Current Status of Intraluminal Stenting Devices – What’s New - SD

Introduction

I am Gail Hadley.
I'm the technical director
of the Vascular Ultrasound Core Lab
of Massachusetts General Hospital,
and today we're going to be talking about the current status
of intraluminal stents.

Current Status of Intraluminal Stenting Devices

Today we're gonna talk about the current status
of intraluminal stenting devices
and what's new accumulated experience
and technological advances have increased the utility
of catheter-based treatment for vascular disease.
It is currently accepted as an alternative to open repair
for many vascular beds and a variety of vascular pathology.
This includes the extracranial
and intracranial carotid arteries.
Most commonly, however,
stents are placed in the internal carotid artery
where the prevalence of disease is located at the
carotid bifurcation.
In the carotid bulb.
It's also utilized for CCA disease vertebral artery disease
and the intracranial carotid arteries
most rapidly increasing in utility is the use of stents
for peripheral arterial disease, particularly
the femoral popliteal region.
And in some cases, studies are being done
to identify the utility of stents
for chronic limb ischemia and stenting.
The tibial arteries,
aortic iliac disease has long been treated using
endovascular procedures, primarily PTA,
and now they're using stents as a scaffold
to keep the arteries open.
Currently there are ongoing studies
to stent the hypogastric arteries for erectile dysfunction.
Stents are also being used in the visceral vessels,
both renal arteries and the mesenteric vessels.
And for venous obstruction, both the iliacs
and the upper extremity veins primarily
stents have grown in such
a rapid pace that things like this.

Landmark NIH Clinical Trial

The landmark NIH clinical trial comparing two stroke
prevention procedures, show surgery
and stenting equally safe and effective.
This just came out a few months ago
and this was everyone was holding their breath
until we could actually identify which was better
for the treatment of carotid art artery disease,
carotid artery stenting or carotid endarterectomy.
So as you can see, this truly is a landmark PA page,
a landmark paper stenting devices.

Types of Stents

You need to be familiar, familiar with the types of devices
as this terminology will be used
and may impact the actual outcome
of your ultrasound findings.
First, they'll talk about the mechanism of expansion.
This includes balloon expandable stents
and self expandable stents.
Balloon expandable come crimped within the catheter.
And then a balloon is used to expand the stent
to fit the configuration of the artery
where a self expandable stent is already pre sized
to the vessel and the physician will select the
size that they would like.
Also, the composition varies
and this changes rapidly in terms everyone's trying
to find the best stent possible, which provides flexibility,
but also enough strength to keep the vessel open
and keep plaque from encroaching upon the vessel.
This would include coatings, uh, coated stents
with things like PTFE, metal stents, nitinol,
stainless steel, all
of which give different ultrasound characteristics.
And in some cases there are even biodegradable stents.
The design and shape varies from stent to stent.
There's a close cell, an open cell,
there's different mesh types coils,
and then the covered stent graft.
The clinical significance of the variety
of different stents unknown at this time.
And many of these are still in clinical trials
to identify which design is best suited
for which vascular bed.

Impact on Duplex Scan Findings

Now the real question for us is
what impact does this have on the findings
from the duplex scan?
And you can see that papers are recently emerging
trying to identify if there is a situation
where the design makes a difference.
Here we have open cell versus closed cell stent design,
differences in blood flow velocities after carotid stenting.
This was just presented at the 2008 vascular annual meeting
in San Diego, and
what they identified is they looked at carotid artery
stents, those patients that had closed cell
and those patients that had open cell.
And what they identified is
that there was a statistically significant difference
between the peak systolic velocities from within the stent
of a close cell versus the open cell.
Now you can see here a close cell design has smaller spaces.
That's called the free cell space versus the open cell,
which has a much larger free cell space.
So again, these yielded a much higher velocity.
Now is this going to impact the
different criteria that we use?
And this has just recently been identified,
so we don't know the answer to that at this time.

Other Current and Evolving Technologies

Other current and evolving technologies,
as we just talked about variable configurations,
some are covered, some are partial, some are drug eluting.
The shapes are different again,
depending upon the vascular bed that's being treated.
The most important thing to understand about the variety
of different stents is here's a covered stent
and that's with PTFE
and the scaffolding is actually outside.
So the stent graft
or the graft portion of it is actually lining the stent.
Now, if you're using this in the superficial femoral artery,
that will actually exclude any branches here,
whereas in more open design it actually keeps the
collateral vessels patent.
So if this goes down, then
the hope of recanalization is present.
It is unknown if this really makes a difference in terms of
to the patient clinically.
Now the most important thing to the sonographer is to know
that if you don't know the type of stents, you just need
to be prepared that they all, depending upon what
their composite is,
will give different ultrasound characteristics.
Here we have a wall stent that shows up very, very nicely.
You can see the integrity of the stent.
If we were to take the color off,
you can actually even see the opening here
where the external carotid artery is coming over
and the stent for the internal carotid artery.
Here we have an iliac stent.
This one actually causes a lot of artifact.
So on the image alone, it's very difficult
to tell if there's anything going on
inside the lumen of this stat.
This is where a transverse view would be much more helpful.
And also your spectral doppler findings.
Here is A-P-T-F-E covered graph
and it's a spiral coated graft.
So in between the spiral coating,
we have acoustic shadowing from the PTFE.
So there's no PTFE here, PTFE, it goes down.
So it's very hard to even get a good doppler signal from
these areas because of the acoustic shadowing.

