Unlike traditional x-rays and CT (computed tomography) scans, which use radiation, MRI uses powerful magnets and radio waves. The MRI scanner contains the magnet. The magnetic field produced by an MRI is about 10 thousand times greater than the earth's.

The magnetic field forces hydrogen atoms in the body to line up in a certain way (similar to how the needle on a compass moves when you hold it near a magnet). When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals. For example, healthy tissue sends back a slightly different signal than cancerous tissue.

The MRI scanner is located within a specially shielded room to avoid outside interference from other radio signals, including those from FM radio stations.

For the test, you will be asked to lie on a narrow table that slides into a large, tunnel-like tube within the scanner. You usually leave your clothes on. MRI can easily be performed through clothing. However, because the magnet is very, very strong, certain types of metal can cause significant errors, called artifacts, in the images.

Some MRI's require contrast dye, which helps the doctor better see certain body structures. If contrast dye is used, it will be injected into a small vein of the hand or forearm.

A technologist will operate the machine and observe you during the entire study from a nearby room.

Several sets of images are usually required, each taking from 2 to 15 minutes. Single MRI images are called slices. The images can be stored on a computer or printed on film. Depending on the sequences performed and the possibility of the need for a contrast dye, a complete scan may take up to a hour or more. Newer scanners with more powerful magnets, updated software, and advanced sequences may take less time.

No special tests, diets, or medications are usually needed. An MRI may be performed immediately after other imaging studies.

If you have a fear of confined spaces (claustrophobia), tell your doctor before the exam. You may be prescribed a mild sedative, or your doctor may recommend an "open" MRI, in which the machine is not as close to the body.

The strong magnetic fields created during an MRI can interfere with certain implants, particularly pacemakers . Persons with cardiac pacemakers cannot receive an MRI and should not enter an MRI area.

Because of the strong magnets, certain metallic objects are not allowed into the room.

• Items such as jewelry, watches, credit cards, and hearing aids can be damaged.
• Pins, hairpins, metal zippers, and similar metallic items can distort the images.
• Removable dental work should be taken out just prior to the scan.
• When the MRI magnet is turned on, pens, pocketknives, and eyeglasses may fly across the room. This can be dangerous, so such items are not allowed into the scanner area.

If you have any of the following metallic objects in your body, you should not get an MRI:

• Inner ear (cochlear) implants
• Brain aneurysm clips
• Certain artificial heart valves
• Older vascular stents
• Recently placed artificial joints

You will be asked to sign a consent form that says you do not have any of these items in your body.

Before an MRI, sheet metal workers or any person that may have been exposed to small metal fragments should receive a skull x-ray to check for metal in the eyes.

An MRI exam causes no pain. Some people may become anxious when inside the scanner. If you have difficulty lying still or are very anxious, you may be given a mild sedative. Excessive 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. Ear plugs are usually given to help reduce the noise.

An intercom in the scanner allows you to speak to the person operating the exam at any time. Some MRIs have televisions and special headphones that you can use to help the time pass.

There is no recovery time, unless sedation was necessary. After an MRI scan, you can resume your normal diet, activity, and medications.

A chest MRI provides detailed pictures of tissues within the chest area. It may be used to:

• As an alternative to angiography , or to avoid repeated exposure to radiation
• Clarify findings from previous x-rays or CT scans
• Diagnose abnormal growths in the chest
• Evaluate blood flow
• Show lymph nodes and blood vessels
• Show the structures of the chest from multiple angles
• Stage (provide information about the size, severity, and spread of) chest tumors
• Tell the difference between tumors and normal growths

Normal results mean there are no new growths, or problems or changes in the size or position of organs in the chest cavity. If you had an MRI that shows how things in the body work (functional MRI), organs would appear to function normally.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

The sensitivity of MRI depends, in part, on the experience of the radiologist.

An abnormal chest MRI can be due to many different conditions or disorders, including:

• Abnormal blood vessels in the lungs (pulmonary vessels)
• Abnormal growth of cells in the esophagus
• Abnormal lymph nodes
• Aortic dissection
• Aortic stenosis
• Atrial myxoma
• Atrial septal defect
• Bacterial pericarditis
• Bronchial abnormalities
• Bronchiectasis
• Cardiac tamponade
• Chronic obstructive pulmonary disease (COPD)
• Coarctation of the aorta
• Constrictive pericarditis
• Cystic lung lesions
• Ischemic cardiomyopathy
• Lung tumors
• Mitral regurgitation - acute
• Mitral regurgitation - chronic
• Mitral valve prolapse
• Other masses or tumors of the chest
• Pericardial effusion
• Pericarditis after heart attack
• Pleural abnormalities, including thickening or pleural effusion
• Pulmonary edema
• Restrictive cardiomyopathy
• Skin lesion of histoplasmosis
• Swollen glands and enlarged lymph nodes in the chest
• SVC obstruction
• Thoracic aortic aneurysm
• Thymus tumor

There is no ionizing radiation involved in MRI, and there have been no documented significant side effects of the magnetic fields and radio waves used on the human body to date.

The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. This dye should not be given if you are pregnant because of potential harm to the fetus.

There are risks of over-sedation, if a sedative is prescribed. The person operating the machine will monitor your heart rate and breathing as needed.

People have been harmed in MRI machines when they did not remove metal objects from their clothes or when metal objects were left in the room by others.

MRI is more accurate than CT scan or other tests for certain conditions, but less accurate for others. MRI is a superior technique in most cases where telling differences in soft tissues is necessary. It can show organs without blockage by bone and foreign bodies. It can show the tissues from multiple viewpoints and is a noninvasive way to evaluate blood flow.

Currently, MRI is not considered a valuable tool for spotting or monitoring slight changes in lung tissue, since the lungs contain mostly air and are difficult to image.

Disadvantages of MRI include:

• High cost
• Long duration of the scan
• Sensitivity to movement

MRI is usually not recommended for acute trauma situations, because traction and life-support equipment cannot safely enter the scanner area and the exam can take quite a bit of time.