Evaluation of an integrated variable flip angle protocol to estimate coil B1 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-1 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-1 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-1 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-1. For xenon-129, the B-1 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-1 maps showed good agreement with a separately acquired B-1 map of a phantom and were effectively used to correct coil-induced signal inhomogeneity. Conclusion: A simple, versatile, and effective VFA B-1 mapping method was implemented and evaluated. Inclusion of the B-1 mapping method in hyperpolarized imaging studies can enable more robust signal quantification.