Friday, January 29, 2010

Doppler imaging of the carotids is not as difficult as you think:

The main purpose of studying the carotid arteries (the common carotid artery (CCA), the internal carotid (ICA) and the external carotid (ECA) and lastly the Vertebral artery - is to assess the possibility of stroke due to stenosis and embolism. Any of these conditions in the carotid system, can be fatal at worst or at best result in serious disability including hemiplegia, or partial paralysis.
The following are the main steps to evaluating the carotid arterial system:-

1) differentiate the CCA from the IJV (internal jugular vein)- this is easy as the Jugular veins shows a typical venous flow on Color Doppler imaging and has thin walls which are not visualized clearly on B-mode ultrasound imaging.

2) differentiate the ICA from the ECA. Here the going becomes a little difficult, as the bifurcation of the CCA into the ICA and ECA is not always clearly visualized (see image above).
For more details of step -2 (see: )
The temporal tap sign : see Spectral Doppler waveforms of ECA below) is vital to clearly distinguish the ECA from the ICA. Another useful point is the fading of the color flow signal in the ECA during diastole.
See this color Doppler video clip of the "blinking" on and off of the external carotid artery as it pulsates: This is unlike the steady flow in the internal carotid artery (seen in transverse section through these vessels):

3) Thirdly the next step in carotid sonography is visualization of the carotid arterial walls and the measurement of the combined thickness of the inner layer-the intima and the middle layer- the media. This is called IMT or intima media thickness.
Here are some Color Doppler ultrasound video clips of the normal carotid vessels:

The first is a video of the spectral Doppler waveform tracing of the common carotid artery:
This color Doppler spectral ultrasound video shows the typical pulsations of the common carotid artery with moderately high PSV or peak systolic velocity and diastolic flow maintained throughout the period of left ventricular relaxation. The accompanying audio signal displays the typical acoustic characteristics of a moderately low resistance flow.

 The above spectral Doppler ultrasound video clip (with sound) of the normal ECA (external carotid artery) shows a high resistance flow with high peak systolic velocity and very low diastolic velocity, almost touching the baseline. Note the sharp sound of the audio as the external carotid artery pulsates.
Lastly, we have a Color Doppler spectral waveform video (with audio) of the internal carotid artery. Note the low peak systolic velocity and high and persistent diastolic flow, that is typical of the low resistance flow of the normal internal carotid artery:
    Note the gentle hushed sound of the low resistance flow in this normal internal carotid artery.

Color Doppler ultrasound imaging of the normal vertebral artery:
This vertebral artery (right) shows antegrade flow. Note the breaks in the display of the vertebral artery caused by shadows from the vertebral transverse processes. This artery was imaged from the C-3 to C-6 cervical vertebrae. Based on the location of the segment of the vertebral artery, it is divided into 4 parts:
V-1: from the origin of the vertebral artery to the transverse process of C6 (6th cervical vertebra).
V-2: from the transverse process of C-6 vertebra to the C-2 (2nd cervical vertebra). (The vertebral arteries travel to the cranium via the foramen transversarium of the cervical vertebrae. These foraminae are present in the transverse processes of the cervical vertebrae).
V-3: from the C-2 vertebrae to the dura within the skull.
V-4: from the dura (intracranial) to the confluence of the vertebral arteries to form the basilar artery.
It is the V-2 segment of the vertebral artery that is readily visualized on Color Doppler ultrasound imaging.  Yet often the vertebral artery can be difficult to visualize even in the V-2 segment due to the anatomical location of the artery (vertebral bone and cervical musculature). Occlusion or stenosis of the vertebral artery usually occurs at the proximal (V-1) segment. Hence color Doppler ultrasound study of the intertransverse segment (V-2) may often be normal. In stenosis of the subclavian artery proximal to the origin of the vertebral artery, there may be reversed flow through the vertebral artery (a condition known as subclavian steal syndrome). Herein, the arm on the side of subclavian artery stenosis is supplied via reversed flow from the vertebral artery of the opposite  side.                                                                                                             In the normal individual (color Doppler video clip above), the easiest way to spot retrograde flow is to study the direction of flow in the common carotid artery and the ipsilateral vertebral artery. If both these vessels show same direction of flow (and hence the same color on Doppler flow study), the direction of flow is normal or antegrade.                                                                                                                                    
This spectral Doppler video clip (with audio output) shows the normal vertebral artery (right side).
Some useful links on this topic:  (free article and images)

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