Practice Perfect 985
Duplex Arterial Ultrasound for Podiatrists
Part 2: Images and the Report

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Welcome back to Part 2 of our mini-series on arterial duplex ultrasonography. In Part 1, we covered the basic physiology to provide a foundation to help with what we will discuss today: the actual images and the report. Let’s get right to it!

The ultrasound itself is obtained by an ultrasonographer trained in this technology. With the patient lying supine, they will start at the common iliac artery, working their way distally, and ending at the dorsalis pedis and posterior tibial arteries. They will obtain a reading at the proximal and distal ends of each artery. They will obtain the images we showed in last week’s Practice Perfect 984 – Duplex Arterial Ultrasound for Podiatrists Part 1: Physiology, so the physician is able to see the B mode (looking at the anatomy itself as well as the compressibility of the artery) and the color mode (to see the flow). To make things easier to read here, I’m going to focus on the spectral Doppler, which shows the color mode and the waveforms.

Figure 1. Normal spectral Doppler image.

When examining the images, we are looking at several pieces of information: the appearance of the artery itself, the waveform shape and line thickness (also known as the spectral window), and the peak systolic velocities. In the image above, the peak systolic velocity is 184 cm/s.

What the Doppler Image of the Artery Tells Us

When examining the artery itself, using a longitudinally oriented probe, we can see a generally confluent reddish-orange color with very slight blue and no narrowing. Remembering the Doppler effect, blood flowing toward the probe will appear red, while blood moving away will appear blue. However, if there is turbulent flow due to narrowing and/or the presence of an atherosclerotic plaque, the flow in the artery will become multidirectional, which is picked up by the Doppler probe as shown in Figure 2. These focal color changes are termed aliasing.

Figure 2. Turbulent flow in an artery, showing the multicolor appearance of blood flowing in several different directions relative to the ultrasound probe, consistent with aliasing.
 


On the bottom of Figure 1, you can easily see the normal waveforms, which are mostly biphasic (one tall peak above the line, which is the velocity of blood traveling toward the probe, and a smaller peak showing blood traveling away from the probe).

Examining the Waveforms Themselves

We want to see tall biphasic or triphasic waveforms, with thin lines, and a monomorphic appearance (ie, similar waves appearing throughout the study) as shown in Figure 3. This figure also defines the other aspects of the waveform as they relate to normal arterial pulsations.

Figure 3. Normal waveform or spectral window.



However, narrowing of a vessel will cause turbulent flow, changing the shape of the waveform and creating thicker lines and a wider peak (termed spectral broadening) as in Figure 4.

Figure 4. Ultrasonographic appearance of a spectral window with widened, thicker peaks is consistent with spectral broadening in PAD.
 


One may also see a bruit, as in Figure 5, another indicator of turbulence. This is the visual representation of what one may auscultate in an artery with narrowing, secondary to PAD. For those still learning these skills, the next time you have a patient on hemodialysis, place your stethoscope on the patient’s fistula and listen to the bruit (or thrill) generated by the turbulent blood flow.

Figure 5. Bruits picked up on ultrasound (3 red arrows), demonstrating turbulent flow in a left anterior tibial artery. Note also the spectral broadening (very wide bases of the waveforms) and the mild aliasing on the color Doppler image.



Another important waveform change is the tardus parvus waveform (Figure 6), which is seen in vessel stenosis distal to the narrowed area secondary to low velocity and variable flow. The “tardus” portion of the wave is indicated by the red line in Figure 6, which is a prolonged systolic acceleration (ie, slow upstroke). The “parvus” aspect is the small systolic amplitude and rounded peak. Tardus parvus is Latin for “slow and small.”

Figure 6. An example of the tardus parvus waveform demonstrated in a distal superficial femoral artery (SFA) with severe PAD. Note also the very low peak systolic velocities indicated by the number on the right side of the image (~ 52 cm/s is very low for the SFA).
 

Understanding Peak Systolic Velocities

Up to this point, we’ve examined the appearance of the artery on the color Doppler image and the waveforms themselves (the spectral window). The final part of the ultrasound puzzle is to examine the reported peak systolic velocities (PSV). These will be reported separately from the images (which are typically not included in the report unless you examine the images yourself) for each artery in the lower extremity tree. The key to this part is to compare the area of stenosis with an adjacent more proximal segment to obtain a velocity ratio and use that to estimate the severity of PAD (Table 2).

The typical normal velocity ranges are shown in Table 1. If you recall from the hemodynamic principles that a narrowing of the artery lumen will lead to increased flow, then we should expect to see an increased PSV at the area of stenosis and a relative decrease in PSV just beyond the stenosis (which is why we see the tardus parvus waveform described above).

Table 1. Absolute normal peak systolic velocities compared with elevations seen at areas of stenosis.



Table 2. PSV ratios and interpretations to estimate the severity of a stenosis.

Let’s finish up this discussion with a few helpful rules of thumb:

Clinical Pearls

- Low cardiac output may lower all velocities.
- Collateral vessels may bypass an occlusion, causing underestimation of
disease severity.
- Diabetic or calcified vessels may cause acoustic shadowing, reducing accuracy.

In next week’s edition, we’ll review a few examples of arterial ultrasounds to help bring all these concepts together.

Best wishes.

Jarrod Shapiro, DPM
PRESENT Practice Perfect Editor
[email protected]