Renal Artery Duplex Scan - SD

Introduction

I am Gail Hadley.
I'm the technical director
of the Vascular Ultrasound Core Lab at
Mass General Hospital.
This presentation will discuss the details on how
to conduct a renal artery duplex scan.

Incidence and Risk Factors

The incidence of renal artery stenosis is reported
to occur in approximately 5%
of those patients with hypertension.
However, over the last number of years,
there has been increased number of reports
that have identified those patients
that have high risk factors for increased incidents
of renal artery stenosis, Olin
and others were the first to identify the association
of other atherosclerotic disease with renal artery stenosis.
So if a patient would have carotid artery disease,
iliac artery disease, they have an increased risk 10
to 40% of having a renal artery stenosis,
peripheral arterial disease.
So basically your atherosclerotic patient of those patients
with renal vascular hypertension, 75%
of those will be atherosclerotic in nature.
The prevalence of disease is along the osteum
or proximal portion of the renal artery.
Now there's a remaining 25% of patients
who will have fibromuscular dysplasia known as FMD.
The difference between the, it's a different plaque
or it's a different process.
It's really not plaque than atherosclerosis.
It's usually not as easily identified.
However, the location of the vessel makes you suspicious
because it occurs in the mid to distal renal artery.
While atherosclerosis occurs more often in the osteum
and the proximal renal artery.
So it occurs more often than previously described.
Early detection is essential to avoid loss of renal function
and the proximal renal artery
and origin of the vessels are actually the easiest
to identify using ultrasound.

Visceral Vessel Anatomy

Now, in order to conduct a visceral vessel scan
and be able to identify accurately the renal arteries,
you need to have a thorough working knowledge of
the visceral vessel anatomy.
So if we look here at the anatomical drawing,
we see the abdominal aorta, we see the celiac axis.
Then the next vessel
or branch off the aorta is the SMA,
the superior mesenteric artery
traveling inferiorly.
And then just below the SMA, the right
and left renal arteries arise slightly anterior
and then coursing laterally to the kidneys.
So you can see that
although there are numerous vessels in the abdomen
that it's just a very narrow field of which we need to work
to be able to identify the right and left renal arteries.
It's important to note that the right renal artery is
generally longer than the left renal artery.
And you can see that here in this diagram.
When we look at the image
by placing the transducer subxiphoid in a transverse view,
it's best to use the SMA as your landmark.
Usually you can see the bright echoes surrounding the SMA
and then just immediately inferior
to the SMA.
You will see the left renal vein crossing anteriorly
over the aorta.
We see the aorta.
Now you don't see the renal arteries,
but they're located slightly posterior to the renal veins.
But now you've narrowed down your field
of view in which you need to be able to locate
and search for the renal arteries.

Renal Artery Anomalies

Now, renal artery anomalies.
In order to search for the renal arteries, you do need
to be aware of any anomalies that you might encounter.
The most common is multiple renal arteries.
A lower pole is generally the ones seen most often,
however, you can have an early bifurcation.
As we see in this diagram here, there's very,
very small trunk of the main renal artery.
Then we have this early bifurcation
at other multiple renal arteries.
You can see that we actually have
independent origins off of the aorta.
These can sometimes be very small and difficult to identify.
Here we have an image of the aorta
and we see renal artery number one and right renal artery.
Number two, if the renal artery,
the accessory renal artery is identified, it is important
to thoroughly evaluate that renal artery for stenosis.
Another anomaly could be a retro aortic left renal vein.
As we noted in the image, we have
the left renal vein crossing anteriorly over the aorta
as one of our landmarks.
If the left renal vein is retro aortic, it would cross
the aorta beneath the aorta on the image.
And if you were to try
and find the renal arteries down here,
you would never find them 'cause they're actually up here.
But this actually is one of the less common anomalies
that occur in the renal arteries.
Now if you see kidney anomalies, horseshoe kidneys,
pelvic kidneys, these are often associated with
variances in the anatomy of the renal artery.
So one should be suspect if they see an anomaly
of the kidney to suspect that the renal artery might not be
in its normal location equipment.

