Improvements in selective nuclear magnetic resonance excitation using nonlinear gradients.

The potential role of nonlinearities in the magnetic field gradient in magnetic resonance imaging for producing sharper boundaries for the excited spin slice region is investigated. A theoretical analysis based on solving the Bloch equations numerically is presented. Simple criteria for measuring the performance of selective excitation are defined and then used to evaluate the results for selective 90 degrees and 180 degrees pulses.