Magnetic resonance imaging (MRI) is a test that uses a magnetic field and pulses of radio wave energy to take pictures of the head. In many cases, MRI gives information that can't be seen on an X-ray, ultrasound, or computed tomography (CT) scan.
For an MRI of the head, you lie with your head inside a special machine (scanner) that has a strong magnet. The MRI can show tissue damage or disease, such as infection or inflammation, or a tumour, stroke, or seizure. Information from an MRI can be saved and stored on a computer for more study. Photographs or films of certain views can also be made.
In some cases, a dye (contrast material) may be used during the MRI to show pictures of structures more clearly. The dye may help show blood flow, look for some types of tumours, and show areas of inflammation.
MRI of the head may be used to look for the cause of headaches.
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Why It Is Done
Magnetic resonance imaging (MRI) of the head is done to:
- Look for the cause of headaches.
- Help diagnose a stroke or blood vessel problems in the head. Problems with blood vessels may include an aneurysm or abnormal twisted blood vessels that are present at birth (this is called an arteriovenous [AV] malformation).
- Check blood flow or blood clots to the brain. MRI can show bleeding in or around the brain.
- Check symptoms of a known or suspected head injury.
- Check symptoms such as change in consciousness, confusion, or abnormal movements. These symptoms may be caused by brain diseases, such as Huntington's disease, multiple sclerosis (MS), Parkinson's disease, or Alzheimer's disease.
- Check for "water on the brain" (hydrocephaly).
- Look for tumours, infections, an abscess, or conditions of the brain or brain stem, such as encephalitis or meningitis.
- Check the eyes, the nerves from the eyes to the brain (optic nerves), the ears, and the nerves from the ears to the brain (auditory nerves).
- Look for problems of the pituitary gland.
- Investigate or follow a finding seen on another test.
How To Prepare
Before your MRI test, tell your doctor and the MRI technologist if you:
- Are allergic to any medicines. The contrast material used for MRI does not have iodine. If you know that you are allergic to the contrast material used for the MRI, tell your doctor before having another test.
- Have any other health conditions, such as kidney problems or sickle cell disease, that may prevent you from having an MRI using contrast material.
- Are or might be pregnant.
- Have any metal implanted in your body. This helps your doctor know if the test is safe for you. Tell your doctor if you have:
- Heart and blood vessel devices such as a coronary artery stent, pacemaker, ICD (implantable cardioverter-defibrillator), or metal heart valve.
- Metal pins, clips, or metal parts in your body, including artificial limbs and dental work or braces.
- Any other implanted medical device, such as a medicine infusion pump or a cochlear implant.
- Cosmetic metal implants, such as in your ears, or tattooed eyeliner.
- Have an intrauterine device (IUD) in place. An IUD may prevent you from having the MRI test done.
- Become very nervous in small, tight spaces. You need to lie very still inside the MRI magnet. You may need medicine to help you relax.
- Wear any medicine patches. The MRI may cause a burn at the patch site.
You may need to arrange for someone to drive you home after the test if you are given a medicine (sedative) to help you relax.
You will be asked to sign a consent form that says you understand the risks of the test and agree to have it done. Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form .
How It Is Done
A magnetic resonance imaging (MRI) test is done by an MRI technologist. The pictures are read by a radiologist. But some other types of doctors (such as a neurologist or neurosurgeon) can also read an MRI scan of the head.
You may be able to have an MRI with an open MRI machine that doesn't enclose your entire body. But open MRI machines aren't available everywhere. The pictures from an open MRI may not be as good as those from a standard MRI machine.
You will need to remove all metal objects (such as hearing aids, dentures, jewellery, watches, and hairpins) from your body because these objects may be attracted to the powerful magnet used for the test. If you have had an accident or you work around metal, there is a chance that you have metal pieces in your head, eyes, skin, or spine. An X-ray may be taken before the MRI to see if you can have the test.
You may need to take off some of your clothes. You will be given a gown to wear during the test. If you keep your clothes on, empty your pockets of any coins and cards (such as credit cards or ATM cards) with scanner strips on them because the MRI magnet may erase the information on the cards.
During the test
During the test you will lie on your back on a table that is part of the MRI scanner. Your head, chest, and arms may be held with straps to help you lie still. The table will slide into the space with the magnet. A device called a coil may be placed over or wrapped around your head.
Some people feel nervous (claustrophobic) inside the MRI magnet. If this keeps you from lying still, you can be given a medicine (sedative) to help you relax.
Inside the scanner you will hear a fan and feel air moving. You may also hear tapping or snapping noises as the MRI pictures are taken. This is normal. You may be given earplugs or headphones with music to reduce the noise. It is very important to hold completely still while the scan is being done. You may be asked to hold your breath for short periods of time.
