3. Instrumentation and physical principles of carotid (Duplex) ultrasound

What you need to know before you start to practice carotid ultrasound.

3.1 Which ultrasound scanner and transducer should you use?

Carotid ultrasound is performed with a linear array probe. Such probes have a large aperture and provide high spatial resolution in the near field. Handheld ultrasound systems have been found to be comparable to mid and high range ultrasound systems (for point of care purposes). In general, it is advised to use scanners with high spatial and temporal resolution since soft plaques can easily be missed if image quality is poor.

It is recommended to optimize your settings and to define presets specifically for carotid Duplex ultrasound. Aside from a good (high resolution B image) you will need color Doppler functionality and pulsed wave (spectral Doppler) to assess flow and to quantify the degree of stenosis. Many systems also offer automatic spectral Doppler tracking (automatic velocity measurements) or automated intima media thickness calculations. In addition, you will find vendor specific color and B-Mode flow options such as: power Doppler or B mode flow. Recently there has been large interest in contrast enhanced carotid (CEUS) ultrasound. This modality provides improved flow visualization, helps to differentiate true structures from artifacts, plaque delineation (ulcerations?) and to detect neovascularization of plaque. Specific settings are required for contrast-enhanced ultrasound (CEUS).

 

 

3.2 Which settings are important when scanning the carotid arteries (carotid ultrasound)?

• Adjust the brightness and contrast of the monitor first! The setting you chose depends on the lighting of the room. The brighter the room the brighter you will also need to adjust your monitor.
• Use a high imaging frequency. The higher the imaging frequency the better is the spatial resolution (however you will loose penetration).
• Adjust the depth to make sure that the artery fills the screen and that structures posterior to the carotid artery can still be seen (depth setting is usually 2-3cm)
• Adjust the focus point at the level of the arteries

 

• Adjust the gain (avoid over- or under gaining)
• Adjust the time gain compensation
• Image contrast: a higher contrast helps to delineate “boarders” but the higher the contrast the less tissue information you get (you have less shades of gray in the image)
• Tissue harmonic imaging: is a technique where the harmonic properties (resonance) of tissue are used to create the image. Its main advantage is that you have fewer artifacts. This modality is the preferred mode of imaging. Don’t turn it off!
• Compound imaging: combines images from several steering angles to one image. Using multiple images to look at a specific region reduces artifacts and noise form the image.

Note: many scanners have an image optimization button (auto optimization) that adjusts the gain and the contrast. While this button is often helpful, don’t rely to heavily on this function.

 

 

3.3 What is the angle of insonation in carotid ultrasound?

The angle of insonation is defined as the angle with which the ultrasound beam hits the surface of a certain structure (i.e. the carotid artery wall). The more perpendicular the angle (90°) of insonation is the more ultrasound rays are reflected back to the ultrasound transducer and the better the image quality will be.

Angle of insonation

 

 

3.4 What is pulsed wave (PW) spectral Doppler in carotid (Duplex) ultrasound?

Pulsed wave Doppler

 

 

3.5 What is Color Doppler (Duplex)?

Color Doppler ultrasound uses the same principles as pulsed wave Doppler. Within a region of interest (ROI – Color Doppler box) the system looks at many different “sample volumes or pixel areas” and calculates the velocity and direction of flow (for each individual area). This information is then encoded in color according to a color map scheme (which can be chosen by the operator) and displayed for each imaging frame (dynamic color flow imaging). Usually flow towards the transducer is depicted in red and colors away from the transducer in blue (unless the color encoding is inverted).

The color map also assigns a different shade of color to the absolute velocity of flow. Usually the lighter the color (yellow or light blue) the higher the velocity. Note that a color shift occurs if velocities surpass the aliasing velocity (also called Nyquist limit). Color Doppler allows us to directly visualize flow (visualize vessels) and helps us to see areas where the flow velocity is elevated (aliasing).

 

 

3.6 What is aliasing in ultrasound and how do we use it during carotid Duplex sonography?

Aliasing

 

 

3.7 Carotid ultrasound – artifacts

Artifacts are alterations in the image, which are not true structures but produced by physical phenomena of ultrasound and its interaction with tissue. Artifacts can occur in both good and poor image quality and typically occur with implants (i.e. metallic stents) It is important to know which potential artifacts can occur to avoid misinterpretation and misdiagnosis. Here are the most common forms of artifacts:

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