Heart magnetic resonance imaging (MRI) is a imaging method that uses powerful magnets and radio waves to create pictures of the heart. It does not use radiation (x-rays).

The test may be done as part of a chest MRI.

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.

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 into a large tunnel-like tube.

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. 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. Each image takes about 2-15 minutes. A complete scan may take up to 1 hour. Newer scanners may complete the process in less time.

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

Before the test, tell the radiologist if you are currently receiving dialysis, as this may affect whether you can have IV contrast.

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.

A heart 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 MRI scanners have televisions and special headphones that you can use to help the time pass.

There is no recovery time, unless sedation was necessary. (You will need someone to drive you home if sedation was given.) After an MRI scan, you can resume your normal diet, activity, and medications, unless otherwise instructed by your doctor.

MRI provides detailed pictures of the heart and blood vessels from many views.

It may be used to diagnose:

• Heart muscle damage after a heart attack
• Birth defects of the heart
• Heart tumors and growths

MRI is sometimes used to avoid the dangers of angiography, repeated exposure to radiation, or the use of iodine-based dye (contrast).

It may provide additional information when an echocardiogram is unclear.

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

A heart MRI may reveal the following disorders:

• Heart valve disorders
• Pericardial effusion
• Tumor invasion of blood vessels or pericardium (around the heart)
• Atrial myxoma
• Congenital heart abnormalities (such as pulmonary atresia)
• Fibrosis or scarring of heart muscle
• How much heart muscle is dead (after a heart attack)

There is no radiation involved in MRI. The magnetic fields and radio waves used during the same have not been shown to cause any significant side effects.

Allergic reactions to the dye used during the exam are rare. The most common type of contrast (dye) used is gadolinium. It is very safe. 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 usually not recommended for traumatic injuries, because traction and life-support equipment cannot safely enter the scanner area, and scans can take a long time.

MRI is more accurate than CT scan or other tests for certain conditions, but less accurate for others. MRIs can be costly, take a long time to perform, and are sensitive to movement. Persons with claustrophobia or who are anxious may have difficulty lying still for the scans.

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.

Introduction. In: Mettler FA Jr. Essentials of Radiology. 2nd ed. Philadelphia, Pa: Saunders Elsevier; 2005:chap 1.