Magnetic Resonance Imaging (MRI)

Functional MRI detects metabolic changes that occur when the brain is active. Thus, it can show which areas of the brain are active when a person does a specific task, such as reading, writing, remembering, calculating, or moving a limb. Functional MRI can be used in research and clinical settings, for example, to plan brain epilepsy surgeries.

With perfusion MRI, doctors can estimate blood flow in a particular area. This information can be useful during a stroke Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction). Symptoms occur suddenly... read more to determine whether blood flow to parts of the brain is decreased. It can also be used to identify areas where blood flow is increased—for example, in tumors.

Diffusion-weighted MRI detects changes in water movement in cells that are not functioning normally. It is used primarily to identify early stroke Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction). Symptoms occur suddenly... read more . It is also used to detect certain brain disorders and to determine whether tumors have spread to the brain, or to differentiate a brain abscess from a tumor. Use of this technique to image areas other than the brain is limited. Diffusion-weighted MRI is often combined with other techniques to evaluate tumors, mostly in the brain.

Magnetic resonance spectroscopy uses radio waves that are emitted almost continuously rather than in pulses as in conventional MRI. Magnetic resonance spectroscopy is used to detect brain disorders, such as seizure disorders Seizure Disorders In seizure disorders, the brain's electrical activity is periodically disturbed, resulting in some degree of temporary brain dysfunction. Many people have unusual sensations just before a seizure... read more , Alzheimer disease Alzheimer Disease Alzheimer disease is a progressive loss of mental function, characterized by degeneration of brain tissue, including loss of nerve cells, the accumulation of an abnormal protein called beta-amyloid... read more , brain tumors Overview of Brain Tumors A brain tumor can be a noncancerous (benign) or cancerous (malignant) growth in the brain. It may originate in the brain or have spread (metastasized) to the brain from another part of the body... read more , and brain abscesses Abscess of the Brain A brain abscess is a pocket of pus in the brain. An abscess may form in the brain when bacteria from an infection elsewhere in the head or in the bloodstream or from a wound enter the brain... read more . It can distinguish between the dead debris inside an abscess and multiplying cells inside a tumor.

This technique is also used to evaluate metabolic disorders in muscles and the nervous system.

MRA, like conventional angiography Angiography Angiography is a type of medical imaging that uses x-rays and a contrast agent to produce images of blood vessels. In angiography, x-rays are used to produce detailed images of blood vessels... read more and CT angiography CT angiography Computed tomography (CT) is a type of medical imaging that combines a series of x-rays to create cross-sectional, detailed images of internal structures. In computed tomography (CT), which used... read more , can provide detailed images of blood vessels. However, it is safer and easier to do, although more expensive. Often, MRA can be done without injection of a contrast agent.

Magnetic resonance angiography can show blood flow through arteries and veins or blood flow only in one direction and thus show only arteries or only veins. As in CT angiography, a computer is used to remove all tissues except the blood vessels from the image.

Often, a gadolinium contrast agent is injected into a vein to outline blood vessels. The examiner carefully times the scanning so that the images are taken when gadolinium is concentrated in the blood vessels being evaluated.

MRA is used to evaluate blood vessels of the brain, heart, abdominal organs, arms, and legs. It is used to detect the following:

Magnetic resonance venography is an MRA of veins. It is often used to detect a blood clot in a vein that carries blood away from the brain (cerebral venous thrombosis) and to monitor the effect of treatment on this disorder.

Echo planar imaging produces sequences of images in only seconds. It can be used to image the brain, heart, and abdomen. Because it is fast, movement by the person being examined does not blur the images as much. Also, the technique can provide information about how tissues are functioning.

However, it requires special equipment and is more likely to misrepresent certain structures than conventional MRI because of the nature of the technique.

Space in the MRI scanner is small and enclosed, making some people feel claustrophobic, even people who usually are not anxious about confined spaces. Some people with obesity have difficulty fitting within the scanner.

Some MRI scanners (called open MRI scanners) have an open side and a larger interior. In them, people may feel less claustrophobic, and people with obesity may fit more easily. The images produced in open MRI scanners may be inferior to those produced by enclosed scanners depending on the magnet strength, but they can still be used to make a diagnosis.

People who are anxious about MRI can be given an antianxiety drug, such as alprazolam or lorazepam, 15 to 30 minutes before scanning.

Usually, MRI is not used if people have

These devices include some cardiac pacemakers, defibrillators, cochlear implants, and magnetic metallic clips used to treat aneurysms. The magnetic field used in MRI can cause an implanted device to move, overheat, or malfunction. The device is more likely to be affected if it was implanted within the previous 6 weeks (because scar tissue, which can help hold the device in place, has not yet formed). These devices can also distort MRI images.

Some devices, such as common dental implants, an artificial hip, or rods used to straighten the spine, are not affected by MRI.

Before MRI is done, people who have any implanted devices should tell their doctor, who can determine whether imaging is safe.

The MRI magnetic field is very strong and always on. Thus, if a metal object (such as an oxygen tank or an IV pole) is near the entrance of the scanning room, the object may be pulled into the scanner at high speed. The person being evaluated may be injured, and separating the object from the magnet may be difficult.

Gadolinium contrast agents can cause headache, nausea, pain and a sensation of cold at the injection site, distortion of taste, and dizziness.

These agents are much less likely to cause severe reactions than the iodinated contrast agents used in conventional and CT angiography.

However, nephrogenic systemic fibrosis—a severe, life-threatening disorder—has occurred in a small number of people with advanced chronic kidney disease Chronic Kidney Disease Chronic kidney disease is a slowly progressive (months to years) decline in the kidneys’ ability to filter metabolic waste products from the blood. Major causes are diabetes and high blood pressure... read more ; however, most of those cases are linked to a type of contract agent called group I gadolinium-based contrast media (GBCM), which is no longer administered in the United States.