High Dynamic Range Turbo-FLASH B1 Mapping for Characterizing Multi-Transmit Multi-Receive RF Arrays, Validated at 7 T...

Transmit (B₁ ⁺) and receive (B₁ ^-) sensitivities of parallel-transmit/receive RF array are a prerequisite for modern ultra-high-field MRI. Their accurate mapping remains challenging for coils with high transmit dynamic range, particularly in regions with low combined-mode B₁ ⁺ magnitude, where standard reconstruction is impaired by noise. This work presents a new B₁ ⁺ and B₁ ^- model-based fitting approach for interferometric presaturated TurboFLASH (satTFL) data. With an optimized acquisition scheme, this method improves B₁ ⁺/B₁ ^- mapping accuracy.

Voxel-wise B₁ ⁺ and B₁ ^- are expressed as the product of a complex unit-norm vector (phasor) and a scalar (efficiency). Phasors are estimated through singular value decomposition of satTFL signals, and efficiencies are recovered with fitting to the signal equation. This method was evaluated in phantom simulations for 7 T and 11.7 T 8Tx/32Rx RF arrays and compared with standard approaches. As it supports a variable number of pre-saturated scans, different interferometric schemes were optimized for B₁ ⁺ accuracy. Finally, schemes yielding acceptable B₁ ⁺ error with minimal acquisition time were tested in vitro at 7 T and 11.7 T and in vivo at 7 T.

The proposed method provided lower median B₁ ⁺ and B₁ ^- errors than conventional satTFL methods and fewer outliers; the number of voxels with error > 30% is reduced from several dozens to zero. In vitro results were consistent with the simulations, and in vivo maps demonstrated good quality.

The proposed method improves B₁ ⁺ and B₁ ^- mapping robustness to noise and enables more flexible satTFL acquisition scheme, allowing better trade-offs between accuracy and scan time.