William A. Zoghbi, MD discusses basic principles of echocardiography and doppler.
Hello. I'm Dr William Zogby, and I wanna welcome you to our weekly cardiovascular imaging conference. I think this is the start of the academic here, and we usually go through the basics of all the images that we acquire. Beat, echocardiography, nuclear ct, emery and all. And I'd like you also to interact with us. So please send your questions, uh, texted to the baking the e b a k e y toe 3767 And ask your question. I would be happy to answer those questions. So let's get started. We're gonna talk today about Doppler basics and physics. I know it may be boring to some of you, but I think it's important for us to go from time to time to the basics of how the image is constituted. The evolution off echoed opera techniques and how we use them nowadays. But mostly I think it's good to refresh ourselves as toe the basics, because then we can understand the artifacts some of the issues that we're dealing with. So I think it's a good basis for us to review together. So echocardiography for most of you is really a mature technology that has evolved into techniques from M 022 D three d echocardiography and the various Doppler evaluations that we have to deal with. And I think each one of them is amazing in a way because all of them depend on these small packets of ultrasound that are sent. They reflect back, tow us. And then we asked the question, Is it how how far is it, how intense the frequency change when we send it back? And these are the basis of imaging and ultrasound. So, uh, an ultrasound you have to think about the wavelength and the wavelength, and the frequency of that is the number of cycles of these over per second. The longer the wavelength, obviously the lower the frequency, the shorter, the higher the frequency. Remember the speed of ultrasound and biological tissue of 1540 m per second. And that's how we calculate the depth of a particular object that we image and properties of reflection refraction are similar to light. So if we take the transducer, we send usually small packets of ultrasound and they are in the ultra range, so we cannot hear them because they're in the megahertz range if they reflect the surface, whatever that surfaces. This is where the beam is going. Some of it is refracted, and we're not going to see that some of it is reflected in a symmetrical fashion, and we don't see that because it's not captured by the transducer. So what you see is that little tiny little tiny fraction that comes back perpendicular in the same direction that you sent it out and the images processed. So that's how the images are processed. We send burst off packets of ultrasound for you to be able to reflect it back and ask the question of how deep is it and everything else. And if it is more reflected, they will have a higher intensity and therefore more dense. If you turn this back from an A mode to a bi mode, so you reflect them 90 degrees back. Then you'll get a mawr reflected area, and that's what we use nowadays for two D and three D and for you, for a mode right. This M is for motion, So if you have a certain dot and then the scale is time, if there is no motion, it will be a straight line. If this motion goes up and down, let's say the Metro Bob's going a little bit up and then down you're gonna see it going up and down. And that's what The Nemo, the beauty of an M mode is, has a great time resolution so you can take a look at really every little tiny detail of its motion. And we use that nowadays for a few things. A few wanna address a motion of a particular subject. Uh, you know, the motion of the metro of a few things here and there. But for measurements, obviously we prefer to go to two D, and this is a to the examination. The question is, how how do you make this image? And these are actually from steering the beam from the ultra sound. These crystals that are piece of electric crystals that are deformed with electricity and send the ultrasound waves so you could have so many scan lines along the line there with some interpolation. But notice also that because off the ribs the way they are, we have to image in between them. So for echocardiography, it is more of a V shape scan as opposed to linear where in the other parts of the body we don't have much interpretation of other, uh, areas that are reflected Too much ultrasound notice also that they spread further. If you go further down, they'll spread further. And this is some of the part issues of the lateral resolution that occurs with this kind of scanning. Uh, we think also of to kind of resolutions, at least for two D actual resolution in the line of the transducer beam or the ultrasound beam, the higher the frequency, meaning that tighter these waves are, the better the resolution so you can differentiate one dot from another and the, uh, slower the frequency, the lower the frequency. These two dots may not may not be differentiated, and there will be one so one. One important message here is used as much higher frequency transducer for you to have the best resolution actual resolution. Also, this is the best resolution, so meaning if I want to look at the septum or, let's say an interational septum defect. The best is perpendicular to that structure. So a sub costal is really the best way to do that. So the more perpendicular you are toe an object, the better the resolution. This is lateral resolution will depend on the beam with As you go further down, you can focus some of these beams and hopefully differentiate thes two dots from one to another. The further out you go. Certainly you're gonna have mawr of a globular like look, because you're not able to differentiate. You know these two dots from each other, and you could see that in the further, uh, distance. These are some of the important instrument controls, overall gain, time gain, compensation, lateral gain, compensation, reject compression and then mechanical index. All these 1st 12345 Our post processing meaning, that is, after the ultrasound wavelet has come back to