Magnetic resonance imaging (MRI) is a safe and painless test that produces clear cross-sectional or three-dimensional images of the body’s tissues, even through bone and other structures. Because of its safety and clarity, the MRI is a valuable tool that can aid in the diagnosis of a wide range of conditions.
The standard MRI usually requires no advance preparation, though some patients may need to adjust their medications if contrast medium (dye) will be used in the scan. The contrast medium helps increase the visibility of organs and tissues for a more detailed image. If a contrast medium is used, it is injected into the patient’s vein prior to the test.
An MRI relies on a large magnetic field, therefore, certain people should avoid the test. These include patients with implanted pacemakers, implantable cardioverter defibrillators (ICDs), artificial joints or some of the older mechanical valves. Patients with some mechanical valves and coronary or other stents should consult with a cardiologist or cardiac surgeon to determine when they can undergo MRIs.
In all cases, patients should inform the physician or technician of any other metal objects present in their bodies. Also, pregnant women should generally avoid MRI because the risk to the developing fetus is unknown. However, there may be some instances when an MRI is warranted in high-risk pregnant patients.
An MRI may require up to an hour in a tight space. Patients who are nervous in small spaces may wish to ask their physician about receiving a sedative during the procedure. If patients receive a sedative, they may need someone to take them home from the test. Otherwise, patients are free to go about their daily activities, such as driving, after the test is completed.
The MRI is a valuable tool for physicians. Among other conditions, it can help detect or diagnose cardiovascular problems, including:
Aortic disease
Blockages within the cerebral arteries (which supply oxygen-rich blood to the brain) or the coronary arteries (which supply oxygen-rich blood to the heart)
A weakened heart muscle (cardiomyopathy)
The severity of a heart attack
Heart valve disease
Disease in the arteries outside the heart, or peripheral arterial disease
Heart defects present at birth (congenital)
Depending on the results of the MRI, additional tests may be necessary.
About MRI tests
The magnetic resonance imaging (MRI) test uses a powerful magnetic field to create images of structures and organs within the body. It is a safe and usually noninvasive test that can help physicians diagnose a wide range of diseases and conditions without subjecting the patient to radiation or radioactive isotopes. If a contrast medium (dye) is not used, the test can be completed without the use of needles as well.
An MRI works by placing the patient in a chamber surrounded by a magnetic field. The center (nucleus) of every atom of the patient’s body responds to the magnetic force in characteristic ways, allowing a computer to produce clear cross-sectional or three-dimensional images.
In some MRIs patients may receive an injection of a contrast medium (dye) into a vein in the arm or hand shortly before the procedure. This dye can increase the clarity of the images by making the body’s tissues more responsive to the machine’s magnetic and radio waves. In many cases, an MRI is first taken without the use of the dye and a second scan is taken with the dye. These contrast dyes are different from the ones used in other imaging tests in that they are not based on iodine and therefore rarely cause allergic reactions. Because of this, contract-enhanced MRI tests are considered generally safer than contrast-enhanced x-ray tests in unstable patients. However, patients with kidney disease are generally advised to avoid contrast-enhanced MRI because of the risk of potentially serious kidney disease.
In some cases, patients may also undergo a stress MRI. This test operates on the same principles as a standard stress test, except that the heart is always stimulated with a special drug (e.g., dobutamine) rather than exercise. This variation has been shown to be particularly useful in predicting recovery after procedures meant to treat coronary artery disease, such as bypass operations or angioplasty.
Some common variations of the standard MRI include:
Spin echo imaging, or the black blood approach. This depicts the tissue of the heart as bright and the blood as dark. It allows the physician to get a good picture of the anatomical structure of the heart and the coronary arteries. It is especially useful when diagnosing atherosclerosis because it allows physicians to get a clear view of the structure of the living coronary arteries. It is also used to detect areas of heart muscle that have been damaged by heart attack.
Cine MRI, or “bright blood” approach. This depicts the blood as bright and the cardiac tissue as dark. It allows the physician to evaluate the ventricles, heart valves and other heart structures.
Phase velocity mapping. This allows physicians to directly measure blood flow and is helpful to diagnose heart valve disease or narrowing of the coronary arteries.
Using one of these techniques, an MRI can detect conditions including:
Aortic disease, including aneurysm, coarctation and dissection
Aneurysms
Blood clots
Cardiomyopathy
Congenital heart defects
Coronary artery disease
Pericarditis/pericardial fluid
Vascular diseases
MRI scans also can be used:
To assess patients’ progress after a heart attack
To evaluate blockages in the coronary arteries (which could rupture and cause a heart attack)
To identify a stroke
To detect blockages of the carotid arteries that could lead to stroke
To detect an atrial myxoma (heart tumor)
MRI has also emerged as an important tool for stroke assessment. Because there are different kinds of stroke (e.g., ischemic caused by a blood clot and hemorrhagic caused by bleeding into the brain), it is very important to correctly identify the nature of a stroke as quickly as possible and develop an effective treatment plan. MRI and computed tomography, or CT scans, are vital tools in the rapid assessment of stroke, with most hospitals relying on the CT scan. However, some recent studies have suggested that MRI is much more accurate and its widespread use in the setting of acute stroke may improve patient outcomes.
