Alternative names: Nuclear magnetic resonance – cranial; Magnetic resonance imaging – cranial; MRI of the head; MRI – cranial; NMR – cranial; Cranial MRI; Brain MRI; MRI – brain; MRI – head
Definition: A head MRI (magnetic resonance imaging) is an imaging test that uses powerful magnets and radio waves to create pictures of the brain and surrounding nerve tissues.
It does not use radiation.
How the Test is Performed
Head MRI is done in the hospital or a radiology center.
You lie on a narrow table, which slides into a large tunnel-shaped scanner.
Some MRI exams require a special dye, called contrast material. The dye is usually given during 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 watches you from another room. The test most often lasts 30 to 60 minutes, but may take longer.
How to Prepare for the Test
You may be asked not to eat or drink anything for four to six hours before the scan.
Tell your health care provider if you are afraid of close spaces (have claustrophobia). You may receive medicine to help you feel sleepy and less anxious. Or your provider may suggest an “open” MRI, in which the machine is not as close to the body.
You may be asked to wear a hospital gown or clothing without metal ties (such as sweatpants and a t-shirt). Certain types of metal can cause blurry images.
Before the test, tell your provider if you have:
- Brain aneurysm clips
- An artificial heart valve
- Heart defibrillator or pacemaker
- Inner ear (cochlear) implants
- Kidney disease or are on dialysis (you may not be able to receive contrast)
- Recently placed artificial joint
- A blood vessel stent
- Worked with sheet metal in the past (you may need tests to check for metal pieces in your eyes)
The MRI contains strong magnets. Metal objects are not allowed into the room with the MRI scanner. This includes:
- Pens, pocketknives, and eyeglasses
- Items such as jewelry, watches, credit cards, and hearing aids
- Pins, hairpins, metal zippers, and similar metallic items
- Removable dental work
How the Test will Feel
If you need dye, you will feel the needle pinch in your arm when the dye is injected into the vein.
An MRI exam causes no pain. If you have difficulty lying still or are very nervous, you may be given a medicine to relax. Too much movement can blur the images and cause errors.
The table may be hard or cold, but you can ask for a blanket or pillow. The machine makes loud thumping and humming noises when turned on. You can ask for ear plugs to help reduce the noise.
An intercom in the room allows you to speak to someone at any time. Some MRIs have televisions and special headphones that can help you pass the time or block the scanner noise.
There is no recovery time, unless you were given a medicine to relax. After an MRI scan, you can go back to your normal diet, activity, and medicines.
Why the Test is Performed
An MRI provides detailed pictures of the brain and nerve tissues.
A brain MRI can be used to diagnose and monitor many diseases and disorders that affect the brain, including:
- Birth defect
- Bleeding (subarachnoid bleed or bleeding in the brain tissue itself)
- Family history of aneurysms
- Infection, such as brain abscess
- Tumours (cancerous and noncancerous)
- Hormonal disorders (such as acromegaly, galactorrhea, and Cushing syndrome)
- Multiple sclerosis
An MRI scan of the head can also determine the cause of:
- Muscle weakness or numbness and tingling
- Changes in thinking or behavior
- Hearing loss
- Headaches when certain other symptoms or signs are present
- Speaking difficulties
- Vision problems
A special type of MRI called magnetic resonance angiography (MRA) may be done to look at blood vessels in the brain.
What Abnormal Results Mean
Abnormal results may be due to:
- Abnormal blood vessels in the brain (arteriovenous malformations of the head)
- Tumour of the nerve that connects the ear to the brain (acoustic neuroma)
- Bleeding in the brain
- Brain infection
- Brain tissue swelling
- Brain tumours
- Damage to the brain from an injury
- Fluid collecting around the brain (hydrocephalus)
- Infection of the skull bones (osteomyelitis)
- Loss of brain tissue
- Multiple sclerosis
- Stroke or transient ischemic attack (TIA)
- Structural problems in the brain
MRI uses no radiation. To date, no side effects from the magnetic fields and radio waves have been reported.
The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. However, gadolinium can be harmful to people with kidney problems who are on dialysis. If you have kidney problems, tell your provider before the test.
The strong magnetic fields created during an MRI can make heart pacemakers and other implants not work as well. It can also cause a piece of metal inside your body to move or shift.
MRI is used more often than CT scan. This is because it can easily find defects in the soft tissue. Also, it is safe during pregnancy.
Tests that may be done instead of an MRI of the head include:
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
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Chernecky CC, Berger BJ. Magnetic resonance imaging (MRI) – diagnostic. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. St Louis, MO: Elsevier Saunders; 2013:754-757.
Wilkinson ID, Graves MJ. Magnetic resonance imaging. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison’s Diagnostic Radiology. 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2015:chap 5.