Time-of-flight angiography at 7T using TONE double spokes with parallel transmission

Abstract

Purpose: To demonstrate that fast-k_z spokes can be used in parallel transmission to homogenize flip angle ramp profiles (known as TONE) in slab selections, and thereby improve Time-Of-Flight angiography of the whole human brain at 7T.

Methods: B₁⁺ and B₀ maps were measured on seven human brains with a z-segmented coil connected to an 8-channel pTx system. Tailored two-spoke pulses were designed under strict hardware and SAR constraints for uniform slab profile before transforming their subpulse waveforms for linearly-increasing flip-angle ramps. Increasing angulations along the feet-head direction were prescribed in 2-slab and 3-slab TOF acquisitions. Excitation patterns were simulated and compared with RF-shimmed (single spoke) ramp pulses. Excitation performances were assessed in ~10-min TOF acquisitions by visually inspecting Maximal Intensity Projections angiograms.

Results: The flip-angle ramp fidelity achieved by double spokes inside slabs of interest was improved by 30-40% compared to RF-shimmed ramps. This allowed better homogenizing signal along arteries, and depicting small vessels in distal areas of the brain, in comparison with RF-shimmed ramp pulses or double-spoke uniform excitation.

Conclusion: Ramp double spokes used in conjunction with parallel transmission yield better blood saturation compensation and more finely resolved TOF angiograms than mere double spokes or ramp single spokes at 7T.

Keywords: Angiography at UHF; Blood saturation; RF pulse design; TOF-MRA; TONE; pTx.