Equipment

It is very, very critical to have
state-of-the-art equipment.
You cannot do this with smaller units.
You need to have a lot of power.
You're gonna look at small vessels at very deep depths.
So you need low frequency transducers, warm acoustic gel.
Additional tools use everything that you have.
It includes color doppler, color power,
angio harmonic imaging.
You will need to optimize the image in order to be able to
identify these small renal arteries recording device.
And it's always critical to optimize your image so
that you maintain frame rates.

Patient Preparation

Patient prep. In order to successfully
scan the patient's renal arteries, they need
to have an aggressive prep.
They should remain NPO eight to 12 hours prior to the exam.
They should be asked to have a low fiber meal the evening
before, have no cigarettes and no chewing gum.
They should have the scan done early in the morning.
That way the patient isn't utmost of the day taking in a lot
of air and creating gas even though they've been NPO
when the patient arrives.
You should explain the breath holding techniques
because it's critical that when you need the patient
to hold their breath when you found a stenosis,
that they don't take too big of a breath
and completely move the vessel out of your field of view.
Dimethicone, it does not get rid of large amounts of gas,
but dimethicone might be able to help with small pockets
of gas that come in once you've started the exam.
There is no contraindication to simethicone, so it is safe.
You should always do a bilateral exam,
renal arteries and kidney.

Exam Protocol

So a complete evaluation
of the renal arteries is all the way from the osteum off
of the aorta to the renal hilum.
And if you find any abnormality of the renal artery,
it should be confirmed from two views.
So when we look at the images here, the two views that were
are used most commonly
to identify the renal arteries would be
the anterior approach.
However, the problem
with the anterior approach is you can see the renal arteries
will frequently show up at 90 degrees off
of the aorta in your image
and that does not leave for good doppler angles.
So in order to assess the patient
with the best doppler angles,
the lateral decubitus position is the better position.
It's also helpful to place a pillow under the patient's side
that's that they're lying on
because with the pillow, you can now put the arm
above the head and open up and try
and move the rib cage away from the kidney.
Here's some examples.
Here's the anterior approach with the patient
resting comfortably.
Here's the lateral debe position.
You can notice that the arm is extended up over the head
to open up the space between the hip and the rib cage.
It's also important to notice the technologist position.
This is a demanding exam
and the technologist should
remain comfortable protocol.
You need to obtain a longitudinal image of the aorta,
obtain spectral doppler from the level of the SMA.
You need to do direct evaluation of both renal arteries,
then conduct spectral doppler of the entire renal artery.
Indirect evaluation includes hylo analysis,
renal parenchymal, waveform evaluation,
and kidney length measurements.
Exam components include the beam mode image,
which provides the anatomical information,
the pulse doppler, which provides the
hemodynamic information.
The pulse doppler is
where we get our diagnostic information from
and additional tools such as color power
harmonic imaging.
Here we can see a complete exam.
