What is MRI and how does it work?

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. Instead, MRI uses a powerful magnetic field, radio waves, a rapidly changing magnetic field, and a computer to create images that show 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 MR scanner. The signals are specially characterized using a changing magnetic field, and computer-processor to create very sharp images of tissues as "slices" that can be viewed in any orientation.
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 MRI technologist or radiologist at any time using an intercom system or by other means. What is MRI used for?
MRI has become 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. 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.
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 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 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. 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 systems may damage an external hearing aid or cause a heart pacemaker or electrical stimulator, or neurostimulator, to malfunction or cause patient injury. 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.
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 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, or liver disease, you must inform the MRI technologist and/or radiologists before receiving a gadolinium-based contrast agent. If you are unsure about the presence of these conditions, please discuss these matters with the 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 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.
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 defibrillatorCatheter that has metal components that may pose a risk of a burn injuryA ferromagnetic metal clip placed to prevent bleeding from an intracranial aneurysmAn implanted or external 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 stripsElectronic devices such as beepers or cell phonesHearing aidsMetal jewelry, watchesPens, paper clips, keys, coinsHair barrettes, hairpinsAny article of clothing that has a metal zipper, buttons, snaps, hooks, underwires, or metal threadsShoes, belt buckles, safety pins
Objects that may interfere with image quality if close to the area being scanned include:Metallic spinal rodPlates, pins, screws, or metal mesh used to repair a bone or jointJoint replacement or prosthesisMetal jewelry including those used for body piercingSome 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 fragmentMetallic 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)An example of the MRI examination.
The MRI examination is performed in a special room that houses the MR system or "scanner." You will be escorted into the room by a staff member of the MRI facility and asked to lie down on a comfortably padded table that gently glides you into the scanner.
In general, in preparation for the MRI examination, you may be required to wear earplugs or headphones to protect your hearing because, when certain scanners operate, they may produce loud noises. These loud noises are normal and should not worry you.
For some MRI studies, a contrast agent called "gadolinium" may be injected into a vein to help obtain a clearer picture of the area being examined. At some point during the examination, a nurse or technologist will slide the table out of the scanner in order to inject the contrast agent. This is typically done through a small needle connected to an intravenous line that is placed in an arm or hand vein. A saline solution will drip through the intravenous line to prevent clotting until the contrast material is injected at some point during the exam.
The most important thing for the patient to do is to relax and lie still. Most MRI exams take between 15 to 45 minutes to complete depending on the body part imaged and how many images are needed, although some may take as long as 60 minutes or longer. You will be told ahead of time how long your scan is expected to take.
You will be asked to remain perfectly still during the time the imaging takes place, but between sequences some minor movement may be allowed. The MRI technologist will advise you accordingly.
When the MRI procedure begins, you may breathe normally, however, for certain examinations it may be necessary for you to hold your breath for a short period of time.
During your MRI examination, the MR system operator will be able to speak to you, hear you, and observe you at all times. Consult the scanner operator if you have any questions or feel anything unusual.
When the MRI procedure is over, you may be asked to wait until the images are examined to determine if more images are needed. After the scan, you have no restrictions and can go about your normal activities.
Once the entire MRI examination is completed, the images will be looked at by a radiologist, a specially-trained physician who is able to interpret the scans for your doctor. The question of claustrophobia
Some patients who have MRI may feel confined, closed-in, or 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. 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. 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.
You should inform your radiologist if you are breast-feeding at the time of a scheduled MRI study and 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.