Cardiac MRI: A New Way of Looking at the Heart
Magnetic Resonance Imaging, or MRI, is a method to evaluate the tissues and organs of the human body without using radiation (such as X-ray). A powerful magnet is used to take the images. The images are then sent to a computer, where a physician can view and interpret them. The magnetic field used in MRI scanners is approximately 10,000 times stronger than Earth's magnetic field. These strong magnetic fields are not harmful. However, they may have an effect on any foreign bodies, such as some implanted medical devices, for example cardiac pacemakers, and on some aneurysm clips. If you are going to have a MRI scan, you should inform the technologist of all metal devices in your body to make sure they are MRI-safe.
People usually associate MRI with the scan of the brain or the spine. It is not commonly known that in the last two decades applications of the MRI increased, making it possible to perform imaging of the heart. First attempts to look at the heart with MRI in the laboratory were performed in the early 1980s. The first clinical applications of MRI to measure the function of the heart happened in late 1980s-early 1990s. "Doctors and scientists knew, even back then," said Dr. John R. Lesser, MD, FACC, director of Cardiovascular MRI at Abbott-Northwestern Hospital and a clinical cardiologist at the Minneapolis Heart Institute, "that MRI could be ideal for detailed studies of the heart, and that it could potentially bring a big improvement in ways of looking at the heart compared to other available technologies."
Collaborations between manufacturers, such as Siemens, and major research centers around the world have brought advancements in technology to allow MRI images of the heart. As a result, MRI scanners specifically designed for imaging of the heart are becoming more available. Such scanners allow doctors to perform very detailed, precise, high-quality evaluations of the heart's chambers, valves and blood vessels, to measure the function of the heart and blood flow, to distinguish portions of the heart (myocardium) that are viable from the ones that have scar tissue, to determine the thickness of the outer layer of the heart (pericardium), and much more. Abbott Northwestern Hospital has a state-of-the-art 1.5T Sonata Siemens scanner that is capable of doing such examinations. "MRI scan of the heart usually takes between 20 to 40 minutes, depending on how many images we need to collect," said Dr. Lesser. Patients enter the scanner on a sliding table. Being inside the scanner is like being inside a narrow tube. "The majority of patients tolerate the scan very well, but those that think they might feel claustrophobic usually get a little bit of a mild sedative medication before the procedure," explained Dr. Lesser.
Patients come to the Minneapolis Heart Institute with a variety of complaints such as chest pain, shortness of breath, tiredness, being unable to walk as fast as he or she used to, high blood pressure and other symptoms that may indicate heart disease. Several different tests may be used to diagnose the heart problem and guide the direction of treatment. Each new technology that comes into patient care has to stand trial of comparison against other technologies to establish its position, application and place in the diagnosis and treatment process. MRI has already successfully accomplished that task and has become recognized as the "gold standard" for visualizing the shape, size and structure of the heart and major arteries, and measuring the function of the heart and its mass. This type of evaluation is often necessary in patients with coronary artery disease, high blood pressure and congestive heart failure to help doctors make decisions about the course of disease and directions of treatment. The heart is scanned in motion over several minutes to see how it contracts and relaxes. Usually, 10-15 different projections of the heart are taken to make sure that all views of the heart are recorded. One of the typical pictures of the heart taken in these series is a view of all 4-chambers of the heart (see Figure 1). "After all the images are taken, we draw contours around the outer layer of the heart and the inner layer of the heart to obtain measurements of the heart's function," said Dr. Lesser.