We have the aorta,
here's the right renal artery extending out
to the hilum into the kidney, and then we have the kidney.
We've turned on color and you can actually again see aorta
and the renal artery coursing towards the kidney.
If you do not get a complete exam,
this must be noted in your report.
So starting at the subxiphoid approach,
we have the sagittal view
of the abdominal aorta at the level of the SMA
using a 60 degree doppler angle.
With the cursor place center stream,
we obtain the aortic peak systolic velocity.
This is what will be used for the renal aortic ratio
evaluating the renal arteries from two views,
we will have multiple doppler angles.
However, if the doppler angles are less than 60 degrees
and the exam is consistent with stenosis from both views,
the accuracy goes up.
If you have a normal velocity from one view,
and in this case we can see from the lateral
to cube approach, the right renal artery has a velocity
of 1 31.
If we come from the anterior approach,
you can see the 90 degree course of the vessel from the
aorta attempting to intonate.
That portion of the vessel
does not give us very good doppler angles.
And in trying to measure this at 70 degrees, we end up
with a peak systolic velocity of 2 77.
So in this case, the two views do not confirm stenosis,
and since you cannot really make a stenosis normal,
we're talking about hemodynamically significant stenosis
of over 60%.
This patient is normal
and this is an error based on doppler technique.
The image from multiple views is also very helpful in
identifying where you wanna place your sample volume.
Here is sort of a coronal view of the aorta.
We call this the banana peel approach.
We have the origin of the right renal artery.
As you can see that provides excellent doppler angles.
Here is the left renal artery,
and again, so both renal arteries,
the proximal segments provide good doppler angles
to evaluate the arteries.
Here's the anterior approach from the same patient.
Again, the right renal artery, 90 degree doppler angles,
but the left renal artery actually due
to its course provides rather nice doppler angles.
So this needs to be assessed individually from each patient.
A complete exam, here's the peak systolic velocity
or spectral doppler from the abdominal aorta.
Then we move into the proximal renal artery,
then we move into the mid renal artery
and into the distal renal artery.
It's important to note
that the signals throughout the renal artery are
essentially the same.
And what's critical about
that is if you see a change in these wave forms,
it might actually indicate that you've missed something
and then you would need to go back
and identify why there was a change in
the shape of the waveform.
They should remain uniform and consistent throughout.
So exam components, walking the sample volume
through the entire course of the vessel.
This is to identify the maximum peak systolic velocity
Doppler interrogation is performed in
the sagittal view only.
You're using a small sample volume place, center stream
or center to flow Jett.
In the renals, this can be difficult.
Doppler angles of less than 60 degrees use angle correction
with the cursor aligned parallel to the vessel wall
and calculate the maximum PSV from designated locations
and any areas of abnormalities.
And since we're looking
for hemodynamically significant lesions here, often
with the belly, you will get some
brews color brews from the soft tissue.
Whenever you see a color brewery, it should be evaluated
with your doppler.

