New Innovations
Cardiac Imaging: Integrated, Applications-Driven use of Cardiac CT and MRI
Cardiac imaging methods such as Cardiac Magnetic Resonance Imaging (MRI) and Cardiac Computed Tomography (CT) are allowing physicians to take a closer look at the heart and great vessels at little risk to the patient. We compare and contrast Cardiac CT and MRI for you, as each procedure has its benefits.
What is Cardiac MRI?
MRI uses large magnets and radio-frequency waves to produce high-quality still and moving pictures of the body's internal structures; no X-ray exposure is involved. The scan monitors energy changes in tissues reacting to magnetic forces. A computer analyzes these changes and creates a composite image of the tissues. The images can be shown in two or three spatial dimensions in either static or dynamic cine mode.
MRI is especially useful for evaluating the cardiovascular system. In addition to being non-invasive (not requiring an incision) and risk-free, MRI can image a large portion of the body, such as the chest, in one session. Because MRI acquires information about the heart as it is beating; it can create moving images of the heart throughout its pumping cycle. This allows MRI to display abnormalities in cardiac chamber contraction and to show abnormal patterns of blood flow in the heart and great vessels. Using MRI, physicians can obtain images of the chest and cardiovascular system from many angles. This allows better assessment of complex anatomic abnormalities than with other imaging techniques. Due to the development of new imaging techniques, MRI has the capability to identify areas of the heart muscle that are not receiving adequate blood supply from the coronary arteries. Aided by use of non-iodine-based enhancing agent (Gadolinium-DTPA), it can also clearly identify areas of the muscle that have become damaged as a result of infarction (heart attack).
This combination of unique capabilities has made MRI a commonly used diagnostic imaging procedure for evaluating specifically: ischemic heart disease, "thick-wall" myocardial (heart muscle) disease, right ventricular abnormalities, pericardial (sac around the heart) disease, cardiac tumors, valvular disease, thoracic aortic disease, pulmonary artery disease, and congenital heart disease before and after surgical repair. For cardiac MRI, the Cleveland Clinic Foundation uses state-of-the-art systems (Siemens Magnetom Symphony and Sonata scanners).
What is Cardiac CT?
A traditional CT scan is an x-ray procedure which combines many x-ray images with the aid of a computer to generate cross-sectional views of the body. Cardiac CT uses advanced CT technology with or without intravenous iodine-based contrast to visualize cardiac anatomy, including the coronary arteries and great arteries and veins. To this end, the Cleveland Clinic Foundation uses state-of-the-art multi-detector CT systems (Siemens Somatom Volume Zoom and Sensation16 scanners); with multi-detector scanning, it is possible to acquire high-resolution three-dimensional images of the heart and great vessels.
Cardiac CT is especially useful in evaluating the myocardium, coronary arteries, pulmonary veins, thoracic aorta, pericardium, and cardiac masses, such as thrombus of the left atrial appendage.
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Cardiac MRI and CT Examination Safety
MRI and CT pose no risk to the average patient if appropriate safety guidelines are followed.
Neither test is advisable if the patient is pregnant, although they may be needed if the health of the woman is at stake. Because cardiac CT involves use of iodine-based contrast and x-ray, patients should tell their doctor if they are allergic to iodine and/or shellfish or any medications, undergoing radiation therapy, or over 60 years old or have a history of kidney problems (they may be required to have a blood test to evaluate your kidney function prior to receiving any contrast agent.).
MRI often uses non-iodine-based contrast, called Gadolinium-DTPA. However, being non-iodine-based, it is, therefore, less toxic to the kidneys. It can be administered to patients with iodine or shellfish allergies. The large magnets used in MRI may pose a problem for some patients. Tell your physician if you have contraindications: an implanted permanent pacemaker or defibrillator components, cerebral aneurysm clip (metal clip in a blood vessel in the brain), implanted insulin pump or narcotic pump or implanted nerve stimulators (TENS) for back pain, metal in the eye or eye socket, or cochlear (ear) implant for hearing impairment.
