We have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (echo PIV). This method takes advantage of the non-linear backscatter characteristics of ultrasound contrast microbubbles when exposed to certain ultrasonic field. Preliminary in vitro, animal and clinical studies have shown significant promise of this method for measuring multiple velocity components with good temporal and spatial resolution. However, there is still difficulty in maximizing the non-linearity of bubble backscatter using conventional Gaussian-pulse excitation techniques because significant harmonic components may not be produced at modest pressure amplitudes and the higher incident pressure amplitudes required to induce non-linear behavior may cause bubble destruction. We present here a potential solution to this problem through the use of multi-frequency excitation. A rectangular pulse with multiple harmonics is used to drive the bubble. The backscatter process is studied through a modified Rayleigh-Plesset equation. Results show that the rectangular wave is effective in improving the visibility of microbubbles with ultrasound backscattered efficiency significantly higher than the widely used Gaussian waveform. Use of rectangular pulses with 4 and 2 harmonics showed no significant difference in bubble backscatter behavior, indicating that a two-frequency excitation may be sufficient to induce non-linear behavior of the microbubbles practically at modest incident pressures.