the transducer and you're processing this by the increasing the gain a reject and compression. This is the energy that is sent the mechanical index and output power off the ultrasound that is regulated to keep safety on board. But also that's the one that we have to think keep in mind when we use for imaging off contrast. So we don't destroy these tiny micro bubbles. And this is the mechanical index peak negative pressure divided by the square root of the frequency. So for ultrasound bubbles, obviously the higher the mechanical index, the more stretched these bubbles have and the more destruction you have some of these bubbles going forward. A few other things that we noticed during examination. Reverberation Attenuation. This is attenuation, meaning that the wave let's are coming here. Most of them are reflected back and then you have a shadow behind it Completely. Shadow there because if the ultrasound is not penetrating further down, you're not going to be able to reflect much for you to be able to see. This also happens the same thing for Doppler. This is the reverberation. Actually, this is not shadowing this interesting finding here, a times in certain structures or mechanical prosthesis. At times the ultrasound reverberates between the two, you know, metal areas off of the prosthetic valve and you see this shadowing in a way. But it has a little different look of reverberations, and this is this is what you see now. At times, you you see a lot of crowding of the ultrasound, particularly in the near field of the transducer itself. And you wonder whether this is a Trump is clinically or this is an artifact. And for you to differentiate that if you don't have contrast, did you change the transducer because they have to do with the casing, have a different window, go up a little bit and go down towards the apex itself. Change your window to try to avoid that. But nowadays, actually not infrequently will use contrast to try to help differentiate this area. So you could tell here that it would be very hard to tell whether this is a rhombus, but you could see that very nicely. One contrast is used to the market it so technologies over the years have evolved so much. Uh, we don't want to date ourselves back, but we can remember the video days to digital days very high frame rates, which is amazing, uh, multi frequency transducers in the past. We have to change for each frequency. Change the transducer harmonic imaging we'll talk about because it improves images as well as contrast destroyed. Doppler digital image storage and real time three d echo certainly have evolved significantly over time, and you could really enjoy the all these developments nowadays. Harmonic imaging. So you send a certain frequency, and usually they're, you know, a certain, let's say, 3.5 megahertz frequency. This is called the fundamental. But whenever you hit an object or it may resonate. So you have multiples of that frequency two times fundamental three times or even, uh, ultra frequency or lesser, almost half of that. And why is it important? It's always an interesting discovery that, actually native tissues will emit harmonics, which is a multiple of that ultrasound frequency that you're sending and actually by using a harmonic imaging, meaning I send a certain frequency. But I image at multiple of that I could decrease some of these reverberations that you see and improve image resolution. It's also better for visualization of contrast, because when contrast is hit with these fundamental frequencies, they vibrate and emit resonant frequencies. So if I image using my resonant frequency, enhance that signal coming back from contrast itself, Okay, so this is an example of harmonic versus fundamental in the same patient. Here, you can tell whether this is an artifact or not. In here, you could tell that this is an a pickle hyper trough, a supposed to a lot of crowding here that you can't see too well. Thea other use obviously, is for contrast itself. Notice now this is a contrast has been injected. It's filling the ventricle. But I cannot differentiate well from the contrast per se from the my KoreAm here per se and notice that if I turn on harmonic and this individual, I enhanced significantly the contrast itself compared to the muscle itself. And this way you have this nice contrast enhancement to be able to evaluate ventricular function. Whether there's Trumbo's etcetera, etcetera So you have to keep those in mind when you image a patient and you use contrast to use the right settings for it. And another thing is that for optimization and we'll talk about that. Another contrast is you wanna make sure that the mechanical index, the M I is not too high. Otherwise you're gonna destroy quite a bit of this contrast and may not see things as well. Switching onto Doppler for Doppler, you know, there are a few concepts that you need to know. Pulse wave Doppler, the inspector of color or continuous waved opera, and this is in Salzburg, Austria. When I visited many times and this is actually Theburbs Place of Christian Doppler, who was the inventor off Doppler effect. Just imagine and back in the 18 hundreds to be able to do that from looking at the stars and the Doppler shift off color that we're seeing and coming up with the doctor equation. It's really amazing to be able to do this. And the principle is simple, is actually is that if you have a certain frequency and that frequency is, uh is moving or you moving in relation to that frequency, there is either compression or decompression of these way forms so and be either higher frequency or lower frequency, and it relates also to the velocity of that of that object. So the best example usually take is a siren on your, you know, standing on the street and you know the ambulance comes over and you could tell if you close your eyes that indeed that ambulances coming towards you so you could tell that because the pitch goes up Rainier higher, and then you could tell even that it is going away from you because the frequency will come down. Interestingly enough, if