Principles Tune Up: How Sharp Are You? - SD

Principles Tuneup: Vascular Doppler Ultrasound

On this presentation, we will be reviewing principles that are involved with vascular Doppler ultrasound. We'll do it in a question and answer format where I will present a question to you, give you an opportunity to think about the answer and then present the answer.

There will be a range of difficulty in the questions, and my desire is that no one that views this program will get them all wrong and that no one will get them all right so that everyone can learn something, but nobody is discouraged by not knowing enough.

I've tried to gauge it at what I would think would be the midpoint of the audience ability, and I trust that after we go through this q and a exercise, you will know what you already knew, but also will have learned some new things.

In this presentation, we're going to review principles, and I'm doing it in a q and a format, we'll call it principles tuneup. And this is a self-evaluation for you to determine how sharp you are with regard to these principles that are relevant to vascular Doppler ultrasound.

We will cover five major topics in this review, The Doppler effect, blood flow, and some facts about blood flow that are relevant to what we measure with the Doppler effect in vascular work. We'll see how we present the Doppler information on our instruments in the form of color Doppler displays and in the form of spectral Doppler displays. And then we'll look at some things that can go wrong in all of this, and they are called Doppler artifacts.

We'll be covering about 60 questions in about an hour, and they will have varying difficulty and they are not ordered in difficulty. So we will encounter some easy ones and some difficult ones as we go through this. Not in any increasing order of difficulty, but random order of difficulty.

But we'll start out with one that is pretty simple, very fundamental, and that is that we are using ultrasound to accomplish this. So we at least want to be sure that we know what ultrasound is.

Definition of Ultrasound

And ultrasound must be some form of sound because that is in the word. And so ultrasound is sound, that is to what? The prefix ultra means beyond. So we're talking about sound that is beyond in some sense, and specifically we're talking about sound that is beyond the frequencies that we as humans can hear, namely above the highest frequencies that we can hear.

And so the complete definition here would be ultrasound is sound that is too high in frequency for humans to hear. Sound is a propagating or traveling variation in pressure. It is a pressure wave or a sound wave. And by a wave we simply mean a traveling variation in something.

Most of the waves that we're familiar with, like light and radio and TV and microwaves, x-rays and so on, are traveling variations in electric and magnetic fields. But sound is a traveling variation in pressure and frequency for any wave is just how many complete variations in whatever it is. That's varying pressure in this case, how many complete variations or cycles occur in a second?

Humans can hear sound frequencies. We say up to about 20,000 cycles per second or 20 kilohertz. That's really not true for most of us. We peak out in our hearing at about age 15 and it's all downhill after that. We're aging before we even get our driver's license, and we lose about three or four hertz off the upper limit of our hearing every week.

But we'll pretend as we often do for definitions that we all hear up to 20 kilohertz and we then have a nice precise definition. So that will be our definition of ultrasound. It is too high, namely above 20 kilohertz, for us to hear.

Well, we want you to be more specific than that. You heard a train horn, and that was Amtrak, but specifically you just heard the doppler effect.

Applications of Ultrasound in Medicine

Now with ultrasound, we do two things primarily in medicine. We can do therapeutic ultrasound and we can do diagnostic ultrasound. In diagnostic ultrasound, we can do two things primarily, and that is anatomic imaging. And secondly of interest. In this discussion, we can detect and measure and evaluate motion, which can be the motion of moving tissue, which mostly would be the heart, that is our mechanical pump.

And so it is continually contracting and relaxing and that motion certainly is of interest in evaluating the function of the heart. And then the result of that, of course, is blood flow. And in all areas of the body we're interested in blood flow.

So for evaluating the motion of the myocardium as the heart pumps or evaluating the motion of the resulting blood flow, we can do that with ultrasound and we will use the Doppler effect to accomplish that. You just heard the Doppler effect.

The Doppler Effect

The doppler effect is a change in frequency caused by a change in wavelength caused by motion. In the case of the demonstration that we just had, it was the motion of the train engine and the horn on that engine as it passed by. This caused a change in the frequency that we heard, although the frequency of the horn that was being produced was not changing, but what we heard was changing because of the motion of that source of sound, namely the moving horn that caused a change in wavelength.

As you can see on this diagram, because of the motion of this source of sound s it is moving to the left. And because the speed of sound is determined by the medium through which it is traveling and namely air in this case, the sound travels through air at about 740 miles per hour, the motion of the sound source does not increase the speed of the sound. It is still traveling at the same speed.

So in fact, the source is moving into its own sound. And the result of that is that the pressure peaks, which are indicated by these lines, are pushed closer together. The distance between these peaks is the wavelength, and you probably already know that shorter wavelengths are associated with higher frequencies.

So as this source is approaching us, we are experiencing a shorter wavelength. Therefore, a higher frequency is also true as you can see, that after the source passes by and the wavelength is stretched out here, that we're going to experience a longer wavelength and therefore a lower frequency.

Now our interpretation as humans of the frequency of sound that we hear is its pitch. And so the way we normally experience the Doppler effect is that we will hear a higher pitch of the sound of a moving sound source as it approaches us, and then as it passes by, it'll shift to a lower pitch, the classic race car sound.

So here we have completed these blanks. The Doppler effect is a change in frequency caused by a change in wavelength caused by motion, but the motion is the motion of what. Well, we just saw that in our example, it was the motion of the sound source, but actually the Doppler effect can occur three different ways.

One is the motion of a moving source of sound, but we could have a stationary source of sound. And if we are a moving listener or observer or receiver, we will hear a different frequency as we approach that stationary source and as we go away from it.

Thirdly, we can have a moving reflector. Moving reflector by definition is a combination of a receiver and a source because a reflector by definition takes whatever it receives and sends it back. So a reflector is both a receiver and a source.

Now we have to clarify something here. In fact, when we're talking about a receiver, there is no change in wavelength. We have a stationary source. Now, as you can see in the upper right hand diagram, so the sound waves or cycles, the pressure cycles are moving out away from that source uniformly, not non uniformly, as in the upper left hand diagram where we had the motion of the source.