Duplex Scanning of Stents

So now let's talk about the duplex scanning of stents.
It's been the standard of care
for the follow-up post open repair.
So it's only natural to apply the same technique
to stented vessels.
It's repeatable and it's non-invasive, so
that makes it accepted by the patient.
So what do we know about scanning
stents at this point in time?
Well, as it's been even evolving
technique for evaluation of stents,
we have identified at this point that there is a need
for study specific protocols.
There's a need for revised reporting of the findings.
It needs to be more inclusive of
what we see within the stent, what the stent looks like,
where the stent is located.
And we also need to address the issue
of stent specific criteria.
And this would be different for each vascular bed.
And as we just discussed,
does the stent design make a difference
and would we even need to maybe include specific criteria
for different stent designs?
And at this time, we really don't
know the answer to that question.
It just important to note that not all image
and spectral doppler changes reflect pathology
that is clinically re relevant when talking about
the patient with stent.

Information Needed for Scanning

So what do we need to know first?
This is our wishlist because often we're not gonna have all
this information, but if you uh, have access
to this information, it would be helpful to your scan.
First the location of the stent, the type of procedure,
did the patient have any prior procedures,
particularly any prior stents, the number of stents,
how were the stents placed?
Were they remote from one another or are they overlapping?
When stents are placed in an overlapping fashion,
it's required that they overlap by a minimum
of five millimeters
and this will certainly provide an interesting appearance
that could almost look like a defect on your
ultrasound scan.
While in fact it is nothing more than the structure
of the two stents overlapping one another.
Procedural success, what's reported
as a successful procedure.
Usually patients have of 30% or less residual stenosis.
This might impact some of the velocities
that we see through these stents.
And then we also need to know if it's comprised
of a covered graft
because not only does it obliterate branches when scanned
early on, a covered stent graft may actually not be seen.
And so you'll just get big uh, one big acoustic shadow
until the air actually resolves.
And this could take up to a month.
Here is that stent where we can actually see that
the external carotid artery is coming through the stent.
And this is an important area to investigate
because it is reported that this often causes stenosis.
Here we have the overlapping stents
where you can see here's the lumen of stent number one,
and here's the lumen of stent number two.
And what you wanna look for is the circumferential nature
of both the stents and the transition.
As we move from one stent to the other,

Complications to Look For

what are the complications that we need to look for?
Well, we do know that all stents fracture in all vascular
beds, they can disintegrate and they can migrate.
Fractures are graded. And right now currently the gold
standard for grading a fracture is the utility
of duplex remains unknown.
Currently it has just been identified
that carotid stents fracture.
Right now it's being reported at a rate of about 2%
and these tend to be occurring
after about the three year period.
There are currently ongoing studies though
to identify the true incidents.
The most common site for stent fracture is the SFA
restenosis thrombosis
and occlusion are also complications post stent.
The most common sites for rest stenosis,
generally the proximal and distal edges.
The interesting thing about occlusions
that if it's in the SFA
and there are branch vessels that partial
occlusion can occur where a distal portion
of the stent may be kept open by a branch vessel.
You can get aneurysms at stent edges, particularly in those
that employ some sort of cryop plasty
where there might actually be a weakening of the wall.
You also have to look for catheter injuries.
Dissections are the most common
and these can occur at locations remote from the stent such
as pseudo aneurysm at the catheter insertion site.
Embolic protection devices are most often used
when placing a carotid artery stent.
And it's important to know if a device was used
because you need to look for
possible injury from the actual embolic protection device.
This is a proximal occlusion device
and you actually have balloons
that are inflated in the proximal common
and the proximal ECA.
These need to be evaluated for possible complications
from the placement of the balloons.
Here's the emboli that is to be trapped
with the embolic protection device.
And what's important about this is to note that
it's generally thought that it's not recommended
to use stents in those patients that have ulcerating plaques
because you actually have to pass this device
through the stenosis in order to get beyond it.
So it can catch the plaque during the procedure
or the emboli if they have an ulcerating plaque.
Before you even place the device,
you might dislodge the emboli distally to the brain
or in the case of the renal arteries to the kidney

