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) (Formula presented.) 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 (Formula presented.) 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 (Formula presented.) shim setting to calibrate the relative (Formula presented.) maps yielding absolute (Formula presented.) 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 (Formula presented.) shims were used to qualitatively validate (Formula presented.) shim predictions. Results: The phantom validation revealed good agreement for (Formula presented.) 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 (Formula presented.) 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 (Formula presented.) 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 (Formula presented.) maps of the human body at UHF, which enables the investigation and development of 3D (Formula presented.) shimming and parallel transmission methods to further enhance body imaging at UHF.