When can Doppler be used in place of integrated backscatter as a measure of scattered ultrasound intensity?

The purpose of this work was to determine under what circumstances the intensity of Doppler audio signals can be used as a substitute for the more direct and complex measure of ultrasonic backscatter (integrated backscatter) which requires radio-frequency ultrasound signals. Using a rotating rubber disk phantom and a microbubble echo-contrast flow phantom, we have shown that the intensity of audio Doppler signals is independent of the constraints typically associated with Doppler ultrasound (velocity and angle), but like integrated backscatter depends on the transmit intensity, gain of the ultrasound receiver, attenuation and the nature of the scatterers. Using Doppler ultrasound for backscatter measurements is ideally suited for the expected application of the technique: the assessment of echo contrast in cardiac chambers, blood vessels and tissue perfusion (i.e., any flow system). Compared to integrated backscatter, the Doppler audio method has the advantage of using standard clinical ultrasound machines, requires less sophisticated data storage and processing equipment and the positioning system for the region of interest (the Doppler sample volume) is built into all pulsed-wave Doppler machines. Further, the low-velocity filter removes all nonmoving scatterers (like the intense echoes from heart valves and the walls of blood vessels), thus allowing study of only those echoes originating from the blood pool. This combination of features is what attracted us to the Doppler method for quantitating ultrasonic backscatter in flow systems.