Duplex Scanning Protocol

duplex scanning protocol for all stents,
you must scan the entire stent.
It's important that you use gray scale and do a survey scan
because you need to look at the actual structure
and integrity of the stent.
And as I say that one recognizes that
that's can sometimes be a daunting task
because in this particular case, the entire
SFA is stented.
You need to examine the inflow and outflow vessels.
You need to image from multiple views,
particularly at any site where there appears to be a defect
or an normality, investigate abnormal image
and doppler findings.
And it's critical to determine which you're gonna weight
more heavily and that's gonna depend upon the quality
of the image, the quality of your doppler findings.
Are you able to get a good doppler angle?
You also need to assess waveform patterns since the
diagnostic criteria is not standardized at this point,
you need to look for uniform stable velocities
as you transition from native vessel into stent
and then out of the stent back into native vessel.
You need to do complete spectral doppler
of both the native vessel and the stent,
particularly at the stent edges or the transition zones.
And if you have the luxury of having a prior study
or a baseline study assess for any interval changes,
particularly in those cases
where the peak systolic velocity was elevated at baseline.
You wanna look for any progression of disease,
you need to be curious.
And here you can see that this is a stent,
but due to the artifact caused by the stent, it's difficult
to see what's going on inside the vessel
or inside the stent.
This is where you need to try different views.
This would probably be much better visualized in terms
of the lumen from a transverse view.
Here's one where you have plaque that appears
to be actually pushing in on the stent.
And this stent is a very flexible stent.
You can tell because it,
it's not fracturing when the plaque is encroaching upon it.
It's actually like a little bit of a slinky pressing in
the natural of a history of this is unknown.
But what's interesting is we have a peak systolic velocity
of about 53 centimeters a second with a normal waveform.
So in this particular case,
the image is weighted more heavily
because there's definitely a defect here.
Now there are current reports saying that those patients
where a greater than three millimeter defect is identified
between the stent
and the arterial wall are those that are at highest risk
for going on to re-stenosis.
It's important to remember that
because of the structure of the stent, we need
to use a 90 degree angle to actually image the stent
to get the best image possible.
But at the same time, you need
to do your spectral doppler from a different view using a
longitudinal approach
and using doppler angles of 60 degrees or less.
Here's the transition zone we were talking about.
This is one where it's difficult to say it appears
to be some aliasing here in color.
It's a small stent, it looks like it's tapered.
So is there a stenosis here
or is this actually considered patent?
And just a normal transition from the stent.
And because the image quality due
to the small space we're talking about here, it's difficult
to tell you In this case,
you're probably gonna rely more heavily on
the spectral doppler.

Carotid Stents

Now we're gonna specifically talk about each vascular bed,
and in this case we're gonna talk about carotid stents.
What's important to remember is that you do need
to scan the entire stent.
Here's the patient's angiogram, pre stent, post stent,
and you can see how it crosses over the bifurcation.
And up into the ICA, it's really important that we evaluate
the ICA beyond the actual stent placement.
Now remember, because this is
where your embolic protection device would've been placed,

Contraindications for Carotid Artery Stenting

the first thing we need to know though is first,
what are the contraindications for carotid artery stenting?
Extensive calcification, tortuous vessels,
a disease distal ICA?
Remember, you need to have a nice landing zone in order
to place the embolic protection device access difficulties.
They still need to be able to get a catheter up to the area
that they're going to be treating
and placing the stent ulcerating plaques.
As we can see here, we don't wanna be putting
catheters through this area.
You need a lumen large enough
to actually pass the embolic protection device.
So sometimes when you have a string sign, uh,
that is not a patient that would be a candidate
for stenting necessarily in the carotid
arteries, this is different.
However, in the peripheral arteries, they've identified
that those patients greater than 80 years
of age do not fare well with stenting procedures
and those patients who have any serious comorbid conditions.
And I just wanted to bring this up
because it is important
to include these findings in your pre-procedure report when
they're determining what is the best type
of treatment for this patient.

Standardizing Exams for Carotid Stents

So at this point, we know we need to standardize our exams,
that we have critical recording sites, we need to assess
for any interval changes to be much more sensitive
to any changes that are occurring.
Peak systolic velocity
and end diastolic velocities alone are not enough.
You need an algorithm for interpretation due to the lack
of standardized criteria
and most importantly, always internal QA
to see if you're finding the same thing
that's being reported nationally
and that you need to address these issues
and come up with a standardization
to report these findings in your interpretation.

