An important method of differentiating the ECA from the ICA is the "temporal tap" sign. Tapping the temporal artery (a branch of the ECA) at the level of the zygomatic arch, just anterior to the external ear, produces fluctuations in the spectral waveforms of the ECA. This is seen in the above ultrasound images showing vertical dips and peaks during the diastolic flow. (Compare the Doppler images on left (without the tap) with the image on right (during temporal tap)). 
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