Although magnetic resonance imaging (MRI) has been available clinically for over a decade, its application to the cardiovascular system is still in its infancy. The complex anatomy and motion of the cardiovascular system presents special technical challenges which are not found in other organ systems. The purpose of these studies is to develop and improve magnetic resonance (MR) techniques for assessing the cardiovascular system. Our main focus has been 1) to develop new MR techniques that provide information about the cardiovascular system that cannot currently be obtained, 2) to apply these techniques to learn more about cardiovascular physiology, and 3) to incorporate these techniques into a clinically relevant cardiac exam that can be performed on patients to assess cardiovascular disease and its response to treatment. Over the last year, we have studied the dynamics of flow in the aorta with phase contrast MRI. Peak diastolic flow in the ascending aortas of patients with atherosclerosis is abnormally low (about 30% of normal), probably due to a decrease in total arterial vascular compliance. These studies also revealed increased flow separation (up to 5-fold) in the ascending aorta of women as compared to men with the same aortic diameter. The cause for the increased flow separation is uncertain, but is probably related to a difference in arterial impedence. We have also assessed regional, transmural myocardial strain using a technique based on phase contrast MR images which are sensitive to the velocity of myocardial motion. Image analysis showed that normal myocardium has a gradient of radial velocities which linearly increase in magnitude from epicardium to endocardium. Areas of ischemia/infarction show a decrease in gradient (hypokenesis), no gradient (akenesis), or a reverse in gradient (dyskenesis). Technical developments over the last year include the development of a cardiac gradient coil with approximately a 2-fold increase in maximum gradient strength. We have also developed a technique for image acquisition that compensates for physiologic heart rate variability on a beat-to-beat basis. Because the entire R-R interval is sampled, images now directly show atrial contraction.
