MRI: An Important New Tool in Diagnosing Cardiac Problems
Medical College of Wisconsin Cardiologist
Raymond Migrino, MD, FACC
Magnetic Resonance Imaging or MRI has been around for decades, but only recently has it been used regularly to diagnose cardiac problems. MRI technology has improved to the point where it can capture an image of a beating heart. Raymond Migrino, MD, FACC, discusses how MRI can help diagnose almost any kind of cardiac disease or condition.
Q. What is MRI? MRI stands for Magnetic Resonance Imaging, and it is a technique to obtain very detailed pictures of body parts, including the heart, using the magnetic properties of the body. Basically, our bodies are composed of billions of protons or hydrogen atoms, and these protons have their own little magnetic spins and we exploit these magnetic spins to take pictures of the body.
Q. How does it work? Essentially, we put a patient on a large magnet in an MRI machine and the magnet then aligns the protons in the body in a certain direction, according to its magnetic field. Then a low-frequency radio wave is directed at the patient, which causes a slight change in the spins of these protons. This change produces a signal that we then detect in order to create detailed pictures of the heart.
Q. How is MRI used to diagnose cardiac problems?MRI can now take very detailed pictures of the heart, and that allows physicians to make accurate diagnoses of disease conditions. MRI is able to get a level of resolution and to finely detect changes in the heart that is not approached by any other non-invasive modality of imaging. We can basically use MRI for detecting almost any type of cardiac problem.
The most common use is in the field of coronary artery disease. There, we can use it to detect how much damage a heart has undergone after a heart attack. We can determine the level of ventricular function or heart function after a heart attack. We can also pinpoint which areas of the heart sustained the heart attack and how much damage there is. We can determine how much living muscle there is to a very fine degree.
Q. How new is the use of MRI to diagnose cardiac problems?The use of MRI for diagnosing cardiac problems is relatively new compared to other uses because taking a picture of the heart is more challenging. It’s a moving target, beating at around 60 to 100 beats per minute. It’s not like a knee or brain that doesn’t move when you’re imaging it.
The theoretical basis for MRI has been known since the 1950s, but the imaging of body parts began in 1977. The first image of a moving heart was in 1987, and from then onward there has been an exponential increase in the capabilities of MRI. In our institution, there has been a cardiac MRI program run by Radiology for a few years.
Q. Can it diagnose all cardiac problems? MRI can be beneficial in the assessment of various types of heart disease from problems relating to coronary disease, heart muscle abnormalities, tumors, valve disease and problems of the blood vessels. MRI is helpful in answering most questions, but not all questions. As an example, some types of electrical abnormalities of the heart, also called arrhythmia, need specific tests to define the electrical disturbance that MRI cannot detect. Also, some people would still need to have heart catheterization (injecting contrast to the artery supplying the heart via plastic catheter) to measure the degree of obstruction of the artery. MRI is still not able to replace this procedure.
Q. What are some of the unusual cardiac problems that cardiac MRI can diagnose? Aside from the other uses mentioned before, there are specific diseases where MRI can provide more comprehensive answers than other imaging modalities, like arrhythmogenic right ventricular dysplasia, an abnormality in the fat deposit in the right ventricle that causes an electrical abnormality. If a patient has this type of disease, MRI might be the preferred test.
Another exciting area of increasing utility is in heart failure. MRI, because it can characterize the type of damage in the heart, is used for studying causes of heart failure. It can help determine whether the cause of the heart failure is idiopathic cardiomyopathy, which is a heart failure that we don’t know the cause of; or whether this is related to a myocarditis, which is an inflammation of the heart muscle; or whether it’s due to an abnormal thickening of the heart muscle called hypertrophic myopathy.
With MRI, we can now characterize these disease conditions better. Before, we were able to say that the heart muscle is thick and the function is bad. Now with MRI, we can go beyond thickness and poor function to see if there are areas of abnormal collection of fibrous tissue or scar that will make the patient predisposed to a bad outcome or to an electrical arrhythmia event. We’re able to help the physician in terms of characterizing the abnormality and determining the patient’s risk. That helps in determining what course of treatment and how aggressively people should be treated.
Q. What are the advantages of using MRI to diagnose cardiac problems?I think unique to MRI, over any other type of cardiac testing, is the ability to characterize the tissues. If there is a cardiac mass, MRI can distinguish the nature of that mass. No other modality can approach the ability of MRI to do that. The most important advantage and most useful feature of MRI is the ability to quantify the amount of scar tissue a specific wall of the heart has after a heart attack. The measurement of the amount of scar tissue is relevant in terms of deciding, for example, whether the patient will need bypass surgery or will need angioplasty or will just benefit from medical therapy. This area of infarct or scar imaging and viability imaging is perhaps the greatest advantage of MRI over any other modality.
MRI right now is able to look at the structure of the heart, the function of the heart, the blood flow of the heart, and it’s also able to look at the coronary arteries. It can produce very detailed pictures akin to a one-stop shop. Whereas before you would need an echocardiogram to look at function, a nuclear scan to look at flow and a coronary angiogram to look at anomalous vessels, now you can do all that with one exam.
It’s also a very reproducible test, a very accurate test. That means that if you used MRI to follow up a patient’s course of treatment, there’s a better likelihood that the differences between the two studies are caused by the changes in the disease itself rather than by the errors introduced by the method of measurement.
Another advantage is that is uses no radiation and does not require any special preparation.
Q. What are the disadvantages or limitations of MRI?The major limitation of an MRI is that there are certain patients who cannot get an MRI. These are patients with metal implants in their bodies such as pacemakers, defibrillators and certain kinds of stents, but not all (some stents are MRI compatible). They cannot have an MRI because of the magnetic field involved.
