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 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. 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 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 4 to 6 hours before the scan.
Tell your doctor if you are afraid of close spaces (have claustrophobia). You may receive medicine to help you feel sleepy and less anxious. Or your doctor 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 fasteners (such as sweatpants and a t-shirt). Certain types of metal can cause blurry images.
Before the test, tell your health care provider if you have:
- Brain aneurysm clips
- Certain types of artificial heart valves
- Heart defibrillator or pacemaker
- Inner ear (cochlear) implants
- Kidney disease or dialysis (you may not be able to receive contrast)
- Recently placed artificial joints
- Certain types of vascular stents
- 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
An MRI exam causes no pain. If you have difficulty lying still or are very nervous, you may be given a medicine to relax you. Too much 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 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 of the brain
- 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 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.
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 patients with kidney problems who require 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.
Tests that may be done instead of an MRI of the head include:
- Head 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
Reference: U.S. National Library of Medicine