Cardiac MRI: Uses and Limitations

Magnetic resonance imaging (MRI) is a test that has been useful for decades in diagnosing problems of the brain, spine, joints, and other stationary organs. In recent years, thanks to newer techniques, MRI is also proving very useful in evaluating the organs that move—including the heart and major blood vessels.

What Is MRI?

MRI is an imaging technique that takes advantage of the fact that the nucleus of certain kinds of atoms (most typically, the single proton that forms the nucleus of a hydrogen atom) will vibrate, or "resonate," when exposed to bursts of magnetic energy. When the hydrogen nuclei resonate in response to changes in a magnetic field, they emit radiofrequency energy. The MRI machine detects this emitted energy and converts it to an image so that various body structures can be viewed in astonishing detail.

Hydrogen nuclei are used because hydrogen atoms are present in water molecules (H₂O), and therefore they are present in every tissue in the body. The images obtained by MRI scanning are generated as 3D projections and are remarkably precise and detailed. 

Furthermore, these 3D MRI images can be "sliced" and each slice can be examined in detail, in any plane. In some ways, it is almost like doing exploratory surgery on a computer screen.

Subtle differences in the hydrogen atoms between various parts of an organ—variations caused, for instance, by differences in blood flow or in the viability of the organ—emit dissimilar amounts of energy. These energy disparities can be represented by different colors on the MRI display. So, for instance, the MRI offers a potential means of detecting areas of cardiac tissue that have poor blood flow (as in coronary artery disease, CAD) or that has been damaged (as in a myocardial infarction).

What Can Cardiac MRI Do Today?

Thanks to technological advancements, MRI has become very useful in the evaluation of many cardiovascular conditions. The advancements that have especially enhanced cardiac MRI have been gating techniques, which eliminate most of the motion artifact caused by the beating heart; and the use of gadolinium, a contrast agent injected into the bloodstream, that helps MRI differentiate various tissue processes in the heart and blood vessels.

MRI is routinely useful today for evaluating the following cardiovascular conditions:

Aortic disease: Thanks to the precise, detailed images that can be generated, MRI has revolutionized the evaluation of diseases of the aorta. These include aortic aneurysm (a bulge in the aorta), aortic dissection (a tear in the aorta), and coarctation (narrowing of a short section of the aorta). MRI scanning has become a routine and nearly indispensable aid to surgical repair of disorders of the aorta.

Myocardial Disease: MRI can help to characterize the nature and extent of diseases of the heart muscle (myocardium), such as cardiomyopathy (chronic disease of the heart making it harder to pump blood).

It can help to determine whether myocardial disease is caused by ischemia (inadequate blood supply to the heart), inflammation, fibrosis (a thickening and scarring), or some other process such as amyloidosis (abnormal protein deposits) or sarcoidosis (patches of red, swollen tissue). MRI can also help to evaluate the extent and nature of hypertrophic cardiomyopathy (an abnormally thick heart muscle).

Another use of cardiac MRI is in the evaluation of potential hibernating myocardium, heart muscle affected by coronary artery disease that looks dead but that has the potential of recovering its function. MRI testing can help identify people with apparent permanent heart muscle damage who can actually benefit from a stent or bypass surgery. 

Structural cardiovascular abnormalities: MRI can also locate and characterize the rare cardiac tumor. And in children with complex congenital heart disease, MRI can help to identify and sort out the various anatomic abnormalities, and to plan potential surgical approaches to correcting them.

Pericardial disease: MRI can help to measure the extent of a pericardial effusion, and to evaluate constrictive pericarditis.

Potential Future Uses of Cardiac MRI

Several applications of cardiac MRI are being studied that should eventually enhance the usefulness of this technique even further. These include:

• Detecting acute coronary syndrome (ACS): MRI has the potential to assist in quickly making the diagnosis of ACS (sudden, reduced blood flow to the heart) when a person has chest pain so that therapy can begin sooner.
• Diagnosing coronary artery blockages: Using MRI to visualize the coronary arteries is possible, but there are several limitations that prevent its routine use today. MRI is quite accurate in detecting blockages in the larger sections of the coronary arteries, but either misses or over diagnoses blockages in the smaller sections. Newer technologies are under development that may improve this result.
• Diagnosing cardiac syndrome X (microvascular coronary artery disease): MRI has been used to detect abnormal blood flow to parts of the heart muscle, despite an absence of “typical” CAD. This finding gives objective evidence that cardiac syndrome X (chest discomfort) is present.

What Are the Advantages of MRI?

• MRI has the potential of replacing or supplementing at least four other cardiac tests: the echocardiogram (ECG or EKG), the MUGA (multigated acquisition) scan, the nuclear stress test, and diagnostic cardiac catheterization.
• MRI does not involve exposing the patient to ionizing (potentially harmful) radiation.
• The images generated by MRI are remarkably complete, detailed, and precise—far more so than other cardiac imaging tests.

What Are the Disadvantages of MRI?

• Being placed in the MRI scanner can induce significant claustrophobia in about 5% of the people who have these tests.
• It is difficult to monitor patients while they are in the MRI scanner—for instance, the ECG is significantly distorted—so this technique is not suitable for patients who are critically ill.
• Patients with certain kinds of medical devices such as pacemakers, implantable defibrillators, and some artificial heart valves may not be able to have MRI safely. However, in recent years pacemakers have been developed that allow MRI scanning.
• The MRI image becomes distorted by metal, so the image can be suboptimal in patients with surgical clips or stents, for instance.
• MRI technology is extremely complex and expensive. Access to cardiac MRIs can vary depending on location, insurance coverage, and referral policies. For MRI to come under widespread usage, the cost will have to come down substantially.

A Word From Verywell

Cardiac MRI scanning is a noninvasive imaging test that can be very helpful in defining the anatomy and function of the heart and major blood vessels. As MRI technology improves and becomes less expensive, it will be incorporated more routinely into cardiovascular diagnosis.