Scalable spokes pTx pulses for 2D turbo-spin-echo imaging at 7 T

Turbo spin echo (TSE) is important clinically. Unfortunately, 7 T TSE suffers from B₁ ⁺-induced signal dropouts. Magnitude-based parallel transmit (pTx) pulse design algorithms cannot enforce phase patterns complying with the Carr-Purcell-Meiboom-Gill conditions (90° phase shift between excitation and refocusing). We introduce scalable spokes pTx pulses for 7 T TSE imaging.

We define scalable spokes pulses as having time-symmetric RF waveforms, antisymmetric in-plane gradients, and rephased subpulse slice-selection gradients. They produce flip angles that are approximately proportional to the applied voltage with voltage-independent phase patterns.

Scalable spokes pulses were designed for a phantom. Scaling behavior was characterized via Bloch simulations. Performance in terms of TSE echo homogeneity was assessed by extended phase graph simulations using in vivo field maps. Performance was validated for TSE acquisitions in a phantom and in vivo. Hippocampal TSE imaging was performed for four subjects comparing circularly polarized (CP), RF shimming, and scalable spokes pulses.

Scalable spokes pTx pulses show similar scaling behavior to previously proposed 3D k_T-point pulses. Scalable three-spoke pulses decrease flip-angle RMS error across subjects compared to CP mode pulses (11% vs. 23% for 120° pulses). TSE images with these pulses recover signals in cerebellum and temporal lobes.

Scalable spokes pTx pulses produce flip angles that vary approximately linearly with peak voltage while maintaining consistent spatial patterns of phase. Together with their spatial flip-angle homogeneity, these pulses enable high-fidelity 2D slice-by-slice TSE imaging at 7 T, albeit with reduced slice coverage with our choice of homogeneity target under the current vendor-provided specific absorption rate constraints on 7 T MRI scanners.