Mean echo power as a measure of flow reduction for bubble occlusion therapy

Ultrasound can be used to phase transition perfluorocarbon (PFC) droplets into gas bubbles, a process known as acoustic droplet vaporization (ADV). In vivo blood flow reductions can then be achieved using the ADV bubbles. An in situ ultrasonic measurement of this blood flow reduction is desirable. Since, in a simple model, initial flow reduction is proportional to the number of bubbles, and the mean echo power (MEP) in a B-mode image is related to the number of bubbles (off resonant scattering), it is hypothesized that the MEP can be used to estimate the relative flow reduction. To test this hypothesis, three canines were anesthetized and a laparotomy performed to externalize the left kidney and renal artery. An ultrasonic flow probe was placed on the renal artery. PFC droplets were injected intravenously and a focused 3.5 MHz single-element transducer was used to vaporize the PFC droplets, just downstream of the flow probe. The flow in the renal artery was recorded with the flowmeter as the vaporized droplets became lodged in the renal capillary beds. Simultaneously, B-mode cine loops were recorded. The cine loops were processed to determine the echogenicity, as measured by the MEP, in the kidney cortex. It was found that the MEP and renal arterial flow did not correlate quantitatively. However it was found that the MEP could be used to determine when the renal arterial flow had recovered to a steady state.