CAROTID ULTRASOUND IMAGING
Carotid ultrasound uses Doppler to screen patients for blockage or narrowing of their carotid arteries (stenosis), which if present may increase their risk of having a stroke.
WHY IS THE PROCEDURE PERFORMED?
If a patient has high blood pressure or a carotid bruit in the neck that is heard with the stethoscope.
In preparation for coronary artery bypass surgery.
To locate a hematoma, a collection of clotted blood that may slow and eventually stop blood flow.
Check the state of the carotid artery after surgery to restore normal blood flow.
Verify the position of a metal stent placed to maintain carotid blood flow.
Evaluate tumours and congenital vascular malformations.
THE PROCEDURE ITSELF
You will be positioned lying face-up on an examination table that can be tilted or moved. Patients may be turned to either side to improve the quality of the images.The sonographer or radiologist then places the transducer on the skin in various locations, sweeping over the area of interest or angling the sound beam from a different location to better see an area of concern.
LENGTH OF THE PROCEDURE
30 to 45 minutes
BASICS
Combined with Doppler ultrasound, this test can show how blood is moving through arteries. Plaque buildup in one or both of the carotid arteries in the neck and to see whether the buildup is narrowing carotid arteries and blocking blood flow to the brain.
WHAT IS SEEN
The most common location for an inner carotid artery (ICA in the image) stenosis is in the first 3cm of its course, and this is fortunate because it is more difficult to image the deeper you go, with potential false-positive results.
STENOSIS PROFILE
Blood flow normally follows well-defined patterns. Normal laminar flow is described as flowing faster in the centre of the stream and slower at the sides. The flow stays in parallel lines with most of the flow being at the same speed. In spectral imaging, there is normally a clear spectral window. (No stenosis profile in the image above).
In disturbed or turbulent flow, there is less organised and parallel flow. So the waveform spectral windows displays significant spectral broadening generally proportional to the disease severity. (Mild and severe stenosis in the image above).
The kind of spectral broadening seen in diseased arteries is also observed when a vessel turns abruptly, kings back on itself or branches, so the presence or absence of plaque should be also located in a vessel with suspected turbulent flow.
THE SPECIFIC CHANGES IN THE DOPPLER PROFILE
A decrease in the diameter of the vessel causes a significant change in the velocities within the vessel, because the length of the vessel and the thickness of the blood do not change.
Stenosis is considered significant when the diameter reduction reaches 50%, corresponding to a 75% decrease in area.
Proximal to the stenosis, the velocities in the vessel are usually dampened, flow disturbances may be present.
Within the stenosis itself, the Doppler frequencies increase and present as spectral broadening.
Poststenotic turbulence takes place at the exit from the stenosis creating flow reversals and currents that appear as disturbed Doppler flow patters above and below the baseline.
PITFALLS
Aliasing (appears on the image as a disorganised mosaic of colour. Occurs when very high-flow states are present, or a low pulse repetition frequency is used.)
Confusing external and internal carotid arteries. (A slight tap in the temporal area will cause oscillations in the normal ECA waveform but not in the ICA waveform.)
No internal carotid artery waveform present (an ECA branch may be mistaken for the ICA)
Torturous internal carotid artery (they can be difficult to image when they follow a torturous course through the neck, winding and intertwining)
Calcification (pulsed Doppler signals cannot penetrate calcifications and reflect bright echoes back to the transducer.)
Low-flow velocities
Sources:
Clinical sonography, Roger C. Sanders, Tom C. Winter