Fast Online 3D SPACE and FLAIR Imaging at 7T Using Multiple Subject-Specific Parallel Transmission Pulses Based on...

In 3D SPACE and FLAIR sequences, designing single pTx RF pulse for excitation and refocusing pulses by scaling its amplitude can introduce artifacts. This study addresses this by designing multiple subject-specific RF pulses online within clinically viable times, using subpopulation universal pulses (UPs) as initialization.

Subpopulation UPs were calculated using $B_1^{+}$ and $B_0$ maps from 150 training subjects. During experiments, refocusing flip-angles were grouped into five clusters. Starting from the subpopulation UP yielding the lowest error, a reference RF pulse was optimized for the cluster with mean flip-angle closest to 90°. The remaining refocusing-cluster and excitation RF pulses were optimized using the reference phase response, with refocusing RF pulses scaled to desired flip-angles. Time-symmetric RF and anti-symmetric gradient waveforms were enforced as a constraint. For FLAIR, a new T2-prepared inversion scheme was proposed, based on designing a symmetric 180° inversion pTx pulse and splitting it to obtain 90° pulses.

Compared to $B_1^{+}$ -shimming and single-RF pulse optimization, the proposed method minimized artifacts resulting from RF pulse scaling, particularly in high $\Delta B_0$ regions. Online pulse calculations required ∼20 s and an additional ∼9 s for FLAIR. Across 120 test subjects, the multi-RF approach demonstrated an 18% improvement in the simulated echo magnitude within high $\Delta B_0$ regions and 13% improvement within the brain. pTx inversion pulses achieved an 86% SAR reduction compared to adiabatic ones.

The proposed method enables efficient subject-specific RF pulse design for 3D SPACE and FLAIR.