The applicants propose to further develop techniques for real-time interactive imaging of the coronary arteries. Based on results of the previous funding cycle and the current status of the field, the applicants propose that the principal technical challenges facing coronary artery MRA today are improved spatial resolution, improved technical reliability, and the need for sub-10 sec acquisition times. To address these issues, the applicants propose to use contrast material for improved depiction of the intraluminal signal. Further, would develop a novel dual-flip - angle acquisition method which, when combined with complex subtraction, is expected to allow the small acquisition time of 2D imaging and the high sensitivity to contrast along the slab direction of 3D imaging. The specific hypothesis of this proposal is that dual flip angle imaging can be used to provide high contrast sub-mm spatial resolution in imaging of the coronary arteries. Three projects will be studied: 1. Dual Flip Angle Imaging Physics. The applicants propose to develop a technique in which the vascular signal can be isolated by using complex subtraction of data acquired at different flip angles. They hypothesize that this method will allow the accurate depiction of a contrast-enhanced blood vessel whose diameter in the slice-select (Z) direction is smaller than the slice thickness. This will be studied by determination of optimum flip angles and consideration of various means for complex subtraction; 2. Real-Time Implementation. They will develop gradient echo and multi-shot EPI sequences which toggle between low and high flip angles on consecutive repetitions. Complex subtraction will be performed online, and acquisition and reconstruction adapted to allow for arbitrary cardiac phases; and 3. In Vivo Evaluation. The techniques will be studied in vivo. They hypothesize that coronary artery images formed from data obtained using a predefined cardiac phase known to have motion within some acceptable tolerance are less blurred than images obtained using a temporally fixed cardiac phase. To determine the cardiac phase suitable for high resolution imaging, lateral coronary artery motion will be measured on a vessel-specific basis at 40 Hz realtime. The methods will be studied in a pig model of coronary artery stenosis and compared with x-ray angiography.