New Technologies in Vascular Ultrasound - SD

Introduction

Good afternoon.

I'm Philip Bendick, the technical director of the Peripheral Vascular Laboratory at William Beaumont Hospital in Royal Oak, Michigan, just outside the city of Detroit in the southeastern part of the state.

Today I'm going to be talking to you about new technologies in vascular ultrasound and how we are applying them to improve our diagnostic capabilities.

This afternoon we're gonna talk about new technologies in vascular ultrasound and how we can apply these technologies to improve our diagnostic skills.

Historical Context of Ultrasound in Vascular Testing

If we look at ultrasound in the early days, instrumentation was extremely crude and we were not able to do much with either the instrumentation or the displays that they generated.

If you look at vascular testing in the early days, ultrasound is conspicuous by its total absence in an early vascular laboratory.

There were no other competing technologies out there, it would be an entity unto itself and allow efficient vascular testing.

Current Options for Vascular Imaging and Diagnosis

Today there are multiple options for imaging and diagnosing disease in the vascular tree.

The most common, which of which is contrast angiography.

It has been used for a number of years for this purpose and is our gold standard against which we compare other tests.

Despite the fact that radiation is involved, there are contrast complications and the potential for other complications secondary to the arterial catheterization involved and the fact that angiography only provides a luminal shadow gram, you only get to see the lumen.

You don't get any information about the vessel itself in the vessel wall, such as you get with ultrasound, where you can clearly see not only the lumen but the structure in the vessel wall itself.

In fact, the national heart and lung blood Institute has called these two dimensional lumino grounds in that angiography doesn't give you really any blood flow data, does not characterize the lesions and does not provide any wall architecture.

Competing Techniques: Magnetic Resonance Angiography

There are other new techniques out there competing with contrast angiography.

Magnetic resonance angiography is one.

It has its own set of limitations in that it requires significant patient cooperation, has limitations with any metal implants that might be present.

There's always the problem of possible venous contamination on the image and other things.

Competing Techniques: CT Angiography

The other large competitor is CT angiography and this actually is playing a much more prominent role in angiography today as there is the potential for contrast nephrotoxicity and there is radiation involved.

So it is limited to some extent to those patients who have adequate renal function.

The biggest thing people worry about with CT angiography is the use of the ionizing radiation in that a single thoracoabdominal CT angiogram can be equivalent to approximately 300 chest x-rays.

So it is not something to be used indiscriminately or lightly and is particular concern in the pediatric population in this day and age.

That leaves duplex ultrasound as a potential use in the vascular system for imaging and diagnosis.

Advancements in Vascular Ultrasound Technology

And I want to talk about how the technology and vascular ultrasound has kept up with that in conventional angiography and MR and CT angiography in three sections.

One is probe technology, two is changes in software and computing power that have allowed certain functions to occur, and the other is the increase in the functions available through the neology on the machine that is now possible in this day and age.

So again, to allow exquisite images like this where we have good gray scale, we have good colored doppler filling, we have excellent spectral doppler resolution, all of those are made possible by technologic improvements that we have achieved in the last few years starting with probe technology.

Probe Technology

Traditionally probes are a simple 2D or a simple 1D array, a linear array of elements which will allow you to focus in the lateral dimension and allow a decent focal zone at one point in the beam.

But where you are not focused in the thickness plane, you are losing a lot of resolution.

The new so-called matrix array or two dimensional probes have a two dimensional matrix of smaller elements which allows precise focusing along the length of the beam so you can see all those targets very clearly.

This is an example using an ultrasound phantom with a conventional linear array probe within the focal zone.

You see the targets very, very clearly when you apply a matrix array probe.

You can see the dramatic improvement in the ultrasound phantom where you can see the targets throughout a much wider region of interest and you can see the exquisite detail that it allows us when imaging atherosclerotic disease in a blood vessel.

With this type of matrix array probe, it avoids a lot of the slice thickness artifacts.

The second technology is actually the crystals themselves.

Historically we've had basically a series of crystals all lined up, none of which are entirely perfect, so there's not perfect alignment in 90 degree shift in that alignment when an electronic signal or a pressure is applied, but we can get a signal into and out of the probe with new so-called single crystal technology where all the elements in the probe are grown in the laboratory from a single crystal.

They all behave exactly the same way.

They've been cloned one from another, so they all line up exactly the same and they all go through their 90 degree face shift exactly in the same manner when an electrical signal or a pressure is applied and this leads to a much cleaner, much stronger electrical signal for processing in the instrument itself.

