From Richard N. Fogoros, M.D.,
Your Guide to Heart Disease.
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Magnetic Resonance Imaging (MRI) has long been useful for diagnosing problems of the brain, spine and joints. Over the past decade, MRI has proven useful in diagnosing certain uncommon cardiovascular problems such as aortic dissection, cardiac tumors, and congenital heart disease. And MRI has proven a valuable research tool for studying more common cardiac disorders such as ischemia and cardiomyopathy. Until recently, however, it has been impractical to use MRI where it would be the most useful – in the routine evaluation and management of patients with coronary artery disease.

All that appears about to change. New techniques are becoming available that promise to deliver the holy grail of cardiology – a means to non-invasively image the coronary arteries – and to do it with far more precision than is achieved by today’s gold standard, coronary angiography.

What is MRI?

MRI is an imaging technique that takes advantage of the property of certain atomic nuclei (in this case, the single proton that forms the nucleus of a hydrogen atom) to 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.

Hydrogen nuclei are used because hydrogen atoms are present in water molecules (H2O), and therefore are present in every tissue in the body.

The images obtained by MRI scanning are remarkably precise and detailed. With current MRI machines, these images are generated as 3-D projections. And once a 3-D MRI image is obtained it can be “sliced” and examined in detail, and in any plane – almost like doing exploratory surgery on a computer screen.

Also, subtle differences in the hydrogen atoms between various parts of a tissue – differences caused, for instance, by differences in blood flow or in the viability of the tissue – emit different amounts of energy. These energy differences show up as different shades of gray on the MRI. Thus, the MRI offers a potential means of detecting areas of cardiac tissue that have poor blood flow (as in coronary artery disease) or that has been damaged (as in a heart attack).

However, there are many technical problems in imaging moving structures like the heart with MRI. Movement during data-acquisition significantly distorts the image, and when the structures you are trying to see are small (such as the coronary arteries) the movement problem becomes extremely difficult to overcome. Technology is progressing rapidly, however, and commercial MRI machines that can perform high-quality cardiac MRI are right around the corner.

How is cardiac MRI useful today?

While MRI machines abound in the United States, cardiac MRI, because of its complexity, has largely been limited to university hospitals where there is a strong research interest. Accordingly, much of the work with cardiac MRI has been done in the research setting.

Because of the significant remaining limitations of the MRI technique (which will be discussed below), only a few uses of cardiac MRI have become more-or-less routine. MRI has proven very useful in evaluating patients with aortic dissection prior to surgery. The detailed images offered by MRI tell the surgeon precisely where the “tear” in the wall of the aorta begins, and the full extent of the dissection. 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 anomalies, and to plan potential surgical approaches to treatment.

While such applications of MRI are very helpful, these clinical situations are relatively rare. So cardiac MRI has yet to become a commonly used tool in clinical medicine.