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Recently, there has been a great interest in the capabilities of high-resolution ultrasound imaging. One of the applications is imaging blood vessels to assess tumor growth and response to therapy. Due to their nonlinear response, microbubble contrast agents scatter ultrasound energy at frequencies higher and lower than the imaging frequency. To maximize ultrasound scatter, the imaging frequency should be near the microbubble resonant frequency. Previously, this was not possible with high-frequency imaging systems because most contrast agents resonate at 1-10 MHz and the systems pulse at higher frequencies. We have developed a unique dual-frequency confocal transducer which excites microbubbles at low frequencies, near their resonance, and detects their emitted high-frequency energy at greater than 15 MHz. With this imaging approach, we have attained an average improvement in contrast-to-tissue ratio of 12.3 dB over standard b-mode imaging for MI between 0.5 and 0.65 with spatial resolution near that of the high-frequency element (30 MHz). This method is less susceptible to tissue motion corruption than power-Doppler because it does not rely on signal-decorrelation. Because of this, dual-frequency imaging can be used for flow imaging without respiratory gating.