Magnetic Resonance Angiography (MRA) of the brain is a non-invasive medical imaging technique used to visualize the blood vessels within the head. It is a specialized application of Magnetic Resonance Imaging (MRI) that focuses on the circulatory system. The procedure provides detailed, three-dimensional images of the arteries and veins. This allows physicians to assess their structure and the flow of blood through them without the need for traditional, more invasive catheter-based angiography.
How Magnetic Resonance Angiography Works
MRA utilizes a powerful magnetic field and radio waves to generate images of the body’s internal structures, a process identical to a standard MRI. The core difference lies in the specific sequences and techniques employed to highlight flowing blood while suppressing the signal from stationary brain tissue. A common technique is known as Time-of-Flight (TOF) MRA, which capitalizes on a phenomenon called flow-related enhancement.
In TOF MRA, the magnetic pulses are rapidly repeated, causing the magnetization of the protons in the stationary tissue to become partially saturated, which results in a low signal and a dark background. However, “fresh” blood constantly flows into the imaging area, bringing protons that have not been exposed to the preceding radiofrequency pulses. These unsaturated protons produce a strong signal, making the flowing blood appear bright against the darker backdrop of the surrounding brain tissue. This intrinsic contrast allows for detailed visualization of the vascular structures, such as the Circle of Willis, often without needing an external contrast agent.
Phase contrast MRA is another technique that measures the velocity of blood flow by detecting the phase shift in the proton signal caused by movement. This method is useful for quantifying the speed and direction of blood flow through specific vessels. The images captured by these specialized methods are then processed by a computer, frequently using a Maximum Intensity Projection (MIP) algorithm. This creates a clear, three-dimensional reconstruction of the brain’s vascular tree, enabling precise analysis of the size, shape, and orientation of the blood vessels.
Conditions Diagnosed by Brain MRA
The primary role of a brain MRA is to evaluate and diagnose various cerebrovascular diseases that can compromise brain function. One of the most common applications is the detection of cerebral aneurysms, which are abnormal, balloon-like bulges in the wall of an artery. These aneurysms are dangerous because they can rupture, leading to a hemorrhagic stroke.
MRA is also used to identify stenosis, which is the narrowing or blockage of arteries due to the buildup of plaque or other material. The visualization of the blood flow helps physicians assess the degree of this narrowing in the arteries supplying the brain, such as the carotid or vertebral arteries. For patients who have experienced an ischemic stroke, MRA can pinpoint the blocked or severely narrowed arteries that caused the reduced blood flow to brain tissue.
Another significant indication for the scan is the assessment of vascular malformations, such as arteriovenous malformations (AVMs). AVMs are abnormal tangles of blood vessels where arteries connect directly to veins without the normal capillary network in between, which can lead to hemorrhage or seizures. MRA can also aid in diagnosing vasculitis, an inflammation of the blood vessels, and can be used to monitor known vascular conditions over time to track their progression or stability.
Preparation and Procedure for the Scan
Preparation for a brain MRA typically involves removing all metallic objects, as they can interfere with the powerful magnetic field of the scanner. Patients are instructed to remove jewelry, watches, hairpins, and any clothing with metal fasteners, and they may be asked to wear a hospital gown. Before the procedure, patients must inform the technologist about any internal metallic implants, such as pacemakers, certain aneurysm clips, or neurostimulators, as these may be absolute contraindications due to safety concerns.
During the procedure, the patient lies on a movable table that slides into the large, cylindrical MRA machine. A specialized device called a coil is often placed around the head to optimize the quality of the images. The machine produces loud knocking and humming noises as the magnetic gradients are rapidly turned on and off, so earplugs or headphones are provided to protect the patient’s hearing.
While the scan is non-invasive, a contrast agent containing gadolinium may be administered intravenously to enhance the visibility of blood vessels. This contrast-enhanced MRA is used for better delineation of specific vascular structures or to assess blood vessel walls. The patient must remain completely still for the duration of the scan, which can last between 45 minutes and an hour, to ensure the captured images are clear and not blurred by movement.