Diagnostic Criteria

The diagnostic criteria is based upon the calculation
of the renal aortic ratio
that's taking the maximum peak systolic velocity from the
renal artery and dividing it by the maximum PSV
in the aorta at the level of the SMA.
Now the limitations to this technique,
abdominal aortic aneurysms will give you an erroneous
spectral doppler velocity.
Or if the aortic PSV is less than 40,
this will give you a falsely elevated renal aortic ratio.
So it is important to note
that when the abdominal aorta is not normal
or has low velocities
or extended high velocities greater than 1 25,
the renal aortic ratio is not calculated.
Now here's a normal renal artery.
Looking at the waveform pattern, we can see
that we have continuous forward flow throughout diastole.
This is considered with decreased pulsatility
that's observed throughout the renal artery always providing
flow going towards the kidney.
Now the diagnostic criteria, renal aortic ratio
of less than 3.5 is considered normal.
Now the associated absolute peak systolic velocity is A PSV
of less than 180 is considered normal.
A diameter reduction of less than 60% is consistent
with an RAR of less than 3.5.
However, the absolute PSV is
180 to 200.
The key thing is to note
that you will find no post stenotic turbulence in patients
that have a less than 60% stenosis.
60 to 99% diameter reduction is consistent
with an RAR of greater than 3.5 A PSV
of greater than 180
and the presence of post genau turbulence.
Now, there's additional criteria
that was done at Bowman Gray,
and this is actually one of the larger series,
and they identified that a peak systolic velocity
of greater than 200 centimeters a second
with PST is consistent with a 60
to 99% stenosis.
So there continues to evolve new criteria
for the diagnosis of renal artery stenosis occluded,
no detectable flow,
and most often you will see either no flow
or a monophasic low velocity signal out at the hilum
or within the parenchyma of the kidney.
Now talking about post stenotic turbulence, it's important
to identify this transition through a stenosis.
So this is going to represent
what we see proximal to the lesion.
This is the maximum peak systolic velocity from within the
lesion and here you can see as blood flow is trying
to restore itself back to laminar.
We have random and chaotic flow, which exhibits
which can be seen here in this waveform,
and that's known as posts stenotic turbulence.
Here's a very real patient that has a renal artery stenosis.
You can see the elevated peak systolic velocity,
diffuse spectral broadening.
Here's the post stenotic turbulence
and then beyond the turbulence.
Once you move out from the stenosis,
you will see the delayed systolic upstroke
and monophasic waveform known
as your TARDIS parvis waveform.
Here's a normal patient.
The PSV is less than 200 centimeters a second.
The waveform shows decreased pulsatility
throughout, and this is right at the origin
of the renal artery.
As we move to the mid renal artery, we see a PSV of 1 54.
Again, the waveforms are a match, so we know
that this is an essentially normal renal artery.
Up through the mid portion here,
we have a left renal artery.
The stenosis, the little bit of color aliasing
that's occurring is actually being covered
by the sample volume.
This is a very, very tight lesion. Here's the aorta.
You can see where aliasing in systole,
this exceeds 500 centimeters a second,
and even in the presence of aliasing,
you can't calculate the RAR,
but even if you used 500,
this patient's RAR would exceed the 3.5 scale.
Here we have a 60 to 99% stenosis.
This patient has a peak systolic velocity
of greater than 600,
and again, with post stenotic turbulence,
you can actually see the brewery down here in
down here on the spectral tracing.
Here's the sample volume.
So again, this is at the proximal portion
of the renal artery.
Here you can also see the post genomic turbulence
and color brewery.
Now occluded renal arteries,
the renal arteries are not often seen when they're occluded.
Occasionally you might be able
to see a little stump off the aorta.
This is the right renal artery
and there's no doppler signal.
But in the presence of being able
to identify an occluded renal artery, it's important
to go out to the kidney in the hilum
to see if his kidney is actually viable via collaterals.
And here in this case, we will look at the kidney length,
which was nine centimeters, small but still viable.
It had peak systolic velocities that ranged from 25
to 30 centimeters a second out at the hilum,
and this was very, very reproducible.
Here we can see the various wave forms
from throughout the kidney.
All of this speckling is due
to the increased power level use trying
to detect these low flow arterial signals from the kidney.
Here's our nine centimeter kidney,
and you can see actually by looking at the echogenicity
of the kidney that it actually looks like a
relatively healthy kidney.
Here's the angiogram with the collateral vessels as detected
by the ultrasound
and by intonating these vessels,
this gave the surgeon an opportunity to do an aortic
to renal bypass and actually save this patient's kidney.
Now, fibromuscular dysplasia occurs in the mid
to distal renal artery.
It gives you this sort of strand
or beaded appearance of pearls.
Now this generally does not cause as high of an increase
as atherosclerotic disease, so you do need to be aware of
that, that if you see this presentation
or you see the high velocity waveform from a young female in
the mid to distal renal artery to suspect FMD.
Now the one thing that's interesting
because this is different than the pathology
of atherosclerosis, that the hilar
and parenchymal signals are not affected, meaning
that you won't see a delay in the systolic upstroke.

