Duplex ultrasound is named as such because it produces two components: images and waveforms. Technically, it’s a triplex because it can also be performed live!
The two-dimensional (2D) imaging component of duplex ultrasound is in black and white, and allows visualization of arterial plaque. Color Doppler imaging confirms that plaque is not an artifact. As well, the velocities measured within the Doppler waveforms give a percentage of obstruction.
When should you order an arterial duplex ultrasound?
There are three main reasons for ordering an arterial duplex ultrasound:
- In the case of an abnormal ankle-brachial index (ABI).
- If there is suspicion of an aneurysm.
- If there is suspicion of a pseudoaneurysm.
Arterial duplex ultrasound allows you to view the arterial walls and lumen. It also records waveforms and velocities of blood flow to determine the level and extent of arterial disease. In contrast, an ABI test only gives you information about a circulation problem and the degree of disease—but not the location.
What will you see on duplex ultrasound?
Before we get into the different available modes, let’s review what a healthy artery looks like on a duplex ultrasound. On a 2D image of a normal artery, the lumen will be black and the walls will be smooth. Color imaging can be used to confirm that the vessel is healthy; on a healthy vessel, the color should fill the lumen from wall to wall with no aliasing (e.g., bright spots), and you should see no evidence of plaque.
Next, the transducer is moved distally along the vessel to look for plaque. If plaque is present, it can be visualized as changes within the lumen.
Calcific plaque is echogenic (e.g., bright) and has acoustic shadowing, which is a black shadow that is produced because ultrasound cannot penetrate plaque. Some patients with diabetes and atherosclerosis have calcification that can also produce acoustic shadowing.
How to use different duplex ultrasound modes
Using the color flow Doppler mode
If any plaque is seen protruding into the lumen, the area is further examined with color flow Doppler imaging. Color flow provides a visual representation of blood flow around arterial stenosis. Aliasing occurs as blood velocity increases within stenosis, and is visualized as bright areas within the color flow.
Color aliasing in the presence of stenosis can appear as a few spots on ultrasound. However, as stenosis worsens, it will present in multiple areas.
A string sign is a classic sign on color flow imaging which indicates that minimal flow is getting through arterial stenosis. Often, a color bruit can be seen above the string sign. This is produced by massive vibrations from tight stenosis.
A Doppler bruit is also possible. Both Doppler and color bruits are similar to audible bruits which can be heard with a stethoscope over stenotic carotid and femoral arteries.
Using the power Doppler mode
An additional feature called the power mode can be used to detect blood flow in tight areas of stenosis. It is sensitive enough to differentiate tight stenosis from occlusion. As well, the color maps for a power Doppler ultrasound are adjustable with each machine.
Power Doppler ultrasounds can be used to investigate if there is a presence of flow around the plaque, which would prove that the vessel is not occluded.
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