What Is Magnetic Resonance Imaging (MRI)?

A magnetic resonance (REZ-oh-nans) imaging scan is usually called an MRI. An MRI does not use radiation (X-rays) and is a noninvasive medical test or examination. The MRI machine uses a large magnet and a computer to take pictures of the inside of your body. Each picture or "slice" shows only a few layers of body tissue at a time. The pictures can then be examined on a computer monitor.

Pictures taken this way may help caregivers find and see problems in your body more easily. The scan usually takes between 15 to 90 minutes. Including the scan, the total examination time usually takes between 1.5 to 3 hours.

A substance called gadolinium is injected into a vein to help the physicians see the image more clearly. The gadolinium collects around cancer cells so they show up brighter in the picture. Sometimes a procedure called magnetic resonance spectroscopy (MRS) is done during the MRI scan. An MRS is used to diagnose tumors based on their chemical make-up.

How does MRI work?

The MRI machine is a large, cylindrical (tube-shaped) machine that creates a strong magnetic field around the patient. This magnetic field, along with a radiofrequency, alters the hydrogen atoms' natural alignment in the body.

A magnetic field is created and pulses of radio waves are sent from a scanner. The radio waves knock the nuclei of the atoms in the body out of their normal position; as the nuclei realign back into proper position, they send out radio signals.

These signals are received by a computer that analyzes and converts them into an image of the part of the body being examined. This image appears on a viewing monitor. Some MRI machines look like narrow tunnels, while others are more open.

MRI may be used instead of a CT scan in situations where organs or soft tissue are being studied, because with MRI scanning bones do not obscure the images of organs and soft tissues, as does CT scanning.

Other related procedures that are used to assess the heart may include:

• Resting or exercise electrocardiogram (ECG)
• Signal-averaged electrocardiogram (ECG)
• Holter monitor
• Cardiac catheterization
• Chest X-ray
• Computed tomography (CT scan) of the chest 
• Electrophysiological studies
• Myocardial perfusion scans
• Radionuclide angiography
• Ultrafast CT scans