Indirect Findings

Indirect findings are those that are caused
by the secondary changes from the hemodynamically
significant stenosis.
As you get the energy and pressure loss across the lesion,
this will change the waveform shape and the velocities.
This is important to recognize
because it helps the physician identify
and maybe predict a response to treatment.
Patients have been treated for renal vascular hypertension,
and although they've improved the vessel wall
and remove the lesion, the patient remains hypertensive.
So essentially they have surgical success,
but a clinical failure.
So the newer studies are trying to identify those patients
that are amenable to revascularization
and have better outcomes.
This includes doing hilar waveform pattern analysis,
renal parenchyma, resistive index,
diastolic systolic ratios,
and looking at kidney size
renal hilar analysis, the TARDIS parvis waveform.
The acceleration time
of less than a hundred milliseconds is normal.
Anything greater than that is consistent with occlusion
or stenosis.
Renal parenchymal flow, assess both upper and lower poles.
Identify the peak systolic velocities.
Identify the absence or presence of flow kidney length.
Any kidney less than nine centimeters a second
is considered abnormal.
However, if there's a asymmetry between the right
and left kidney, that is considered abnormal.
So a patient could have a normal size kidney
of 10 centimeters,
but if the contralateral kidney is 12.5,
the smaller kidney is actually considered abnormal.
Now, kidney length is important
because it's found that those patients that have kidneys
that are less than seven centimeters
actually do not respond well to surgical intervention
or any other reconstruction between eight
and nine is variable depending upon the amount
of medical disease present.
Now the identification of medical renal disease is
identified by either doing the diastolic systolic ratio
or measuring the resistive index.
A DSR of greater than 0.3 is considered normal,
and RI of less than 0.7 is within normal limits
and greater than 0.8 is consistent with renal disease.
Markedly abnormal diastolic systolic ratio and
or a markedly abnormal resistive index are suggestive
that the patient would not farewell with intervention
kidney length measurements and support.
To note that when measuring the kidneys due to the
awkward plane that they lie in, it is best
to take three independent pole to pole measurements.
Average those to determine
that patient's kidney length measurements.
Unilateral small kidney is associated with
renal artery stenosis.
To measure the hilar waveform, you want
to change the sweep speed so that you can identify
where the actual peak occurs.
Here's the first onset of systole,
and here's the first peak, the second arrow points
to the maximum peak.
If you were to measure the maximum peak,
this patient's acceleration time would be abnormal
while the acceleration time is actually normal.
Another component to look at is the early systolic peak.
This is very suggestive of a normal compliant vessel.
Here's just a diagram showing you what you're measuring.
Here's your timeframe down here on your scale,
and you're measuring the onset of systole to the first peak
parenchyma wave forms.
These are helpful in measuring the resistive index
and identifying the absence
or presence of medical renal disease.
So you want to assess both the upper
and lower poles by using color doppler.
Here you can see
that you have nice filling throughout the kidney,
the clinical utility.
It helps identify those patients who might fare better
with revascularization.
If you have a DSR of less than 0.8,
these patients do not fare well with intervention
and generally are considered clinical failures.
This also is apparent in those patients
that have an end diastolic velocity
of less than five centimeters a second
or a resistive index of 0.7.
Intrarenal waveforms can be affected by the degree
of stenosis, the etiology of the lesion.
As noted earlier, fibromuscular dysplasia generally does not
affect the waveforms out in the parenchyma
and appear to to be essentially normal.
The resistance in compliance, the age of the patient,
arterial pressure,
however, they are most predictive when markedly abnormal.
If you find an abnormal waveform out at the hilum
and a delayed acceleration time, this is highly predictive
of proximal renal artery stenosis.
However, if the waveform is normal, it really does not
imply whether the vessel is patent or stenosis.

Limitations of Renal Artery Duplex

The limitations of renal artery duplex include
duplicate renal arteries.
14 to 25% of patients are reported
to have multiple renal arteries,
and this is generally the primary source
of a false negative exam.
Renal artery duplex is not helpful
to identify branch vessel disease in the presence
of a large abdominal aortic aneurysm.
The renal arteries are generally splayed out
around the renal artery, the aneurysm,
and it makes it difficult to see the origins
and proximal segments of the renal arteries
post-op abdominal surgery.
Generally, the patient has too much air post-op
and it can take up to three to six months
before you could get optimum images of the renal artery.
Renal duplex does not identify lesions of less than 60%.
In those patients with small kidneys that have been a
atrophic for a long time,
they may have an associated small renal artery
and it's not occluded and it's not from renal disease.
This can make the differential difficult.
Bowel gas is the nemesis of renal artery duplex.
That's why it's important to employ an aggressive prep the
evening before
and the limitations of using multiple doppler angles,
particularly those over 60 degrees,
just showing you some of the limitations.
Here's the duplicate renal artery.
Here's a patient that's gassed out.
If you can't see the aorta like in this particular image,
simethicone will not help you with this patient.

Interpretation Considerations

Interpretation considerations,
exam quality and completeness.
The doppler angles used.
If the patient's abnormal,
were the findings confirmed from two views?
Did the patient have the presence of medical renal disease?
Do the direct and indirect findings correlate?
And is your focal increase in velocity focal
or is it extended throughout the entire renal artery?
Corresponding findings,
the waveform should be symmetrical from both renal arteries
and the renal parenchyma.
The hilar analysis, both the acceleration time
and early systolic peak, are they present the kidney size?
Are they normal and symmetrical?
Is there no focal increase in velocity?
The waveforms remain uniform
and consistent throughout their course
and there's no turbulence.

Conclusion

Thank you very much.