B1-MRF: Large dynamic range MRF-based absolute B 1 + mapping in the human body at 7T

This study aims to map the transmit magnetic field ( $B_1^{+}$ ) in the human body at 7T using MR fingerprinting (MRF), with a focus on achieving high accuracy and precision across a large dynamic range, particularly at low flip angles (FAs).

A FLASH-based MRF sequence (B1-MRF) with high $B_1^{+}$ sensitivity was developed. Phantom and in vivo abdominal imaging were performed at 7T, and the results were compared with established reference methods, including a slow but precise preparation-based method (PEX), saturated TurboFLASH (satTFL), actual flip angle imaging (AFI) and Bloch-Siegert shift (BSS).

The MRF signal curve was highly sensitive to $B_1^{+}$ , while T₁ sensitivity was comparatively low. The phantom experiment showed good agreement of $B_1^{+}$ to PEX for a T₁ range of 204-1691 ms evaluated at FAs from 0° to 70°. Compared to the references, a dynamic range increase larger than a factor of two was determined experimentally. In vivo liver scans showed a strong correlation between B1-MRF, satTFL, and RPE-AFI in a low body mass index (BMI) subject (18.1 kg/m²). However, in larger BMI subjects (≥25.5 kg/m²), inconsistencies were observed in low $B_1^{+}$ regions for satTFL and RPE-AFI, while B1-MRF still provided consistent results in these regions.

B1-MRF provides accurate and precise $B_1^{+}$ maps over a wide range of FAs, surpassing the capabilities of existing methods in the FA range < 60°. Its enhanced sensitivity at low FAs is advantageous for various applications requiring precise $B_1^{+}$ estimates, potentially advancing the frontiers of ultra-high field (UHF) body imaging at 7T and beyond.