Rapid radiofrequency field mapping in vivo using single‐shot STEAM MRI |
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| Authors: | Gunther Helms Jürgen Finsterbusch Nikolaus Weiskopf Peter Dechent |
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| Institution: | 1. MR‐Research in Neurology and Psychiatry, Faculty of Medicine, University of G?ttingen, G?ttingen, Germany;2. Department of Systems Neuroscience, University Medical Center Hamburg‐Eppendorf, Hamburg, Germany;3. Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College, London, UK |
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| Abstract: | Higher field strengths entail less homogeneous RF fields. This may influence quantitative MRI and MRS. A method for rapidly mapping the RF field in the human head with minimal distortion was developed on the basis of a single‐shot stimulated echo acquisition mode (STEAM) sequence. The flip angle of the second RF pulse in the STEAM preparation was set to 60° and 100° instead of 90°, inducing a flip angle‐dependent signal change. A quadratic approximation of this trigonometric signal dependence together with a calibration accounting for slice excitation‐related bias allowed for directly determining the RF field from the two measurements only. RF maps down to the level of the medulla could be obtained in less than 1 min and registered to anatomical volumes by means of the T2‐weighted STEAM images. Flip angles between 75% and 125% of the nominal value were measured in line with other methods. Magn Reson Med 60:739–743, 2008. © 2008 Wiley‐Liss, Inc. |
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| Keywords: | radiofrequency mapping stimulated echo human brain |
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