Magnetic Resonance Angiography (MRA) of Cerebral Vessels

Currently, Magnetic Resonance Angiography (MRA) of cerebral vessels has emerged as a leading diagnostic modality for many vascular diseases of the brain, often relegating older methods such as conventional X-ray angiography (which requires arterial puncture) and, in some specific vascular indications, standard computed tomography (CT) scans, to secondary or complementary roles.

 

Clinical Indications for Cerebral MRA

An MRA of the cerebral vessels may be prescribed by a doctor in various clinical situations to diagnose or evaluate conditions affecting the brain's blood supply. Common indications include:

• Suspicion or known Arteriovenous Malformations (AVMs) of the brain or other vascular anomalies.
• Evaluation of cerebral and spinal adhesive arachnoiditis if vascular involvement or complications are suspected.
• Assessment of encephalopathy where a vascular cause (e.g., chronic ischemia, vasculitis) is considered.
• Investigation of persistent or atypical headache or migraine, especially if concerning features suggest underlying vascular pathology.
• Evaluation of conditions associated with increased intracranial pressure and hydrocephalus to assess venous outflow or related vascular issues.
• Diagnosis and monitoring of ischemic brain disease (cerebrovascular disease), including assessment of arterial stenosis or occlusion.
• Acute evaluation in ischemic stroke or transient ischemic attack (TIA) to identify occluded vessels (e.g., carotid artery, middle cerebral artery) and assess cerebral ischemia.
• Investigation of unexplained nosebleeds (epistaxis) if a vascular anomaly within the head (e.g., carotid-cavernous fistula, aneurysm) is suspected as a rare cause.
• As part of the workup for certain neurological conditions like Parkinson's disease if vascular parkinsonism is in the differential, or to rule out vascular lesions mimicking symptoms.
• Assessment of vascular supply or compression in cases of pituitary microadenoma, macroadenoma, nonfunctioning pituitary adenomas (NFPAs), or hyperprolactinemia syndrome.
• Screening for, diagnosis of, and follow-up of saccular aneurysms (berry aneurysms) and assessment after subarachnoid hemorrhage.
• Evaluation of symptoms suggestive of somatoform autonomic dysfunction if vascular dysregulation is considered, though MRA is not primary for this.
• Following traumatic brain injury (TBI) (concussion, contusion, brain hemorrhage, axonal shearing lesions) to assess for vascular injury such as dissection, pseudoaneurysm, or traumatic occlusion.
• Diagnosis of vertebrobasilar insufficiency (VBI) presenting with vertigo, to evaluate the posterior circulation arteries (vertebral and basilar arteries).
• Detection of dural venous sinus thrombosis.
• Evaluation of vasculitis affecting cerebral vessels.

MRA Techniques and Contrast Enhancement

Several MRA techniques can be employed to visualize cerebral vessels, some of which do not require intravenous contrast agents, while others utilize them for enhanced detail.

 

Time-of-Flight (TOF) MRA

TOF MRA is a common non-contrast technique that relies on the principle of flow-related enhancement. It distinguishes flowing blood from stationary tissue by exploiting the unsaturated spins of protons in fresh blood entering the imaging slice, which appear bright compared to the saturated spins of static tissue. 2D TOF is often used for larger areas like the carotid arteries, while 3D TOF provides higher resolution for intracranial vessels.

 

Phase-Contrast (PC) MRA

PC MRA is another non-contrast technique that uses differences in the phase of proton spins induced by blood flow along magnetic field gradients. It can provide quantitative information about blood flow velocity and direction, in addition to anatomical images. It is particularly useful for imaging venous structures or assessing flow dynamics.

 

Contrast-Enhanced (CE) MRA

In CE-MRA, a gadolinium-based contrast agent (e.g., Omniscan, though specific agents used vary) is injected intravenously. The contrast agent shortens the T1 relaxation time of blood, making vessels appear bright on T1-weighted images. This technique provides excellent vessel delineation, is less susceptible to flow artifacts than TOF MRA, and allows for imaging over larger areas with faster acquisition times. It is often used for evaluating extracranial carotid and vertebral arteries, venous sinuses, or when non-contrast techniques are suboptimal.

High-field MRI scanners, such as those with a magnetic field strength of 3.0 T (Tesla), offer improved signal-to-noise ratio and spatial resolution for all MRA techniques, potentially enhancing the visualization of smaller vessels and subtle pathologies. It's also possible to combine MRA with standard MRI sequences using intravenous contrast to increase the visual difference between healthy brain tissue and tumors or areas of inflammation.

