Phase-sensitive B₁ mapping: Effects of relaxation and RF spoiling

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Quantitative MRI techniques
• Biophysics

Background:

• Accurate B1 field mapping is crucial for quantitative MRI.
• Existing B1 mapping techniques can be sensitive to radiofrequency (RF) spoiling imperfections and magnetization relaxation.
• These factors can lead to inaccuracies in B1 field estimation.

Purpose of the Study:

• To develop a novel phase-based B1 mapping technique.
• To account for the effects of imperfect RF spoiling and magnetization relaxation (T1 and T2*) in B1 mapping.
• To achieve fast and robust B1 field quantification.

Main Methods:

• A multi-gradient-echo sequence with two successive orthogonal RF excitation pulses was employed.
• A theoretical expression was derived to relate MR signal phase to B1 field and B0-related frequency shift.
• The method incorporates imperfections in RF spoiling and T1/T2* relaxation into the signal model.

Main Results:

• Computer simulations and experimental results confirmed the significant impact of RF spoiling imperfections on B1 mapping accuracy.
• The proposed technique demonstrated high accuracy by accounting for RF spoiling, relaxation, and frequency shift effects.
• The method was successfully tested on phantoms and a healthy volunteer.

Conclusions:

• A new, accurate, and robust phase-based B1 mapping technique has been developed and validated.
• Accounting for imperfect RF spoiling allows for shorter repetition times (TRs), significantly reducing scan time.
• This technique offers a faster and more reliable approach to B1 field mapping in MRI.