Excitation of arbitrary shapes by gradient optimized random walk in discrete k‐space

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Abstract
A new technique for the excitation of arbitrary shapes is proposed. It is based on a parallel sequence of small tip angle RF pulses and gradient pulses. The small tip angle rotations co‐add yielding a 90° excitation pulse within the selected excitation profile while outside the profile, the rotations cancel each other. A full theory of the completely arbitrary regional volume excitation (CARVE) method is presented and experimentally verified. In CARVE, k‐space is discrete because the RF is applied in pulses. The discrete character of k‐space permits an arbitrary trajectory for the k‐space walk. The optimal random trajectory is found by minimizing the gradient load using simulated annealing. It is shown, both theoretically and experimentally, that such a trajectory is much better than any other systematic or random trajectory in k‐space.