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Hey, everybody. My name is Cos Bergen.

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I'm an associate professor of radiology and the chief of ultrasound here at UAB.

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We're going to talk a little about vascular ultrasound today, and I think we're

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going to try to demystify it. My goal is not to turn everybody into a vascular

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sonographer, but to help you understand what you're looking at, why the waveforms

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behave the way they do, and how to recognize some of the major pathology that

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you're going to encounter in day-to-day practice.

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By the end of today's talk, you really should understand the basic components of a

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duplex ultrasound. We're going to talk very briefly about Doppler physics that make

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all that work and talk about some important waveform patterns and apply some of

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these hemodynamic principles to some of the common exams that you're going to see.

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So just a brief intro to vascular ultrasound before we dive into pathology.

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Everything we do is based on blood flow and how that flow is going to change in

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response to disease.

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So why do we do this? Well, vascular ultrasound is one of the most useful tools

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that we have. It's non-invasive. It doesn't use radiation. We can do it bedside.

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And importantly, it really provides some real-time physiologic information.

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So unlike your CTAs and your MRAs, we're not just seeing that anatomy.

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We can really impact that hemodynamics.

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So first, let's talk about duplex ultrasound.

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Well, the interesting thing about this is this combines a number of different

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modalities. So it combines the B-mode gray-scale imaging, it combines the color

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Doppler, and then it combines the spectral Doppler waveform.

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Technically, because we're doing three things, it actually is called triplex

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imaging, but everybody just still calls it duplex ultrasound.

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So this tells us from the B-mode standpoint what the vessel looks like.

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Color Doppler gives us direction where blood flow is, and then spectral Doppler

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gives us directionality plus how fast it's flowing as well.

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So the Doppler principle, it's the foundation of vascular ultrasound.

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The moving red blood cells cause a frequency shift in those reflected sound waves

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from the beam. And so the important concept is angle.

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We only can measure movement towards or away from the transducer.

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And so when you get to 90 degrees, you can't really see any measurable Doppler

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shift. And actually, once angles go over 60 degrees, the velocity calculation

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becomes essentially very inaccurate.

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And so that's why proper angle correction matters.

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So you're always going to want to look for an angle that's below 60 degrees.

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So color Doppler is pretty important because it's going to display direction and

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relative velocity. And so one common misconception is that red always means artery,

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blue always means vein. So that's not going to be true.

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Red simply means flow in one direction relative to the transducer, typically

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towards the transducer, and blue means the opposite direction, away from the

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transducer. So always check your color map on the side of the image because the

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sonographers can actually invert that scale, sometimes intentionally and sometimes

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not. So you want to check that when you look at your images.

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One important artifact to talk about in Doppler imaging is aliasing.

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It occurs essentially when the velocity exceeds that system's ability to sample

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them. So there's something called the Nyquist limit, where once the Doppler shift

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exceeds that limit, you start to have aliasing.

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And so there are ways to adapt to this, and so you need to have your pulse

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repetition frequency actually double that Doppler shift to be able to accurately

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measure it. There are ways to increase the pulse repetition frequency, so

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increasing your scale, so making a wider range of frequencies will increase that as

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well, and increasing the depth will also increase the aliasing.

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So the closer you are, the less you're going to be likely to see the aliasing.

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So here's some examples of what this look like.

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This is classic color aliasing that you can see here in the color image.

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We have this very different colored signal in that area.

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You can see it is a much brighter signal in a different direction.

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And so instead of showing this typical smooth color transition that you might see

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with a waveform, the signal actually wraps around.

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So you can see a little bit to that waveform at the top and bottom of the screen

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because it is exceeding that frequency.

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And so this is reflecting the turbulence in that flow in that area.

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So there are actually ways to correct it.

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One of the ways that you can do that is actually just shift the baseline.

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So you can imagine if there's wraparound, if we adjust where that baseline is,

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you're going to be less likely to see that wraparound.

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We talk about increasing the scale and that pulse repetition frequency.

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You can also use a lower frequency transducer, so something that is going to do a

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little better job of penetrating will also improve your aliasing as well.

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You can also increase the Doppler angle, which actually decreases that frequency

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shift, but you can imagine this, as we talked about earlier, has some impacts of

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potentially making things less accurate.

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So all of these have trade-offs, which is why aliasing is not necessarily easily

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fixed all the time. So finally, before we jump into waveforms themselves, we're

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going to talk about arteries versus veins.

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So when you're looking at Doppler waveforms, you want to try to figure out what

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you're looking at because a lot of times these things are going to run directly

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right next to each other and you want to know which is which.

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So arteries are going to be pulsatile, and so you have more fluctuation in the

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waveform because they're reflecting the pressure that's generated by the cardiac

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cycle. Veins, on the other hand, are what we would call phasic, where you do have

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some up and down movement, but the pressure fluctuation is a lot less.

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And so this is probably the fastest way to identify the vessels.