Also, people who are very claustrophobic who may feel confined in the MRI machine, may not want the test, although this is very rare. We can usually give relaxants for those patients to be able to tolerate the MRI. Aside from that, there really is no other contraindication of MRI.
Another disadvantage in some situations can be the length of time of the exam. It can take about 30 minutes to an hour to perform a cardiac MRI, depending on the question being asked. It’s a more time consuming examination than CT, for example.
Q. Is MRI different than other technologies used for cardiac imaging, like the 64-slice CT? MRI and CT utilize two different ways of taking a picture. The CT scan basically shoots radiation through the patient and then a detector detects the radiation after it goes through the patient, and that’s how the picture is created. The advantage of the CT is that it’s a shorter examination. Usually it’s performed under a minute.
The disadvantages of CT are one, that there is radiation involved. Less exposure to radiation is better, especially in children and young adults.
Second, the CT requires an iodine-based contrast agent that has the potential side effect of adversely affecting kidney function, especially in patients with underlying kidney disease.
From an imaging standpoint, the temporal resolution of a CT (how quickly pictures are obtained relative to the beating heart) is not quite up to the standards of an echocardiogram or MRI yet. That means a CT can evaluate heart function but not as well as MRI or an echo. In terms of looking at coronary narrowing, which is the driving force behind cardiac CT, it seems like the CT and MRI have about similar accuracy in terms of detecting narrowing. Of course, CT has the advantage of being a faster test – that is, within one minute you get all the information vs. being in an MRI scanner for 30 minutes to an hour to get the same information. That’s a distinct advantage of CT.
A CT scan, however, does not have the ability to discern details about the nature of specific tissues. In terms of the characterization of specific tissues and disease processes, CT is not as good as MRI.
There are some specific indications where a CT would be better than an MRI. For example, if you have a patient who is claustrophobic, of course CT would be better. In an emergency room setting where a patient is having chest pain, and you’d like to get a very fast answer, a CT scan would be better because you can get the answer rather quickly.
There are other cardiac imaging techniques that can determine function and viability and perfusion just like MRI, but in terms of the spatial resolution or the ability to detect details in the pictures, MRI is superior. Aside from that, the other techniques are sort of one trick ponies. They can do certain things very, very well, but not all of them. An MRI has a more broad capability of determining function, structure, blood flow, tissue characteristics and plaque burden. MRI, if you will, is an all-in-one approach for diagnosing heart problems.
Nuclear scans are another way of defining scar tissue and defining the coronary effects of a heart attack. Nuclear scans can detect the amount of infarct or the amount of decreased blood flow to a particular segment of the heart. The two disadvantages of a nuclear scan are one, the radiation involved and two, it is not as detailed in terms of the spatial resolution as an MRI. Needless to say, despite these two disadvantages, the nuclear scan has had a long history of being helpful in diagnosing coronary disease. MRI currently is complementary to nuclear scan. It has not replaced nuclear scans, so nuclear is alive and well.
Another method is an echocardiogram or an ultrasound of the heart. The ultrasound also looks at heart function and structure and can also detect coronary disease and the amount of infarct or scar just like an MRI. The advantage of an echo over MRI is that it’s easier to obtain. The disadvantage of an echo is that if you really want very specific characterization of tissues, the echo may not provide that. Currently, the use of MRI has been to supplement echo because echo is just more available and cheaper, so it is usually the first modality of imaging. If there are questions that the echo cannot answer, usually they’ll request an MRI for a better, detailed evaluation of the heart structure and vascular structures.
Q. Will MRI replace other forms of cardiac diagnostic testing?It’s another tool, not a replacement. The trend though, is to use cardiac MRI more often because it is becoming more available. Cardiac MRI requires a dedicated specialist and dedicated technicians who are attuned to the technique, which is not available in every institution. As the knowledge of cardiac MRI increases, and as the number of specialists and technicians increase, I think there may be an exponential increase in the use of cardiac MRI.
Right now, it is sort of an alternative to those other imaging modalities. I can foresee, though, that it may become the first line of imaging modality just because of its intrinsic advantages.
Currently, the imaging of the coronary arteries for MRI on a clinical basis is limited to looking at anomalous origins of the coronaries as well as looking at bypass grafts. It’s not really ready for prime time yet in regards to replacing coronary angiograms, the cardiac caths, but a lot of research is going into that. At this time for clinical purposes, it’s not ready yet.
Q. When should a patient ask about cardiac MRI? Crucial to understanding when any test is appropriate is initiating a conversation with your doctor. If a patient is about to undergo a test, he or she should understand exactly what the test is for, what it will answer and what are the treatment modalities that will arise out of the results. If the test result is this, what will happen and if the test result is that, what will happen? The patient should ask, “What are my alternatives?” I think it will benefit patients to have that kind of thinking, whether the test is CT or MRI or nuclear or echo. I think a better informed patient population will always be beneficial.
There are some tests where the doctor might say, for example, because of the patient’s body type, we can’t see the pictures too well, either with an echo or a nuclear scan. Or it may be that some of these tests are borderline and we can’t be sure. If that’s the case, the patient may ask whether the cardiac MRI and its improved ability to detect details of heart structures would be able to answer those questions.
Q. What else should people know about cardiac MRI?In cardiology, there are different imaging modalities and the current trend is to be conscious about which modality would better serve your diagnostic question, and not be rigid about utilizing only one modality. You want to have them all at your disposal, and fortunately we have them all here at Froedtert & the Medical College of Wisconsin.
Source: Every Day
Date: May - July 2006 Issue
|Medical Reviewer: ||Raymond Migrino, MD, FACC|
|Medical College of Wisconsin Cardiologist||