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3D Free‐breathing multichannel absolute B1+ Mapping in the human body at 7T

Dietrich, Sebastian; Aigner, Christoph S.; Kolbitsch, Christoph; Mayer, Johannes; Ludwig, Juliane; Schmidt, Simon; Schaeffter, Tobias; Schmitter, Sebastian

FG Medizintechnik

Purpose To introduce and investigate a method for free‐breathing three‐dimensional (3D) B1+ mapping of the human body at ultrahigh field (UHF), which can be used to generate homogenous flip angle (FA) distributions in the human body at UHF. Methods A 3D relative B1+ mapping sequence with a radial phase‐encoding (RPE) k‐space trajectory was developed and applied in 11 healthy subjects at 7T. An RPE‐based actual flip angle mapping method was applied with a dedicated B1+ shim setting to calibrate the relative B1+ maps yielding absolute B1+ maps of the individual transmit channels. The method was evaluated in a motion phantom and by multidimensional in vivo measurements. Additionally, 3D gradient echo scans with and without static phase‐only B1+ shims were used to qualitatively validate B1+ shim predictions. Results The phantom validation revealed good agreement for B1+ maps between dynamic measurement and static reference acquisition. The proposed 3D method was successfully validated in vivo by comparing magnitude and phase distributions with a 2D Cartesian reference. 3D B1+ maps free from visible motion artifacts were successfully acquired for 11 subjects with body mass indexes ranging from 19 kg/m2 to 34 kg/m2. 3D respiration‐resolved absolute B1+ maps indicated FA differences between inhalation and exhalation up to 15% for one channel and up to 24% for combined channels for shallow breathing. Conclusion The proposed method provides respiration‐resolved absolute 3D B1+ maps of the human body at UHF, which enables the investigation and development of 3D B1+ shimming and parallel transmission methods to further enhance body imaging at UHF.