Robust nonadiabatic T preparation using universal parallel-transmit k -point pulses for 3D FLAIR imaging at 7 T.

PURPOSE

The fluid attenuated inversion recovery sequence is a pillar technique to detect brain lesions in MRI. At ultrahigh field, the lengthening of T often advocates a T -weighting preparation module to regain signal and contrast between tissues, which can be affected by transmit RF field inhomogeneity. In this note, we report an extension of a previous fluid attenuated inversion recovery study that now incorporates the T preparation with parallel transmission calibration-free universal pulses to mitigate the problem.

METHODS

The preparation consisted of a 90°-τ-180 -τ-90° module to implement an effective inversion in the CSF and a saturation in the brain tissues. Care was taken for the pulses to have the desired phase relationship in every voxel by appropriate pulse design. The RF pulse design made use of the k -point parametrization and was based on a database of 20 B and ΔB maps previously acquired on different subjects at 7 T. Simulations and experiments on 5 volunteers, not contained in the database, were performed for validation.

RESULTS

Simulations reported very good inversion efficiency for the preparation module with 8% variation, with respectively 4 and 6 times less power and specific absorption rate than for the adiabatic version. Experiments revealed fluid attenuated inversion recovery images free of B artifacts.

CONCLUSION

This work contributes further to the panel of 3D sequences validated and now available with universal pulses at 7 T. The drop in power and specific absorption rate demand compared with adiabatic pulses in the T preparation leads to more freedom for the design of the readout train.