Now we have a stationary source and you can see that the pressure peaks travel out uniformly so that we would hear the same frequency, which in fact is the frequency that the source is emitting no matter where we were listening from, as long as we are a stationary listener.

But if we are a moving listener coming from the right for example, then you can see we're going to encounter more cycles in a second than we would if we were stationary. Therefore, as we approach the source, we will hear a higher frequency than what was actually emitted.

And then as we continue now away from the source, we are riding along with the sound in the same direction in which it is spreading out. And so we will encounter fewer cycles every second and we will hear a lower frequency.

So the doppler effect can occur in these three ways. It is the third one that really is the relevant one for us because we will be dealing with moving reflectors, namely either moving tissue or moving blood cells.

Where the Doppler Effect Occurs in Flowing Blood

But precisely where does the Doppler effect occur when we have flowing blood in a vessel? And I have four possibilities here. And then another possibility is none of those at the erythrocyte boundary, at the leukocyte boundary, at the transducer surface inside the blood cells.

And the answer is none of the above because the erythrocytes and the leukocytes, namely the red blood cells and the white blood cells are moving along with the blood plasma, it's all moving along together. The cells are not swimming through the blood.

So this situation is different than the train engine moving through air here. Everything's moving along together. So there is no relative motion between the cells that scatter the sound and the liquid that is suspending them. It's all moving along together.

And so there is no doppler effect caused by cells moving through the liquid because they aren't. So where is the doppler effect occurring? Well, the doppler effect occurs where you have relative motion and namely that's going to occur at the vessel boundary where we have a boundary between stationary tissue and then we have blood flowing when we move inside the vessel.

And of course at the vessel wall, it's not moving fast at all, it's moving very slowly. And then we will go through various layers of increasing flow speed of the blood as we approach the center of the vessel. This is the nature of fluid flow. It's we call it laminar flow, Latin for layer and it's layers sliding over each other with ever increasing flow speed as we progress toward the center of the vessel.

Then as we continue on toward the far wall, we will have layers of decreasing flow speed until we get to the far wall where it's hardly moving at all. It is the relative motion between these layers that is causing the doppler effect to occur as we go from one layer to the next.

Role of Blood Cells in Doppler Ultrasound

So what then is the role of the cells in Doppler ultrasound? Well, without the cells, we wouldn't get any echoes back. The doppler effect would in fact be occurring as we go through the various layers, but there would be no way to detect it because there wouldn't be any echoes coming back.

But because of the presence of the cells in the blood, we get echoes back that are doppler shifted, and the doppler shifts that we encounter depend on where those cells are, how fast those layers are moving that are generating the echoes from the cells in them.

The Doppler Equation

Well, what is the relationship between the frequency change caused by the motion of these reflectors, these blood cells and the speed of those blood cells? The Doppler instrument, the Doppler ultrasound instrument is going to detect the frequency changes. It's going to be observing the difference between the returning echo frequency and the frequency of the ultrasound that was sent in.

But what we wanna know is not a frequency difference. We wanna know a physiologic quantity, namely the speed of the blood flow or the speed of the motion of the tissue in the case of the heart. And thanks to Professor Doppler who back in the 18 hundreds determined what the cause of the Doppler effect is.

And we have honored him and his work by naming the effect after him. We call it the Doppler effect, that he studied various details about this doppler effect and he determined the relationship between the change in frequency caused by the motion that is the Doppler effect and the motion that caused it.

And this is the Doppler equation that relates those two. There are really three Doppler equations, one for each of the situations that I mentioned earlier, whether you have a moving source, a moving receiver, or a moving reflector. It is the third one that we're interested in. And this is the equation for a moving reflector.

This is the Doppler shift frequency. By definition, it is the returning echo frequency minus the transmitted operating frequency that we sent in to the body. We commonly call it just the Doppler shift, and it is the difference between those two frequencies.

Professor Doppler figured out that that doppler shift is related to the speed of the reflectors in this equation, but there are some things that are missing. So this is called what I already gave you the answer to that. This is called the Doppler shift and what goes in the numerator here, the relationship between the doppler shift and the speed of the tissue motion or the blood flow.

There's something else up here and it is two times the operating frequency, the frequency we send in. So the dopper shift does depend on the operating frequency. The factor of two here is a result of the fact that we're dealing with a moving reflector. And remember I reminded you that a moving reflector is a combination of a moving observer and a moving source, and that's why we have a double doppler effect here.

What goes in the denominator? Well, that is the speed of sound and tissue. Well, the instrument is going to observe the doppler shift. It knows the operating frequency, it knows the speed of sound and tissue. So the only thing that's left is the speed of the motion, which it then can present to us in various ways.

The Doppler Angle

Well, there's one complication in this. I made an assumption And that involves something here that is called what You can see that it's an angle. It is often designated by the Greek letter theta as I have shown here. And it is the angle between the direction of the sound beam and the direction of the motion. In this case, the motion of blood in a vessel.

That angle is called the Doppler angle. It's another thing that Professor Doppler studied and determined that the doppler shift does depend on that angle. So what I assumed previously was that the sound beam is parallel to the flow as if the transducer were inside the vessel.

Now in vascular work, it's almost never a parallel situation. There may be some abdominal situations where the beam would be parallel to the flow and cardiac doppler work. It can be, apical view of the heart where the beam is going through the ventricle and the atrium in a way that is approximately parallel to the flow of blood from the atrium into the ventricle.

In that situation, the doppler angle can be ignored. In other words, it can be assumed to be zero. And if the actual angle is within 20 degrees of zero, then the error will be 6% or less. And the error is always a negative error. In other words, the value that you get for the speed of motion is going to be smaller than it should be if you assume a zero angle. And it is not zero.

