title: 7 T body MRI: B1 shimming with simultaneous SAR reduction.
abstract: The high frequency of the radiofrequency (RF) fields used in high field magnetic resonance imaging (MRI) results in electromagnetic field variations that can cause local regions to have a large specific absorption rate (SAR) and/or a low excitation. In this study, we evaluated the use of a B1 shimming technique which can simultaneously improve the B+1 homogeneity and reduce the SAR for whole body imaging at 7 T. Optimizations for four individual anatomies showed a reduction up to 74% of the peak SAR values with respect to a quadrature excitation and a simultaneous improvement of the B+1 homogeneity varying between 39 and 75% for different optimization parameters. The average SAR was reduced with approximately 50% for all optimizations. The optimized phase and amplitude settings from an elliptical phantom model were applied to four realistic human anatomy models to evaluate whether a generic application without prior knowledge of the detailed human anatomy is possible. This resulted in an average improvement of the B+1 homogeneity of 37% and an average reduction of the maximum and average SAR of 50 and 55%, respectively. It can be concluded that this generic method can be used as a simple method to improve the prospects of 7 T body imaging.
authors.affiliation: Department of Radiotherapy, University Medical Center Utrecht, PO Box 85500, HP Q.00.118, 3508 GA Utrecht, The Netherlands. B.vandenBergen@UMCUtrecht.nl
authors.forename: Bob
authors.initials: B
authors.lastname: van den Bergen
authors.forename: Cornelis A T
authors.initials: CA
authors.lastname: Van den Berg
authors.forename: Lambertus W
authors.initials: LW
authors.lastname: Bartels
authors.forename: Jan J W
authors.initials: JJ
authors.lastname: Lagendijk
mesh_terms: D001822:Body Burden; D003198:Computer Simulation; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008954:Models, Biological; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods; D014915:Whole-Body Counting