What is the role of Magnetic Resonance Angiography (MRA) in diagnosing and managing dissecting aortic aneurysms?

Role of Magnetic Resonance Angiography in Dissecting Aortic Aneurysm

Magnetic Resonance Angiography (MRA) is a highly sensitive and specific imaging modality for diagnosing aortic dissection, but is often limited by availability in emergency situations and challenges with unstable patients. 1

Diagnostic Capabilities of MRA

• MRA clearly demonstrates the extent of aortic dissection and depicts the distal ascending aorta and aortic arch in more detail than transoesophageal echocardiography 1
• The sensitivity of MRA for diagnosing aortic dissection approaches 90%, with the ability to accurately localize entry and reentry tears 1
• MRA enables proper classification of the disease into proximal and distal aortic dissection, which is crucial for selecting appropriate management 1
• MRA can accurately assess adverse signs such as pericardial effusion and aortic regurgitation 1

Advanced Functional Assessment

• Flow in the false and true lumen can be quantified using phase contrast cine magnetic resonance imaging or tagging techniques 1
• With state-of-the-art MRI, the proximal coronary arteries and their involvement in the dissecting process can be clearly delineated 1
• MRA can detect intramural hemorrhages, showing a thickened wall (>7 mm) with areas of high signal intensity due to methemoglobin formation 1

Advantages Over Other Imaging Modalities

• MRA does not require ionizing radiation or nephrotoxic contrast agents, making it suitable for patients with renal impairment 2
• MRA provides excellent visualization of the extent of dissection, with the true lumen typically located anteriorly and the false lumen posteriorly 1
• In the future, MRA may replace conventional angiography for visualizing the full anatomical situation including collateral flow 1

Limitations of MRA in Aortic Dissection

• MRA is often not available on an emergency basis, limiting its use in acute settings 1
• Examination of hemodynamically unstable patients may be difficult, though it can be performed in centers with experience in monitoring cardiac patients on mechanical ventilation 1
• Conventional spin-echo MRI and cine MRI require substantial imaging time, though newer rapid scanning techniques permit image acquisition within a single breath-hold 1
• Signal loss can occur in slow or turbulent flow conditions, and spatial resolution may be unsatisfactory in small vessels 3

Clinical Applications and Recommendations

• MRA is classified as a Class I recommendation (with level of evidence C) for the diagnosis of aortic dissection according to European guidelines 1
• MRA is particularly useful in the preoperative staging of aneurysms and assessment of proximal main renal artery involvement 3
• For young individuals requiring repetitive studies and long-term follow-up, MRA is preferred due to the absence of radiation exposure 2
• MRA with three-dimensional surface rendering provides high-resolution images that can be reviewed with three-dimensional form and perspective, which surgeons often prefer over invasive angiography before surgical intervention 4

Integration with Other Imaging Modalities

• Transthoracic echocardiography followed by transesophageal echocardiography is recommended as the first-line approach, with MRA or CT as complementary techniques 1
• CT is often preferred in emergency situations due to its rapid image acquisition, multiplanar capacity, and wide availability 2
• When combined with preoperative segmental Doppler arterial studies, MRA can help formulate an accurate surgical plan 5

Common Pitfalls and Caveats

• Awareness of artifacts, knowledge of normal anatomy, and use of axial images plus additional planes as needed are essential to avoid misinterpretation 1
• Because intramural hemorrhage of the thoracic aorta may be associated with true dissection of the abdominal aorta, both areas should be evaluated 1
• A "blind spot" exists in visualizing small circumscript dissected segments within the distal part of the ascending aorta and anterior portion of the aortic arch due to interposition of the trachea and left main stem bronchus 1