Potential risks with MRI tests
In general, the magnetic resonance imaging scan (MRI) is a very safe test with an excellent track record for safety. The test is typically performed in an outpatient setting at a hospital or clinic, unless it is used as part of a rapid diagnosis during a medical emergency.
If the test is done on an outpatient basis, patients should closely follow their physician’s recommendations in preparing for the test. Usually, patients will be allowed to eat without restrictions and to take their usual medications. Once at the facility, patients may be asked to change into a gown or to wear clothing without fasteners, such as a sweatshirt and sweatpants.
It is important for the patient to remove all personal metal objects (e.g., rings, earrings, necklaces) before the test. In fact, patients may want to leave these objects at home on the day of the test. In addition, drug-eluting foil patches, such as nicotine patches, should also be removed.
There should also be no metal objects inside the room in which the MRI is being performed. Patients with implanted metal devices or other metallic materials inside the body should avoid MRIs. These objects include (but are not limited to):
Pacemakers
Implantable cardioverter defibrillators (ICDs)
Artificial hips, knees or other joints
Inner ear implants
Titanium implants in the mouth
Aneurysm clip of the brain
Neuromuscular stimulators
Implanted drug infusion pump
If patients suspect that they have a metal device or fragment (such as from an injury) inside their bodies, they should not have an MRI. In the cases of patients with artificial heart valves, stents or who have undergone open-heart surgery, MRIs may be administered after a certain time period has elapsed. These patients should consult their physicians before having an MRI.
Pregnant women should also avoid having an MRI unless absolutely necessary, because risk to developing fetus is unknown. Finally, patients with tattoos or permanent makeup are encouraged to consult with their physician before an MRI is performed. These patients might feel some mild discomfort or a burning feeling on their skin due to a reaction between the metallic substances commonly found in the darker inks of the tattoo and the magnetic field generated during the test. Furthermore, large or very dark tattoos can also cause “artifacts,” or false shadows to appear on the film produced from the test.
If the MRI involves injection of a contrast medium, there is the possibility of a reaction. Patients should inform their physician and the technician if they feel any nausea, headache or pain at the site of injection.
Before, during and after MRI tests
The magnetic resonance imaging (MRI) chamber is a relatively tight space, and individuals undergoing a scan are required to lie still for up to one hour. Some people may feel uncomfortable or claustrophobic in this environment. In many scanners, music is available to help patients relax, but those who become anxious in small spaces may want to discuss with their physician the possibility of receiving a mild sedative during the procedure. Newer “open” MRI scanners have been designed to increase patients’ comfort levels, but they are subject to more background interference in the images they produce. Open MRIs are not an option for the diagnosis of some conditions.
The patient is placed on a long bed that slides into the MRI chamber. He or she may be given a hand-held buzzer to stop the test in case of some type of emergency. Once inside the chamber, the patient should relax, breathe normally and lie still for the duration of the procedure. The MRI equipment will make various noises during the scan as it takes multiple views of the target site. These noises may be a bit unsettling at times, but they are a normal part of the MRI procedure. The technician and the patient are able to communicate during the test through a microphone and speaker system. This allows the technician to provide instructions and the patient to express any concerns.
Those taking the test should be able to leave immediately following the MRI with no side effects. However, if a sedative is administered, patients will be monitored until the effects of the sedative have passed and they may need someone to drive them home after the test.
Results of the test are recorded on a series of films. The films are reviewed by a radiologist and a full report will be completed with the results. The report will be sent to the ordering physician as well as any other physicians designated by the patient. It is important for the patient to indicate which physicians, such as their cardiologists, should receive a copy of the report. The MRI results will help the patient’s physician determine the need for further tests and treatment.
Variants of the MRI test
Two variants of magnetic resonance imaging (MRI) are showing promise in the field of cardiology. One variant, known as a magnetic resonance angiogram (MRA), is used to view blood flow through the coronary arteries or carotid arteries without the use of catheter–based techniques. The MRA can be used to take three–dimensional images of the blood flow and to detect damage in the vessel walls, including stenosis, or reclosure of an artery after treatment. The test takes less time than a carotid or coronary angiogram, requires a shorter recovery period and does not entail the same risks as an invasive procedure. It has shown benefit in detecting whether veins have renarrowed after coronary artery bypass surgery through a noninvasive technique. Further developments of the test may improve its accuracy and usefulness.
A second variant of the MRI is the phase-contrast MRI or PC–MR. This noninvasive technique involves the injection of a contrast medium to produce very clear images. It can assess heart function as well as measure blood flow. Although other noninvasive techniques such as positron emission tomography (PET) are able to measure blood flow, they cannot assess heart function. Therefore, the PC–MR may be ideal for identifying the earliest stages of heart disease. Furthermore, this technique has proven valuable in the diagnosis of some cardiac motion problems, such as abnormal movement of the left ventricle.
Questions for your doctor regarding MRI tests
Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about magnetic resonance imaging (MRI):
Why do I need to have an MRI?
What type of MRI do I need?
What information will an MRI provide about my heart?
Do I need any special preparations for the test?
Will any dyes or injections be used during my MRI?
How long will the test take to complete?
What is the difference between an MRI and a CAT scan?
When and from whom will I receive the results of my test?
I have had heart surgery in the past. Am I still a candidate for an MRI?
Can I request a sedative if I am anxious about the test?
What happens if I have an allergic reaction to the dye?
Should MRIs be performed regularly to monitor my heart condition?
