Color Doppler

Color Doppler overlays spatially localized estimates of motion on an ultrasound image. In blood-flow imaging, the estimated quantity is related to the Doppler ultrasound shift of echoes from moving red blood cells. The color scale encodes motion relative to the beam, not the blood's oxygen content. The displayed legend and sign convention must be read with the image.^[ref]

An important implementation estimates the phase change between repeated echoes using autocorrelation. Chihiro Kasai and coauthors demonstrated an autocorrelator combined with pulsed acquisition, testing scanners with phantoms as well as examining hearts. This made many localized estimates available fast enough for a changing two-dimensional image; it did not make each colored pixel an independent direct measurement of transported blood volume.^[ref]

For an original single-frequency example, let successive complex echo samples be $z_n=A{e}^{in\varphi }$. Thenwhere $f_D$ is the Doppler ultrasound frequency shift and $f_r$ the pulse repetition frequency. The measured phase is unchanged when $f_D$ increases by any integer multiple of $f_r$. This is aliasing even for a noiseless signal.

With $f_r=4000$ Hz and transmitted frequency $f_0=2.5$ MHz, assume speed of sound $c=1540$ m/s and motion parallel to the beam. True velocity 0.80 m/s gives $f_D=2f_{0}v/c\approx 2597.4$ Hz. Its principal sampled value is approximately $-1402.6$ Hz, corresponding to displayed beam velocity $-0.432$ m/s under the same conversion. A color sign change can therefore occur without physical reversal of fluid flow.

The example isolates sampling. Mixed moving scatterers, finite spatial resolution and signal processing add further limitations. A color image is useful for locating a jet or a changing flow pattern, while a stroke volume requires an appropriate spatial and temporal integration of velocity, not the apparent size or brightness of a colored patch.