High dynamic range B1+ mapping for the evaluation of parallel transmit arrays

• Published in: Magnetic resonance in medicine Vol. 93; no. 3; pp. 1298 - 1305
• Main Authors: Felder, Jörg, Zimmermann, Markus, Shah, N. Jon
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 01.03.2025; John Wiley and Sons Inc
• Subjects: Arrays; B1+$$ {\mathrm{B}}_1^{+} $$ mapping; Channels; Data acquisition; ESPIRiT; high SNR; Image acquisition; Image processing; Image reconstruction; Image transmission; Imaging Methodology; Technical Note; Transmission; ultrahigh‐field
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.30349

Purpose Demonstration of a high dynamic‐range and high SNR method for acquiring absolute B1+$$ {\mathrm{B}}_1^{+} $$ 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 B1+$$ {\mathrm{B}}_1^{+} $$ 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 B1+$$ {\mathrm{B}}_1^{+} $$ 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 B1+$$ {\mathrm{B}}_1^{+} $$ maps can be achieved. Conclusions The proposed method is especially suited for the acquisition of B1+$$ {\mathrm{B}}_1^{+} $$ 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.