A magnetic resonance imaging (MRI) scan of the head is a noninvasive method to create detailed pictures of the brain and surrounding nerve tissues.
Unlike x-rays and computed tomographic (CT ) scans, which use radiation, MRI uses powerful magnets and radio waves. The MRI scanner contains the magnet. The magnetic field produced by an MRI is about 10 thousand times greater than the earth's.
The magnetic field forces hydrogen atoms in the body to line up in a certain way (similar to how the needle on a compass moves when you hold it near a magnet). When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals.
Single MRI images are called slices. The images can be stored on a computer or printed on film. One exam produces dozens or sometimes hundreds of images.
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How the test is performed:
You may be asked to wear a hospital gown or clothing without metal fasteners (such as sweatpants and a t-shirt). Certain types of metal can cause inaccurate images.
You will lie on a narrow table, which slides into the middle of the MRI machine. If you fear confined spaces (have claustrophobia), tell your doctor before the exam. You may be given a medicine to help you feel sleep and less anxious, or your doctor may recommend an "open" MRI, in which the machine is not as close to the body.
Small devices, called coils, are placed around the head. These devices help send and receive the radio waves, and improve the quality of the images.
Some exams require a special dye (contrast). The dye is usually given before the test through a vein (IV) in your hand or forearm. The dye helps the radiologist see certain areas more clearly.
During the MRI, the person who operates the machine will watch you from another room. Several sets of images are usually needed, each taking 2 - 15 minutes. Depending on the areas being studied and type of equipment, the exam may take 1 hour or longer.
How to prepare for the test:
Depending on the area being studied, you may be asked not to eat or drink anything for 4 - 6 hours before the scan. Other preparations are usually not needed.
The strong magnetic fields created during an MRI can interfere with certain implants, particularly pacemakers. Persons with cardiac pacemakers cannot have an MRI and should not enter an MRI area.
You may not be able to have an MRI if you have any of the following metallic objects in your body:
- Brain aneurysm clips
- Certain artificial heart valves
- Inner ear (cochlear) implants
- Recently placed artificial joints
- Some older types of vascular stents
Tell your health care provider if you have one of these devices when scheduling the test, so the exact type of metal can be determined.
Before an MRI, sheet metal workers or any person that may have been exposed to small metal fragments should receive a skull x-ray to check for metal in the eyes.
Because the MRI contains a magnet, metal-containing objects such as pens, pocketknives, and eyeglasses may fly across the room. This can be dangerous, so they are not allowed into the scanner area.
Other metallic objects are also not allowed into the room:
- Items such as jewelry, watches, credit cards, and hearing aids can be damaged.
- Pins, hairpins, metal zippers, and similar metallic items can distort the images.
- Removable dental work should be taken out just before the scan.
How the test will feel:
An MRI exam causes no pain. Some people may become anxious inside the scanner. If you have difficulty lying still or are very anxious, you may be given a mild sedative. Excessive movement can blur MRI images and cause errors.
The table may be hard or cold, but you can request a blanket or pillow. The machine produces loud thumping and humming noises when turned on. You can wear ear plugs to help reduce the noise.
An intercom in the room allows you to speak to the person operating the scanner at any time. Some MRIs have televisions and special headphones that you can use to help the time pass.
There is no recovery time, unless you need sedation. After an MRI scan, you can resume your normal diet, activity, and medications.
Why the test is performed:
MRI provides detailed pictures of the brain and nerve tissues. It also provides clear pictures of parts of the brain that are difficult to see clearly on CT scans.
MRI can also show:
- Blood flow
- Blood vessels
- Fluid that surrounds the brain and spinal cord
MRI can be used to diagnose and monitor many diseases and disorders that affect the brain, including:
- Abnormal brain development
- Bleeding in the brain (subarachnoid or intracranial hemorrhage)
- Brain infection
- Brain tumors
- Hormonal disorders (such as acromegaly, galactorrhea, and Cushing syndrome)
- Multiple sclerosis
An MRI scan of the head can also help:
- Determine the cause of headaches
- Determine the cause of vision problems, hearing loss, speaking difficulties, muscle weakness, or numbness and tingling
- Diagnose a new stroke
- Evaluate changes in thinking or behavior
- Tell the difference between tumors and normal tissues
MRI is sometimes used to avoid the dangers of angiography or of repeated exposure to radiation.
What abnormal results mean:
The sensitivity of an MRI depends, in part, on the experience of the radiologist.
Abnormal results may be due to:
What the risks are:
MRI contains no ionizing radiation. To date, there have been no documented significant side effects of the magnetic fields and radio waves used on the human body.
The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. The person operating the machine will monitor your heart rate and breathing as needed.
MRI is usually not recommended for acute trauma situations, because traction and life-support equipment cannot safely enter the scanner area and the exam can take quite a bit of time.
People have been harmed in MRI machines when they did not remove metal objects from their clothes or when metal objects were left in the room by others.
Tests that may be done instead of an MRI of the head include:
- Cranial CT scan
- Positron emission tomography (PET) scan of the brain
- Skull x-ray
A CT scan may be preferred in the following cases, since it is faster and usually available right in the emergency room:
- Acute trauma of the head and face
- Bleeding in the brain (within the first 24 to 48 hours)
- Early symptoms of stroke
- Skull bone disorders and disorders involving the bones of the ear
Wilkinson ID, Paley MNJ. Magnetic resonance imaging: basic principles. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 5.
Saunders D, Jäger HR, Murray AD, Stevens JM. Skull and brain: methods of examination and anatomy. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 55.