But in vascular work, we're commonly working in much larger angles. Typically, for example, in the carotid artery, we'll have an angle of about 60 degrees. And in most vascular work we're going to have angles probably between 30 and 60 degrees, and we certainly cannot ignore the angle there.

So what is the angle dependence? It appears in the Doppler equation how we already saw the Doppler equation, but we didn't have the Doppler angle dependence in there. So now we'll insert it and it is the co-sign of that angle. Now we don't need to worry about the trigonometric details of that, but what it does mean is that as the doppler angle increases, the doppler shift will do what? Increase, decrease, remain constant, gets stronger.

Well, it will decrease as indicated by this diagram. If you are familiar with the trigonometry in cosine, you'll remember that as the angle goes up, the cosign does go down. But this describes it in a graphical way where we see that if the beam is parallel to the flow, we will get the maximum doppler shift.

If the beam comes in at 10 degrees off from that, then we will get a slightly smaller doppler shift. At 20 degrees it'll be down by 6%. As I said earlier, at 60 degrees, it is just half the doppler shift that we would get at zero degrees.

So if all of these different doppler shifts are corresponding to the same flow just observed at different angles, then clearly when we make an observation of doppler shift to get the correct calculation of flow speed, we have to know what that angle was. Well, that's just another way of saying that the dopper angle is in the dopper equation.

And as a sonographer does a Doppler scan, the incorporation of the angle into that process is an important part of carrying out that scan and the operator is responsible for helping the instrument with that.

Well, I've rearranged the Doppler equation a little bit here. Just to illustrate something, the ratio of the doppler shift to the operating frequency is approximately equal to what? Well, we can see that the ratio of the doppler shift to the operating frequency, we just brought this over from the right hand side where it was in the numerator for in algebra.

As we cross equal sign it would go into the denominator and we could see that this ratio then would be approximately equal, to that ratio. In fact, if we had an angle of 60 degrees, then the cosine will be one half and two times one half just becomes one. So at 60 degrees this ratio will be exactly equal to this ratio.

But if the angle is not 60 degrees, it'll still be approximately equal to the speed ratio. Well remember this is the speed of motion. We'll say blood flow, and this is the speed of sound and tissue. So what would that ratio be? Typically our doppler shifts are approximately what fraction of the operating frequency then?

Well, it would be the ratio of the two speeds, approximately the same fraction as the ratio of the flow and sound speeds. So what's a typical flow speed? Well, we can have a range, a large range, but a typical mid range flow speed we could say would be 154 centimeters per second, over the whole body, that would be a little higher than normal in the carotid artery, but it's a typical number, mid range over the large range of speeds that we encounter in flow in the body.

And what's the speed of sound and tissue? 1540 meters per second is the common number that's known for that. But that's also 154,000 centimeters per second to put it in the same units as our flow speed units. And so you can see that that ratio is one over a thousand.

And so typically our dopper shifts are going to be one 1000th of the operating frequency. We're operating in a frequency range of about two to 10 megahertz for diagnostic imaging and Doppler purposes over that frequency range. We get good resolution. We get the penetrations we need and so our ratio is going to be about one over a thousand.

Therefore we have operating frequencies in the megahertz range. We will get dopper shifts in the kilohertz range.

Blood Flow and Doppler Measurements

So this is our physiologic situation where I've illustrated it with a gorge, but it illustrates the physiologic disease situation where this would be the plaque, this would be the stenosis. And in the stenosis we will have high flow speed because we have to have the same volume flow rate occurring as we go through this narrow channel.

As we go through the gorge, we're not creating or destroying water. So every gallon of water that goes down the Broad River also has to go through the gorge. And because it's a narrow channel, it will have to pass through at a high speed to accomplish, to maintain the volume flow rate.

So we will have a high flow speed in the stenosis and then as we broaden out into the wide river, we will have turbulence in the transition from high to low speed. We have the same situation as we have plaque buildup in an artery and we have a stenosis, a narrowing of the lumen.

We will use doppler ultrasound to evaluate that situation. And the doppler ultrasound is very useful in this situation because it can give us four types of flow information. What are the four types of flow information that doppler ultrasound will give us that will be so helpful in this kind of situation?

Well, here we have an example where we're presenting color doppler and spectral doppler in the popliteal artery behind the knee. And looking at this illustration, we see the four types of information we get.

First of all, we know there is flow in this vessel. That's a question sometimes is it a patent vessel or not? We have color doppler here. We have spectral doppler occurring here. So we know that something is moving in this vessel and of course it has to be blood. So that's the first thing that we have here is the presence of flow.

Secondly, we know the direction of flow. We expect the flow in the artery to be toward the feet away from the heart and over part of the cardiac cycle. At least that's true, but here we have a flow reversal where the blood is now flowing backwards in an artery toward the heart.

Now that would not be normal over the entire cardiac cycle, but it certainly can be normal to have flow reversals in portions of the cardiac cycle. In fact, this reversal in this location is quite normal.

Thirdly, with proper incorporation of the angle here and the operator has placed this line parallel to the vessel walls, the typical way of doing this to tell the instrument this is the direction that we think the flow speed is instrument already knows the direction of the beam is indicated here. So it will determine the angle between the two, which in this case is 60 degrees and it will calibrate the vertical axis then based on that.

In other words, it's inserting the cosine of 60 degrees into the dopper equation and solving the dopper equation, calibrating the vertical axis appropriately so that in this example we see that we have a peak systolic flow speed of 70 centimeters per second and that will be correct if the angle incorporation is correct.

Now, speed information is certainly useful because as we've already seen, if we have a narrowed lumen or if we have a stenotic valve, a calcified valve that's not opening completely. In both cases we will have a smaller area through which the blood has to flow and therefore it will have to go through faster just like putting your thumb over the end of a garden hose.

So as disease progresses in a vessel or in the heart, we expect associated with that for there to be an increase in the flow speed. And finally if we have a hemodynamically significant stenosis, then we will have turbulent flow downstream from that. And so it's important to determine what kind of flow we have.

