CT and MRI are both being used more frequently to
help diagnose disease of the heart and great vessels.
High-speed helical (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 (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 multiple detectors CT (MDCT) 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. MDCT showed a high negative
predictive value for coronary stenosis, but it had a relatively variable
degree of positive predictive value in several studies. Cardiac CT have
a additative role in diagnosis in coronary artery disease combined with
coronary angiography. In addition, post-bypass surgery evaluation, left
ventricular function, and valvular morphology and function were also able
to be evaluated.
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. MRI is already being considered 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.
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.
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. Careful further
clinical studies of appropriate size and design will be needed to clarify
the role of coronary CT and cardiac MR angiography, especially in the
context of stable and acute coronary syndrome.
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