Symmetric pulses to induce arbitrary flip angles with compensation for rf inhomogeneity and resonance offsets

Abstract With conventional radiofrequency pulses, flip angle variation with RF field strength can limit the types of experiments that can be performed with an inhomogeneous RF coil, such as a surface coil. As an alternative, composite and adiabatic pulses can induce constant rotations even when the RF amplitude varies by >I 0-fold. Unfortunately, these pulses generally afford compensation for RF inhomogeneity at the expense of greatly increased RF power and reduced bandwidth (off resonance performance). In this paper, a procedure is described for constructing symmetric composite and adiabatic pulses which provide compensation for RF inhomogeneity while maintaining wide and symmetric bandwidths. Using this procedure, a symmetric adiabatic pulse, BIR-4, is derived which can induce a constant rotation of any desired flip angle despite RF inhomogeneity and resonance offset. In comparison to adiabatic half-passage, the 90° BIR-4 has a wider bandwidth over a similar operational range of RF amplitudes (or power). These new pulses are evaluated by numerical calculations and by surface-coil phantom experiments.

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