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Knowledge of the transmission field ( B 1    + ) of radio-frequency coils is crucial for high field ( B 0   = 3.0 T) and ultrahigh field ( B 0  ≥7.0 T) magnetic resonance applications to overcome constraints dictated by electrodynamics in the short wavelength regime with the ultimate goal to improve the image quality. For this purpose  B 1    +  mapping methods are used, which are commonly magnitude-based. In this study an analysis of five phase-based methods for three-dimensional mapping of the  B 1    +  field is presented. The five methods are implemented in a 3D gradient-echo technique. Each method makes use of different RF-pulses (composite or off-resonance pulses) to encode the effective intensity of the  B 1    +  field into the phase of the magnetization. The different RF-pulses result in different trajectories of the magnetization, different use of the transverse magnetization and different sensitivities to  B 1    +  inhomogeneities and frequency offsets, as demonstrated by numerical simulations. The characterization of the five methods also includes phantom experiments and  in vivo  studies of the human brain at 3.0 T and at 7.0 T. It is shown how the characteristics of each method affect the quality of the  B 1    +  maps. Implications for  in vivo B 1    +  mapping at 3.0 T and 7.0 T are discussed.

Characterization of Phase-Based Methods Used for Transmission Field Uniformity Mapping: A Magnetic Resonance Study at 3.0 T and 7.0 T