High dynamic range B 1 + mapping for the evaluation of parallel transmit arrays.

Purpose: Demonstration of a high dynamic-range and high SNR method for acquiring absolute $B_1^{+}$ maps from a combination of gradient echo and actual-flip-angle measurements that is especially useful during the construction of parallel-transmit arrays.

Methods: Low flip angle gradient echo images, acquired when transmitting with each channel individually, are used to compute relative $B_1^{+}$ maps. Instead of computing these in a conventional manner, the equivalence of the problem to the ESPIRiT parallel image reconstruction method is used to compute $B_1^{+}$ maps with a higher SNR. Absolute maps are generated by calibration against a single actual flip-angle acquisition when transmitting on all channels simultaneously.

Results: Depending on the number of receiver channels and the location of the receive elements with respect to the subject being investigated, moderate to high gains in the SNR of the acquired $B_1^{+}$ maps can be achieved.

Conclusions: The proposed method is especially suited for the acquisition of $B_1^{+}$ maps during the construction of transceiver arrays. Compared to the original method, maps with higher SNR can be computed without the need for additional measurements, and maps can also be generated using previously acquired data. Furthermore, easy adoption and fast estimation of receiver channels is possible because of existing highly optimized open-source implementations of ESPIRiT, such as in the BART toolbox.