Evaluation of an interleaved acquisition scheme for improved robustness of channel-wise relative B₁ ⁺ mapping at 7 T

The purpose was to evaluate, whether an interleaved acquisition scheme for a fast relative $B_1^{+}$ mapping method at 7 T reduces the likelihood of errors from exceeding the linear flip angle (FA) regime compared to a conventional sequential acquisition.

Simulations of a channel-wise relative $B_1^{+}$ mapping sequence were performed for sequential and interleaved acquisition schemes at different reference voltages (≙ different FAs). The simulations were performed for a phantom, the heart and the prostate and were based on 7 T ground-truth (GT) $B_1^{+}$ data acquired with an actual FA imaging sequence (phantom) or obtained from electromagnetic field simulations (heart/prostate). Acquisition schemes were evaluated based on their signal linearity by calculating a normalized mean FA error between simulated signal intensities and GT $B_1^{+}$ data. Additionally, validation measurements of relative $B_1^{+}$ maps with the sequential and interleaved acquisition schemes were acquired for the phantom.

Validation measurements showed a good agreement with the simulation results for both acquisition schemes and displayed stronger deviations to the GT $B_1^{+}$ data for the sequential scheme. The quantitative evaluation yielded higher FA errors for the sequential acquisition scheme for all three regions and all simulated reference voltages. At the same level of error, mean signals were higher for the interleaved acquisition scheme in all cases. Differences between interleaved and sequential acquisition schemes were most pronounced in the steady-state.

An interleaved acquisition of channel-wise relative $B_1^{+}$ maps extends the range of the linear FA regime, reducing the likelihood of errors and increasing the robustness of the approach.