you're far away and perpendicular to the trajectory of this moving object that is emitting a sound. You may not differentiate it because it's almost perpendicular to that frequency itself. So that's That's the basic principle here, right? So you can tell by the opera you can tell whether it's a velocity that's coming towards you or going away from you. And also, this frequency shift relates to the cosine of the angle of what your image ing with the motion of that particular object. And most of the time here, it's either blood in the cavity and more as of late, more muscle and tissue Doppler. So in the past, you know, the the echo machines used to display the change in frequency, and we had to kind of convert it if you will. But most often nowadays, and actually pretty much everywhere you will see a velocity scale and cosine theta is assumed to be. The angle of the ultrasound with emotion is assumed to be zero, so this would be one so therefore it relates to the frequency shift and the initial frequency the emitting frequency, in addition to the velocity of tissue and the medium, the biological media. So most of the time it is acceptable if you have, you know, an angle below 20 degrees. Once it becomes greater than 20 degrees, I think you will underestimate the frequency shift and therefore the velocity. Okay, so let's talk a little bit about pulse. Dapper, this is the spectral display. And the beauty of Paul stopper is I have an area of interest that I'm interested in finding out what the velocities are. So I send a packet of ultrasound like this one here, and I have I won't listen to it, see what happened to it and let it go. I know what the distance is gonna be. Meaning the time that it takes for it to get there on. I wait for it to come back. So double the distance from the from the transducer all the way down. And then I listen to that and I listen with the question of how much frequency shift has occurred between the fundamental frequency and what came back. So if it is higher frequency, you know the velocity is coming towards me of that object. If it is lower is gonna be going away from me. So I can range gate. I can find out where it is and the problem with it. Since it is almost robust topic, you're not gonna be able to resolve very high velocities Example of pulsed opera. This is Paul Stopper and the LV outflow tract. Most of these red cells are going at the same frequency and saying velocity. Then there is some dispersion here because not all of them are going at the same speed and then deceleration. So this is your pulse Doppler and the LV outflow tract. This is mitral inflow. And with all the applications that we have, the astrology outflow regressed in volume rehearsed infractions. Everything else. So this is spec. What we call spectral broadening and the way we solve for this equation is the Fast four year, which is probably among the best equations to solve this complex frequency shift that occurs. Eso alias thing is the phenomenon where I cannot resolve what these velocities are, and they look like basically bands of frequency shift, and I really cannot tell what what they are. And it depends on the pulse repetition frequency. It's almost like a stroke risk OPIC thing. If you're in a nightclub and a moving up shit and you can't tell whether this individual is going in the right direction or opposite because you're seeing only few pieces and time overall. Okay, So, um, now, if the velocity of the frequency shift is not too high, you could do a baseline shift, bring it up and down so you can see almost double of that, uh, Nyquist limit that you're dealing with. And that's what we do nowadays. So this is an example of a leasing meaning a mitral inflow. Velocities are low. I can differentiate them and look at the time of their distribution Here, this is mitral regurgitation. As you know, Mitra regular station jet is very high velocity. I'm not gonna be able to differentiate this, so it looks like a very a liest Basically signal. Not gonna be ableto help me, but I know that at least it will exceed that velocity by that much color. Doppler is, in a way, a pulse Doppler technology, but not enough time to be able to resolve a fast Fourier transform. So we have a nest imation of actually what the velocity shift is occurring at these tiny, tiny little windows, and therefore we can tag them with a direction coming towards me. More red yellow. Going away from me from the transducer is bluish. And if we have alias thing, basically you're gonna have a mosaic of these colors, you know? But as you could tell, the alias ing frequency and recommendations usually is to use about 60 cm's per second Nyquist limit. So you're going to see, you know, a higher incidence of alias ing with color Doppler technology used for identifying where abnormal flows and this is mitral regurgitation. We'll go through a session for that and where the flows are and maybe detection, where some of the abnormality of flow is notice that, you know it still has the same principle of Doppler. So therefore, let's take an example. If there is a change in direction off flow, you're going to go through a period of blackness meaning no frequency shift. So this is an ascending aorta. Read. Reddish color is coming towards you with a certain velocity perpendicular to flow becomes no frequency shift because cosine theta of 90 degrees zero, then it starts getting blue. So this is how you can tell difference between alias ing versus a direction of change in directions. What are the factors that will influence scholar Dr Gains? Obviously, color maps frame great. And a Nikos limit. This is just a higher gain. And you could exaggerate in aortic regurgitation and mitral inflow coming through here. Uh, so Nyquist limit is very important because if I decrease my Nyquist limit in this mitral regurgitation to the twenties, I'm going to detect almost any flow that is occurring here and will exaggerate what mitral regurgitation looks like. This is the same patient with more appropriate Nyquist limit, usually above 50 between 50 