Jürgen Finsterbusch1, Martin G Busch, Peder E Z Larson. 1. Department of Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Hamburg, Germany; Neuroimage Nord, University Medical Centers Hamburg-Kiel-Lübeck, Hamburg, Germany.
Abstract
PURPOSE: Segmented 2D-selective radiofrequency excitations can be used to acquire irregularly shaped target regions, e.g., in single-voxel MR spectroscopy, without involving excessive radiofrequency pulse durations. However, segments covering only outer k-space regions nominally use reduced B1 amplitudes (i.e., smaller flip angles) and yield lower signal contributions, which decreases the efficiency of the measurement. The purpose of this study was to show that applying the full flip angle for all segments and scaling down the acquired signal appropriately (signal scaling) retains the desired signal amplitude but reduces the noise level accordingly and, thus, increases the signal-to-noise ratio. METHODS: The principles and improvements of signal scaling were demonstrated with MR imaging and spectroscopy experiments at 3 T for a single-line segmentation of a blipped-planar trajectory. RESULTS: The observed signal-to-noise ration gain depended on the 2D-selective radiofrequency excitation's resolution, field-of-excitation, and its excitation profile and was between 40 and 500% for typical acquisition parameters. CONCLUSION: Signal scaling can further improve the performance of measurements with segmented 2D-selective radiofrequency excitations, e.g., for MR spectroscopy of anatomically defined voxels.
PURPOSE: Segmented 2D-selective radiofrequency excitations can be used to acquire irregularly shaped target regions, e.g., in single-voxel MR spectroscopy, without involving excessive radiofrequency pulse durations. However, segments covering only outer k-space regions nominally use reduced B1 amplitudes (i.e., smaller flip angles) and yield lower signal contributions, which decreases the efficiency of the measurement. The purpose of this study was to show that applying the full flip angle for all segments and scaling down the acquired signal appropriately (signal scaling) retains the desired signal amplitude but reduces the noise level accordingly and, thus, increases the signal-to-noise ratio. METHODS: The principles and improvements of signal scaling were demonstrated with MR imaging and spectroscopy experiments at 3 T for a single-line segmentation of a blipped-planar trajectory. RESULTS: The observed signal-to-noise ration gain depended on the 2D-selective radiofrequency excitation's resolution, field-of-excitation, and its excitation profile and was between 40 and 500% for typical acquisition parameters. CONCLUSION: Signal scaling can further improve the performance of measurements with segmented 2D-selective radiofrequency excitations, e.g., for MR spectroscopy of anatomically defined voxels.