What are the indications for Magnetic Resonance Angiography (MRA)?

Indications for Magnetic Resonance Angiography (MRA)

MRA is indicated as the preferred noninvasive imaging modality for evaluating intracranial aneurysms (untreated, endovascularly treated, and non-clipped surgically treated), arterial dissections, vascular malformations, cerebrovascular steno-occlusive disease, and peripheral arterial disease when planning revascularization, with particular advantage in pediatric populations and patients with renal insufficiency where radiation and nephrotoxic contrast must be avoided. 1

Cerebrovascular Indications

Intracranial Aneurysms

• MRA is the preferred long-term follow-up imaging modality for untreated intracranial aneurysms, endovascularly treated aneurysms, and non-clipped microsurgically treated aneurysms. 1
• MRA can detect aneurysmal pathology ≥3 mm in maximal diameter with high sensitivity. 1
• For pediatric patients with stable aneurysms not requiring treatment, perform MRA every 5 years for life. 1
• For complex pediatric aneurysms (giant >25 mm, daughter sacs, fusiform/dissecting morphology), perform MRA at 1,2, and 5 years, then every 5 years for life. 1
• MRA is a noninvasive alternative to catheter angiography for detecting aneurysms as the underlying cause of subarachnoid hemorrhage, with 57% of pediatric SAH due to aneurysm. 1

Arterial Dissections

• MRA is excellent for assessing vascular injury, particularly intramural hematoma, and parenchymal changes associated with dissection, including concurrent ischemia and hemorrhage. 1
• The presence of para-aneurysmal luminal stenosis on MRA helps distinguish fusiform from dissecting pseudoaneurysms. 1
• MRA can identify and distinguish mural and luminal thrombosis. 1

Cerebrovascular Steno-Occlusive Disease

• MRA is routinely used in cerebral arterial occlusive diseases with high accuracy in grading carotid artery stenosis, having largely replaced contrast angiography in many institutions. 1, 2
• Three-dimensional time-of-flight MRA is recommended for arterial evaluation of steno-occlusive disease. 2
• For patients with impaired renal function (eGFR <30 mL/min/1.73 m²), non-contrast MRA techniques should be used for carotid evaluation to avoid nephrotoxicity. 3

Vascular Malformations

• MRA is indicated for detecting and characterizing arteriovenous malformations, with dynamic contrast-enhanced sequences particularly valuable. 1, 2
• MRA can demonstrate high-flow vascular malformations, though developmental venous anomalies, cavernomas, and capillary telangiectasias may be better evaluated with other sequences. 1

Nontraumatic Intraparenchymal Hemorrhage

• MRA may be used to delineate vascular anatomy and demonstrate underlying vascular malformations or aneurysms as the cause of hemorrhage, though it lacks the temporal information of catheter angiography. 1

Cerebral Vasculitis

• MRA is useful in detecting small-vessel vasculitis, including childhood primary angiitis of the central nervous system. 1

Peripheral Vascular Indications

Peripheral Arterial Disease (PAD)

• MRA is a first-line technique for imaging peripheral vascular disease with sensitivity and specificity in the 90-100% range for detecting stenoses >50%. 1
• MRA is more cost-effective than duplex ultrasound and safer than catheter arteriography. 1
• In diabetic patients, MRA is particularly helpful for runoff evaluation due to superior ability to detect flow in small, calcified vessels, approaching the sensitivity of digital subtraction angiography. 1

Pre-Revascularization Planning

• Contrast-enhanced MRA is accurate for preoperative planning and has largely replaced angiography as a pure diagnostic tool in patients with normal renal function. 1
• Time-resolved imaging of calves and pedal arteries provides accurate identification of infrageniculate and pedal arteries as potential touchdown sites for bypass surgeries. 1

Post-Revascularization Surveillance

• MRA is helpful in detecting recurrent disease after angioplasty, though improved outcomes from such surveillance have not been definitively documented. 1
• MRA provides excellent evaluation of lower extremity bypass conduits with imaging quality similar to digital subtraction angiography. 1

Abdominal Vascular Indications

Abdominal Aortic Aneurysm (AAA)

• MRA may be substituted for CTA in pre-intervention AAA evaluation when CT cannot be performed (e.g., iodinated contrast allergy). 1
• For patients with severe renal insufficiency, the imaging center must be capable of performing MRA without gadolinium contrast. 1

Renal Artery Stenosis

• Steady-state free precession MRA demonstrates sensitivity of 78-90% and specificity of 91-94% for detecting renal artery stenosis. 3

Special Population Considerations

Pediatric Patients

• MRA is advocated over CTA for serial follow-up imaging of pediatric intracranial neurovascular pathology to avoid cumulative radiation exposure and its carcinogenic risk. 1
• Radiation confers significant risk of carcinogenesis in children, who are more sensitive to harmful effects of radiation. 1
• The need for sedation is a disadvantage for MRA use in young children. 1

Patients with Renal Insufficiency

• Non-contrast MRA techniques (time-of-flight, phase-contrast, fresh-blood imaging, balanced steady-state free precession) are increasingly adopted for patients with renal insufficiency to avoid nephrotoxic contrast agents. 1, 3
• Time-of-flight MRA is sufficiently sensitive to screen for culprit intracranial and extracranial lesions in cerebrovascular disease. 3

Technical Advantages of MRA

• MRA is noninvasive, involves no ionizing radiation, and circumvents systemic reactions sometimes caused by contrast agents in conventional angiography. 4
• MRA can visualize all arterial feeders, contribution, and collaterals simultaneously in the same image. 1
• MRA provides both anatomic and physiologic information in a single examination. 4
• MRA can be conducted in an outpatient setting with fewer restrictions than conventional angiography. 4

Important Limitations and Pitfalls

Technical Limitations

• Time-of-flight MRA may give false-negative results in patent aneurysms with low or turbulent flow. 1
• In recently ruptured aneurysms, T1 hyperintensity from subacute hematoma may mislead diagnosis of aneurysmal persistence or recurrence. 1
• Phase-contrast MRA may be the best modality in acute/subacute settings to circumvent confounding by T1 hyperintensity from blood clot. 1
• Overestimation of stenosis severity is common with time-of-flight techniques, particularly in high-grade stenosis. 3
• Metallic surgical clips near vessels can cause signal loss artifacts that falsely suggest stenosis. 3

Patient-Related Limitations

• MRA requires exclusion of patients with pacemakers or other metallic implants. 1
• Loss of signal occurs in arterial segments within metal stents or adjacent to metallic clips or prosthetic joints. 1
• Cardiac arrhythmia can impair image quality with newer noncontrast techniques, limiting evaluation of distal calf and pedal arteries. 1

Comparative Limitations

• CTA remains the preferred modality for following clipped aneurysms. 1
• MRA has not yet replaced catheter angiography as the gold standard, though it has largely replaced it for pre-intervention planning in many institutions. 1
• MRA lacks the temporal information available from catheter angiography. 1