Recommended Protocol for Vascular MRI
For vascular MRI, the optimal protocol depends on the anatomical region and clinical indication, but generally combines non-contrast MRA of the head with contrast-enhanced MRA of the neck, using specific technical parameters to maximize diagnostic accuracy while minimizing contrast exposure.
Protocol Selection by Clinical Indication
Cerebrovascular Disease (Stroke/TIA)
The preferred MR vascular imaging approach combines non-contrast head MRA with contrast-enhanced neck MRA 1. This combination is rated as "usually appropriate" (rating 8/9) by the American College of Radiology for both acute stroke and TIA evaluation 1.
- For acute stroke (<6 hours): Perform MRA head and neck without and with IV contrast in conjunction with brain MRI, as vascular imaging is essential to evaluate stroke mechanism and guide endovascular therapy decisions 1
- For TIA evaluation: MRA head and neck (with or without contrast) should be obtained alongside brain MRI to identify stenosis/occlusion and determine appropriate secondary prevention 1
- Time-critical scenarios: Non-contrast MRA alone (rating 8) is acceptable when contrast is contraindicated or in patients with renal failure 1
Aneurysm Screening
Non-contrast MRA of the head is the ideal screening study of choice (rating 8) for patients at risk of unruptured aneurysms, including those with polycystic kidney disease or family history of subarachnoid hemorrhage 1.
- MRA head without and with IV contrast is an acceptable alternative (rating 8) 1
- CTA provides comparable diagnostic accuracy but involves radiation exposure 1
Extracranial Carotid Disease
For asymptomatic patients with cervical bruit or risk factors, duplex ultrasound, non-contrast neck MRA, or contrast-enhanced neck MRA are all rated as "usually appropriate" (rating 8) 1.
- If initial screening is positive, follow-up with CTA or contrast-enhanced MRA is recommended 1
- CTA and contrast-enhanced MRA are comparable non-invasive alternatives, each with distinct advantages 1
Technical Specifications
MRA Head (Non-Contrast)
- Scanner: 3.0T MRI preferred, minimum 16-channel head coil 1
- Sequence: 3D time-of-flight (TOF) technique with fat suppression 1, 2, 3
- Acquisition: Cardiac-gated using diaphragm navigator or breath-hold acquisitions 1
- Advantages: No contrast required, excellent for intracranial circulation 4, 2, 3
MRA Neck (Contrast-Enhanced)
- Scanner: 1.5T acceptable, 3.0T preferred with minimum 16-channel head/neck coil 1
- Contrast: Gadobutrol 0.1 mL/kg (0.1 mmol/kg) administered via power injector at ~1.5 mL/second 5
- Sequence: T1-weighted, fat-suppressed, contrast-enhanced black blood imaging 1
- Timing: Bolus-tracking method with arterial phase acquisition 1
Body/Aortic MRA
- Coverage: Aorta and major branches from carotid bifurcation to abdominal/pelvic arteries in coronal acquisition 1
- Sequences:
- Spatial resolution: 1.2×1.3×2 mm³ 1
Contrast Considerations
When to Use Contrast
- Neck MRA: Contrast-enhanced technique is preferred over non-contrast for extracranial vessels 1
- Breast MRI: Contrast is essential for malignancy assessment (0.1 mmol/kg as bolus via power injector) 5
- Cardiac MRI: Two separate 0.05 mmol/kg injections (stress and rest) at 4 mL/second 5
When to Avoid Contrast
- Renal impairment: GFR <30 mL/min/1.73m² carries highest risk for nephrogenic systemic fibrosis 5
- Acute kidney injury: Screen all patients; avoid contrast unless diagnostic information is essential 5
- Contrast allergy: Non-contrast MRA techniques are viable alternatives 1, 4, 2, 3
- Pediatric patients: Non-contrast techniques avoid gadolinium deposition concerns 4
Advanced Non-Contrast Techniques
Recent technical advances have expanded non-contrast MRA capabilities beyond traditional TOF imaging 2, 3:
- Quiescent-interval slice selective (QISS) MRA: Provides good visualization comparable to contrast-enhanced techniques 6
- 4D-flow MRI: Offers hemodynamic parameters (wall shear stress) not available with contrast methods 6
- Arterial spin labeling (ASL): Non-contrast alternative for cerebral blood flow mapping in stroke evaluation 7
- Balanced steady-state free precession: Emerging technique for time-resolved vascular imaging 3
Common Pitfalls and Optimization
Image Quality Optimization
- Avoid perpendicularity: Angle the color box in longitudinal scans to prevent flow signal loss 1
- Timing is critical: For contrast-enhanced studies, use bolus-tracking to ensure peak arterial opacification 1, 8
- Reformatting essential: Use multiplanar and maximum-intensity projection reconstructions; relying on axial slices alone underestimates stenosis in 43% of cases 8
Technical Limitations
- Heavy calcification: Can limit interpretation of both MRA and CTA; dual-energy CTA may help when available 8
- Metallic artifacts: Ferromagnetic occlusion devices in older Fontan patients can preclude satisfactory MRA 1
- Acquisition time: MRA requires longer scan times than CTA, making it less suitable for acute arterial injury assessment 1
Patient-Specific Adjustments
- Renal function screening: For patients >60 years or with hypertension/diabetes, estimate GFR before contrast administration 5
- Injection site: Use contralateral arm if continuous pharmacologic stress agent infusion is being administered (cardiac MRI) 5
- Flow rate adjustment: Pediatric patients may require manual injection rather than power injector 5
Comparison with Alternative Modalities
CTA remains superior for coronary artery visualization and acute arterial injury assessment 1, but MRA offers:
- No radiation exposure 1
- Superior soft tissue contrast 1
- Functional capabilities (flow quantification, perfusion) not available with CTA 6, 7
- Viable non-contrast options for patients with contraindications 4, 2, 3
Conventional angiography is now primarily reserved for cases requiring endovascular intervention rather than diagnostic purposes alone 8.