A magnetic resonance imaging (MRI) scan of the chest is a noninvasive imaging method that uses powerful magnets and radio waves to create detailed pictures of the chest (thoracic) area.

Unlike x-rays and computed tomographic (CT) 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.

Single MRI images are called slices. The images can be stored on a computer or printed on film. One exam produces dozens or sometimes hundreds of images.

See also: Magnetic resonance imaging (MRI)

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 inaccurate images.

You will lie on a narrow table, which slides into the middle of the MRI machine.

Some exams require a special dye (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 will watch you from another room. Several sets of images are usually needed, each taking 2 - 15 minutes. Depending on the type of equipment, the exam may take 1 hour or longer.

You may be asked not to eat or drink anything for 4 - 6 hours before the scan.

Some exams require a special dye (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.

If you fear confined spaces (have claustrophobia), tell your doctor before the exam. You may be given a medicine to help you feel sleepy and less anxious, 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. People with cardiac pacemakers cannot have an MRI and should not enter an MRI area.

You may not be able to have an MRI if you have any of the following metallic objects in your body:

• Brain aneurysm clips
• Certain artificial heart valves
• Inner ear (cochlear) implants
• Recently placed artificial joints
• Some older types of vascular stents

Tell your health care provider if you have one of these devices when scheduling the test, so the exact type of metal can be determined.

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

Because the MRI contains a magnet, metal-containing objects such as pens, pocketknives, and eyeglasses may fly across the room. This can be dangerous, so they are not allowed into the scanner area.

Other metallic objects are also 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 before the scan.

An MRI exam causes no pain. Some people may become anxious 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. You can wear ear plugs to help reduce the noise.

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

There is no recovery time, unless you need sedation. 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.

A chest MRI may be done for the following reasons:

• 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
• See if chest cancer has spread to other areas of the body (this is called staging. Staging helps guide future treatment and follow-up and gives you some idea of what to expect in the future)
• 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.

Results depend on the part of the body being examined and the nature of the problem. Different types of tissues send back different MRI signals. For example, healthy tissue sends back a slightly different signal than cancerous tissue.

An abnormal chest MRI can be due to:

• 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

Consult your health care provider with any questions and concerns.

MRI contains no ionizing radiation. To date, there have been no documented significant side effects of the magnetic fields and radio waves used on the human body.

The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. The person operating the machine will monitor your heart rate and breathing.

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.

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.

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

Wilkinson ID, Paley MNJ. Magnetic resonance imaging: basic principles. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 5.