2.2 Cardiac MRI and Image Analysis
Cardiac MRI examinations were performed using a 3T MRI scanner (MAGNETOM Skyra, Siemens Healthineers, Erlangen, Germany) with a phased array 18-channel body matrix coil and 12-channel of the 32-channel spine matrix coil. VENC-MRI was a sequence of cardiac MRI protocol. A retrospective electrocardiography (ECG) gated, two-dimensional flow-sensitive phase-contrast gradient-echo sequence was performed for VENC-MRI perpendicular to the LAA ostium. VENC-MRI parameters were as follows: TR/TE, 4.5 msec/2.9 msec; flip angle, 15°; 30 frames per heartbeat; encoding velocity set, 200 cm/sec; PAT factor, 2. In the analysis of LAA hemodynamics, VENC-MRI data were analyzed by two cardiovascular image experts (S.H.H with 10 years of experience in cardiovascular imaging, and Y.H.O with 23 years of experience in cardiovascular imaging) using a commercial software workstation (Terarecon iNtuition; TeraRecon, Foster City, CA, USA). Both reviewers were blinded to the patients’ clinical information. The reviewers traced and determined the outer contours of the LAA ostium and descending aorta on VENC-MRI in consensus to evaluate both the LAA and aortic blood flow in a cardiac cycle. Based on the VENC-MRI data, the development of blood flux peak from the LAA to the LA chamber was defined as a LAA emptying. The maximum blood flux of LAA emptying was defined as the LAA emptying flux (in ml/s).