Standardizing the Protocol

So starting off with standardizing the protocol,
basic carotid 1 0 1,
good technique lining up your cursor parallel
to the wall using consistent doppler angles.
It's important to note that we have the same segment
of vessel here and you can see the differences in the
velocities 41 11 here we're not even measuring the correct
velocity because the doppler ankle is not aligned correctly.
So it is important to standardize this
for each patient at each segment of the vessel.
Where it's most important is right on when you're on the
threshold of a 50% stenosis.
And it's most important with the stented patient again,
because the criteria currently
is different in being reported differently
around the country, it's being reported differently from
different stent designs.
So you need to incorporate this into your procedure.

Image Protocol

The image protocol, as we mentioned,
we're gonna use a 90 degree approach to interface so
that we can actually see the structures of the stent.
You want to do this separate from your spectral doppler
again, because you're going to employ different angles,
you wanna use gray scale, you don't want color
to overwrite the actual structure of the stent.
You wanna be able to see the relationship to the walls
and the opposition at the stent edges.
You wanna use multiple views in order
to appreciate all this.
In this particular case, you can see in the transverse view,
here's the lumen of the stent.
Here we have the actual difference between
the arterial wall and the stent.
And if you wanna measure the largest diameter
to follow this, this is being recommended.
Um, by some authors, it again adds a lot of time.
And if you have a normal patent stent, it's something
to consider but not necessary.
Here's one where it fits the criteria
of it's greater than three millimeters.
So is this something where there's at risk
for this plaque progressive?
Other areas that you need
to be concerned about is when you're going
to again weight the image more heavily than you are the
doppler in this case,
your spectral doppler has a peak systolic velocity
of about 84 centimeters a second.
And this is right here at the site
of this plaque proximal to the stent.
So it is encroaching
and this would actually be considered stent re stenosis if
that becomes worse.
Here's the transverse view of the stent
and it needs to be reported that this is at the stent edge
because the actual stent itself is patent.
We like to refer to stents in our reports as patent, not
as normal because these patients do all have a residual
stenosis as you can see here.
Here's the stent, here's the wall.
So there's your residual stenosis
and is discussed earlier,
less than 30% is considered a successful procedure.
So caution using absolute velocity criteria in the presence
of a normal image.
If you get elevated velocities anywhere through this stent,
you have to recognize there is a diameter change here
as we're moving along a little bit more plaque here as well
and that the velocities might elevate slightly.
But this we look at the color doppler
represents a patent stent.
Other things we need to look at here, you can see
the plaque along the wall
and these are areas
that are at highest risk for progression.
So even though there's no doppler change here,
is it something that you wanna measure so
that you can follow over time?
Here we have an acoustic shadow.
Remember the calcification in the plaque is still there.
So even though this patient has a stent in place,
we're really not able to assess this area.
And that's something that needs to be mentioned in your
report because certainly as we look at the utility of duplex
to identify stent fractures in this particular case,
you'd not, you would not be able to rule out stent fracture.
Here we have a beautiful image of the entire stent
and you can actually see the complete transection along the
posterior wall here of this carotid stent.
Now these can even get more complicated.
Here is a patient that had a carotid stent fracture,
then they placed a second stent.
It actually looks like a third stent,
but here's a stent, here's a stent, here's the fracture
and they bridged it with another stent placed
inside the two stents.
So this is obviously not gonna fit into any
of our nice neat little boxes for stenosis criteria.
You can see the second stent here, it doesn't even appear
that circumferential
and that just could be due to the situation that was here
that might be considered normal for this patient.
Here's one of the the original stent,
but we do get a peak systolic velocity of 2 88.
This patient's been followed for three years
and this has been stable running
around 300 centimeters per second.

Velocity Thresholds for Stented ICA

So velocity thresholds for the stented ICA,
we originally applied the native criteria
and unfortunately that yielded many high
false positive rates.
These patients would go on for angiography as part
of the clinical trial for any lesion found over 50%.
So if the PSV was over 1 25, they would get an angio,
they would see that the stent was patent.
So it was then observed and questioned.
Do the stents actually alter the velocity?
And here we have a patent stent
with a peak systolic velocity
of about 139 centimeters a second
and an end diastolic velocity
of about 47 centimeters a second.
This is a stable uniform velocity.
And now we know that this is considered a patent stent
and not a greater than 50% stenosis.
So now we need to consider does stent design matter?
Does this patient maybe have a closed cell stent in which
it's been reported, causes a little bit higher velocity?
It's also been reported that the materials
that in composition of the stent, if it's thicker
that they might actually increase the velocities,
it obviously might also change the compliance of the vessel.
The current interpretation algorithm that we use,
and these are the parameters
that are most currently accepted.
A patent stent is defined as one
with a peak systolic velocity
of less than 150 centimeters a second, they're uniform
and they're stable throughout
and the waveform pattern is consistent
and this is all in the presence of a normal image.
So when we look at the utility of carotid, you do still need
to look at the whole picture.
Those are just some working parameters
that are being used today.
But here we have a patient that shows up post procedure
with a PSV of 171 centimeters a second.
You can see here this is a wall stent.
It appears to be in gray scale, widely patent.
The patient returns at six month goes
to a different institution,
is recorded at peak systolic velocity, the maximum
through the stent of 1 75.
This is now reported as a 50 to 79% stenosis.
The patient goes on for angiography
and the stent is widely patent.
So in retrospect, if you go back
and look at this, first we have a normal
image at both visits.
There's no interval change between the post and six months.
Those are considered stable velocities.
And what's most interesting,
as we even know in the native circulation,
this patient has a contralateral occlusion which could also
contribute to the extended high velocities.
So you need to look at the whole picture,
not just the numbers because you will get false positives.

