The second major impediment to coronary MRA is respiratory motion. Early approaches to suppressing respiratory motion involved the use of breath-hold techniques. Two-dimensional (2D) breath-hold coronary MRA relied on acquiring contiguous images, with the goal of surveying the proximal segments of the coronary arteries during serial breath-holds. More recently, three-dimensional (3D) breath-hold techniques for coronary MRA have also been implemented [6-10]. Breath-hold approaches offer the advantage of rapid imaging and are technically easy to implement in compliant subjects. For coronary MRA techniques that utilize the first-pass enhancement of intravenously injected extracellular contrast agents, breath-holding is a requirement at the present time. However, breath-holding strategies have several limitations. Some patients may have difficulty sustaining adequate breath-holds, particularly when the duration exceeds a few seconds. Additionally, it has been shown that during a sustained breath-hold there is cranial diaphragmatic drift , which may be sub stantial in many cases (~1cm). Among serial breath-holds, the diaphragmatic and cardiac positions frequently vary by up to 1 cm, resulting in registration errors [6,12]. Misregistration results in apparent gaps between the segments of the visualized coronary arteries, which could be misinterpreted as signal voids from coronary stenoses. Finally, the use of signal enhancement techniques, such as signal averaging or fold-over suppression is significantly restricted by the duration of the applicable breath-hold duration. Using breath-holding techniques, the spatial resolution of the images is also governed by the patient's ability to hold his/her breath. Thus, while breath-hold strategies are often successful with motivated volunteers, their applicability to the broad range of patients with cardiovascular disease is more limited.
To overcome limitations associated with breath-holding, different methods such as MR navigators  have been developed to allow for free-breathing coronary MRA. With vertical positioning of the navigator at the dome of the right hemidiaphragm (lung-liver interface), the diaphragmatic craniocaudal displacement can be monitored. These data can be used to gate coronary MRA acquisitions. The gating process can be either prospective (i.e. before data acquisition) or retrospective (i.e. following data acquisition, but before image reconstruction). Although navigator approaches greatly improve patient comfort and do not require significant subject motivation, their use prolongs the scan duration since coronary MRA data are collected during 50% of the RR intervals on average . To overcome problems associated with narrow gating windows and prolonged scans, coronary MRA with prospective navigator correction has been implemented and has been shown to maintain or improve image quality both for 2D and 3D approaches to coronary MRA [15-17], while scanning time can be shortened. However, it is of utmost importance that the navigator is positioned in close temporal proximity to the imaging part of the sequence . Typical examination times with free-breathing 3D real-time navigator approaches are ~7min.
Currently, inversion-recovery techniques seem to be emerging as the method of choice for contrast-enhanced coronary MRA [19-22]. However, the inversion-pre-pulse precedes the navigator thereby reducing the magnetization at the location of the navigator, which may adversely affect navigator performance. Therefore, countermeasures have been proposed  and successfully applied .
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