RF pulse optimization for Bloch-Siegert B ₁⁺ mapping.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency Pulse Design
• Biomedical Engineering

Background:

• The Bloch-Siegert (B-S) method offers fast and accurate B₁⁺ mapping but suffers from high specific absorption rate (SAR) and long echo times (TE).
• These limitations hinder scan times and cause signal loss due to B₀ inhomogeneity, particularly at high magnetic fields (e.g., 7 T).

Purpose of the Study:

• To develop an optimized Bloch-Siegert radiofrequency pulse for B₁⁺ mapping.
• To maximize B₁⁺ measurement sensitivity while considering specific absorption rate (SAR) and T₂ constraints.

Main Methods:

• A novel method for optimizing the Bloch-Siegert radiofrequency pulse was designed.
• The optimized pulse aims to enhance B₁⁺ measurement sensitivity for a given SAR and T₂.
• Performance was evaluated using phantom and in vivo brain imaging at 7 T.

Main Results:

• A 4-ms optimized pulse demonstrated 35% lower SAR compared to a conventional 6-ms Fermi pulse.
• The optimized pulse achieved a 22% improvement in B₁⁺ map angle-to-noise ratio.
• Validation confirmed the pulse's effectiveness in phantom and in vivo 7 T brain imaging.

Conclusions:

• Optimized Bloch-Siegert pulses offer a significant improvement over conventional methods for B₁⁺ mapping.
• Reduced SAR and enhanced sensitivity lead to more efficient and accurate MRI, especially at high fields.
• This method holds promise for improving clinical MRI applications.