It allows us to increase the bandwidth of the probes.

It gives much better signal to noise ratio and it improves the axial resolution and penetration of the resulting ultrasound beam when it's in the tissue, all of which are incorporated into the system basically to improve image quality overall.

So we see things with much more clarity than we did even five to 10 years ago.

Imagine the ability that you have with a matrix array probe with these new technologies.

When you look again at a just a simple tendon, a flexor tendon in a digit, could not have seen something with this kind of detail just a few years ago.

Software and Computing Power Changes

In terms of software algorithms, there's a number of changes that have gone on in the instrumentation that have automated a number of functions previously done manually.

One is optimizing the image.

There's so-called adaptive beam forming and scanning protocols can be programmed into each machine and automated as well.

If we look at auto optimization of the image, if we look at default settings, they're usually fairly good.

This is a common carotid artery in a thyroid and cross section and it just comes up as the default when the machine is turned on and the carotid application is selected.

However, when you automatically optimize this image, it is able to clean up some of the noise.

It reduces some of the the speckle.

It cleans up the noise associated with the vessel walls, so we see the overlying muscle, we see the thyroid, we see the common carotid artery.

All of these structures are seen much more clearly and distinctly.

When the image is automized opt optimized automatically you can do these things manually, but now the machine will do it at a punch of a single button and it does save significant time.

In addition to grayscale, the automatic optimization feature will also work for spectral and color doppler.

Taking a display such as you see here and converting it with simple push of a button into a good color doppler image and a good spectral display with appropriate gain at scale, the system will also automatically angle correct for you if it has a quality enough image to detect the vessel walls and put that angle correct cursor parallel to the walls automatically.

Again, another time saving feature that can be used.

Adaptive beam forming is something the engineers have worked on for a number of years.

If you look at how traditionally ultrasound systems have been designed, there are a number of assumptions that go into that design such as constant sound speed, attenuation, et cetera, which really are not true in real life because patients are not built like clones or crash test dummies.

Patients are all different.

What adaptive beam forming does is it takes clinical information based on the signal being returned to the probe from the reflections in the tissue.

Instead of making these basic simplifying assumptions processes that clinical information to actually form the beam appropriate to each individual patient, assuming that each patient is going to be very, very different in their characteristic, which is in fact the case so that with adaptive beam forming, you can look at the color doppler imaging within the kidney or if you so desire, you can look at the mass and differentiate that mass from the the renal parenchyma itself in that tissue.

So it allows that feature.

It also allows a very, very wide dynamic range so that you can see the bright echoes of surrounding tissue.

You can see the advent tissue, yet at the same time you can see those very weak echoes from, in this case hypoechoic atherosclerotic plaque at the carotid bifurcation.

So it allows a lot better improvement in image quality.

One of the great time savers that have been added to the modern machines is the ability to load in your own individual scanning protocol and automate that entire sequence.

Basically, it allows the machine to think the way you tell it to so that it will automatically set up your imaging control in whatever mode you desire.

In the next step of your scanning protocol, it will automatically steer and select spectral and or color doppler as needed.

As part of your protocol, it will initiate and complete the required measurements that are part of your protocol and it will automatically insert annotation and comments in as required in your protocol.

These can all be pre-programmed into the machine for your institutional specific detail.

Looking at some examples in the vascular world, if you look at conventional scanning for a venous duplex examination, about half the time is of the sonographer is spent on actual scanning and about half the time is spent on what I call busy work at the keyboard, inputting annotation or angle correcting, doing whatever is necessary.

It's even worse for carotid duplex because of the number of measurements involved, the velocities in that only about a third of a stenographer's time is actually spent doing scanning and two thirds of their time is this busy work at the keyboard.

The idea of automating the scanning protocol is automating the busy work features to allow more scanning time for the sonographer so that if you use an automated scanning protocol, actually 85% of the time in the study is devoted to scanning for about a 35 to 40% time saving, and it's even more so and more dramatic in the carotid where 80 to 90% of the time now can be sent spent on scanning for about a 50% time savings, allowing the sonographer to think clinically instead of technically in doing all of that keyboard busy work.

New Neology Functions

The neology functions that are available now are the exciting part of the new technology because it allows us to see things more clearly or see things we could not see before.

And this is a partial list.

The ones I want to talk about, the improvements in harmonic and compound scanning, the ability to image flow, 3D, 3D and four D imaging, infusion imaging, and also some future functions down the road which are available but more in a research status than a clinical status at this point.

