Parallel-transmission spatial spectral pulse design with local specific absorption rate control: Demonstration for robust uniform water-selective excitation in the human brain at 7 T.

purposeTo propose a novel method for parallel-transmission (pTx) spatial-spectral pulse design and demonstrate its utility for robust uniform water-selective excitation (water excitation) across the entire brain. THEORY AND

methodsOur design problem is formulated as a magnitude-least-squares minimization with joint RF and k-space optimization under explicit specific-absorption-rate constraints. For improved robustness against off-resonance effects, the spectral component of the excitation target is prescribed to have a water passband and a fat stopband. A two-step algorithm was devised to solve our design problem, with Step 1 aiming to solve a reduced problem to find a sensible start point for Step 2 to solve the original problem. The efficacy of our pulse design was evaluated in simulation, phantom, and human experiments using the commercial Nova head coil. Universal pulses were also designed based on a 10-subject training data set to demonstrate the utility of our method for plug-and-play pTx.

resultsFor k

conclusionDemonstrated useful for 4D (3D spatial, one-dimensional spectral) pTx spatial-spectral pulse design, our proposed method provides an effective solution for robust volumetric uniform water excitation, holding a promise to many ultrahigh-field applications.