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

Abstract : Purpose: To demonstrate that fast-kz 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.
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Submitted on : Wednesday, June 5, 2019 - 10:35:42 AM
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Gaël Saïb, Vincent Gras, Franck Mauconduit, Nicolas Boulant, Alexandre Vignaud, et al.. Time-of-flight angiography at 7T using TONE double spokes with parallel transmission. Magnetic Resonance Imaging, Elsevier, 2019, 61, pp.104-115. ⟨10.1016/j.mri.2019.05.018⟩. ⟨hal-02147803⟩

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