(figure 1)
MRI is being successfully applied in congenital heart disease, where doctors frequently find abnormal shapes and sizes of the heart and of the great vessels. In patients with congenital heart disease, 3-D visualization is frequently the key element that allows estimating the time for surgery and to define prognosis of the patients. When a doctor suspects that a patient may have a blood clot or blockages of arteries in the legs or in the lungs, a MRI angiogram is performed. For this, a small amount of a contrast dye is given to a patient during MRI scan to help see the vessels and to construct images of an angiogram in three dimensions. "Patients usually tolerate the dye very well - all one may feel is a little bit of tingling in the arm when we inject the dye. Very few patients complain of nausea or a headache. It is a very safe procedure," explained Jana Lindberg, RT, lead cardiovascular MRI technologist at Abbott Northwestern Hospital. After the scan is finished, doctors see the entire network of vessels - the vascular tree - and are able to make an accurate diagnosis (see Figure 2). "The 3-D capabilities and outstanding quality of the images that we can achieve with the state-of-the-art scanner are very helpful in the decision-making process," said S. Murthy Tadavarthy, MD, an interventional radiologist at Abbott Northwestern Hospital.
(figure 2)
Building on past research in heart disease and the field of MRI, doctors and researchers at the Minneapolis Heart Institute Foundation seek new ways to utilize MRI to better understand heart disease. Laboratory research and animal studies allow investigation of the disease process and new treatments before they are applied in humans. "We plan to use MRI to help us understand as deeply as possible what really happens in the coronary arteries when heart disease is in full progress and when it is in its earliest stages," said Robert S. Schwartz, MD, FACC, FAHA, medical director, Minnesota Cardiovascular Research Institute, and clinical cardiologist at the Minneapolis Heart Institute.
Several research studies are already under way at the Minneapolis Heart Institute Foundation using MRI as the technique that allows distinguishing different degrees of damage to the heart in patients that have chest pain and other symptoms despite having had bypass surgery, angioplasty, or stenting. Many of these patients are enrolled in trials of new therapies with angiogenic growth proteins, which are thought to stimulate the growth of new blood vessels to supply blood and oxygen to weak areas in the heart. "We think that cardiac MRI provides the best measurements of effect of these new therapies and allows us to apply treatments to the areas that need it the most," said Timothy D. Henry, MD, director of research, Minneapolis Heart Institute Foundation. "We already used cardiac MRI to monitor effects of an angiogenic protein in the past," shared Dr. Henry, "and it made us realize how much detailed information MRI scans can provide." These research studies with MRI often involve administration of a drug, adenosine, which dilates blood vessels to create a stress on the heart. The use of adenosine is a standard part of nuclear medicine studies, and is now being successfully adapted to be used in MRI. "Adenosine exists in the body and by giving it to our patients, we simply increase its concentration. The half-life of adenosine is less than 10 seconds, and once the short (3-4 minutes) infusion is finished, it is gone from the body in a minute or two," explained Betsy V. Wilson, RN, MS, senior research nurse clinician in cardiovascular imaging, Minneapolis Heart Institute Foundation. "We have built extensive expertise in cardiac MRI here at Abbott Northwestern Hospital," said Dr. Lesser, "and being involved in research studies makes it possible to quickly adapt new techniques to bring the best and latest technology to our patients."
Dr. Lesser and Dr. Scott W. Sharkey, MD, FACC, clinical cardiologist, Minneapolis Heart Institute, use MRI scans to study patients with a newly described disease. Patients may exhibit symptoms of a heart attack, but their coronary arteries look normal, and the heart displays normal function within several weeks or months. The results of that research were presented at the American Heart Association Scientific Sessions - 2002 in Chicago.
Dr. Barry J. Maron, MD, FACC, director of the Hypertrophic Cardiomyopathy Center, Minneapolis Heart Institute Foundation, has already applied cardiac MRI to study patients that carry a mutant gene for hypertrophic cardiomyopathy but do not have any clinical symptoms. Another study has looked at blood flow to very thick areas of the heart, which are common in hypertrophic cardiomyopathy. The data of that research have been presented at the American Heart Association Scientific Sessions in 2001 and 2002 and at the Scientific Sessions of the Society for Cardiovascular Magnetic Resonance in 2002.
There are many exciting directions of research where MRI could be a very useful, even superior technique. Measuring blood flow to the parts of the heart that are affected by blocked arteries, performing non-invasive angiograms of the coronary arteries with MRI to investigate the plaque in the arteries, and studying the small vessels of the heart immediately after a heart attack are examples of what the researchers at the Minneapolis Heart Institute Foundation have in mind for the future. |