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: 1 + and 0 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 feethead 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 7 T.

Purpose The sensitivity of pseudo‐continuous arterial spin labeling (PCASL) to off‐resonance effects (ΔB0) is a major limitation at ultra‐high field (≥7T). The aim of this study was to assess the effectiveness of different PCASL ΔB0 compensation methods at 7T and measure the labeling efficiency with off‐resonance correction. Theory and Methods Phase offset errors induced by ΔB0 at the feeding arteries can be compensated by adding an extra radiofrequency (RF) phase increment and transverse gradient blips into the PCASL RF pulse train. The effectiveness of an average field correction (AVGcor), a vessel‐specific field‐map‐based correction (FMcor) and a vessel‐specific prescan‐based correction (PScor) were compared at 7T. After correction, the PCASL labeling efficiency was directly measured in feeding arteries downstream from the labeling location. Results The perfusion signal was more uniform throughout the brain after off‐resonance correction. Whole‐brain average perfusion signal increased by a factor of 2.4, 2.5, and 2.1, respectively, with AVGcor, FMcor and PScor compared to acquisitions without correction. With off‐resonance correction, the maximum labeling efficiency was ~0.68 at mean B1 (B1mean) of 0.70 µT when using a mean gradient (Gmean) of 0.25 mT/m. Conclusion Either a prescan or a field map can be used to correct for off‐resonance effects and retrieve a good brain perfusion signal at 7T. Although the three methods performed well in this study, FMcor may be better suited for patient studies because it accounted for vessel‐specific ΔB0 variations. Further improvements in image quality will be possible by optimizing the labeling efficiency with advanced hardware and software while satisfying specific absorption rate constraints.

Continuous arterial spin labeling using a separate neck labeling coil is an attractive approach for imaging perfusion at 7T. This approach is hindered by specialize hardware including a labeling coil and a separate amplifier located outside the magnet room. On-coil amplifiers have demonstrated higher RF power efficiency with minimal cable loss and patient-coil interaction. This work shows that a separate labeling coil using on-coil amplification can be used for flow driven adiabatic inversion in a flow phantom suggesting that it may provide a suitable approach to simplify the hardware package required as well as to improve CASL studies at 7T.