New phase-based B1 mapping method using two-dimensional spin-echo imaging with hyperbolic secant pulses.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Pulse Sequence Design

Background:

• Accurate B1 field mapping is crucial for quantitative MRI.
• Existing B1 mapping methods can be complex or require sequence modifications.

Purpose of the Study:

• To introduce a new phase-based B1-mapping method using hyperbolic secant (HS) pulses.
• To leverage phase information from conventional 2D spin-echo imaging for B1 estimation.

Main Methods:

• Employed HS pulses for π/2 excitation and π refocusing in spin-echo imaging.
• Optimized pulse length ratio (2:1) for phase independence from offset frequency.
• Utilized dual-echo acquisition with opposite frequency-sweep directions to cancel unwanted phase variations.
• Validated the method with phantom and in vivo experiments using surface and volume coils.

Main Results:

• B1 maps generated by the proposed method showed strong agreement with those from established techniques.
• The method demonstrated accurate B1 field strength measurements in both phantom and in vivo studies.

Conclusions:

• The proposed B1-mapping method is easily integrated into standard spin-echo sequences without modification.
• It exhibits insensitivity to B0 inhomogeneity and chemical shift.
• The technique is robust across a practical range of B1 field strengths, enhancing its clinical utility.