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Whole-Heart Coronary MRA

Background MRI was first reported to visualize the ostia of the coronary arteries in the late 1980’s (1, 2). Tremendous progress has been made in coronary MRA ever since. One of the milestones was the acquisition of k-space data in a segmented mode (3, 4), allowing collection of an entire data set within multiple heartbeats. Early coronary MRA studies used two-dimensional (2D) technique to acquire single-slice images within a single breath hold (4-6). Three-dimensional (3D) acquisition has now become the major method of choice not only because of its higher signal-to-noise ratio (SNR) than 2D methods, more importantly, the acquisition of contiguous thin slices improves the delineation of torturous coronary arteries and facilitates post-processing of images (7-9). The development of navigator-echo methods a decade ago provides a way to monitor the diaphragm motion in real-time. Respiratory motion of the heart was correlated to that of the diaphragm with a correction factor for respiratory gating (10-12). Imaging time can be extended from previous one breath-hold to several minutes with free breathing. Higher spatial resolution and/or slab coverage can be achieved as a result. A recent multicenter study of 109 patients using navigator-echo gating method showed the sensitivity, specificity, and accuracy for patients with disease of the left main coronary artery or three-vessel disease to be 100%, 85%, and 87%, respectively (13). In these studies, a volume-targeted scan was usually prescribed to cover each of the major coronary arteries. Due to the intrinsically tortuous course and the epicardial situation of major coronary arteries, it has always been of interest to scan the whole heart to depict the entire coronary artery tree. An early study collected multiple overlapped 3D slabs to cover the whole heart using retrospective respiratory gating (14). Long segments of all three major coronary arteries were reported. Thick slab coverage (40 slices with 2 mm thickness) was also achieved within a single breath-hold by using a segmented echo-planar imaging (EPI) method. An extravascular contrast agent was administered to boost the blood signal and imaging contrast (15). However, EPI techniques suffer from low SNR and spatial resolution. Recently, blood-pool contrast agents have been developed to allow whole-heart coverage of navigator-echo guided coronary MRA using conventional gradient-echo sequence (16). However, these agents have not yet been approved for clinical applications. With the recent improvement in gradient systems, steady-state free precession (SSFP) imaging techniques have been extensively used in cardiac MRI (17, 18). The increased SNR of SSFP over conventional gradient-echo sequences makes it feasible to image with higher spatial resolution or imaging speed. Recent developments in parallel imaging techniques accelerated data acquisition by reconstructing images from undersampled data sets (19, 20). With advanced design of data acquisition and image reconstruction schemes, parallel imaging techniques can be used to improve the spatial resolution and/or reduce the imaging time without inducing imaging artifacts. Advances of these techniques make it possible to collect the full whole heart data set within a reasonable imaging time (around 10 minutes). A recent study performed whole-heart