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

Abstract

AbstractPurposeThis study aims to map the transmit magnetic field () 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).MethodsA FLASH‐based MRF sequence (B1‐MRF) with high 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).ResultsThe MRF signal curve was highly sensitive to , while T1 sensitivity was comparatively low. The phantom experiment showed good agreement of 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 regions for satTFL and RPE‐AFI, while B1‐MRF still provided consistent results in these regions.ConclusionB1‐MRF provides accurate and precise 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 estimates, potentially advancing the frontiers of ultra‐high field (UHF) body imaging at 7T and beyond.