Magnetic Resonance Imaging (MRI)
What is MRI and how does it work?
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 or "ionizing" radiation. Instead, MRI uses a powerful magnetic field, radio waves, rapidly changing magnetic fields, and a computer to create images that show whether or not there is an injury, disease process, or abnormal condition present. For the MRI procedure, the patient is placed inside of the MR scanner—typically a large, tunnel or doughnut-shaped device that is open at both ends. The powerful magnetic field aligns atomic particles called protons that are present in most of the body's tissues. The applied radio waves then cause these protons to produce signals that are picked up by a receiver within the MR scanner. The signals are specially characterized using the rapidly changing magnetic field, and, with the help of computer processing, very clear images of tissues are created as "slices" that can be viewed in any orientation.
An MRI examination causes no pain, and the magnetic fields produce no known tissue damage of any kind. The MR scanner may make loud tapping, knocking or other noises at times during the procedure; using earplugs prevents problems that may be asssociated with this noise. You will be able to communicate with the MRI technologist or radiologist at any time using an intercom system or by other means.
What is MRI used for?
MRI is the preferred procedure for diagnosing a large number of potential problems or abnormal conditions in many different parts of the body. In general, MRI creates pictures that can show differences between healthy and unhealthy tissues. Physicians use MRI to examine the brain, spine, joints (e.g., knee, shoulder, hip, wrist, and ankle), abdomen, pelvic region, breast, blood vessels, heart and other body parts.
How safe is MRI?
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. As a patient, it is vital that you remove all metallic belongings in advance of an MRI exam, including watches, jewelry, and items of clothing that have metallic threads or fasteners. Additionally, makeup, nail polish, or other cosmetics that contain metallic particles should be removed if applied to the area undergoing the MRI examination.
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 comprehensive screening procedure and protocol that, when carefully followed, ensures 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, due to the presence of an unacceptable implant or device, the exam may have to be canceled. For example, the MRI exam will not be performed if a ferromagnetic aneurysm clip is present because there is a risk of the clip moving or being dislodged. In some cases, certain medical implants can heat substantially during the MRI examination as a result of the radiofrequency energy that is used for the procedure. Therefore, it is very important to inform the MRI technologist about any implant or other internal object that you may have.
The magnetic field of the MR system may damage an external hearing aid or cause a heart pacemaker, electrical stimulator, or neurostimulator, to malfunction or cause patient injury. If you have a bullet or other metallic fragment in your body (e.g., any metallic foreign body) there is a potential risk that it could change position, possibly causing injury.
In addition, a metallic implant or other object may cause signal loss or distort the MR images. This may be unavoidable, but if the radiologist knows about it, allowances can be made when obtaining and interpreting the MR images.
For some MRI studies, a contrast material called gadolinium may be injected into a vein to help improve the information seen on the MR images. Unlike contrast agents used in x-ray studies, a gadolinium-based contrast agent does not contain iodine and, therefore, rarely causes an allergic reaction or other problem. However, if you have a history of kidney disease, kidney failure, kidney transplant, liver disease, or other conditions you must inform the MRI technologist and/or radiologist before receiving a gadolinium-based contrast agent. If you are unsure about the presence of these conditions, please discuss these matters with the MRI technologist or radiologist.
How should I prepare for my MRI exam?
You will typically receive a gown to wear during your MRI examination. Before entering the MR system room, you and any accompanying friend or relative will be asked questions (i.e., using a screening form) regarding the presence of implants and will be instructed to remove all metal objects from pockets and hair. Additionally, the accompanying individual will need to fill out a screening form to ensure that he or she may safely enter the MR system room. If you have questions or concerns, discuss them with the MRI technologist or radiologist.
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 ferromagnetic metal vascular clip placed to prevent bleeding from an intracranial aneurysm
- An implanted or external medication pump (such as that used to deliver insulin or a pain-relieving drug)
- A cochlear (inner ear) implant
- A neurostimulation system
- A catheter that has metallic components that may pose a risk of a burn injury
Note: Some items, including certain cardiac pacemakers, neurostimulation systems and medication pumps are acceptable for MRI. However, the MRI technologist and radiologist must know the exact type that you have in order to follow special procedures to ensure your safety.
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, underwire, or metallic 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
- Metallic jewelry including those used for 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)
- Makeup, nail polish or other cosmetic that contains metal
- 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)
The question of anxiety or claustrophobia
Some patients who undergo MRI examinations may feel confined, closed-in, or frightened. Perhaps one in twenty may 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.
Pregnancy and MRI
If you are pregnant or suspect you are pregnant, you should inform the MRI technologist and/or radiologist during the screening procedure before the MRI examination. 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 safer for the fetus than imaging with x-rays or computed tomography (CT). For additional information see MRI During Pregnancy (www.RadiologyInfo.org/en/safety/index.cfm?pg=sfty-mri-pregnancy).
Breast-feeding and MRI
You should inform your radiologist if you are breast-feeding at the time of a scheduled MRI study if you may need to receive an MRI 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 radiologist will provide additional information to you regarding this matter.
For more detailed MRI safety information, visit www.MRIsafety.com, which provides up-to-date and crucial MRI safety information, especially for screening patients with implants and medical devices.
This page was reviewed on June 13, 2013