Peak RF Power Constrained Pulse Design for Multi-Band Parallel Excitation

Xiaoping Wu¹, Kamil Ugurbil¹, Pierre-Francois Van de Moortele¹

The use of multi-band (MB) RF excitation along with subsequent unaliasing via parallel imaging principles leads to significant acceleration in volume coverage along the slice direction; this approach is becoming increasingly common and has recently been demonstrated with significant success in functional and diffusion-weighted imaging studies of the brain. Conventionally, the total RF energy and peak RF power required in slice accelerated MB imaging increase linearly and approximately quadratically, respectively, with the MB factor that defines the number of simultaneously excited slices. This increase can easily limit the maximum MB factor, especially when spin echo acquisitions are required and/or high magnetic fields are employed. Here we introduce a novel formulation for optimum peak power constraint using MB pTx pulse design based on spoke RF pulses with simultaneously targeting improved B1+ inhomogeneity. The formulation incorporates the interaction of the base pulses of individual bands by taking into account the final summed pulses. The new formulation is validated using B1+ maps simulated in a human whole body model and is shown to result in larger reduction of peak RF power than the conventional formulation for pTx pulses used for optimizing single-slice or sequential multi-slice excitation.