Is it normal laminar flow, is it disturbed from that or is it the ultimate in disturbance which is turbulence? And doppler can give us that information also.

So we have a lot of information here and that's why we're talking about doppler ultrasound. Do we have flow or not? Which way is it going reverse flow in a vertebral artery for example can tell you that you have an occlusion somewhere else. Reverse flow from ventricle into atrium in the heart tells you you have a valve that's not closing properly because it's there to prevent that.

How fast is the flow that's associated with the presence or absence of disease and even the progression of disease? And finally, what kind of flow do we have? A lot of information and that's why doppler ultrasound is so useful to us.

Effects of Plaque Buildup on Flow

Well, as the local lumen area decreases when plaque buildup is progressing, what happens to the flow speed in the lumen and what happens to the volume flow rate in that vessel where the plaque is progressing and you have four choices there and the possibility that more than one of 'em is correct?

Well you may have chosen number three, that would be a good answer. As the luminous decreased, we expect the flow speed through it is gonna have to increase and we also would expect that the resistance to flow in this vessel is increasing with the presence of that plaque and that would cause a decrease in the volume flow rate in that vessel as the resistance to flow increases.

So that all makes sense, however, there are two answers that are correct. How could the flow speed in the lumen decrease? How could answer number two be correct?

Well this is an off quoted graph that illustrates what happens as plaque builds up and the lumen area decreases. The horizontal axis is a decrease in lumen diameter and area as you go from right to left. The red line is the volume flow rate and the blue line is the flow speed through the lumen.

What do we expect? Well, as the disease progresses, the plaque builds up the lumen area and diameter decrease. We expect the flow speed to increase through that narrow channel. We expect the volume flow rate to go down as the resistance to flow goes up with the plaque progression and for quite a long time there really is a negligible effect on the flow because we are only decreasing the diameter of the lumen in the small portion of the entire circulation.

But at some point, and that's usually an area decrease of about 70%. Beyond that, we really have what's called a hemodynamically significant stenosis where it starts to affect, namely decrease the volume flow rate to whatever organ is being supplied by this vessel.

So we are interested primarily in an area like this where we begin to get into that hemodynamically significant region and therefore we're interested in flow speed increases, like this. However, as the blue line illustrates, at some point we must go through a maximum because we know that ultimately the flow speed has to decrease because it's gonna have to go to zero at occlusion.

So there must be a maximum as we see here and beyond that namely beyond about 85% decrease in area, we are going to have a decreasing flow speed as disease progresses. Therefore both of those answers were true.

Flow Patterns in Spectral Display

Well, when we have an increasing resistance, either because we have the presence of plaque, in other words disease or because something normally physiologic is going on to increase resistance, what effect will that have on the flow patterns that we have in spectral display?

Let me remind you that the spectral display is a quantitative presentation of doppler shifts on the vertical axis and time on the horizontal axis. On the vertical axis. Normally the calibration will be in flow speed units like centimeters per second with this solution of the dopper equation with the angle incorporation, what kind of flow pattern would we expect if we have proximal high resistance?

In other words, it's upstream from where we're making the measurement. Is it pattern number A, letter A or is it pattern B? And the answer is it is pattern A and it's called the tus parvus waveform. And those are two terms that describe a slowed acceleration in systole, that's tus and a blunted or reduced peak systolic flow speed value. And that's the parvus portion of the name.

What if we have a high flow resistance downstream or distal to the observation point? What kind of pattern would this produce? 1, 2, 3 or four? Well, with high distal resistance We're going to have the flow reversal and we will have the shutdown in late diastole and we do not have either of those in these three patterns. So the answer in this case would be number four.

Now I mentioned that this, high or low resistance can be a result of what's going on. Physiologically doesn't necessarily have to be disease, for example, this could be the flow pattern that we would have in the common femoral artery for an unexercised leg. But if we exercise the leg, then we would expect this pattern to go from high resistance, which it is to a low resistance pattern, which all three of the others are low resistance patterns.

As we exercise the leg, the muscles are calling for oxygen downstream, arterials open up the downstream resistance to flow goes down and we go from a high resistance pattern to a lower resistance pattern just because of physiologic change.

Presentation of Doppler Information

Well, doppler is going to give us useful information when we are looking at vascular diagnosis. How will this information be presented? Well, we've encountered it already. There are three ways that we can present this information and the ultrasound instrument will do them for us.

We can listen to the doppler shifts because they fall in the audible range. For us, the color doppler gives us the big picture, the global view. We usually do that first and then if there's an area of interest, then we will go locally into a small sample volume and present the domer information quantitatively. In the spectral display form we normally do color first because that's our big picture, that's our global view.

And if everything looks fine then we're okay. If we see an area of interest where the flow looks abnormal, then we can go in with the spectral display to that small sample volume, guiding it to the area of interest and quantitatively evaluating the flow there.

Color Doppler Displays

On a color Doppler presentation, the colors are assigned to the Doppler shifts arbitrarily. We must have a decoder on the display to tell us what the various colors mean. And this is the color map that is always on a display. We have an example of one here.

How do we decode the colors on this color doppler map? What would yellow indicate? Yellow is at the top of the map. So what does that tell us? Yellow on this map would indicate a large positive dopper shift.

These maps always have the baseline, which is zero doppler shift as black around the baseline the maps are always dark colors and as we move away from the baseline they become brighter and often we'll change color as is the case on this map. So on this map, a zero dopper shift is black as it always is.

A small positive dopper shift would be a dark red. And as we progress then with increasing positive dopper shift, in other words, something's moving toward the transducer with increasing speed, then it's go gonna go from a dark red to a brighter orange to a bright yellow. The color map extremes are always bright colors.

Now if we have a negative dopper shift motion away from the transducer at slow speed, a small dopper shift, that's going to be a dark blue on this map. And as the negative dopper shift increases, we go to a bright blue green or cyan color and that would be our large negative shift.