and 70. And you could see the mighty regular station little swirling, but certainly looks very different. So you want to make sure that you have the appropriate Nyquist limit that you don't use a low one. That somebody else may have brought it down significantly. An overestimate. And, you know, we've had some examples in the operating room where people where we were cold to say, Well, this patient may have significant mitral regurgitation, and indeed, there was a narcos limit difference that, you know, you save somebody's intervention. Uh, now, frame rate is important here because you have you have a significant section to process all this information. And we wanna make sure that you have the frame rate that is appropriate. So that in a way you can get toe almost real time, right? You don't want the frame rate to be too low, too low, meaning, you know, below 15. Probably it certainly, tenant below is too low. You're gonna have some almost robotic OPIC view of regurgitation. So you want to optimize it, then? Part of the optimization is to make sure that your width of color is really not too wide. So the frequency, Yes, Scan depth. Now here. I cannot play around with that scandal. That's what I need to assess might regurgitation with of color region and still use your judgment as to what area I really need to see the line density. And that's usually we don't play that around this. How many times a scan line is used for processing of color before going into another one? Hi, Pulse. Repetition frequency. Same principle in a way. I know at times you were not going into it nowadays because we have continuous wave Doppler, but basically it's multiple areas of interest. Uh, on basically The advantage of that is for you to be able to use a higher frequency, a high resolve, a higher frequency or higher velocity. Like in the old days of aortic stenosis where we used to This, uh, nowadays, you really I'm not gonna go through it anymore, just for the sake of time. Continuous wave Doppler, as opposed to pulse Doppler, where you send a pulse. You wait for the range of the depth that you want for you to listen to it and listen to that. Here is you have a crystal that is sending continuously or frequency and a crystal that is listening continuously back so you could tell what the difference is gonna be. The difference is I can interrogate all these velocities along that line, and and the highest one will be demarcated. So this is an aortic stenosis jet. You could see that very nicely here. These are the velocities most likely in the LV out floor in the ventricle itself. So remember CW's along any of this here, So at times you may have an obstruction in the middle of the cavity, and this is the highest velocity that you can have. So you have to interpret that accordingly. Tissue Doppler. A signal that we've ignored many, many years now we've had it. I guess 15 plus years ago now we ignored it because it was a more of a noise things than we were much more interested in the Doppler and the velocities off the red cells themselves in the ventricle. But turns out that there's a lot of information in that, Interestingly enough, you can differentiate them very nicely. Why? Because the velocity in the mic radium is much lower than that in the blood flow itself. And also the intensity of the signal is much higher. So you could put your filters one way or another. I wanna either I want to see muscle tissue, Doppler or blood. Uh, you know, regular Doppler. And you know the applications quite a bit of that of diastolic function and so many other things and special tracking is another technology That is really amazing, actually is to be ableto track these speckles of ultrasound that are reflected back to the transducer. And part of the major advantage of this special track is the ability to look at the information not only in the usual way in the past that we used to take a look at, which is a thickening of the My Korean from the person along access where you're perpendicular to the structure. Here you could look at the information and thickening and thinning in almost any window and what this strain strain is, how much the formation has occurred compared to the baseline. So if you have a 10 centimeter that has shrunken down toe eight centimeter, you have a strain of minus 20% because you decrease. If it's stretched, it would be greater than basically a positive number. 20% Onda, if there's no change and strain, is down to zero strain in the past, it was initially developed with Doppler technology. Unfortunately, with Doppler with two samples here, whenever you have a curvature or an angle ation, you will underestimated the beauty of special tracking is angle independent and that's that's where quite a bit of the applications for systolic diastolic function and you name it is important strain rate. We don't use as much because of noise, and hopefully that will improve over time is although you may have the same strain that has occurred from a point of time to the next one. You can have much faster strain rate to achieve that same deformation that tells you a lot about systolic function or diastolic function. It has some of its applications, but it is a bit noisy. And that's why you go throughout the period of the cardiac cycle that you're interested in meaning strain as opposed to strain rate per se. Okay, different types of strain. The longitudinal alone to use most of the time which relates to the descent of the base, but regionally as well as globally, right. The G l s circumferential radio, which is radio shortening. And and these are the things and this is this is an example of a normal strain on strain rate, and you could see that you could address not only the global one, but also regionally. And this is fully automated, Really. A beautiful application off special tracking technology and normally for age less is normally is, uh, lesser than minus 18%. Basically, deformation insistently is miners 18 minus 19 minus 20 etcetera. And this is a very abnormal heart. Obviously, with the G l s just minus 2%. I