This is a case of a 61 years old male patient who attended the vascular laboratory for a colour Doppler ultrasound assessment of the carotid arteries as part of the preoperative assessment for an aortic valve replacement.

As shown in the video, while the lumen of the external carotid artery appears echo free, the lumen of the internal carotid artery is completely obstructed by the presence of echogenic material. Using colour Doppler flow, no colour flow is detected within the lumen of the internal carotid artery despite using a very low colour Doppler flow PRF of 13. Colour flow is however present throughout the external carotid artery. These findings are then confirmed with the longitudinal view.

Presence of echogenic material through the ICA lumen and absence of colour doppler flown using a low colour doppler flow scale are indicative of the presence of an ICA occlusion. 

Pulsed wave Doppler analysis is important as well, however it is not shown in this video. Absence of pulsed-wave Doppler signal within the ICA and high resistance Doppler waveforms within the ipsilateral common carotid artery are suggestive of ICA occlusion. 
The B-mode thrombus features can help to distinguish along with the clinical presentation, whether the occlusion is acute of chronic. In the presence of a chronic occlusion, the thrombus is well attached to the arterial walls, presents a high echogenicity and is not mobile. In an acute occlusion, the thrombus is generally mobile and hypoechoic. In this case, the thrombus is echogenic, well attached to the artery wall, not mobile with evidence of some calcifications noted at the origin of the ICA. 

TAKE HOME MESSAGE: use a low colour doppler flow scale to rule out an occlusion of the ICA.

Videos provided courtesy of ABC Vascular.