Abstract
A new method for designing radiofrequency (RF) pulses with numerical optimization in the wavelet domain is presented. Numerical optimization may yield solutions that might otherwise have not been discovered with analytic techniques alone. Further, processing in the wavelet domain reduces the number of unknowns through compression properties inherent in wavelet transforms, providing a more tractable optimization problem. This algorithm is demonstrated with simultaneous multi-slice (SMS) spin echo refocusing pulses because reduced peak RF power is necessary for SMS diffusion imaging with high acceleration factors. An iterative, nonlinear, constrained numerical minimization algorithm was developed to generate an optimized RF pulse waveform. Wavelet domain coefficients were modulated while iteratively running a Bloch equation simulator to generate the intermediate slice profile of the net magnetization. The algorithm minimizes the L2-norm of the slice profile with additional terms to penalize rejection band ripple and maximize the net transverse magnetization across each slice. Simulations and human brain imaging were used to demonstrate a new RF pulse design that yields an optimized slice profile and reduced peak energy deposition when applied to a multiband single-shot echo planar diffusion acquisition. This method may be used to optimize factors such as magnitude and phase spectral profiles and peak RF pulse power for multiband simultaneous multi-slice (SMS) acquisitions. Wavelet-based RF pulse optimization provides a useful design method to achieve a pulse waveform with beneficial amplitude reduction while preserving appropriate magnetization response for magnetic resonance imaging.
Highlights
Simultaneous multi-slice (SMS) imaging has been implemented for applications such as human brain imaging with echo planar imaging (EPI) and diffusion acquisitions [1,2,3,4,5,6,7]
The waveletoptimized pulse yields a spectral slice profile response nearly identical to the Shinnar Le-Roux (SLR) profile. This similarity is in spite of the fact that the SLR pulse was kept entirely separate from the wavelet optimization algorithm and only the Fourier pulse was used as an input to the optimization
The wavelet-optimized pulse is compared in Fig 4 to an SLR pulse that was designed based on the methods described by Cunningham and Wood [29]
Summary
Simultaneous multi-slice (SMS) imaging has been implemented for applications such as human brain imaging with echo planar imaging (EPI) and diffusion acquisitions [1,2,3,4,5,6,7]. Without applying techniques to reduce peak power, the maximal radiofrequency (RF) pulse amplitude scales linearly with the number of slices simultaneously excited. A multiplexed-EPI technique has been shown to accomplish high levels of image acquisition acceleration for twice-refocused diffusion spectrum imaging by combining dual-band excitation with dual simultaneous image refocusing [4], thereby reducing the peak RF amplitude compared to a conventional four-band refocusing pulse. Time-shifted RF pulses offer a peak B1 reduction for high slice acceleration factors including three bands and six bands simultaneously excited [8]. To address the challenge of high RF energy deposition at ultra-high field strengths such as 7.0 Tesla and above, the Power Independent of Number of Slices (PINS) technique offers a solution for SMS imaging where SAR reduction is needed [9]. The MultiPINS pulse design technique builds upon the PINS technique to further reduce peak power and energy deposition for SMS pulses by mixing a PINS pulse with a more traditional multiband pulse [10]
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