Numerical analysis of multislice MR excitation and inversion with multifrequency selective rf pulses

Multifrequency selective excitation and inversion were recently described and tested for multislice imaging and multivolume selective spectroscopy (Magn. Reson. Med. 6, 364 (1988), J. Magn. Reson. 76, 155 (1988]. The technique is based on assumption that a multifrequency rf pulse, a linear superposition of several selective rf pulses with different frequencies, generates a MR signal which can be separated into the spin responses due to each individual frequency. This assumption is investigated theoretically by analyzing the effect of multifrequency selective rf pulses on the magnetization of a homogeneous phantom as a function of slice separation, pulse shape, and rf amplitude using computer simulations of the Bloch equations. It is found that multifrequency selective excitation with sinc pulses--up to eight slices are investigated--and two-frequency inversion with hyperbolic secant pulses lead to profiles comparable in quality and selectivity to those of conventional single-frequency pulses.