Assessing for Interval Changes

Now when we talk about assessing for interval changes,
you can see here that at six months this is a normal
spectral doppler waveform.
Normal velocities.
The edge of this appears to be up somewhat
and you can see the little bit of plaque here.
This is an area, this is why the images are so important
because when this patient comes back at 12 months,
six months later, you see
that the plaque is actually pushing the edge
of this stent up.
So it's no longer opposed to the wall.
It's also causing a doppler shift.
So we're getting a peak systolic velocity of 2 75
and an end diastolic velocity of 90.
So this definitely fits an interval change
and is consistent with a 50 to 79% stenosis.

Published Reports on Velocity Criteria

Now here are some of the published reports
in which most people started using a peak systolic velocity
of less than one 50 consistent with a patent stent.
And not to go over those
because in my current literature search
we now have more people looking at this.
There are more patients being examined
and we have better correlations.
So different papers have looked at trying
to categorize a greater than 50% instant stenosis,
greater than 70 and greater than 80.
And there are so many papers out there,
it would be difficult to go through all of them.
So what I did is just give you the ranges
to give you an idea of how they can fluctuate.
Some institutions are using A PSV of greater than one 50
consistent with a greater than 50% stenosis.
Some are using greater than 220.
So you can have a relatively normal stent with PS vs
or less than 50% stenosis with PS vs as highest
as 200 centimeters a second.
Some institutions report in the literature
that the end diastolic velocity does not play a role
in these situations.
Others have reported
that a greater than 50% stenosis also incorporating
that the EDV be greater than 49 centimeters a second.
Another type of category you can use is a ratio.
They are ranging for greater than 50% stenosis,
anywhere from 2.5 to 3.8.
And you can see that we're far from having
a standard criteria.
Now one thing that all of these papers here as we report,
we're finding two consistent things.
One is that for greater than an 80% stenosis,
this is pretty consistent across the board
that a peak systolic velocity of greater than three 40 EDV
of greater than one 40.
And the ratios though are ranging,
but you can see it's not by much 4.0
to 4.15 are consistent with a greater than 80.
And so the key thing is that when to treat.
So if you're gonna treat re-stenosis this last category,
it's important that we're finding
that right now these variables are working the best.
And just like with the native vessel, it's that between 50
and 70% category that we have yet
to identify specific parameters.
Here's to show you the differences here we have a transition
area from stent to native vessel.
We're getting a peak systolic velocity
of 317 centimeters a second we about 106 in EDV.
This is consistent with the criteria that we use
for a greater than 70% stenosis.

Other Considerations for Carotid Stents

Now other things we need to consider is
that here's an stent.
And about 40%
of these stents when they cross the ECA will actually
cause a stenosis of the external carotid.
And here you can see that not only do we have a stenosis
of the external carotid,
but we also have a stenosis of the internal carotid.
So this makes this even a little more complicated.

Interpretation Considerations

So interpretation considerations first,
what were the patient's post-procedure baseline findings,
what was the re residual stenosis from the procedure?
Does the patient have any intracranial disease,
any contralateral disease?
What about disease of the subclavian arteries?
These all play a role in the velocities that are seen
through the stent important.
And we are much more sensitive when we look
for changes over time.
When you see a greater than 50% stenosis
or an elevation in velocity, what you really wanna look
for is correlating findings
with a greater than 50% stenosis.
There should be changes in the image,
there should be other changes that indirectly occur
because of the stenosis.
However, this is all gonna be weighted depending upon the
quality of your image
and the quality of the doppler technique.
And then again, always using your internal
validation when possible.
Here I just bring this up
because one thing that we talk about when we talk about
criteria is always in reference to the internal carotid
and it is specific to the internal carotid.
Well what do we do when we see a carotid artery
stent placed in the common?