Al use of ultrasound contrast elastography and virtual histology, we start off with harmonic imaging.

Harmonic Imaging

There have been a lot of improvements in this technology and the ability to transmit at one frequency and receive and process image data in a second, allowing a number of advantages to improve clarity in our imaging, particularly in terms of lateral resolution and rule of thumb in my own vascular laboratory now is anytime you are taking measurements of any type of circular or oblong type structure in a cross-sectional view and you want a transverse measurement turn on harmonic imaging because it will improve that lateral resolution quite dramatically.

It has also improved because of the doubling of frequency, the axial resolution, and you get a second type of contrast resolution, one at low frequency, one at higher frequency to differentiate tissue types within the lumen.

Also of note with harmonic imaging is you lose a lot of the artifact in the near field that you have with conventional imaging.

So your ability now to look at structures make quantitative measurements is improved significantly with the improvements that are seen with harmonic imaging, and that ability to get the contrast data from different types of tissue is improved as well.

So overall effects of harmonic imaging, the near field artifacts are gone.

Axial and lateral resolution are improved dramatically and the contrast resolution is also improved in different.

Compound Scanning

A second feature compound scanning will get rid of some of the artifacts that you see in the image.

One is the refraction artifact.

If the ultrasound beam is going straight through a cross-sectional view, there is no bending of the beam and if you're outside the structure also there's no bending of the ultrasound beam by refraction.

However, if you go through some other part of the, in this case, the somewhat circular structure there is refraction because of curvature of the walls and the changes in sound speed as you go from one type of tissue to another, and this can lead to basically a zone where there is no ultrasound information present, which you see as a large shadow in the image at the edge, and this is simply every fraction artifact when you use compound scanning, when you look not only directly up and down from the elements, but you're viewing from multiple angles to the side, a lot of this refraction artifact is eliminated because now you have removed that from the image and you do not see it any longer.

The combination of compound scanning and harmonics particularly improves dramatically the resolution you see in the image and the clarity of you get in the surrounding tissues from your particular target in this case, the common carotid artery.

It also allows you these, this combination to visualize things very clearly in BMO that you might not have seen so clearly before.

As noted in this particular pseudo aneurysm, this is again looking simply in B mode at the flow and circulation within these structures, applying harmonics and compound scanning to get this kind of detail flow.

Flow Imaging

Imaging is well beyond the stage now of color.

Color doppler is dramatic.

It is sometimes spectacular, very impressive and gives you a good quick overview.

Power doppler imaging also gives you a good overview of what is going on, but there are also technologies now that allow the direct imaging of flow using B mode, and you can see in real time the effects that you get from a stenosis, for example, and you can see the deflection of the flow jet up against the near wall and then the turbulent flow that goes downstream from that point.

Again, another example in a complex lesion, which shows a double residual lumen, both very small in this greater than 70% diameter stenosis, and the advantage of using B mode imaging for flow is it's not as angle sensitive.

It is not a doppler technique, so you can view it in cross-section at essentially 90 degrees as well.

This is that same patient that shows very clearly the two lumens that are present in the small lumens that they are accounting for this high degree of stenosis in this particular case.

Again, this is another example of the BM mode imaging of flow, which is one of my favorites if you will, because it is such a dynamic process, a little bit of atherosclerotic disease, the external carotid with the superior thyroid branch and the presence of this small pseudo aneurysm off the internal carotid artery when it was mis inadvertently punctured with a needle on the way to the internal jugular vein.

And you can see the systolic filling of the pseudo aneurysm sac and then the diastolic emptying of the jets, all of this very clearly in real time in a very dynamic format.

The other advantage you get when you do this kind of direct flow imaging, which is a, a byproduct of the way the signal is processed, is right where this signal is brightest corresponds to the point of maximum flow velocity.

So if you're looking for the maximum flow disturbance and peak systolic velocity related to a stenosis, the direct B mode imaging of flow allows you to find that point in region very, very quickly without any difficulty, and by placing your sample volume directly at that point, you get right to the point of maximum velocity and it speeds up the diagnostic process and your ability to acquire this Very, very important data regarding stenosis.

Three-Dimensional and Four-Dimensional Imaging

Three-dimensional and four dimensional imaging is another new technology that is finally catching on in ultrasound.

It's been an obstetric ultrasound for a number of years, but it has a number of potential applications in vascular ultrasound as well.