mean dramatic from normal to very abnormal digital image storage. I know it's it is for granted. Nowadays you take that for granted. But let me tell you my feel about the importance of that. This occurred around you have 2020 years plus a digital image storage. The beauty of it is remember, echocardiography is very demographic, and it is very dependent on the person who basically takes these images right, and that brings about a lot of variability and brings about said, if I need to measure something, which is the view that I need to measure it at the beauty of this digital imaging and and display as opposed to a single beat, you can have four beats at the same time or even nine beats. That's what I enjoy most of the time is you could quickly take a look at this heart itself and at the same decide which one of these views are, you know, best for measurement on. You could take a look at that. These are short, different short access at close to the metro valve, popular in muscles in the apex without any peppery muscles, so you can decide quickly if a view is foreshortened more than others. You could see a lot of peppery muscle here and court a tendency. Maybe that's not the best for you to measure the diameter of the ventricle by to the irrespective. More importantly, even from the a pickle window to measure your volumes and ejection fraction, which is less for shorten so you can compare them together. That's the beauty of having multiple, uh, multiple views of a digital image. And I think you know, we take this for granted, but I think this is very important. And part of the obviously evolution of echocardiography is the application of three D I wanted to share with you. This is actually among our first images here back in 2000 and three. So we're talking about 17 years ago, Uh, a you know, from a three d point of view. And yeah, you could take a look at that. And certainly things have evolved over time to a single beat image as opposed to stitching. At times, we still have to do stitching for this heart to be able to, you know, take a look at the whole volume of volumetric major applications nowadays is to take a look at volumes of this heart ejection fraction at times mitral stenosis, area A to the tips of the mitral valve and some of the regulars conclusions. Things have evolved significantly over time. We have now high volume rate three D imaging that can help us significantly. And I think we need more and more of these applications for better ejection fraction and volume estimation. We still the same limitations, if you will, for echocardiography will apply to three D. So if I have a poor image by two D, you know you're gonna have a poor image by three D. Uh, but importantly, it it will avoid quite a bit of the four shortening of the apex, and image optimization is still very significant there. To be able to do that, you could do three D global regional strain also, and some of the beauty of applications are for valvular heart disease. You know, nowadays that you use this in the interventional arena interventional image ing, in addition to just toe, identify where some of these abnormalities are. Yes, the time as well as spatial resolution of three D. B. It whatever methodology is a little still less than to the imaging but the beauty of three D years of times. It may localize things in the three D space that two D may not tell you the whole story, so I know, particularly from mitral valve pathology. Three D ultrasound is really very important to tell us, particularly from the trans esophageal approach. This is three D color Doppler. Yes, it is impressive that you could take a look at the whole jet itself. But some of the quantitative parameters are those for Vienna, contractor on some of the peace calculations for color now per meaning, the flow conversions coming into the mitral regurgitation, much less of the volume of the jet itself. And I think last year, among the first clinical experiences I think reported by Roberta Lange and his group was a different way of showing the images with trance illumination rendering it looks quite impressive. It is very nice to be able to see that, and the images almost like you're looking at the mitral valve per se in front of your eyes. Eso it is quite impressive how much have really come along from the days of early echocardiography, a timeline off our evolution from the early Dr Ed Lear's applications on mitral valve a mode technology and mode technology to the Doppler color t e contrast tissue Doppler live three D special tracking and handheld echocardiography basically the same principles. There really quite an evolution based still on the same principle of sending ultrasound waves and processing whatever is reflected from the object and the transducer taken up by the transducer because it excites the transducer and then go back and process. You know, with all the computerized technology process this I want to share with you. Last is actually this is a video back in 1960. So we're talking about my goodness quite a few years ago. Now 60 70 years of Dr Engie Edler doing a an initial examination depicting the transducer. And that's Dr Adler himself. And you'll see the A mode. There you go. So this is the mitral valve and certainly has our visualization of the heart, with ultrasound has improved so much still on the same principle. So certainly we can't be more thankful for Dr Adler to introducing this in the clinic and gradually, with all the computing technology to be able to, um, render these images almost like in real time for us to be able to evaluate heart disease, its application of the technology and management of patients. So I'm going to stop here and let's see if we have some questions. I know it's been through quite a bit here this, but it's good to reflect back and think about some of the basics of ultrasound on how to avoid some of the pitfalls and improve the images, all right, And I think if no questions, it's a pleasure having you and look forward to seeing you in our weekly conferences, so thank you again for joining us.