Looking at this color doppler presentation of flow in a vessel, We ask the question, why is the flow faster here? You can see that on this map as we progress to either a large positive or negative dopper shift, in both cases it goes to a bright white color. Now we know that we're progressing in the positive direction here because it starts out with a red color, but in here it's progressed all the way up to white, which would be a large Doppler shift.

Why would that be the case here? I don't see any narrowing of the lumen. There doesn't seem to be presence of plaque here. So why would it be so fast there? And the answer is that it's not really fast there, because of a turn in the vessel or because of the presence of plaque or reduced lumen.

The key here is to look at the frame rate. The frame rate is the number of pictures that we are presenting every second The unit is hertz. And so we have 5.8 or about six pictures being presented every second. So how much time does it take for the presentation of each frame here?

Well turns out to be about 172 milliseconds. That happens to be about 20% of the cardiac cycle. So what we are observing in that previous image is that if the frame rate gets low as it is here, then we have a significant progression in time from one side of that picture to the other. As it is being written out in 172 milliseconds.

Things are changing in that blood flow so that as we look at an image, we think of it as being an instant in time, but it's not. And if the frame rate is low, that can be a significant change in time as the picture's being written from one side to the other.

And that was the case that we had there where we had a change in time and we were going from some point here in the acceleration to peak systole and then we went through peak systole and then we're starting to decelerate. And so what we saw in the bright portion in the center is where the blood was flowing the fastest before that and after that, in other words, on either side it wasn't quite as fast.

So we are writing the color box not only in space but also in time, particularly at lower frame rates. That becomes a significant factor.

Well, it's a bird, it's a plane, it's super carotid. What happens when we have a tortuous vessel like this? It's not straight, not horizontal. So we're going to have a changing doppler angle here because the blood flow is changing direction and here we see we're not showing the map, but a increase in doppler shift would take us from a dark red to a brighter red to a bright yellow color on this map.

And we ask the question, why is the flow faster here and here are your choices. And the answer is that it may not be faster at all. There, there may be some narrowing here, but I'm not sure that there is and we don't have to invoke that at all.

What would be the dominant factor affecting the doppler shift is not that the flow is faster here, but that the doppler angle is smaller. You can see here that the beam, which is always parallel to the box air, box edges here, we're writing scan lines that are all parallel to this. That would be the beam direction.

And so in this region the beam is at an angle to the flow of probably 60 or maybe even 70 degrees. But then you see as the flow bends around like this, the doppler angle is going down and right in this region the doppler angle is about zero.

So as we move from here around to here, we're going from a doppler angle of about 60 degrees down to about zero. Therefore the doppler shift goes up as the doppler angle goes down. And that would increase the doppler shift as we see here without invoking any change in the Doppler speed necessarily at all.

And so this increase in doppler shift is simply a result of a decreasing doppler angle as the flow turns in this tortuous vessel.

How about in this aneurysm? Is the flow clockwise or counterclockwise? We have both colors blue and red. I can you answer the question? I hope not because you don't have enough information. Do you need more information? Yes you do. Specifically what do you need?

We always need the map. It's the only way we know what the color assignments are. We always have several choices about that in an instrument. So the only way that we know what the colors mean is to have the map that decodes them for us.

And so here's the map that says that positive dopper shifts are red, orange and yellow colors and negative dopper shifts are blue. So the left portion of this image is negative dopper shifts and the right portion is positive transducer is always at the top.

So if we have positive dopper shift on the right and negative on the left, that means upward flow on the right and downward flow on the left. Therefore counterclockwise flow in this aneurysm.

If we flip the map and saw this image, if that's what the map had been, it wasn't I just artificially inverted it here. But if that's what the map had been when this scan was done, then we would have upward flow on the left and downward on the right that would be clockwise flow.

What About when we cut through vessels in a transverse way? In other words, what is the angulation of the transverse scan plane? In this example we're doing a transverse cut abdominally, so we're cutting through the aorta and the inferior vena cava.

Well what can we tell from this? From the anatomic gray scale image? We really can't tell which way we're angled, but we can tell by the flow on that map. Positive doppler shifts Are red colors so the flow in the aorta is coming toward us or toward the transducer and blue is negative.

So the flow in the IBC is away from the transducer, therefore the scan plane transducer and scan plane have to be angled in such a way that the scan plane is angled toward the heart so that the flow is coming toward the transducer in the aorta and away from it in the IVC.

If we took the transducer and angled it over here so that the scan plane was going down that way, then the flow would be away from the transducer in the aorta toward in the IVC and we would see these two colors reverse as you go from this orientation over to this one.

How about this transverse cut through the external carotid artery? Is the flow toward or away from the transducer or the observer? Well it's interesting as you can see that in the ECA we have red and orange and yellow and cyan and blue colors. So what's going on here?

That would be positive doppler shift, positive doppler shifts in the center and negative doppler shifts in periphery. So do we have flow going away from us in the periphery and toward us in the center? How could that be? Well, in fact we do have flow going away from us in the periphery.

So what's going on in the center? Well, that is the fastest flow away from us. We expect the fastest flow in the center and it is the fastest flow away from us that in fact has alias. We have exceeded this limit, which is called the negative nyquist limit.

When we do that, we jump over to the other side and we get positive dopper shifts. But in aliasing the positive doppler shift does not flow toward us. It is the fastest flow away from us that has alias from one side of the baseline to the other.

Changing the color color display shows shifts at 90 degrees, 90 degree Doppler angle. Well here we have a convex array that sends out pulses in different directions out this way, this way, this way, this way, this way. So we have for a straight vessel or in fact it's a tube here with constant flow in it.

We have various colors here because we have a varying doppler angle out here. The doppler angle's about 70 here it would be about 80. Here it's 90 here it's 80 again here it's 70 again. So we have a doppler angle that varies across here giving us varying doppler shifts.

