RF engineers, MR physicists, ultra-high field practitioners.The potential benefits of ultra-high field MRI – higher resolution and sensitivity – are, in practice, hard to realize because of non-uniformities of the transmit fields (B₁⁺) and susceptibility effects that distort the image and lead to through-slice signal dropout. A popular strategy to create uniform flip-angle (FA) distributions using a slice-selective sequence is the spokes pulse [1,2,3]. Using parallel transmission (pTx), the complex weightings of the spokes played on each transmit channel can be optimized to maximize FA uniformity within the slice. Spokes pulses achieve this by sampling k_z (slice direction) at high resolution and by depositing energy at discrete locations in the k_x-k_y plane. However, spokes pulses are extremely sensitive to off-resonance effects so that, in practice, their performance worsens dramatically close to the sinus and ear canal cavities [4]. This is due to the fact that spokes pulses are relatively long and deposit RF energy at discrete locations, so that errors in the position of the spokes in the k_x-k_y plane (e.g., due to B₀ effects) lead to inaccurate excitations. In this work, we introduce a new pulse trajectory that we call the “twisted spokes”. This pulse is able to excite sharp slice-selection profiles while creating highly uniform FA distributions at 7T. Importantly, the pulse is very robust to off-resonance effects, because the pulse duration is kept short and the RF energy is not deposited at discrete k_x-k_y locations but is instead distributed across the k_x-k_y plane.Gradient and RF Waveforms: The twisted spokes trajectory consists of two helical k-space segments oriented along the k_z-axis (z being the slice direction, Figure 1). These segments have a large extent along k_z (k_z,max=150 m^-1, Δz=6.0 mm) to ensure sharp slice-selection and only a minor extent in k_x/y (≈10 m^-1). The exact shape of the twisted spokes segments (e.g., extent along the k-space axes, number of revolutions, etc.) has a large impact on the excitation quality. We control these basic features of the twisted spokes by so-called shape parameters that we optimize using our joint k-space trajectory and RF waveform design method [5]. In this method, we optimize the shape parameters for a given target profile (z-slice) and given B₀/B₁⁺ maps using a nested optimization approach in which the inner loop is a small tip-angle RF pulse design on a constant k-space trajectory. The outer loop is a constrained optimization on the shape parameters. In order to achieve sharp and uniform slice excitation and B₀ robustness at the same time, we optimize the RF waveforms (inner loop) such that the target FA map is realized simultaneously at offset frequencies −50 Hz, 0 Hz, +50 Hz [6,7]. To improve the excitation quality, the last RF design is a magnitude least-squares optimization that does not impose a predetermined spin phase. Evaluation: Our RF pulses were evaluated using Bloch simulations based on B₀ and B₁⁺ maps (8 channels) acquired on a 7T scanner (“Step 2” pTx, Magnetom 7T, Siemens, Erlangen) loaded with a realistic 3D-printed head phantom with three compartments (bone, brain and everything else) [4]. The target was to excite a 6 mm z-slice at 10° FA in about 1.5 ms with good performance regarding in-plane uniformity in the brain (uniformity in the skull is not considered), through-plane slice profile, and robustness to off-resonance effects. Our twisted spokes pulses are compared to four different spokes strategies: (A) 1-spoke (birdcage mode), (B) 1-spoke (MLS optimization), (C) 2-spokes (MLS, no B₀ robustness), and (D) 2-spokes (MLS, B₀ robustness enforced as explained in [4,7]). All pulses satisfy the gradient system (G_max = 40 mT/m, S_max = 150 mT/m) and peak RF power (V_max = 150 V per channel) constraints.In Figure 2, the results of the six RF pulses are shown. The 1-spoke birdcage mode pulse (A) exhibits severe in-plane FA variations which is partly compensated by optimizing the complex weights of each channel (B). Both 1-spoke pulses are intrinsically robust to B₀ effects. The 2-spokes pulses are rather long (~2.6 ms) and either achieve in-plane FA uniformity (C) or off-resonance robustness (D), but not both. Our optimized twisted spokes pulse (F) is short (1.8 ms) and produces a uniform in-plane FA and a good slice profile. At the same time and compared to both 2-spokes pulses, B₀ robustness is dramatically improved. Comparing the non-optimized (E) and optimized (F) twisted spokes RF pulses emphasizes the impact of trajectory shape optimization on FA fidelity, B₀ robustness, as well as average power (P_mean reduced from 27.5 W to 5.3 W).