Robust nonadiabatic T₂ preparation using universal parallel-transmit k_T -point pulses for 3D FLAIR imaging at 7 T

The fluid attenuated inversion recovery sequence is a pillar technique to detect brain lesions in MRI. At ultrahigh field, the lengthening of T₁ often advocates a T₂ -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 T₂ preparation with parallel transmission calibration-free universal pulses to mitigate the problem.

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_T -point parametrization and was based on a database of 20 B₁⁺ and ΔB₀ maps previously acquired on different subjects at 7 T. Simulations and experiments on 5 volunteers, not contained in the database, were performed for validation.

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 B₁⁺ artifacts.

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 T₂ preparation leads to more freedom for the design of the readout train.