Common Carotid Artery (CCA) Stents

I think here you can see one
of the nice things about the common carotid is it
usually visualizes.
We can get good doppler angles.
Here we have the vessel wall here,
we actually have this stent.
You can see that that is being reduced
by about 50% just on visual estimation.
And if you look, we're gonna see a huge step up
as we cross over, we're getting velocities in excess
of 370 centimeters a second.
What we currently use for CCA stents
and we've had correlation with angiography in 37 cases
is a step up of greater than two.
So we're looking at the peak systolic velocity immediately
proximal to the area of maximum increase
and the maximum increase
and the category is just greater than 50.
There is no specific criteria to narrow it down
to anything other than 50 to 99.
And we have correlated very well using that algorithm.

Stent Fractures in Carotid

Now just remember, as I noted,
it's just been recently addressed
that the carotid stents do fracture.
It's so important to look at these here.
It doesn't show up that well in a
longitudinal view down here.
But when we go to a little bit different angle,
you can see the huge gap here between the struts.
You can actually see in color power flow going out
around that stent.
Here we're getting a peak systolic velocity of about 170
centimeters a second.
So that's really not, that would be difficult to say if
that's the stent or if that's from the fracture.
And the natural history
of these are not well described at this point
because we've just identified
that this occurs more commonly than originally thought.

Peripheral Arteries

Now we're gonna talk about the peripheral arteries.
First off, the problems here in treating
this, they're long lesions.
It's diffuse disease. Generally occlusions occur.
Diabetes mellitus, which causes calcification
of the medial wall, impact of the inflow
and outflow vessels play a role 20 to 7, 27
to 40% stenosis.
That's a very, very high stenosis rate. They do fracture.
The significance is unknown at this point that elastic
recoil actually amplifies the reactivity of the SFA
and intimal hyperplasia.
So the variety of balloons and devices specific to pathology
and the location of disease in patient
condition widely vary.
So this makes it even more challenging when
doing our evaluation.

Considerations for Peripheral Arteries

So first two things to consider.
The biggest issues, high rates of re stenosis as high
as a 40% re-stenosis at one year has been reported
and that stents fracture here you can see you also have the
challenges due to the unique area of the adductor canal.
You can see how curvy and tortuous the stent is.
Is that gonna be something that will impact
and maybe cause fracture down the road?
Again, unknown. It will obviously also create some changes
in our velocity as we course through that particular stent.
So considerations for the peripheral arteries,
we do need revised criteria
and just as we found in the carotids
that the current native criteria, if you apply
that will give you false positives.
Stent specific duplex protocols are required.
You need to assess the inflow and outflow vessels.
As we mentioned, the natural history
of stent fractures are unknown,
but we do know that multiple stents increase the risk
of fracture and the varying grades
increase the risk of stenosis.
So depending upon the type of fracture
will place the patient.
One type might place the patient at
greater risk than another.

Current Working Criteria for Peripheral Artery Stents

Here's the current working criteria for
identifying patency
of a stent in the peripheral artery beds patent.
And you can see here I put patent versus normal
because anything less than 50%, we like
to use the terminology patent
'cause these vessels certainly aren't normal.
No peak systolic velocity change.
Uniform stable velocities throughout with stable waveforms,
no spectral broadening the peak systolic velocity.
You can see in native vessels we use 2.0 here we're using a
ratio of less than 2.4
and that's when you use a normal proximal segment
as your denominator 50 to 90% stenosis.
It's consistent with a peak systolic velocity ratio
of greater than or equal to 2.5 when
compared to normal segment.
The PSV is greater than one 90
and you have an abnormal image.
Now this has just been recently identified trying
to categorize the degree of stenosis of greater than 80%.
And what we're finding is this very similar
to the criteria used for vein bypass grafts.
And that's a peak systolic velocity of greater than 2 75
and a velocity ratio of greater than 3.5.
You should also have an abnormal image.
It's important to note that once the stenosis gets over 95%,
you're actually gonna see a decrease in the
peak systolic velocity.
And then occluded vessels, no signal.

