Abstract: In a magnetic resonance imaging system, an RF power amplifier is employed to boost an RF pulse to sufficient strength to excite the nuclear spins in a subject. The non-ideal behavior of the amplifier distorts the shape of an excitation pulse, and this distortion in turn degrades a slice profile. The distortion of the RF signal is manifested by nonlinearity in amplification and in incidental phase modulation. By determining the amount of nonlinearity and the phase modulation resulting from the power amplification, the baseband RF signal can be predistorted or prewarped to offset the distortion resulting from amplification. Improved slice selectivity results therefrom.