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

Abstract

Purpose: To introduce and investigate a method for free-breathing three-dimensional (3D) $B_1^{+}$ 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 $B_1^{+}$ 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 $B_1^{+}$ shim setting to calibrate the relative $B_1^{+}$ maps yielding absolute $B_1^{+}$ 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 $B_1^{+}$ shims were used to qualitatively validate $B_1^{+}$ shim predictions.

Results: The phantom validation revealed good agreement for $B_1^{+}$ 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 $B_1^{+}$ 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 $B_1^{+}$ 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 $B_1^{+}$ maps of the human body at UHF, which enables the investigation and development of 3D $B_1^{+}$ shimming and parallel transmission methods to further enhance body imaging at UHF.

Keywords: $B_1^{+}$ mapping; 7 Tesla; actual flip angle imaging; body imaging; radial phase encoding; ultrahigh field.