Ultrasound
Technique
Ultrasound technology was originally developed as sonar, used for navigation and ranging. The same basic technology is used in medical ultrasonography. An ultrasound probe has many small piezoelectric devices which are able to both transmit and receive ultrasonic waves. Data from this transmit/receive exchange can be used to make gray scale ultrasound images, color Doppler images with color showing flow, and pulsed Doppler analysis showing the waveform of the pulsation and velocity of blood in vessels. In neurovascular imaging, ultrasound is most often used to evaluate the carotids for stenosis. While narrowing can be seen with grayscale imaging, the most important parameter for gauging the degree of stenosis is the velocity of flow in the internal carotid artery. The ratio of velocity between the internal carotid and external carotid can be used for additional information.
A linear ultrasound probe would typically be used for carotid artery sonography.
Color Doppler ultrasound of the common carotid artery shows good vessel patentcy and unidirectional flow.
Grayscale ultrasound illustrating shadowing calcified atherosclerotic plaque in the carotid artery.
Pulsed Doppler ultrasound of the internal carotid artery shows an arterial pulsation waveform with a peak systolic (PS) velocity of 150 cm/s. This velocity suggests a 50-69% stenosis of the internal carotid artery according to an often referenced consensus statement that relates velocities to stenosis. The narrower the pipe the higher the velocity, akin to a nozzle at the end of a garden hose.
Table showing the parameters used in carotid ultrasound for estimating the degree of stenosis. The higher the peak systolic velocity (PSV) in the internal carotid artery the greater the estimated stenosis ("Primary Parameters"). The internal carotid artery to common carotid artery PSV ratio and the internal carotid artery end diastolic velocity (EDV) are additional parameters that are considered.1
References
1. EG Grant et al. Carotid Artery Stenosis: Gray-Scale and Doppler US Diagnosis-Society of Radiologists in Ultrasound Consensus Conference. Radiology. 2003 Nov;229(2):340-6.