Cardiac MRI - another step closer
By DrRich
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.
In December, 2001, researchers at Harvard reported in the New England Journal of Medicine that they were able to use MRI scanning of the coronary arteries to detect (or rule out) disease in the major branches of the coronary arteries. They reported an overall accuracy of 72% with the MRI technique, and a much higher accuracy for disease of the left main coronary artery (which, while relatively uncommon, is the most dangerous place for a person to have coronary artery disease.)
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 being able to do 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 image. 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 of the heart during scanning significantly distorts the image (just as taking a photo of a moving object causes a blurring of the picture), 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 produce high-quality heart images are already being used in many research institutions.
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 difficulties in producing detailed MRI heart scans, 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.
What are some of the potential uses of cardiac MRI?
Once certain limitations are overcome – and that day seems to be rapidly approaching – the uses of cardiac MRI will explode.
MRI has the potential (and has been used in the research setting) to diagnose heart attacks in patients presenting with chest pain. Not infrequently, a patient coming to the emergency room with chest pain will not have the typical ECG changes seen with myocardial infarctions, and the doctors end up waiting for an hour or two for the results of cardiac enzyme tests. If a heart attack is actually occurring, critical time is thus lost before therapy can begin. MRI can detect myocardial infarction immediately, and can reduce the time it takes to begin definitive treatment.
Strides are being made toward being able to diagnose coronary artery disease with MRI. A new MRI processing technique called “black-blood” MRI (so called because it produces an image of an artery in which the blood appears black, and the wall of the artery appears white) seems to be able to distinguish very nicely between normal and atherosclerotic coronary arteries. While further refinements are necessary, such techniques are bringing us very close to the day in which MRI will be able to replace cardiac catheterization for diagnosing coronary artery disease.
MRI can help distinguish between “stable” atherosclerotic plaques and “vulnerable” plaques. Vulnerable plaques are those that are prone to rupture, thus suddenly occluding a coronary artery and causing a myocardial infarction. If vulnerable plaques can be identified (and this is something the cardiac catheterization has no hope of ever doing), those particular plaques can be targeted for intervention (angioplasty, stent, or bypass), while leaving the stable plaques alone.
MRI has already proven useful in the research setting for identifying restenosis after angioplasty. MRI might thus prove an accurate, noninvasive means of following patients after angioplasty.
MRI has the potential of detecting changes in the tiny blood vessels of the heart – the microvascular circulation – that are completely missed by cardiac catheterization. Detecting such changes seem to be useful in predicting the outcome of patients after a heart attack, and may prove to be useful in assessing patients with cardiac syndrome X, diabetes, and certain other conditions.
Ultimately, MRI may replace the x-ray tube in both diagnostic and therapeutic situations. Research is already being done in animals using MRI to image the coronary arteries – instead of using fluoroscopy – for angioplasty procedures.
What about this week's report from Harvard on using MRI for diagnosing coronary artery disease?
This week's report in the New England Journal of Medicine constitutes another step forward, but MRI is still quite a ways from being ready to replace cardiac catheterization for most patients. While an accuracy of 72% is encouraging, it is certainly nowhere near the nearly 100% accuracy achieved with cardiac catheterization and coronary angiography. So, aside from the other disadvantages listed below, today the MRI is not accurate enough to substitute for coronary angiography when you really need to know the status of the coronary arteries. Indeed, while progress is ongoing, the MRI today is scarcely better in overall accuracy than the less inconvenient noninvasive tests that are used every day in cardiology.
What are the major advantages of cardiac MRI?
Note: these advantages are largely potential advantages, and won’t be realized until technology currently being tested becomes more refined and widespread:
- MRI has the potential of replacing at least 4 other cardiac tests: the echocardiogram, the MUGA scan, the thallium scan, and diagnostic cardiac catheterization
- MRI does not involve exposing the patient to ionizing (potentially harmful) radiation, as do most non-invasive cardiac imaging tests (the exception being the echocardiogram)
- The images generated by MRI are remarkably complete, detailed and precise – far more so than other cardiac imaging tests.
What are the disadvantages of cardiac MRI?
- Being placed in the MRI scanner can induce significant claustrophobia in about 5% of patients
- It is difficult to monitor patients while they are in the MRI scanner – for one thing, the ECG is significantly distorted – so this technique is not suitable for patients who are critically ill.
- Patients with certain kinds of medical devices – pacemakers and implantable defibrillators and some artificial heart valves – cannot receive MRI.
- The MRI image becomes distorted by metal – so the image is distorted in patients with surgical clips or stents, for instance.
- MRI technology extremely complex and expensive. For MRI to come under widespread usage, funds will have to be developed to purchase equipment, and many more individuals will need to be trained to use this technology.
Summary
MRI technology holds tremendous promise in the evaluation and treatment of cardiac disease. It is clearly technically feasible for MRI to replace – and significantly improve on – many of the sophisticated imaging techniques that are now routinely performed in cardiology. The potential for MRI to accurately diagnose and direct the treatment of coronary artery disease before it becomes clinically apparent is probably the most exciting prospect. Before this can happen, however, the amazing technology now being developed needs to be made accurate enough and inexpensive enough to achieve broad usage.
Want to see what a cardiac MRI looks like?
This site gives examples of cardiac imaging with MRI scans:
Cardiovascular MRI Teaching File from the University of Texas Southwestern Medical Center
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