System advances the field of heart imaging in order to improve patient care
The University of Maryland Medical Center is one of the first hospitals in the country to diagnose heart disease using a new type of advanced imaging system. The system, called rubidium PET/CT scanning, combines Positron Emission Tomography (PET) with Computed Tomography (CT) angiography. In less than an hour, this noninvasive imaging technology helps doctors make a precise diagnosis, which is the first step for providing excellent patient care.
“What’s exciting about this system is that we can obtain information about the heart’s anatomy and function in the same setting,” says Vasken Dilsizian, M.D., director of Cardiovascular Nuclear Medicine at the University of Maryland Medical Center and professor of medicine and diagnostic radiology and nuclear medicine at the University of Maryland School of Medicine. “This complementary information makes it possible to interpret the data much more intelligently and quickly determine what kind of treatment will most benefit a patient, whether it is medical therapy, angioplasty or surgery,” says Dr. Dilsizian.
“This is definitely an important advance in imaging for cardiovascular disease. It is new and we are still evaluating its use for particular types of patients, but it may become a one-stop solution for the early diagnosis of heart disease,” says Mandeep R. Mehra, M.B.B.S., head of cardiology at the University of Maryland Medical Center and professor of medicine at the University of Maryland School of Medicine. “The PET/CT combination also improves the ability to do research, because it is noninvasive,” adds Dr. Mehra.
Rubidium PET acquires images by detecting the radioactive tracer rubidium. The CT part of the combined imaging system uses rotating X-ray beams to visualize blood flow. The combined system measures blood flow in precise, numerical terms and yields images of coronary arteries comparable to those seen in an invasive technique called cardiac catheterization.
In addition, when combined with another tracer called FDG, which assesses sugar molecules the heart burns as its main fuel, rubidium PET/CT produces a clear picture of viable areas of heart muscle and differentiates them from those that are scarred. This distinction can help doctors decide whether bypass surgery would make it possible for patients with abnormal hearts to recover their normal pumping function after surgery.
Dr. Dilsizian says a rubidium-based PET/CT scanner is much less expensive to operate than PET/CT systems that use another tracer, ammonia, which provide similar measurements and images. The annual cost of rubidium is about one-sixth the yearly expense to operate a cyclotron to produce the ammonia tracer.
PET imaging has been around for years. It is used extensively to diagnose various cancers. Dr. Dilsizian says the cardiac community has been slow to adopt PET imaging because it is more expensive than another commonly used imaging system called Single Photon Emission Computed Tomography (SPECT).
Many hospitals, including the University of Maryland Medical Center, use SPECT imaging during treadmill tests to determine whether a patient needs angioplasty and/or a stent to unblock a clogged coronary artery. These SPECT studies take a minimum of four hours to complete. SPECT is useful for detecting advanced heart disease, but because SPECT images are not as detailed as PET images, Dr. Dilsizian says SPECT may not show blood flow restrictions that could signal early stages of heart disease. SPECT images show blood flow only in relative terms, as colored areas on a screen: green and blue are worse than yellow and red. PET shows the true, underlying disease in numerical, quantitative terms. “Whether the study is done in Baltimore or in Los Angeles, we should all get the same quantitative numbers and not simply rely on the expertise of the interpreter at a particular institution,” says Dr. Dilsizian.
Dr. Dilsizian says a major advantage of PET over SPECT comes when measuring obese patients. Large body size impairs the ability of photons, the particles of light that come from the radioactive tracer, to register correctly on the SPECT imaging camera. This could result in a false positive reading and an incorrect diagnosis, says Dr. Dilsizian. “These images may suggest that there is impaired blood flow in the heart muscle, when in fact the image is not as clear because of the fatty tissue lying over the heart.”
Dr. Dilsizian says another advantage of the PET/CT system is to detect early calcification or the development of soft coronary artery plaques in time to prevent the progression of heart disease.
“In cardiology, we typically wait until the patient has symptoms. We detect and treat significantly narrowed coronary arteries, often with stents, but the prevention part is frequently not well-addressed,” says Dr. Dilsizian. “The whole idea of having a clogged artery that can regress or become smaller with medical therapy was not thought to be possible in earlier years.”
Nature, it turns out, has put in place a reparative process. “There is now evidence that plaque build-up in the coronary arteries, known as atherosclerosis, can regress if aggressively treated with risk factor modification and appropriate medical therapy,” says Dr. Dilsizian. Similarly, changes in the shape of the heart muscle after a heart attack, called remodeling, can be reversed with medical and/or surgical therapy. “We are always in this fluid state both with coronary artery lesions as well as with the heart muscle,” adds Dr. Dilsizian. “We have found a way to identify where an individual patient with cardiac risk factors is in the progressive process of atherosclerotic disease. This can be accomplished with PET/CT imaging.”
PET/CT scanning can also establish baseline blood flow measurements. Individuals who are studied today and are told to change their eating habits, increase their exercise, and take medications to reduce their risk for coronary artery disease, can be scanned a year later to see what has happened. “This type of imaging could help us get ahead of heart disease, and possibly reverse the process of atherosclerosis earlier,” Dr. Dilsizian says. He adds that PET/CT scans can be useful in tailoring medication dosages to treat heart disease by comparing baseline images to scans after medical therapy.
The University of Maryland Medical Center is the first hospital in the Mid-Atlantic region to use both rubidium PET and CT angiography for the comprehensive evaluation of coronary artery disease.
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