Harmonic power mode Doppler using microbubble contrast agents: an improved method for small vessel flow imaging

By virtue of their ability to enhance the weak echoes received from flow in small blood vessels, ultrasound contrast agents render flow in a lower level of the circulation detectable to colour Doppler imaging. We combine contrast imaging with power Doppler, which is a more appropriate colour mode for contrast enhanced Doppler imaging. We demonstrate that the combination of harmonic Doppler and the enhancement of the Doppler signal by the contrast agent produces an in vivo gain of more than 30 dB in the signal-to-clutter ratio. Harmonic power Doppler imaging demonstrates flow in the cortex of the kidney to within 2 mm of the capsule, even in the presence of respiratory motion. Histological correlation of the Doppler images with sections of the cortical vasculature suggest that flow is detected in interlobular arterioles of mean diameter 44 μm. We conclude that the new method holds promise for the clinical detection of small vessel flow in moving solid tissue