Diagnostic Role of Magnetic Resonance Angiography in Aortic Dissection

Diagnostic Capabilities

• 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 2
• MRA can accurately assess adverse signs such as pericardial effusion and aortic regurgitation 2

Advanced Functional Assessment

• Flow in the false and true lumen can be quantified using phase contrast cine magnetic resonance imaging or tagging techniques 2
• With state-of-the-art MRI, the proximal coronary arteries and their involvement in the dissecting process can be clearly delineated 2
• MRA can detect intramural hemorrhages, showing a thickened wall with areas of high signal intensity due to methemoglobin formation 3

Advantages Over Other Imaging Modalities

• MRA provides excellent visualization of the extent of dissection, with the true lumen typically located anteriorly and the false lumen posteriorly 4
• In the future, MRA may replace conventional angiography for visualizing the full anatomical situation including collateral flow 4

Limitations of MRA in Aortic Dissection

• MRA is often not available on an emergency basis, limiting its use in acute settings 2
• Examination of hemodynamically unstable patients may be difficult, though it can be performed in centers with experience in monitoring cardiac patients on mechanical ventilation 2
• Conventional spin-echo MRI and cine MRI require substantial imaging time, though newer rapid scanning techniques permit image acquisition within a single breath-hold 2

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 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 4

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 2
• Because intramural hemorrhage of the thoracic aorta may be associated with true dissection of the abdominal aorta, both areas should be evaluated 3
• 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 5

MRI for Aortic Dissection: Diagnostic Approach

Optimal MRI Protocol Selection

• The American College of Radiology recommends MRA chest without and with IV contrast as the optimal MRI protocol for diagnosing aortic dissection, offering excellent sensitivity (92-98%) and specificity (100%) 6, 7
• MRA chest without and with IV contrast is superior to non-contrast MRA for aortic dissection diagnosis, providing detailed visualization of the dissection flap, entry tears, and branch vessel involvement 6, 7
• MRA demonstrates 100% sensitivity for identifying thrombus formation and pericardial effusion, which are critical complications of aortic dissection 6
• ECG-gated acquisition should be used to minimize cardiac motion artifacts and allow for accurate orthogonal measurement of the aortic dimensions 8
• When the dissection potentially extends beyond the thorax, imaging should include the abdomen and pelvis to evaluate the full extent of the disease 9

Diagnostic Accuracy Comparison

• MRA has demonstrated 100% sensitivity (95% CI 70% to 100%) for aortic dissection diagnosis, compared to 93% for CT and 88% for TEE 10, 11

Practical Implementation

• When ordering MRA for aortic dissection, specify "MRA chest without and with IV contrast" to ensure optimal protocol selection 6, 7
• For comprehensive evaluation, consider "MRA chest, abdomen, and pelvis without and with IV contrast" if there is concern for extension of dissection 9

Diagnostic Imaging for Aortic Dissection

Primary Recommendation

• The American College of Radiology recommends MRA chest without IV contrast using ECG-gated balanced steady-state free precession (bSSFP) sequences as the best imaging test for diagnosing aortic dissection when IV contrast cannot be used 12, 13
• Non-contrast MRA with bSSFP sequences achieves near 100% accuracy for detecting thoracic aortic dissection, intramural hematoma, and penetrating aortic ulcer when compared against contrast-enhanced imaging 14
• This technique provides inherent contrast between blood pool and vessel wall without requiring gadolinium administration 12, 13

Technical Requirements

• ECG-gating is essential to minimize cardiac motion artifacts and ensure accurate measurements, particularly in the ascending aorta where cardiac pulsatility can degrade image quality 13, 14
• Respiratory gating improves accuracy by compensating for motion, though this increases acquisition time to tens of minutes 12
• Acquisition should include transversal, coronal, and parasagittal planes along the aortic arch curvature 12

Diagnostic Performance

• Non-contrast MRA accurately identifies entry/reentry tears, thrombus formation, pericardial effusion, and aortic regurgitation 12, 13
• Intramural hematoma appears as hyperattenuating crescent >45 HU in aortic wall on non-contrast CT chest 15, 16

Clinical Algorithm

• First-line: MRA chest without IV contrast with ECG-gated bSSFP sequences, extending to chest/abdomen/pelvis if dissection may involve abdominal aorta 12, 13, 14
• Acquisition time: 20-30 minutes with gating 12

Important Caveats

• Longer acquisition times (tens of minutes) may be problematic in unstable patients 12
• Artifacts from surgical material can occur with bSSFP sequences, though less problematic than with contrast-enhanced sequences 12

Best Imaging Test for Aortic Dissection Without IV Contrast

Diagnostic Performance Comparison

• Non-contrast CT chest is a reasonable alternative if MRI is unavailable or contraindicated, though it is significantly inferior to non-contrast MRA, with a sensitivity of 83-89% compared to ultrasound's 57-70% for abdominal aortic pathology 17, 18
• Non-contrast MRA has 85% sensitivity and 100% specificity for identifying the site of entry, according to the American College of Radiology 19