Detection of Individual Microbubbles by Burst-Wave-Aided Contrast-Enhanced Active Doppler Ultrasonography

We propose burst-wave-aided, contrast-enhanced, active Doppler ultrasonography for visualizing lymph vessels. This technique forces ultrasound contrast agents (UCAs) to move using the acoustic radiation force induced by burst waves with low amplitude while suppressing their destruction. Using a flow phantom, we measured the average, decrease rate of echo intensity [i.e., pulse intensity integral (PII)], and the velocity of individual contrast agents, which directly affects the performance of imaging and tracking contrast agents under stationary flow conditions. Comparison with pulse-inversion Doppler without exposure to the burst wave demonstrated that the velocity of the contrast agents could be enhanced up to several tens of millimeters per second by the effect of the burst wave, maximizing the echo intensity extracted by a clutter filter. The contrast ratio (CR), defined as the ratio of the contrast echo to the phantom echo outside the channel, did not change appreciably, even when the lower cut-off velocity of the clutter filter was increased up to 10 mm/s. This implies a better robustness against the motion of the tissue. In addition, the performance for detecting contrast agents (i.e., echo intensity) was superior or similar to that of pulse-inversion Doppler, even in undesirable conditions where the flow had a velocity component in the opposite direction to that of the acoustic radiation force. The echo intensity was lower or the same as that in pulse-inversion Doppler, demonstrating the potential for suppressing the destruction of contrast agents and enabling long-term observations. From these results, we expect that the proposed method will be beneficial for visualizing lymph vessels.