This is just a very quick example of a cine loop about a 12 second cine loop through a region of varicose veins, which you can see very clearly up here in the subcutaneous space, the cine loop is running in real time.

This is all the time it takes to reconstruct that cine loop into a 3D data set, which you see being formed right here.

Again, create the entire data set and then you're free to manipulate it and look at it in any plane you wish.

And again, this coronal plane, which is totally unavailable in ultrasound previously with simple two dimensional imaging truly now gives you a much better image and idea of what's going on in terms of the varicosities involved in this particular system.

It would be very difficult to reassemble this particular image mentally just based on the 2D image acquisition that you had previously, but it's very simple with a quick 3D image acquisition SIM system, you also get to see things you might not see otherwise in a two dimensional plane.

You might not see the small ulceration which is present here in the common carotid artery, which led the sonographer once they saw this, to go back and actually document that small ulceration again in 2D where again, you get the good contrast resolution and you're able to see these kinds of features which might be missed if you did not take this exact slice through the diameter of the vessel from this particular projection initially in the 2D examination.

So it does show you those kinds of features, your ability to look at other structures over extended links.

For example, the stent in the superficial femoral artery.

You can reconstruct that entire process and get a good three dimensional view of the stent looking at that very simply, again, by scanning through there and you can look at it in all possible planes and as a composite 3D structure as well.

And again, looking at a stent in a carotid artery, you can do the same type of thing looking at that stent.

The other advantage to this is if you have four, four dimensional probes, you can actually do tomographic imaging corresponding to what CT and MR displays are like.

You select the frames that you want, you decide where your slices are going to be, and you can reassemble that ultrasound data in a true tomographic image similar to what you're used to looking at in CT and mr.

So you can now look at that carotid stent from multiple projections, multiple planes, and with multiple slices through the image from any approach you wish to take.

Fusion Imaging

Fusion imaging is another new technique and technology that's out there that allows you in real time to fuse your and simultaneously view your ultrasound image with the corresponding CT MR or a previous ultrasound study.

You can do this by comparing the ultrasound to CT side by side.

You can use different scales to display the CT and the ultrasound.

You can zoom on the ultrasound and show the entire CT scale.

You can actually use an overlay technique, which allows you to place the ultrasound and the CT images over on top of one another so you can view them simultaneously and look at the registration.

How is this done? Well, you take the basic CT data and you take the basic ultrasound data and you put them together into the same image and the software is able to allow this simultaneous display once you have registered the ultrasound system with the ct.

This particular example is of interest because it shows an aorta B femoral graft, the patency of the graft, the mesenteric branches, the old thrombo atherosclerotic aorta, which is still in place in underneath the bypass graft.

And the interesting thing about this is this is a recent ultrasound study, but it is being compared in an overlay fashion to a CT angiogram that was done three and a half years previously.

So as long as the basic baseline study can be saved in a DICOM format, it can be put into the machine and you can compare today's ultrasound examination to any previous study that might be available looking for interval changes.

I think the potential application of this technology to follow up aortic endograft and aortic aneurysmal disease looking for sequential changes should be fairly obvious to most anybody.

And that is how we are initially applying this type of technology looking for sequential changes in the surveillance programs in vascular diseases by being able to directly compare today's study to one that was done previously.

And again, the fusion imaging can be side by side, it can be gray scale.

It can incorporate all of the features of ultrasound, including color doppler imaging in this case showing the superior mesenteric artery and its origin on the CT scan.

And here you see the area highlighted and surrounded in green is the actual area in scene in the ultrasound image.

So you can zoom one while maintaining the original scale on the second study and allowing that the next generation of neology functions that really are not either FDA approved or ready for application in the vascular world are ultrasound contrast elastography and virtual histology.

Ultrasound Contrast

Ultrasound contrast looks at the tiny bubbles that are placed into the circulatory system.

They're essentially the same size as a red blood cell, so they circulate freely within the blood pool.

The advantage of these is the small encapsulated, heavy carbon bubbles are much more reflective than red blood cells about a factor of a thousand or so.

And so wherever the blood is flowing, you'll be able to get good strong reflective signals from them and track the blood pool very nicely.

It is not FDA approved for vascular applications at this point.

It is only approved in the United States for cardiac applications for left ventricular opacification.

Hopefully this will be turned around in the near future, but until that time it must be done used under some type of research protocol or as an off-label application in the peripheral vasculature.

But the advantages are very obvious.

This is an ultrasound contrast image of the aorta and the renal arteries.