Examples of Stent Re-stenosis in Peripheral Arteries

So here's your classic stent re-stenosis.
You can see this particular stent is one of those
that has a graph coating.
So these bright echoes are where there's no graph coating
and where you don't have the bright echoes, that is
where there's PTFE.
As we come, we see a nice relatively normal
waveform for a peripheral vessel.
143 meters a second, we progress down.
And you can see here that we've jumped up
to 3 81 the waveform.
We've lost any of the reverse flow component.
The image is abnormal.
So even though we don't have spectral broadening,
which you don't always have to have in a significant
stenosis, everything correlates
with a greater than 50% stenosis.
Then we see a little bit of a tardis parvis waveform,
the delayed acceleration beyond the stenosis.
And then as we go into the native vessel, the distal SFA,
we actually see a second area
of stenosis occlusions.
The key thing to remember is when you see an occluded
vessel, a document it with doppler
but also scan the entire vessel because if it is an open
or a close cell design, the branches
can actually keep the distal segment of the stent open
by recre canalizing it.
Here we have where you can see this is a branch
coming right over the top.
This is an occlusion.
And this one actually,
and I'm sorry I don't have this slide,
did reconstitute distally.
So the distal third of this particular stent was patent.
So don't stop your scan there, continue
to scan all the way down into the native vessel
and see where the vessel re constitutes.

Stent Fractures in SFA

As we mentioned to all stents fracture in the SFA,
the most common site is the SFA, particularly in the area
of the adductor canal.
Multiple stents increase the risk.
Here we have a fracture here, we have a fracture here,
we just have some stenosis.
And you can see how this can make it very, very complicated.
And right now the working theory is that
duplex is probably not the exam of choice to rule out
stent fracture in the SFA.
The key thing though that we're trying
to find out is when you do find a
stenosis, are they related?
So if the patient has a fracture, does
that put them at increased risk for stenosis?
And at this point in time, we really don't know.
There are five categories
of fracture anywhere from one time fracture to complete
transection of the actual stent.
Now you also see increased risk for fracture when the length
of the stent is increased as well as the number.
And generally this is seen
with lesions longer than eight centimeters.
There has been no difference in patency rates for those
with drug eluting stents.
So we're not seeing the same thing
that's been noted in the coronary arteries.
Here. Again, stent fracture,
you can see this is protruding out.
This is not appreciated in the image.
This was a completely normal image.
Here we have another defect noted here.
So this is why X-ray still is the exam of choice
to rule out stent fracture.
However, it doesn't mean you shouldn't look
for them in this particular case.
I've changed view here.
So it looks like the x-ray here you can see
the complete transection.
And here we have the duplex showing you
anterior posterior walls.
And you can see that this does not change the
spectral doppler at all.
So this is strictly an image diagnosis
and it's important that you take your time
and do a slow survey to look at the structure
of the actual stent.
The hard point is when you have equivocal
findings, what do you believe?
In this case we're gonna believe the image
because here we have a fracture, here we have a fracture.
The important thing to remember if this were an overlap,
'cause sometimes overlaps can look like this, that you have
to remember that a overlaps one has
to be inside of the other.
That's the first thing, rule of thumb.
They would also have there edges be parallel to one another.
Here's one, here's one.
So this can't be an edge and this can't be an edge.
So that again puts it in the category
of a fracture down here.
You actually see at this site in color
that it is causing somewhat of a stenosis.
So will that go on to progress?
But what you wanna do is make sure you note that
in your findings and your worksheet so that you can assess
that area when the patient comes back
for their next follow-up visit.

Tibial Arteries

As I mentioned, they're now placing stents in the tibular
arteries, which is very challenging simply to try
and keep limbs attached.
So this is being reserved strictly
for chronic limb ischemia.
And I think the one nice thing is that sometimes it's hard
to tell the stent from calcification,
but it does make it much easier to scan the tibial arteries.
And at this point we're still using the same criteria
that I presented for the stented SFA

Visceral Vessels

now visceral vessels, they're actually stenting SMAs
celiac axis and renal arteries.
The one thing to remember is it's not always visible.
So if you don't know the patient's history,
you might not be aware that they have a stent.
Stents are being used predominantly for osteo disease
and proximal disease.
The only place where this probably differs is
for those patients with FMD in the renal artery,
which is gonna occur most often in the mid distal portion
of the renal artery.
The other thing that's important to remember is that like
with anything else embolization occurs if they're talking
about the mesenteric vessels, it could be to the gut
and if we're talking about the renal arteries,
they could embolize to the kidney.
And currently some centers are employing
embolic protection devices.

Renal Artery Stents

When placing stents in the renal artery, it is important
to evaluate the end diastolic velocity
and note if there's been any change,
because if there's been a large amount of embolization
or maybe it's been to one pole
and not the other, you might see a decrease in the end
diastolic velocity.