During the test, you may be alone in the scanner room. The technologist will watch you through a window. You will be able to talk through a speaker.
If contrast material is needed, the technologist will put it in an intravenous (IV) line in a vein in your arm or hand. The material may be given over 1 to 2 minutes. Then more MRI scans are done.
An MRI test usually takes 30 to 60 minutes but can take as long as 2 hours.
How It Feels
You will not have pain from the magnetic field or radio waves used for the MRI test. The table you lie on may feel hard and the room may be cool. You may be tired or sore from lying in one position for a long time.
If a contrast material is used, you may feel some coolness when it is put into your IV.
In rare cases, you may feel:
- A tingling feeling in the mouth if you have metal dental fillings.
- Warmth in your head. This is normal. Tell the technologist if you have nausea, vomiting, headache, dizziness, pain, burning, or breathing problems.
There are no known harmful effects from the strong magnetic field used for MRI. But the magnet is very powerful. The magnet may affect pacemakers, implantable cardioverter-defibrillators (ICDs), artificial limbs, and other medical devices that contain iron. The magnet will stop a watch that is close to the magnet.
Metal parts in the eyes can damage the retina. If you may have metal fragments in the eye, an X-ray of the eyes may be done before the MRI. If metal is found, the MRI will not be done.
Iron pigments in tattoos or tattooed eyeliner can cause skin or eye irritation.
An MRI can cause a burn with some medicine patches. Be sure to tell your health professional if you are wearing a patch.
There is a slight risk of an allergic reaction if contrast material is used during the MRI. But most reactions are mild and can be treated using medicine. There also is a slight risk of an infection at the IV site.
A dye (contrast material) that contains gadolinium may be used in this test. Be sure to tell your doctor if:
- You are pregnant or think you may be pregnant.
- You have kidney problems.
- You've had more than one test that used gadolinium.
Health Canada has safety warnings about gadolinium. But for most people, the benefit of its use in this test outweighs the risk.
If you breastfeed and are concerned about whether the dye used in this test is safe, talk to your doctor. Most experts believe that very little dye passes into breast milk and even less is passed on to the baby. But if you prefer, you can store some of your breast milk ahead of time and use it for a day or two after the test.
Magnetic resonance imaging (MRI) is a test that uses a magnetic field and pulses of radio wave energy to take pictures of the head.
The radiologist may tell you some of the results of the MRI right after the test. Full results are sent to your doctor or specialist in 1 to 2 days.
All structures of the head—the brain, its vessels, spaces, nerves, and surrounding structures—are normal.
No abnormal growths, such as tumours, in or around the brain are present.
No bleeding, abnormal blood vessels (AV malformations), abnormal pockets of fluid, blockage in the flow of blood, or bulges in the blood vessels ( aneurysm) are present.
Tumours in the brain or in areas outside the brain, such as an acoustic neuroma, are present.
Bleeding or swelling (edema) in or around the brain is present.
Areas of infection or inflammatory disease, such as encephalitis or meningitis, are present.
Bulges or weak areas (aneurysms) or abnormal blood vessels (such as an AV malformation) are present.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Having medical devices with metal. The MRI magnet may cause these devices to not work right or to have problems during an MRI scan. This test may not be done if you have:
- Being unable to lie still during the test.
What To Think About
- Sometimes your MRI results may be different from earlier test results because you were tested at a different medical centre or you had a different kind of test.
- Several special MRI methods have been developed to look at the brain.
- Magnetic resonance spectroscopy shows changes in brain chemistry that may occur in certain areas of the brain. These changes may help show diseases that affect the brain.
- Magnetic resonance angiography (MRA) can be used to study blood vessels. Dye may be injected into the blood vessels so blood flow can be seen clearly. MRA can also be used to show the speed and direction of blood flow. To learn more, see the topic Magnetic Resonance Angiogram (MRA).
- Diffusion-perfusion imaging shows the water content and character of the brain. This method can give an early diagnosis of a stroke and may help predict the outcome of stroke. It can also be used to find tumours or inflammation of the brain.
- Functional MRI (fMRI) measures small chemical changes in the brain, which can show the part of the brain being used for functions such as speech, thought, or movement of body parts. Results from fMRI can be used to locate areas of the brain to avoid during brain surgery. These results can also be used to see how well the brain is functioning in people who have certain conditions.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
- Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Current as of: December 9, 2019
Author: Healthwise Staff
Kathleen Romito MD - Family Medicine
Donald Sproule MDCM, CCFP - Family Medicine
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine
Howard Schaff MD - Diagnostic Radiology
Current as of: December 9, 2019