Simulations of NMR experiments employing static and/or radio-frequency field gradients: Application to slice selection by the inhomogeneous radio-frequency field of a standard coil

A general computer program has been developed in order to simulate any nuclear magnetic resonance (NMR) experiment. It includes, in addition to the action of radio-frequency (rf) pulses and of gradients of both the static and the rf magnetic fields, the effects of inhomogeneity of the latter. It has been used here for devising a slice selection procedure on the basis of the inhomogeneity of the rf field delivered by a standard coil (e.g., a saddle-shaped coil). Two sequences have been investigated, a DANTE-like pulse train and a very simple one (named (SP)-P-2 for slice selection with 2pi pulses), (2pi - tau)(n), where 2pi corresponds to the flip angle of the region to be selected, whereas tau has to be chosen according to the relaxation time values; n, the number of cycles, must be sufficiently large, its actual value being uncritical. Simulations show that performances (in terms of selectivity) of both sequences are comparable, while experimental verifications favor (SP)-P-2 for its robustness and for the absence of any signal loss.