Fast B₁⁺ mapping using three consecutive RF pulses and balanced gradients for improved bSSFP imaging

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biophysics

Background:

• Balanced steady-state free precession (bSSFP) is a widely used MRI technique.
• B1+ field inhomogeneity can significantly degrade image quality and quantitative accuracy in bSSFP.
• Accurate B1+ mapping is crucial for effective B1+ inhomogeneity compensation.

Purpose of the Study:

• To develop and validate a novel method for B1+ mapping during the transient phase of bSSFP imaging.
• To enable subsequent B1+ inhomogeneity compensation without extending scan acquisition time.

Main Methods:

• Acquisition of two images with different flip angles during the bSSFP transient phase using a single-shot spiral technique.
• Estimation of B1+ field using magnitude images under assumptions of slow spatial variation and negligible T1/T2 relaxation.
• Assessment of estimation errors using Bloch simulations and experimental validation with phantom and in vivo data.

Main Results:

• The proposed B1+ mapping method demonstrated reduced sensitivity to T1 relaxation.
• Optimal flip angle (α) of approximately 60° minimized estimation error.
• Reduced B1+ induced intensity variations were observed in phantom experiments.
• Estimated B1+ maps were comparable to conventional spin-echo Double Angle Method (DAM) in phantom and in vivo studies.

Conclusions:

• A B1+ mapping technique was successfully developed during the bSSFP transient phase.
• The method allows for subsequent B1+ inhomogeneity compensation.
• The technique can be implemented as a prescan for B1+ mapping or shimming without additional scan time.