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

Area of Science:

• Medical Imaging
• Magnetic Resonance Imaging
• Physics

Background:

• Accurate mapping of the transmit magnetic field (B1) is crucial for quantitative Magnetic Resonance Imaging (MRI), especially at ultra-high fields (UHF) like 7 Tesla (7T).
• Existing B1 mapping methods face challenges with accuracy, precision, and dynamic range, particularly at low flip angles (FAs).

Purpose of the Study:

• To develop and validate a novel MR fingerprinting (MRF) based method (B1-MRF) for accurate and precise B1 mapping at 7T.
• To assess the performance of B1-MRF across a wide dynamic range and particularly at low flip angles.

Main Methods:

• A FLASH-based MRF sequence (B1-MRF) was designed for high B1 sensitivity.
• Phantom and in vivo abdominal imaging at 7T were conducted.
• Results were compared against established methods: preparation-based excitation (PEX), saturated TurboFLASH (satTFL), actual flip angle imaging (AFI), and Bloch-Siegert shift (BSS).

Main Results:

• The B1-MRF sequence demonstrated high B1 sensitivity with low T1 sensitivity.
• Phantom studies showed excellent agreement between B1-MRF and PEX for T1 values from 204-1691 ms across FAs of 0°-70°.
• B1-MRF achieved more than double the dynamic range compared to reference methods.
• In vivo liver imaging revealed consistent B1-MRF results across varying Body Mass Index (BMI) subjects, outperforming satTFL and RPE-AFI in low B1 regions for higher BMI individuals.

Conclusions:

• B1-MRF provides accurate and precise B1 maps over an extensive range of FAs, significantly outperforming existing techniques for FAs < 60°.
• The enhanced sensitivity of B1-MRF at low FAs is highly beneficial for applications demanding precise B1 estimates.
• This method holds potential to advance ultra-high field (UHF) body imaging at 7T and beyond.