We propose to further develop real-time magnetic resonance imaging (MRI) techniques developed in the previous funding cycle toward high resolution MR angiographic imaging. The specific hypothesis is that compared to the current performance level of fluoroscopically-triggered elliptical-centric contrast-enhanced 3D MRA 30 percent improved spatial resolution and 40 percent improved signal can be obtained in imaging many vascular areas. Despite increased interest, contrast-enhanced MRA continues to be limited by spatial resolution. In the previous funding cycle we developed the technique of fluoroscopically-triggered contrast- enhanced MRA and showed that it can reliably provide high spatial resolution, venous-suppressed angiograms in virtually all arterial areas from the carotid bifurcation to the pelvis (with the exception of the coronary arteries). We have also theoretically determined the dependence of spatial resolution in 3D CE-MRA on MR acquisition parameters and contrast bolus characteristics. We wish to further apply this real-time MRI capability, clinical experience, and theoretical development to generate images with an improved level of resolution and signal. Specific projects to be studied are: 1. k-Space Signal Enhancement Methods. The general strategy for resolution improvement is to obtain increased signal level at the peripheral k-space views. This will be done by TR reduction, FOV reduction, prolongation of the contrast bolus, and selection of scan time. 2. Real-Time MRA Sequence Modification. In order to exploit the risetime of the arriving contrast bolus, a centric-in portion of the MRA acquisition will precede the standard elliptical centric- (out) sequence. The characteristics of the view order will be modified in real time in response to the instantaneously measured bolus transit time. The flip angle of the readout will be altered dynamically to match the time-dependent Ernst angle. 3. Embedded Fluoroscopic/3D MRA. Repetition intervals devoted to 2D fluoroscopic imaging will be inserted into the elliptical centric 3D MRA sequence. These will be sorted, reconstructed and displayed during the 3D acquisition allowing he operator to observe contrast behavior and intervene, such as by triggering another temporal frame or advancing the table.
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