Accurate MRF-Based 3D Multi-Channel B₁ ⁺ Mapping in the Human Body at 7 T.

Area of Science:

• Medical Imaging
• Physics

Background:

• Ultra-high field (UHF) magnetic resonance imaging (MRI) at 7 Tesla (T) offers enhanced signal-to-noise ratio but presents significant B1+ field mapping challenges in the abdomen.
• These challenges include large field of view (FOV) requirements, respiratory motion artifacts, and limited radiofrequency (RF) power, restricting flip angles (FAs) crucial for classical mapping methods.

Purpose of the Study:

• To develop and validate a novel 3D multi-transmit channel B1+ mapping technique for the human abdomen at 7T.
• To address limitations of existing methods, particularly concerning low RF power and motion robustness.

Main Methods:

• A hybrid approach combining stack-of-stars acquisition for motion robustness with low flip angle gradient recalled echo (GRE) images to derive transmit (Tx) channel-wise B1+ information.
• Utilized two complementary phase shims for acquiring two Tx channel-combined MRF-based B1+ maps (B1-MRF) to achieve absolute B1+ values across the entire FOV.
• Validated the 3D hybrid method against a 2D reference using phantom and in vivo free-breathing scans in subjects with diverse body mass indices (BMIs).

Main Results:

• The 3D hybrid method demonstrated strong agreement with the 2D reference across phantoms and in vivo scans.
• Showed improved performance in challenging regions characterized by flow, low flip angles, or low signal-to-noise ratio compared to the 2D implementation.
• The technique successfully acquired absolute B1+ information across the entire FOV, even with limited RF power.

Conclusions:

• The proposed 3D multi-transmit channel MRF-based B1+ mapping technique is accurate and robust for abdominal imaging at 7T.
• This method offers higher accuracy and detail than existing techniques, making it suitable for validating faster imaging methods, electromagnetic simulations, and creating B1+ map libraries for AI applications.