Design of calibration-free RF pulses for T 2 $$ {}_2 $$ -weighted single-slab 3D turbo-spin-echo sequences at 7T utilizing parallel transmission

Abstract

Purpose: T ${}_2$ -weighted turbo-spin-echo (TSE) sequences are a fundamental technique in brain imaging but suffer from field inhomogeneities at ultra-high fields. Several methods have been proposed to mitigate the problem, but were limited so far to nonselective three-dimensional (3D) measurements, making short acquisitions difficult to achieve when targeting very high resolution images, or needed additional calibration procedures, thus complicating their application.

Methods: Slab-selective excitation pulses were designed for flexible placement utilizing the concept of k ${}_T$ -spokes. Phase-coherent refocusing universal pulses were subsequently optimized with the Gradient Ascent Pulse Engineering algorithm and tested in vivo for improved signal homogeneity.

Results: Implemented within a 3D variable flip angle TSE sequence, these pulses led to a signal-to-noise ratio (SNR) improvement ranging from 10% to 30% compared to a two-dimensional (2D) T2w TSE sequence employing $B_1^{+}$ -shimmed pulses. $B_1^{+}$ field inhomogeneities could be mitigated and artifacts from $B_0$ deviations reduced. The concept of universal pulses was successfully applied.

Conclusion: We present a pulse design method which provides a set of calibration-free universal pulses (UPs) for slab-selective excitation and phase-coherent refocusing in slab-selective TSE sequences.

Keywords: GRAPE; UPs; kT‐points; parallel transmission; spokes; ultra‐high field.