Online gradient optimisation for spokes parallel transmit – the BOGAT method

Spokes parallel transmit pulses improve the homogeneity of ultra-high field imaging. Typically the gradient trajectory (i.e. spoke kt-space positions) is determined using the iterative Fourier transform approach (FTA). We introduce “BOGAT” (Bayesian Optimisation of GrAdient Trajectory) to efficiently determine globally optimal gradient trajectories. We evaluated BOGAT in phantoms for single-band and multi-band excitation, and retrospectively in 9 volunteers using existing B0 and B1+ maps. BOGAT improves flip angle homogeneity (by 12.8% vs FTA, P<0.001) and reduces SAR (17.2%, P<0.001). Calculations take ~10s extra for a set of multiband pulses, making it feasible to use for online per-subject optimisation.