And in here we get what we expect and that is when the beam is perpendicular to the flow, we get no doppler shift, we get black, which is our baseline, no doppler shift. So the color Doppler display shows no doppler shifts at 90 degrees. And this statement therefore was false.

Why is that? Well, because in conventional color doppler imaging, we are detecting the doppler shifts with auto correlation mathematical process and that yields two things for us, yields the mean doppler shift. We always have a spectrum of doppler shift frequencies because we're encountering millions of blood cells and they're not all moving together by the very nature of fluid flow, not all moving together like a solid would.

And so we always get a spectrum or a range of doppler shifts even in normal flow. The color doppler will use auto correlation to determine the mean or average of those doppler shifts in that spectrum. And it will also calculate a measure of the spread around the mean, which we will call the variance that is the square of the standard deviation aviation in most color displays.

We're simply going to assign different colors to various values of the mean doppler shift. And so this is the spectrum of doppler shifts that are actually coming back to the instrument in color Doppler analysis. It's going to calculate the mean and maybe a measure of the spread, the variance, but usually we're not using variants. It's used some in echocardiography, but in vascular work it's hardly ever used.

And so we are assigning colors to the mean doppler shift. What happens to the mean doppler shift as we go across that previous image and the doppler angle went from 70 up to 90 degrees. What was happening is we were moving across here and this was in the center where the doppler angle was 90 degrees.

We got no doppler shift. And then as we moved over here, now the mean doppler shift is negative and we get the appropriate color for that. So there were five regions on that image. Let's explain all five of them.

Well we've just explained the first one and that is in the center where the dopper angle was 90, we got no dopper shift and it was black. How about over here? Well on this map as I've labeled it, these colors are positive dopper shifts, so that's positive. The blue is negative. And what's happening?

Well we have flow coming from left to right. So as we look to the left, we see the flow coming toward us. In other words, positive doppler shift. As we look to the right, it's going away from us and we have the negative doppler shift. So this is left to right flow And what's out here?

Well that's an aliasing region, whereas the dopper angle went down from 90 to 80 to 70. At this point the dopper shift went up enough that we alias over to the other side and over here another aliasing region.

But what's going on here? We seem it's the same situation but we have a very uniform presentation of color in here now. Well this is power, doppler power instead of assigning color to the mean doppler shift assigns it to something else and it gives us a very uniform presentation as you can see. And in fact we even have color at 90 degrees.

Our question here is, is the flow direction from right to right or to the left? Well remember from before it was to the right, but let's forget what we already knew and in fact let's blot all that out. And from the power dopper image, we can't tell the direction of flow and fact in power we lose three things, the direction of flow, the qualit, the qualitative speed indication of flow and the character of flow in power dopper, the only thing we have in fact is the presence of flow.

But in power dopper we have the presence of flow more reliably than we do in conventional color Doppler. Why is it that we're able to get color where it's very slow at the vessel walls and it was missing in the previous one and particularly here at 90 degrees, we didn't have any doppler shift before. Now we're showing color there.

How is that that we can do that now Power doppler shows shifts at 90 degree angle or we just saw it. The answer is true. Why? How can it do that? Well, because with power doppler we are integrating everything in the spectrum of frequencies that are present.

Remember in conventional doppler it just yielded the mean value. Now it's incorporating everything in the whole spectrum. In other words, it is calculating the area of that spectrum and that's what has color assigned to it.

Well, as we move across from right to left, that area remains the same. And so the color remains consistent as we go across from right to left. Even though the Doppler angle is changing, the area under that curve remains constant and therefore the power doppler presents a consistent presentation.

So why is power doppler more sensitive? Why do we use power doppler? Because it has the ability to detect, to detect doppler shifts and present them in color where conventional will not do it. Namely where the flow is slow, where it's in a small vessel where it's deep or where the dopper angle is very large.

And the reason is because conventional just gives us the mean value, whereas power gives us everything under the curve. And the second reason is because we don't have directional or speed information, the acceleration and deceleration over time.

So we can integrate over time and build up an image of the very weak echoes coming from the flow and get a more sensitive presentation. So for those two reasons, power dopper is more sensitive and that's why we use power doppler.

Even though it only gives us one of the four pieces of information, it does a much better job of giving us that it is much more sensitive at presenting the presence of flow and is therefore sometimes called a perfusion kind of presentation because it can show flow in tiny vessels or slow flows that conventional doppler will miss.

For example, the trickle flow here in a nearly occluded carotid artery. Here's an interesting color Doppler presentation in an artery to the uninitiated, we might think that that's an obstruction and that the flow is actually going like that spatially it looks like that, but that's not what's happening at all.

If you understand the assignment of colors, you know that can't be what's happening. If that were upward flow, it would be coming toward the transducer and it would have to be positive dopper shift, which is are these colors? This is all negative dopper shift. So it's all downward flow. There's the only upward flow we have and the rest is all downward.

Spectral Doppler Displays

Moving to the spectral display, why is it called spectral? Remember it is dopper shift in the vertical axis time on the horizontal and on the vertical axis through solving the dopper equation. We will calibrate this in flow speed units with proper incorporation of doppler angle.

It's called spectral because what we're doing is similar to what is being done here with white light beam going into a prism. And we have the spectrum of colors coming out the other side revealing to us that white light contains all the colors that is a color spectrum.

It is also a frequency spectrum because color is the interpretation in human vision of the frequency of the light that we see. The prism has revealed to us what we would've not known otherwise. And that is that white light contains all the frequencies in colors that we can see just looks white to us, but contained in there is red and green and blue and all the others.

And the prism has said, look, I'm finding these individual frequencies and colors and I'm gonna spread them out separately. So you can see that they're all present into what is called a color spectrum. It is taking apart the white beam and saying this is composed of many things and I'm gonna separate them out.

And that's called analysis that means to take apart. So therefore this is spectral analysis, analyzing or taking apart the white light beam, spreading it out into the spectrum of frequencies that are actually contained in that white light beam.

