SLICE-SELECTIVE TUNABLE-FLIP ADIABATIC LOW PEAK-POWER EXCITATION (STABLE) PULSE

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: Adiabatic pulses are useful in achieving uniform excitation profiles in the presence of B1 inhomogeneity. BIR-4 pulses have been shown to achieve adiabatic excitation with user-selectable flip angles. However, these pulses are nonselective. The BIR-4 pulse design has been extended through the use of gradient modulation techniques to create slice-selective adiabatic excitation pulses. Unfortunately, these techniques require high RF amplitude, typically above the output of the amplifiers available on most commercial scanners. In this work, we developed an alternative approach that achieves adiabatic slice selection with significantly lower RF peak power requirements. Our Slice-selective Tunable-flip AdiaBatic Low peak-power Excitation (STABLE) pulse consists of an oscillating gradient in conjunction with a BIR-4- like RF envelope that is sampled by many short spatial subpulses in order to achieve spatial selectivity. Methods and Discussion: A sech/tanh amplitude/frequency modulation function was used so that the amplitude and phase variations were sufficiently slow to be accurately sampled by the chosen number of subpulses. A phase discontinuity was introduced between the first and second segments and between the third and fourth segments to produce a 90 flip angle. The resultant spectral adiabatic excitation pulse was 21 ms long and had a spectral bandwidth of approximately 80 Hz. The pulse was then subsampled with the number of sublobes chosen as a trade-off between adiabaticity and minimum slice thickness. The final STABLE pulse was comprised of 33, 0.64 ms subpulses scaled by the adiabatic envelope. The STABLE pulse was integrated into a GRE sequence to compare it to a standard sequence with a conventional excitation pulse. Adiabatic threshold is reached at around 45% below nominal peak B1. Above the adiabatic threshold, the excited cross section remains largely invariant.