

3. Visualization of adjacent structures:

   • Demonstrates compression of the trachea and esophagus, which may cause symptoms 1
   • Superior to cardiac catheterization angiography in demonstrating airway compression 1

4. Surgical planning benefits:

   • Provides 3D reconstruction of the great vessels, which many surgeons prefer before operating on arch anomalies 1
   • Helps determine if revascularization of vessels is needed during potential future aortic repair 1

Specific CMR Protocol Elements

The CMR protocol should include:

• ECG-gated sequences covering the heart and thoracic aorta
• Multiple imaging planes (transverse, sagittal, and coronal views)
• 3D dark blood imaging to visualize the trachea
• Contrast-enhanced MR angiography for detailed vascular anatomy
• Volume-rendered 3D reconstructions of the aortic arch and branches

Clinical Implications and Management

• If the initial imaging that detected the aberrant right subclavian artery did not include thoracic aorta evaluation, a dedicated CMR is reasonable (Class 2a recommendation) 1

• Measurement of any Kommerell's diverticulum should include:

  • Diverticulum orifice diameter (radially and longitudinally at the aortic wall)
  • Combined diameter of the diverticulum and adjacent descending thoracic aorta 1, 2

• Surgical intervention may be considered when:

  • Diverticulum orifice diameter is >3.0 cm
  • Combined diameter of diverticulum and adjacent aorta is >5.0 cm
  • Significant symptoms (dysphagia, respiratory symptoms) are present 2

Advantages Over Other Imaging Modalities

While CT angiography could provide similar anatomical information, CMR offers:

• No ionizing radiation exposure
• No iodinated contrast requirement
• Superior soft tissue characterization
• Ability to evaluate blood flow dynamics

CMR has been demonstrated to be an effective, non-invasive method for evaluating congenital aortic arch anomalies and can substitute for other techniques 3.