How do we do that In our instruments? Doper spectrum is analyzed by what Analog to digital converter, digital analog converter. You know what A DHD is? Well it's the fast foer transform named after the french mathematician that developed the foer transform, which is a mathematical process that involves a calculus and integrals.

But about 50 years ago a fast digital implementation was developed on the computer and since then we've had the fast FOIA transformer, the FFT, and that's what we'll do the analysis for us yielding the spectral display and each spectral analysis which can be done in anywhere from 1000th to a hundredth of a second depending on the speed of your processor.

Each spectral analysis result is shown as a vertical line on the spectral display. They're lined up next to each other across a time axis and you have your spectral display.

Here's a, a spectrum of frequencies present in normal flow in a carotid and then in a stenotic carotid with the presence of disease, we see two things, roughly tripling of the peak systolic flow speed because of the reduced lumen and a broadening of the spectrum as the flow has gone from laminar to turbulent flow so that we have complicated flow.

Now many flow directions, different speeds and that yields a much broader spectrum of frequencies. And those are the two differences that we expect between normal and abnormal flow in a vessel.

What's actually presented on the vertical dimension of the spectral display, it is the frequency spectrum. That's what's being presented. Now we'll put all the alphanumerics in here and calibrate the vertical axis by solving the doppler equation. But what is being presented actually are the doppler shifts that are being measured by the instrument.

Here's a matching problem. Match up the three things on the left with those on the right. Well I just gave you the answer to that. One is, is the doppler shift that the instrument is actually measuring. The doppler angle is estimated by the operator that's incorporated into the dopper equation and the result is a calculation of the flow speed.

The dopper shift can be overestimated. No, The dopper shift is not estimated. The dopper shift is measured by the instrument. So it can't be overestimated or underestimated, it's measured. But remember that that doppler shift does depend on the doppler angle as we saw previously.

How about the flow speed can be overestimated. Yeah, because the flow speed is calculated by solving the doppler equation. And if we put in the wrong doppler angle then we're gonna get the wrong answer.

As illustrated here, this is a moving string in a water bath. This is the beam of sound. I know the speed of the string. It's 50 centimeters per second. I have been a little careless in putting my Angle indicator here. Should be right on top of the string. Should be 56 degrees. But I put in an angle too large here it's 66 and because I put in an angle that was 10 degrees too large, the calculated flow speed is 70 when it ought to be 50. That's a 40% error.

So we, if we put an angle too large, we're going to get a result that's too large obviously then if we put in an angle that's too small, we can get a result that's too small. And yes, we can underestimate a flow speed if we underestimate the dopper angle.

So let's look at a couple of examples. Let's say that we have plaque but it's not visualized, which is often the case. We don't see it. So the operator here does not know that plaque P is present. Therefore the operator would likely place the angle indicator parallel to the vessel walls.

But that would be incorrect because the flow is not parallel to the vessel walls. It's actually going as shown here. So that angle estimate is incorrect. It's too large and so the resulting calculated flow speed is too high.

Why did that happen? Because we put an angle in that was too large, therefore the instrument says oh, the angle is larger. That means the doppler shift is smaller and I have to increase it to correct for that. But we put in an angle is too large so it overcorrect it.

Here is another situation. Now we have plaque P that is imaged. The gray portion of the plaque though for some reason is not imaged. So now the operator says, I see that plaque, I see this plaque. So the flow must be going like this and puts in the angle indicator like that when in fact the invisible plaque has caused the flow simply to be parallel to the vessel walls.

So now the angle estimate is smaller than it should be. The angle should be like this. And so the result is a calculated flow speed that is too low.

One of the nice things about color is that allows us to indirectly image plaque. Here is plaque that's not imaged well, which is commonly the case with soft plaque before it gets calcified. But we know indirectly that plaque is present because there's no flow there. And so the color doppler helps us to avoid situations like these two that I've just illustrated where the problem was that we had plaque that was not visualized.

Sources of Spectral Broadening

Finally, list five sources of spectral broadening. Well, turbulent flow would broaden the spectrum. Tortuosity of a vessel would do it, but there are three others. The first two are what we normally think about. But what else Could broaden a spectrum?

Well, let's look at a few examples to see this is a case of spectral broadening true or false? Well, yes, that's a very broad spectrum. There all the way from minus a hundred or so centimeters per second, all the way up to 450 centimeters per second in the opposite direction. It's a very broad spectrum.

This is a case of turbulent flow. Yes it is. We have a plaque here. We have stenosis and we have turbulent flow. We have disease in this carotid.

This is a case of spectral broadening. Yes, that's a broad spectrum. This is a case of turbulent flow. I hope not because that's my carotid. So what, what's happened here?

We have an artificial broadening of the spectrum not because of disease or turbulent flow, but because we simply have a very long sample volume. The sample volume in fact is 10 millimeters long. So it is covering the entire vessel.

So we're just seeing the broad spectrum of normal flow in a vessel where it's very fast in the center, hardly moving at all at the wall. And we have all the other speeds in between. So if we go across a large vessel like that and cover the whole thing, we're gonna get a broad spectrum for normal laminar flow.

The Peak systolic flow speed in this example is 20 centimeters per second. False, It's reading 20. But we didn't incorporate any angle into this. So the instrument thinks the angle is zero, when in fact it is far from zero. It's probably 60 or maybe a little greater than that.

So we have the correct answer would be probably 50 centimeters per second. Whether what other technical error can broaden the spectrum, we just saw that a sample volume that's way too long will do it. Another one is over gaining. If we crank up the doppler gain, we're going to artificially broaden the spectrum.

Why does this moving string show a spectrum bandwidth? You saw this before. Now I have placed the angle indicator correctly on it and it's reading a, an average value of 50. But why is it showing for a string that's moving at 50 centimeters per second? Why is it showing speeds all the way from 40 to 60?

