Speckle in the Doppler signal - correlation of CW-Doppler signals from blood for varying transmit frequency

Speckle is an unwanted noise in ultrasound images, and appears as returning echoes are coherently summed over the transducer surface. In Doppler applications, the total power of the received spectrum varies in a manner similar to speckle. This paper investigates the correlation of continuous-wave (CW) Doppler signals obtained from simultaneously transmitted carrier frequencies, when a slowly flowing blood stream is insonated. The covariance function of received Doppler power versus transmitted frequency is shown to agree well to a theoretical modelled function, as well as experiments made on static, independent scatterers, even though the present measurements have a larger variability. The speckle to noise ratio of the Doppler signal amplitude was found to be 1.87 (S.E. 0.04), compared to the theoretical number 1.91 valid for pulse-echo imaging. It was also found that average of spectral estimates from independent channels reduce the variability of such estimates.