Stent-Specific Criteria for Renal Arteries

So stent specific criteria, again,
this is just now being reported
that we're seeing the same thing elevated velocities
with patent stents here the most important thing
to do is make sure first
that you standardize your technique across the board using
proper doppler technique.
Cursor aligned doppler angles less than 60 degrees when
anatomy doesn't allow you to do that, you need
to at least note this in your report.
You can see how this is a rigid stent,
so it's actually coming off 90 degrees to the aorta.
And from this particular view, it's going to be impossible
to get a doppler angle of 60 degrees or less.
The best thing to do is come from a flank approach.
So it's key to use more than one approach
when you identify a stenosis
or a high velocity in the renal arteries.
The other thing that's important
to avoid any potential errors is to look for the transition
through the stenosis.
Here we have the elevated velocity.
Now we actually have post stenotic turbulence,
and as we get distal to the hilum,
we actually see a tardis parvus waveform.
So this fits that whole pattern as we lose energy
through the significant stenosis.
Now the criteria for native arteries
for a greater than 60% stenosis is well known.
There's the renal aortic ratio, which is a ratio
of greater than 3.5,
and there's also an absolute peak systolic velocity
criteria, which employs a peak systolic velocity
of greater than 200 centimeters A second, and
although it's not on this slide, there must be the presence
of posts, stenotic turbulence,
and I think we can see that here.
Now, this is actually a nine month instant re-stenosis,
but we have obviously elevated velocities.
Well over 400 we're aliasing.
You can see the brewery down here in the spectrum.
And we can also see the turbulence with the colored doppler.
So this definitely fits a greater than 60% stenosis even
using the native criteria.
The key is what happens though when you see an elevated
velocity and there's no stenosis.
So do we need stent specific criteria?
Here's just to show you angiographically
what a stent re-stenosis looks like.
So I'm gonna share with you a retrospective analysis
of patients who underwent duplex ultra sonography
and CTA within six months of each other.
At MGH, this was independently reviewed
by two blinded reviewers.
And if in disagreement,
a third blinded reviewer analyzed the images,
there were 134 renal arteries analyzed
and this is how they plotted out.
If you look here, the little black marks
zero to 59%
and the hot pink represent those velocities from
a 60 to 99% stenosis.
And what you'll see is
that predominantly a peak systolic velocity
of greater than 240 sec 41 centimeters.
A second is where the cut line came
for a greater than 60% stenosis.
So now we have about 40 centimeters a second,
depending upon which native criteria you use
higher than native.
Now we look at the renal aortic ratio.
Again, we can see in black we have normal patent vessels.
And then in hot pink we have the 60 to 99%.
And what we see is that the renal aortic ratio is much
higher than we see in negative vessels here.
4.3 is consistent with a 60 to 99% stenosis.
So if we look here at how this plotted out,
you can look at the sensitivity, specificity,
positive predictive value,
negative predictive value and accuracy.
And you can see they're all well above 90%.
The key point is, is when you see stenosis by arteriography
and degree of stenosis by ultrasound, you'll see
that even in this scenario, three of them were overestimated
and were false positives.
This was also done in conjunction with Mount Sinai.
So this is what, based on this series of patients,
what we're recommending that a peak systolic velocity
of less than 240 centimeters a second is accurate.
In excluding significant instant re-stenosis.
A peak systolic velocity
of greater than 300 centimeters a second
with turbulence is accurate.
For diagnosing a greater than 60% stenosis,
a renal aortic ratio of greater than
or equal to 4.3 predicts a greater than 60%
instant re-stenosis.
However, what was interesting is
that actually peak systolic velocity came out
to be the better single predictor for instant re-stenosis.
Now you can see we have a huge gray zone.
The peak systolic velocity between 2 41
and 2 99 is a gray zone.
So in these particular cases, you need
to assess the B mode image.
You need to assess for turbulence both in color
and with spectral doppler.
You need to see if you can identify a visual estimate
of stenosis or maybe a residual lumen that's identified
with color power, the severity of PST.
You also need to look at what's going on.
Is there a TARDIS parvis waveform beyond the area?
Is it focal in nature
or is this extended throughout the renal artery?
So we need to caution you when you get
to situations like this.
You can see we have good doppler technique, a good image,
we have a little bit of aliasing that we see here.
We have a velocity almost approaching 400
centimeters a second.
And so in this particular case, this is consistent
using the absolute peak systolic velocity criteria
of greater than 60% osis.

SMA Stents

Now SMA stents, what about that?
Now the problem with this,
and this is why we're only gonna touch on this briefly, is
that all the series that are reported in the literature,
as we know this is uncommon.
So the series small numbers of patients.
So we have identified that it, there does appear
to be an increase in the peak systolic lic velocity
and that you may want
to revise the criteria used when you see a mesenteric stent.
Patent stents ranged in one studies.
This was done with a A series.
You'll see that in one series, the velocities
for normal patent stents ranged from 279 centimeters
a second to 476 centimeters a second.
Now the limitation was that there were only 31 subjects
and vessels evaluated in this series.
So it's hard to determine
and know what would be the value to use for
your greater than 60% stenosis.

Conclusion

And on that note, I wanna thank you very much
for your attention.