The string is moving at a constant speed, all parts of the string are all moving at 50. Why are we getting 40 to 60. And the answer is because the beam profile is like this, we have a focused beam. So some portions of the beam are coming in at this angle, some at this angle, some at this angle.

So we have a range of angles involved here and therefore a range of doppler shifts. This is called the aperture effect.

So here are the five contributions to the doppler shift, spectrum flow, character and tortuosity. But we also saw examples of how over regaining can broaden the spectrum, having too long a sample, volume, length will do it. And we saw the effective aperture.

So we normally forget about those three and think about only the two that are really of interest to us. But remember all five will be involved in what we're seeing on a spectral display.

Doppler Artifacts

Finally, artifacts, do we have a double subclavian artery here? Well, In abdominal imaging we're familiar with the mirror image artifact that everything superior to the diaphragm is just a mirror image of what is inferior to the diaphragm. And we're just at the other end of the lung here at the pleura.

And this is mirror image artifact. This is the subclavian. This is a mirror image of it here inside the lung. This is a transverse cut showing the same thing. And this is an example of fetal kidneys in utero at where they should be. And then this appears to be a fetus outside the uterus and of course can't survive out there.

But that is just a mirror image of this. And the mirror is muscle here.

The gray and color shadows are misaligned because of what? Refraction. Misregistration mirror image speed air. Well, it's none of the above. All we're observing is that the beams that were sent out to do the doppler are all at this angle as indicated by the edges of the color box.

And so a scan line here is going in that direction for the color doppler and there's calcified plaque there. So it produces a shadow for the gray scale anatomic image, it's rectangular and the scan line would go vertically down from the same calcified region producing that shadow.

What's the artifact here? We're chopping off the peak systolic portions and they are reappearing on the wrong side of the baseline. And that is the aliasing artifact.

How do we correct for icing? Well, there are two ways to do it, which way was used here? And the answer is a scale change. This eliminated the aliasing. Yes it did. A scale change is a change in the sampling rate. In other words, the pulsing rate.

Every time we send out a pulse we get a sample of the doppler shift and we have to sample often enough to avoid aliasing. So as we crank up the sampling rate, the scale changes. You can see that it went from 1.5 to five here because the extremes of the vertical axis on the spectral display are in fact half of the pulsing rate.

They're called the nyquist limit. And if we crank up the pulsing rate enough, then we get out of the aliasing situation.

Here we have unders sampled. You have to have at least two samples in each cycle of the dopper shift to avoid aliasing. Here we have plenty of samples as we have cranked up the scale and where we wouldn't be aliasing anymore.

The other way to to handle aliasing is shown here. And what control would that be? That's baseline shift. We've shifted the baseline from the center down. This eliminated the aliasing and that is false. The aliasing is still occurring, but all we've done is electronic cutting and pasting.

We moved the baseline down that took what was down here, the aliasing and put it up here, pasted it up there where it really belongs and where it would be if we were not aliasing. We're actually still aliasing, but baseline shift is just electronic cutting and pasting to correct the presentation to what it ought to be. Electronic cutting and pasting.

Which reason is aliasing here? 1, 2, 3, 4. And the answer is two. This is a torturous carotid has a loop in it. So what's going on? Well the beam is steered this way. That would give us an angle here of about 60 degrees. The flow is to the left, so it is away from us. We're seeing negative doppler shifts.

And then as the flow bends around like this, it gets parallel to the beam. The doppler shift goes down, the doppler shift goes up and we jump from this limit over to this one. And we are in an aliasing situation. It's flowed down but it's giving us a positive doppler shift.

As we continue around, the doppler angle starts to go up, the doppler shift goes down and we uns jumping from there back over to here right at that point. And here we have legitimate downward flow with negative doppler shift.

We pass through zero doppler shift black as we go from dark red through black to dark blue from downward flow to upward flow. That is a true flow reversal. There you can see that true flow reversal with dark colors separated by black. Looks very different from aliasing where you have bright colors next to each other at the bar extremes.

Two more questions and we're done. Why is Professor Dopper not famous in his hometown of Salzburg, Austria? This is his home. This is where he was born. There are some art, some apartments on the upper floors and some shops on the ground floor. But the people there when I was there I asked a few people they didn't know about Doppler Harley at all.

Why look who lived here just two doors away. The famous musician Mozart, last case, this is a fetal scan. Color doppler. This would be the heart. This is the descending aorta. Very interesting pattern of reversals. Back and forth, back and forth. Red, blue, red, blue.

Why alternating colors are caused by shadowing 90 degree Doppler angle, refraction, diffraction, none of the above. Could it be that we are just seeing reversals with cardiac cycles? No, because we're getting 33 pictures per second. So we're not looking at many cardiac cycles. That would be a few seconds to involve that many. And we are getting 33 pictures per second.

So it can't be that looking at it blown up here. Seems like the ribs must have something to do with it. And we see the shadows from the ribs and this is the flow be from right to left. And this is a convex array with varying doppler angle as we go across here.

So we would expect here that over on this side we would be looking into the flow, the flow coming toward us and we would expect red. And over here where we're looking away downstream, we would expect blue on the map that we had. But why do we have all these alternating colors?

Well they are a result of refraction around the ribs. The speed of sound in bone is faster than it is in the surrounding soft tissue. So we get a refraction in this case it will bend to the left. So this pulse is going to be observing downstream flow away from us, which would be a blue color and it's gonna present it on that scan line. And there's the blue color here.

We're refracting that way. We're looking into the oncoming flow now, which would be a positive dopper shift or red. It's gonna show it on that scan line. And there's our red, this happens for all of these ribs and we get the alternating colors. Red, blue, red, blue.

So it's a result of the fact that we're close to a 90 degree Doppler angle. We have a little bit of variation to the right and to the left caused by the refraction and giving us those alternating colors.

Conclusion

Well that concludes our review, our principles tuneup. How sharp were you? Well, I trust that you got some of these right And if there were some that you didn't get correctly, I hope now that you understand those and that you sharpness index as increased.