MRI (magnetic resonance imaging) uses radio waves and a strong magnetic field rather than x-rays to provide remarkably clear and detailed pictures of internal organs and tissues. The procedure is valuable in diagnosing a broad range of conditions in all parts of the body, including heart and vascular disease, stroke, cancer, and joint and musculoskeletal disorders. MRI is unique in that it can also create detailed images of blood vessels without the use of contrast material (although there is a trend toward the use of special non-iodinated MRI-contrast material, for example, Gadolinium). MRI requires specialized equipment and expertise and allows evaluation of some body structures that may not be as visible with other imaging methods.
Magnetic resonance imaging is becoming very important. MRI can help physicians to look closely at the structures and function of the heart and major vessels quickly and thoroughly, without the risks associated with traditional, more invasive procedures. Using MRI, physicians can examine the size and thickness of the chambers of the heart, and determine the extent of damage caused by a heart attack or progressive heart disease.
After a heart attack, for example, an MRI examination can help the cardiologist understand how well the heart is pumping, whether the flow of blood is blocked in any chamber or major vessel, whether the heart muscles are damaged, or whether the lining of the heart is swelling. This is critical knowledge needed to administer prompt and effective treatment.
Recently, specialists in MRI have demonstrated its potential for showing not only the structure, but also the function of the heart muscles, valves and vessels. Using MRI, they have created movie-like images of the beating heart that doctors can use to diagnose a variety of cardiovascular problems. More and more, MRI is being used as part of the traditional cardiac stress test to help physicians with earlier diagnosis and treatment of heart disease and to assess the patient’s recovery after treatment.
Because the strong magnetic field used for MRI will pull on any iron-containing object in the body, MRI staff will ask whether you have a heart pacemaker or implanted defibrillator, implanted port, infusion catheter (often referred to by brand names such as Port-o-cath, Infusaport or Lifeport), intrauterine device (IUD), or any metal plates, pins, screws or surgical staples in your body. In most cases, surgical staples, plates, pins and screws pose no risk during MRI. Red dyes used in tattoos and permanent eyeliner may contain metallic iron oxide and could heat up during MRI; however, this is rare. You will be asked if you have ever had a bullet or shrapnel in your body, or ever worked with metal. If there is any question of metal fragments, you may be asked to have an x-ray that will detect any metal objects.
The radiologist or technologist may ask about drug allergies and whether you have undergone any surgery in the past. If you are or might be pregnant, mention it to the MRI staff. Some patients who undergo MRI in an enclosed unit may feel confined or claustrophobic. If you are not easily reassured, a sedative may be administered. Less than one in 20 patients will require medication.
The conventional MRI unit is a closed cylindrical magnet in which the patient must lie totally still for several seconds at a time, and consequently may feel closed-in or truly claustrophobic. However, new patient-friendly designs are rapidly coming into routine use. The “short-bore” systems are wider and shorter and do not fully enclose the patient, while other units are open on all sides.
MRI is a unique imaging method because, unlike the usual radiographs (x-rays), radioisotope studies, and even computed tomography (CT) scanning, it does not rely on radiation. Instead, radio waves are directed at protons, the nuclei of hydrogen atoms, in a strong magnetic field. The protons are first “excited” and then “relaxed,” causing them to emit radio signals that can be computer-processed to form an image. In the body, protons are most abundant in the hydrogen atoms of water—the “H” of H2O—so that an MRI shows differences in the water content and distribution in various body tissues. Even different types of tissue within the same organ, such as the muscle and vessels of the heart, can be easily distinguished.
The patient is placed on a sliding table and positioned comfortably for the MRI examination. Then the radiologist and technologist leave the room and the individual MRI sequences are performed. The patient is able to communicate with the technologist at any time using an intercom. Also, many MRI centers allow a friend or, if a child is being examined, a parent to stay in the room.
Depending on how many images are needed, the exam will generally take from 15 to 45 minutes, although a very detailed study may take longer. You will be asked not to move during the actual imaging process.
For examination of the heart, contrast material may be used to enhance the visibility of the heart’s chambers and major vessels. A small needle connected to an intravenous line 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, about two-thirds of the way through the exam.
When the exam is over the patient is asked to wait until the images are examined to determine if more images are needed.
MRI causes no pain, but some patients find it uncomfortable to remain still during the examination. Others experience a sense of being closed in, though the more open construction of many new MRI systems has helped reduce that reaction. You may notice a warm feeling in the area under examination; this is normal, but tell the radiologist or technologist if it bothers you.
If a contrast injection is used, there may be discomfort at the injection site, and you may have a cool sensation at the site during the injection. The loud tapping or knocking noises heard at certain phases of the imaging exam disturb some patients; earplugs may help.
A radiologist, who is a physician experienced in MRI and other radiology examinations, will analyze the images and send a signed report with his or her interpretation to your primary care physician. Your physician will discuss the findings with you. At some facilities, new technology also allows for distribution of diagnostic reports and referral images over the Internet.
- Images of soft-tissue structures such as the heart and major vessels are clearer and more detailed than with other imaging methods.
- The detail of MRI makes it an invaluable tool in early detection and evaluation of coronary disease.
- Even without the use of contrast material, MRI often shows sufficient detail of the heart to be valuable in diagnosis and treatment planning.
- When it is used, MRI contrast material is less likely to produce an allergic reaction than the iodine-based materials used for conventional x-rays and CT scanning, and does not contain the radioisotopes used in nuclear medicine exams.
- MRI enables the detection of abnormalities that might be obscured by bone tissue with other imaging methods.
- MRI can help physicians evaluate the function as well as the structure of the heart muscles and valves.
- MRI does not require exposure to radiation or the introduction of radioisotopes to the body.
- An undetected metal implant may be affected by the strong magnetic field.
- MRI is generally avoided in the first 12 weeks of pregnancy. Doctors usually use other methods of imaging—such as ultrasound—on pregnant women, unless there is a strong medical reason to conduct an MRI exam.