MR angiography visualizes blood either through inflow effects (time-of-flight)or differences in phase between the flowing and stationary spins (phase contrast). RF excitation pulses are generally selective in only one dimension (i.e., the slice direction) and as a result may affect in-plane spins that will eventually flow into the area of interest. This can lead to increased saturation effects which can reduce the signal available during data acquisition. We have begun to utilize RF pulses that have a restricted excitation profile in not one but two directions. The hypothesis is that the use of 2D selective RF pulses will provide both reduced saturation effects and the ability to image a smaller field of view (FOV), and will thereby result in greater inflow signals and reduced scan time. Methods and Results We investigated the performance of the two dimensional spatially selective pulses by imaging the renal vessels of normal volunteers. Greater inflow signal in the aorta and better delineation of the renal vessel branches was evident . The increase in inflow signal was verified by imaging a stationary phantom placed alongside tubes containing flowing liquid. Measurements from these experiments indicate that for peak flow velocities of 16 cm/s, the average improvement in signal due to the increased inflow effect is approximately 75%, that is, almost doubled. Discussion These experiments demonstrate the ability of 2D spatially selective RF pulses to improve the quality of MR angiography by increasing flow related enhancement. To date, these pulses have been incorporated into 2D and 3D time-of-flight and phase contrast imaging sequences with very encouraging results. Further work will help to identify studies and sequences that will most benefit from the application of this technology.
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