Patient weight limitations (e.g., up to 200 kg) are a practical consideration for any MRI procedure, including MRA.

 

Benefits and Limitations of Cerebral MRA

Benefits:

• Non-invasive or Minimally Invasive: Many MRA techniques (TOF, PC) do not require contrast injection or arterial puncture. CE-MRA involves only an IV injection.
• No Ionizing Radiation: Unlike CT Angiography (CTA) or Digital Subtraction Angiography (DSA).
• Excellent Soft Tissue Detail: Can be combined with standard MRI to evaluate brain parenchyma simultaneously.
• Good Visualization of Arteries and Veins: Including their course and relationship to other structures.
• 3D Reconstruction Capabilities: Allows for comprehensive assessment from multiple angles.
• Flow Information (with PC-MRA): Can quantify blood flow.

Limitations:

• Susceptibility to Motion Artifacts: Patient movement can degrade image quality.
• Overestimation of Stenosis: Non-contrast TOF MRA can sometimes overestimate the degree of arterial narrowing due to turbulent or slow flow artifacts.
• Limited Resolution for Very Small Vessels: Compared to DSA, which remains the gold standard for fine vascular detail.
• Contraindications to MRI: Certain metallic implants (e.g., older pacemakers, some aneurysm clips), claustrophobia.
• Contrast Agent Risks (for CE-MRA): Though rare, allergic reactions or NSF (in patients with severe renal impairment) are potential risks with gadolinium.
• Longer Scan Times: Compared to CTA.
• Cost: Generally more expensive than CTA or ultrasound.

 

Patient Preparation and Procedure

Preparation for an MRA of cerebral vessels is similar to a standard brain MRI:

• Screening: Patients are screened for any contraindications to MRI (e.g., metallic implants, claustrophobia, pregnancy).
• Metal Objects: All metallic items must be removed.
• Fasting: Usually not required for non-contrast MRA. If contrast is planned, fasting for a few hours might be requested, especially if sedation is a possibility.
• Contrast Information: If CE-MRA is performed, an IV line will be placed. Patients should inform staff of any allergies or kidney problems.

During the procedure, the patient lies on a table that slides into the MRI scanner. They will hear loud noises and must remain still. The scan typically takes 30-60 minutes.

 

Comparison with Other Cerebrovascular Imaging Modalities

Modality	Principle	Invasiveness	Radiation	Contrast	Strengths	Weaknesses
MRA (Cerebral)	Magnetic fields, radio waves, blood flow	Non-invasive (TOF, PC) or Minimally invasive (CE-MRA via IV)	No	Often none (TOF, PC); Gadolinium (CE-MRA)	Good vessel detail, no radiation, soft tissue imaging, 3D.	Motion artifacts, overestimation of stenosis (TOF), longer scan, cost, MRI contraindications.
CT Angiography (CTA - Cerebral)	X-rays with IV iodinated contrast	Minimally invasive (IV contrast)	Yes	Iodinated (IV)	Fast, excellent for acute stroke/hemorrhage, good for stenosis/aneurysms, widely available.	Radiation, contrast risks (allergy, nephropathy), less soft tissue detail than MRI.
Digital Subtraction Angiography (DSA - Catheter Angiography)	X-rays with intra-arterial iodinated contrast	Invasive (arterial puncture/catheterization)	Yes	Iodinated (intra-arterial)	Gold standard for vascular detail, allows for intervention (e.g., coiling, stenting).	Invasive risks (bleeding, stroke, vessel injury), radiation, contrast risks.
Carotid/Transcranial Doppler Ultrasound	Sound waves, Doppler effect	Non-invasive	No	None	Real-time flow assessment, portable, inexpensive. Good for carotid stenosis screening.	Operator dependent, limited intracranial window (especially TCD), bone artifact.

 

Role in Diagnosis and Treatment Planning

Cerebral MRA plays a vital role in the non-invasive evaluation of a wide spectrum of cerebrovascular diseases. Its ability to depict arteries and veins, identify abnormalities like aneurysms, AVMs, stenosis, or occlusion, and be combined with conventional MRI for brain tissue assessment makes it an indispensable tool in neurology, neurosurgery, and neuroradiology for diagnosis, guiding therapeutic decisions, and monitoring treatment outcomes.