MR and CT:
Best contenders may soon replace
other cardiac imaging methods
BY CHARLES BANKHEAD
Opening plenary offers dramatic
proof of MR & CT's potential.
Advances in cardiovascular applications of MR and CT promise to make these imaging modalities major players in evaluating disease, speakers at an RSNA plenary session said Sunday.
In particular, functional cardiac imaging with MR appears on the verge of a breakthrough.
"The question is no longer 'if' MR will replace current methods of cardiovascular imaging, but 'when,'" said Dr. Elias Zerhouni, a professor of radiology at Johns Hopkins University.
The growth in MR's potential in cardiac imaging comes from the modality's strength in providing information about cardiac function. MR already can provide accurate information about basic parameters of cardiac function, such as heart size, ejection fraction, said Zerhouni. The power of the imaging modality will come from its ability to provide information about cardiac function at the cellular level.
By use of labeling chemicals, radiologists can employ MR to evaluate specific regions of the heart and to demonstrate and quantify contractile function, which can then be represented graphically in color-enhanced images. Gadolinium entry into myocytes can be used as means to distinguish necrotic, non-salvageable myocardium from stunned tissue after a myocardial infarction. Dobutamine stress tests with MR also can distinguish between stunned and necrotic tissue.
Proton spectroscopy has proven its ability to distinguish creatine kinase levels in normal and infarcted regions of the heart. The information provides insight into the metabolic reserve of the heart and an indication about the likelihood a patient will develop heart failure, said Zerhouni.
MR sodium imaging provides information about the efficiency of sodium and potassium exchange in the heart. Those exchange channels are the first areas to be affected by myocardial infarction, said Zerhouni.
On the horizon, MR appears poised to play a major role in interventional cardiology. Development of miniature intravascular probes, used in conjunction with MR fluoroscopy, have the ability to guide interventional procedures, such as balloon angioplasty, and to provide immediate, accurate assessment of results.
Still in development, computer-enhanced cine MR imaging represents another potentially major advance in the works. Called 4-D heart interactive imaging, the MR technique permits visualization of the beating heart in isolation from the body.
"This technique gives radiologists a tremendously powerful diagnostic technique," said Zerhouni.
Use of CT to identify and quantify calcification within coronary arteries represents a potentially major advance in non-invasive diagnosis of coronary heart disease, said Dr. William Stanford, a professor of radiology at the University of Iowa.
"Calcium deposition in the coronary arteries is not a passive process but rather an organized and highly regulated process," said Stanford. "Calcification occurs early in atherosclerotic plaque development, which provides the opportunity to identify patients before they have a cardiac event."
Electron beam CT evaluation of coronary calcification has demonstrated a strong correlation between the amount of calcium and the amount of plaque present. Calcification also has been shown to correlate with disease progression and by regression or stabilization induced by treatment with lipid-lowering drugs.
CT and MR combined offer the opportunity for "one-stop shopping" for assessment of coronary artery disease, Stanford concluded.
