Evaluation of an Integrated Variable Flip Angle Protocol to Estimate Coil B for Hyperpolarized MRI

Purpose: The purpose of this work is to validate a simple and versatile integrated variable flip angle (VFA) method for mapping B in hyperpolarized MRI, which can be used to correct signal variations due to coil inhomogeneity.

Theory And Methods: Simulations were run to assess performance of the VFA B mapping method compared to the currently used constant flip angle (CFA) approach. Simulation results were used to inform the design of VFA sequences, validated in four volunteers for hyperpolarized xenon-129 imaging of the lungs and another four volunteers for hyperpolarized carbon-13 imaging of the human brain. B maps obtained were used to correct transmit and receive inhomogeneity in the images.

Results: Simulations showed improved performance of the VFA approach over the CFA approach with reduced sensitivity to T. For xenon-129, the B maps accurately reflected the variation of signal depolarization, but in some cases could not be used to correct for coil receive inhomogeneity due to a lack of transmit-receive reciprocity resulting from suboptimal coil positioning. For carbon-13, the B maps showed good agreement with a separately acquired B map of a phantom and were effectively used to correct coil-induced signal inhomogeneity.

Conclusion: A simple, versatile, and effective VFA B mapping method was implemented and evaluated. Inclusion of the B mapping method in hyperpolarized imaging studies can enable more robust signal quantification.