The Future of MRI and CT
MRI and CT are both being used more frequently to help diagnose disease of the heart and great vessels.
Cardiac MRI has many potential applications, such as evaluation of coronary artery anatomy and flow and myocardial perfusion. In the future, it may be possible to use MRI to track progression of atherosclerosis in both the coronary and carotid arteries. Because of its non-invasive nature, repeated MRI examinations could be used to identify those at increased risk for heart attack or stroke. MRI is already being used to evaluate myocardial viability, as an alternative or adjunct to a stress echocardiogram and/or nuclear heart examination, in order to provide additional valuable information, such as the condition of the myocardium and wall motion. With high-speed MRI, the physician can evaluate left ventricular wall motion during physical stress. Last, studies looking at the ability of the MRI to assess myocardial metabolism are also underway.
High-speed helical (a.k.a. spiral), non-contrast enhanced Cardiac CT scans are being used more frequently to detect calcium deposits found in atherosclerotic plaque in the coronary arteries before symptoms develop (a.k.a. Calcium-Score Screening Heart Scan). Although the predictive value of coronary calcium score screening is not fully defined, more coronary calcium indicates more atherosclerosis, and a greater likelihood of arterial narrowing and future cardiovascular events.
Advanced helical CT is also being used more and more to allow the identification of both calcified and non-calcified plaque within the coronary artery walls or to exclude the possibility of significant stenosis due to atherosclerosis. When contrast-enhanced 3-dimensional versions of this type of scanning are performed, early stages of atherosclerosis of the coronary arteries can be seen before the development of arterial narrowing. It allows early detection of potentially unstable "soft plaque" and non-calcified plaque during a potentially reversible phase of development, making it possible to non-invasively monitor plaque regression with various therapies.
Techniques are also being developed to combine the data acquired by Cardiac CT angiography of the coronary arteries and the data regarding myocardial viability acquired by Cardiac MRI. Based on the co-registration of the CT Angiography and MRI data, a spatial relationship can be directly established between the diseased coronary artery distribution and the myocardium at risk. With the advancement of these imaging modalities, patients may expect to realize improved pre-revascularization planning and reduced invasiveness of the diagnostic process.
In the emergency department, helical CT scans are being used to look at blood flow through the carotid arteries and blood distribution in the brain. During an acute stroke, the doctor can see if the patient is actually having a stroke and if so, what vessel is affected, the amount of blockage, and what areas of the brain are not receiving enough blood. In addition, helical CT is providing an alternative to catheterization techniques for angiographic evaluation of the peripheral arteries of the arms and legs.
Can Cardiac MRI and Cardiac CT Replace Other Imaging Technologies?
Despite its advantages, these tests are not a substitute for other imaging techniques in all cardiovascular conditions. Unlike an echocardiogram machine, the MRI and CT scanners cannot be brought to the bedside of an acutely ill patient. Nonetheless, as previously mentioned, Cardiac MRI is already being used more and more as the examination of choice in certain settings (e.g. assessment of myocardial viability). In addition, early success with non-invasive angiography of coronary arteries based on advanced Cardiac CT techniques has led to its routine application in certain settings (e.g. coronary artery bypass graft patency evaluation, detection of anomalous coronary arteries, and clarification of equivocal results from other screening exams for coronary disease). These tests have already become an integral part of the evaluation of patients with various forms of cardiovascular disease as promising new versions continue to develop.
For more information:
See: Imaging Tool Strengthens Ability to Manage Heart Disease Proactively, Cardiac Consult, Summer 2004
The Section of Cardiovascular Imaging in the Division of Radiology at the Cleveland Clinic Foundation is one of the most active centers in the world for applying Cardiac MRI and Cardiac CT technology to cardiovascular conditions. To schedule and appointment for a Cardiac MRI, Cardiac CT, or for a Cardiac Imaging Consultation please call (216) 445-7050.
Click here to learn more about Diagnostic Radiology: Cardiovascular Imaging - MRI/CT
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