Magnetic resonance imaging, or MRI, is a way of obtaining very detailed images of organs and tissues throughout the body without the need for x-rays. Instead, it uses a powerful magnetic field, radio waves, a rapidly changing magnetic field, and a computer to demonstrate whether or not there is an injury or some disease process present. For this procedure, the patient is placed within the MR scanner—typically a large, tunnel or doughnut-shaped magnet that is open at both ends. The magnetic field aligns atomic particles called protons that are present in most of the body's tissues. Radio waves then cause these particles to produce signals that are picked up by a receiver within the scanner. The signals are specially characterized using the changing magnetic field, and computer-processed to create very sharp images of tissues as "slices" that can be viewed in any plane or from any direction.
An MRI exam causes no pain, and the magnetic fields produce no known tissue damage of any kind. The MR scanner may make loud tapping or knocking noises at times during the exam; using earplugs prevents problems that may occur with this noise. You will be able to communicate with the technologist or radiologist at any time using an intercom.
The powerful magnetic field of the MR system will attract iron-containing (also known as ferromagnetic) objects and may cause them to move suddenly and with great force. This can pose a possible risk to the patient or anyone in an object's flight path. Great care is taken to be certain that objects such as "ferromagnetic" screwdrivers and oxygen tanks are not brought into the MR system area. It is vital that you remove any metallic belongings in advance of an MRI exam, including watches, jewelry, and items of clothing that have metallic threads or fasteners.
The powerful magnetic field of the MR system will pull on any iron-containing object in the body, such as certain medication pumps or aneurysm clips. Every MRI facility has a protocol that, when carefully followed, will ensure that the MRI technologist and radiologist knows about the presence of metallic implants and materials so that special precautions can be taken. In some unusual cases the exam may have to be canceled. An example is MRI of the head when a "ferromagnetic" aneurysm clip is present, because there is a risk of the clip moving or being dislodged. The magnetic field may damage an external hearing aid or cause a heart pacemaker or electrical stimulator to malfunction. If you have a bullet or other metallic fragment in your body there is a potential risk that it could change position, possibly causing injury.
A metallic implant or other object may distort the MR images or make it difficult for the MRI system to "tune" to your body. This may be unavoidable, but if the radiologist knows about it, allowance can be made when interpreting the images.
For some MRI studies a contrast material called gadolinium may be injected into a vein to help interpret the exam. Unlike contrast agents used in x-ray studies, a gadolinium contrast agent does not contain iodine and therefore rarely causes an allergic reaction or other problem.
How will I prepare for my MRI exam?
You will typically receive a gown to wear during your MRI study. Before entering the examining room, any friend or relative accompanying you will be asked questions regarding the presence of implants and will be instructed to remove all metal objects from pockets and hair. Additionally, this individual will need to fill out a screening form to ensure that he or she may safely enter the MR system room.
Before the exam you will be asked to fill out a screening form asking about anything that might create a health risk or interfere with imaging. Items that may create a health hazard or other problem during an MRI exam include:
- Cardiac pacemaker or implantable defibrillator
- Catheter that has metal components that may pose a risk of a burn injury
- A metal clip placed to prevent bleeding from an intracranial aneurysm
- A medication pump (such as that used to deliver insulin or a pain-relieving drug)
- A cochlear (inner ear) implant
Items that need to be removed by patients and individuals before entering the MR system room include:
- Purse, wallet, money clip, credit cards, cards with magnetic strips
- Electronic devices such as beepers or cell phones
- Hearing aids
- Metal jewelry, watches
- Pens, paper clips, keys, coins
- Hair barrettes, hairpins
- Any article of clothing that has a metal zipper, buttons, snaps, hooks, underwires, or metal threads
- Shoes, belt buckles, safety pins
Objects that may interfere with image quality if close to the area being scanned include:
- Metallic spinal rod
- Plates, pins, screws, or metal mesh used to repair a bone or joint
- Joint replacement or prosthesis
- Metal jewelry such as that used with body piercing.
- Some tattoos or tattooed eyeliner (these alter MR images, and there is a chance of skin irritation or swelling; black and blue pigments are the most troublesome)
- Bullet, shrapnel, or other type of metal fragment
- Metallic foreign body within or near the eye (such an object generally can be seen on an x-ray; metal workers are most likely to have this problem)
- Dental fillings (while usually unaffected by the magnetic field, they may distort images of the facial area or brain; the same is true for orthodontic braces and retainers)
Some patients who have MRI in an enclosed scanner may feel confined, closed-in, and frightened. Perhaps one in twenty will require a sedative to remain calm. Today, many patients avoid this problem when examined in one of the newer MRI units that have a more "open" design. Some MRI centers permit a relative or friend to be present in the MR system room, which also has a calming effect. If patients are properly prepared and know what to expect, it is almost always possible to complete the examination even if a conventional MRI system is used.
In general, there is no known risk of using MRI in pregnant patients. However, MRI is reserved for use in pregnant patients only to address very important problems or suspected abnormalities. In any case, MRI is very likely safer for the fetus than imaging with x-rays.
You should be sure to inform your radiologist if you are breast-feeding at the time of a scheduled MRI study and are to receive an MR contrast agent. One option under this circumstance is to pump breast milk before the study, to be used until injected contrast material has cleared from the body, which typically takes about 24 hours.
The MRI safety information on this page was developed in cooperation with the Institute for Magnetic Resonance Safety, Education, and Research from content on www.MRIsafety.com.
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