Another sign to identify the ECA (external carotid artery):

ECA- systole:

ECA- early diastole
ECA and ICA - systole- color flow present:

ECA- late diastole- fades out:

ECA- poor flow in early diastole

ECA- no flow during late diastole: absent color flow signal

A very useful sign to identify the normal ECA is looking for flicker or fading out of the Color Doppler flow in the ECA during diastole. This is due to the very low absent flow in the ECA during diastole, as opposed to the ICA and CCA, both of which show significant flow during diastole. As a result, the ECA flickers on during cardiac systole (see Color Doppler images of the ECA and ICA above) and fades out (absent flow) during diastole). 3 of the Color Doppler images above are axial sections through the ECA and ICA while the other 3 images show the ECA and ICA just emerging from the bifurcation of the CCA (common carotid artery). All images by Joe Antony, MD. Machine used here is the Toshiba Nemio-XG ultrasound system.

http://www.ultrasound-images.com/vascular.htm