Magnetic Resonance Imaging as a Noninvasive Standard for the Quantitative Evaluation of Left Ventricular Mass, Ischemia, and Infarctiona

Because magnetic resonance imaging (MRI) acquires data in a spatially unambiguous fashion and the three-dimensional interrelationships of one image plane to another are easily ascertained, there are far fewer technical restrictions imposed on this method than on other imaging techniques. Furthermore, the multiplanar nature of MRI image acquisition, in any plane desired, is a feature unique to this imaging technology. MRI is thus well suited to the highly accurate quantification of global and regional left ventricular (LV) size and function, and can be used as a standard for comparison to other techniques, once validated. Because the determination of LV mass by MRI requires no assumptions about ventricular shape, it should be well suited to the evaluation of both normal hearts and those distorted by infarction. We performed gated MRI on 15 dogs before and after myocardial infarction. LV mass was calculated with 5 short axis planes. The correlation was excellent between actual mass before infarction and after MI. Accuracy was similar for both end-diastole and end-systole. Thus, MRI accurately determines LV mass in both distorted and normal hearts. We have also developed a method for quantification and mapping of regional wall thickening throughout the LV as an index of regional ischemia by utilizing the 3D geometry to calculate the perpendicular wall thickness of a 3D volume element of tissue. This 3D volume element results in less variability of normal wall thickening and provides a better discriminator of ischemic from nonischemic zones in a canine model of acute ischemia, whereas there is considerably greater overlap between ischemic and normal zones with standard planar MRI techniques. The 3D method is more accurate than planar methods in avoiding biases resulting from the oblique course of an image plane through the LV wall, resulting in better distinction of ischemic from nonischemic tissue. Finally, the accurate assessment of regional LV function for the identification of ischemic or infarcted myocardium has been enhanced greatly by a new technique, myocardial tissue tagging, in which an electronic marker is applied to the myocardium which persists through ejection, enabling the accurate tracking of specific areas of the heart as they move and rotate through the cardiac cycle.