Evaluation of an integrated variable flip angle protocol to estimate coil B₁ for hyperpolarized MRI

Area of Science:

• Magnetic Resonance Imaging
• Medical Physics
• Hyperpolarized Contrast Agents

Background:

• Accurate B1 mapping is crucial for quantitative hyperpolarized MRI.
• Current methods like constant flip angle (CFA) have limitations in accuracy and sensitivity.
• Signal variations due to coil inhomogeneity affect image quality and quantification.

Purpose of the Study:

• To validate a simple, versatile, and integrated variable flip angle (VFA) method for B1 mapping.
• To assess the VFA method's performance against the CFA approach.
• To enable correction of signal variations caused by coil inhomogeneity in hyperpolarized MRI.

Main Methods:

• Simulations were conducted to compare VFA and CFA B1 mapping methods.
• VFA sequences were designed based on simulation results.
• The VFA method was validated in human volunteers for hyperpolarized Xenon-129 (lungs) and Carbon-13 (brain) imaging.
• B1 maps were used to correct for transmit and receive inhomogeneity.

Main Results:

• Simulations demonstrated superior performance of VFA over CFA, with reduced T1 sensitivity.
• For Xenon-129 imaging, B1 maps reflected signal depolarization but could not always correct for receive inhomogeneity due to coil positioning.
• For Carbon-13 imaging, VFA B1 maps agreed well with phantom data and effectively corrected coil-induced signal inhomogeneity.

Conclusions:

• A simple, versatile, and effective VFA B1 mapping method has been successfully implemented and evaluated.
• Incorporating this VFA B1 mapping method enhances signal quantification robustness in hyperpolarized imaging studies.
• The method shows promise for improving quantitative accuracy in diverse hyperpolarized MRI applications.