A Theoretical and Experimental Comparison of Different Techniques for B₁ Mapping at Very High Fields

With the increasing use of ultrahigh-field MR with multiple transmit channels, mapping of the B₁(+) field has become a critical factor in many studies, leading to the publication of a large number of sequences for the measurement of the flip angle in recent years. In this article, the accuracy, precision and practicability of some of the most prominent of these techniques are investigated both theoretically, using error propagation computations and Monte-Carlo simulations, and experimentally for different settings. For an exemplary experiment, which is typical for high-field applications, the flip angle uncertainty is calculated and measured for two- and three-dimensional acquisitions for techniques based on both magnitude and phase data. Simulated and measured results show good agreement. An experimental assessment of T₁ and B₀ dependence yields weak variations with these parameters for only a few of the sequences. Measurements on human scanners show crucial influences of specific absorption rate limitations, especially at ultrahigh field.