Mitigating banding artifacts in balanced steady-state free precession using parallel transmission in a single...

Using parallel transmission (pTx) in ultra-high field MRI allows efficient radiofrequency pulse designs that can produce uniform excitations. Here, we demonstrate that pTx can simultaneously mitigate $B_1^{+}$ non-uniformity and off-resonance artifacts in balanced steady-state free precession sequences at 7 T.

The standard hard excitation pulses of the balanced steady-state free precession sequence were replaced with two pTx pulses, played out in alternating repetition times. Based on the k_T-point trajectory, each pulse was designed to produce spatially varying phase distributions that compensate for off-resonance phase variations accumulated over one repetition time duration, while also mitigating $B_1^{+}$ non-uniformities. Depending on the local ΔB₀, the different steady-state signals formed were distributed over two aliased images with a half field-of-view shift relative to one another. Using slice-GRAPPA, these images were disentangled and recombined to produce artifact-free images.

Simulations validated the concept showing that a pair of strategically designed pTx pulses can impose a favorable distribution of spatially varying steady-state. As a proof of principle, experiments performed demonstrated the technical feasibility of the concept in a phantom in the presence of induced phase variations and in vivo.

With a tailored pair of pTx pulses, it was possible to concurrently mitigate $B_1^{+}$ and ΔB₀ non-uniformities in a thin slab through the brain in a single acquisition. Although further work is needed to extend the coverage and robustness of this method before being adopted more widely, it highlights the potential of utilizing pTx capabilities beyond $B_1^{+}$ correction.