A method and apparatus in an ultrasound imaging system is disclosed that enhances the contrast-to-tissue ratio and signal-to-noise ratio of contrast imaging using stepped-chirp waveforms. The first waveform component is employed with a first frequency optimized to initiate the bubble dynamics and the second waveform component is employed with a second frequency optimized to produce an enhanced bubble nonlinear response. The first waveform component and the at least a second waveform component are transmitted as a single stepped-chirp transmit pulse. At least one of a center frequency, an amplitude, a starting phase, and a bandwidth of the waveform components are adjusted to generate the single stepped-chirp transmit pulse. A relative phase, a switch time, and a time delay between the waveform components are also adjusted for maximal enhancement of bubble nonlinear response.