For those people who out there who have ever struggled to get a good quality renal artery ultrasound examination, this is going to be an incredible boon because even in this early crude image, you can see the tight stenosis in the origin of the left renal artery and the post stenotic dilatation distal to that.

You can see the slight narrowing in the right renal artery and the flow characteristics in the distal renal artery and the ability to anatomically localize these vessels get your sample volume where you need to make velocity measurements is going to be very, very straightforward and even very difficult patients in the future with when ultrasound contrast is available.

Looking at aortic stent grafts, again, shown here again, the the ability to identify endo leaks in these particular structures is also very straightforward and easy as the leakage of the contrast into the surrounding aneurysm.

SAC is seen very, very clearly in these types of studies and is not difficult to track at all.

And if you look at a study looking at the ability to detect endo leaks with contrast enhanced ultrasound, the sensitivity in the initial series was 96%.

Specificity was a hundred percent, a hundred percent positive predictive value, and it did not matter what type of endoleak 1, 2, 3, or four was present.

The contrast enhanced ultrasound was sensitive with this same type of sensitivity to all types of endoleak present in these graphs.

Elastography

Elastography, is it a, will it be able to characterize atherosclerotic plaques?

It remains to be seen, but it certainly has the potential to differentiate this totally anti coic lesion.

Will it have different elastic properties and will elastography show them up compared to surrounding tissue in the blood pool?

Then this fairly uniform ISO coic AIS lesion right here, clearly histologically and clearly clinically, these are two very different lesions, whether they will be very different lesions using elastography remains to be seen, but the potential is clearly there.

At present. We can look at things like gray scale median and characterize these lesions, but this is a very crude estimate of what the lesion itself looks like and how it behaves.

And the ability of elastography to look at the stiffness throughout these lesions may be very important in being able to predict which atherosclerotic lesions in the carotid artery become unstable and lead to embolization and stroke-like symptoms.

The other area of obvious potential application is the potential to differentiate acute from chronic deep vein thrombosis.

Here you have a thrombus in the common femoral vein.

Here you have a thrombus in the femoral vein shown in long axis, and it is not clear from the image based on echogenicity and other characteristics we commonly use.

If this is a totally acute or chronic process or a mixture of the two, and the potential for elastography to differentiate acute from chronic thrombus is clearly there, but the studies will need to be done to demonstrate that this in fact can be realized in real time with our ultrasound machines.

Virtual Histology

The last technology I want to talk about is virtual histology, which at present relies on intravascular ultrasound and the high frequency signals that are returned from surrounding atherosclerotic plaque and the ability to look at the energy in those returning back scattered signals and convert that into information about tissue types where green is basically fibrous atheroma, which tends to be very stable disease, white and red tend to be necrotic debris and or calcification, which may lead to plaque instability.

So virtual histology using the back scatter signal from ultrasound may allow us to characterize atherosclerotic lesions and look at not only their composition, but their likelihood that they will become unstable leading to fibrous cap rupture.

And stroke.

Do you have a predominantly lipid laden lesion?

Do you have fibrous atheroma? Do you have necrotic debris?

All of these are possible classifications and stratifications using, as I say, the back scattered signal in a virtual histology environment.

These studies that have been done to date using intravascular ultrasound have showed excellent correlation with histopathology in the lesions, again, showing the fibro fatty lesions, showing the little bits of calcification and the necrotic debris, that is present in so many lesions.

There are multiple ways to process this information to give you this data so you can get a rapid assessment of what is the nature of an atherosclerotic lesion and will it enhance our ability to predict what lesions are likely to become unstable?

And in the case of the carotid cause stroke, the potential for virtual histology is there.

The developmental work has been done with intravascular ultrasound.

It remains to be developed and seen whether this will apply as well to conventional ultrasound systems.

Future of Vascular Ultrasound

So the future of vascular ultrasound, I think very, very good.

The technology continues to improve.

One only has to remember that it was just over 50 years ago that we were looking at something in, in the crude old vascular lab environment, and we have come now to where we have fusion imaging able to correlate in real time ultrasound with CT or MR Studies regardless of the date in which those studies were done.

So I think the future of vascular ultrasound is clearly one that is exciting in terms of technology.

It's what I would call a rosy future in this case, and we're only gonna be limited by our imagination in how we're able to apply this technology in, in vascular ultrasound and improve our diagnostic accuracy in terms of vascular disease.

Again, at this point, I'll be happy to stop and try and answer any questions which you might have.

Thank you very much.