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.