Renal Artery Disease: Ultrasound Protocols and Tips - SD

Introduction

My name is Kathleen Carter.

I'm a vascular sonographer, and I live and work in Norfolk, Virginia, affiliated with Eastern Virginia Medical School.

And I'm gonna talk to you today about technical protocols and tips for renal artery duplex ultrasound.

We're gonna talk today about renal artery disease and the protocol for doing vascular duplex ultrasound, and hopefully give you a few tips to help you in that endeavor.

Overview of Color Duplex Ultrasound

Color duplex ultrasound allows for both anatomic and functional assessment of the flow within the renal arteries.

It does require a bit of skill.

It's a little bit more challenging than some of the other peripheral vascular studies, but in centers that perform many studies, the sensitivity and specificity can be very high.

Parameters Used

We're going to use peak systolic velocity renal index, resistive index, systolic rise time, and the renal aortic ratio to look at potential stenosis within the renal arteries.

Indications

The indications for doing renal vascular duplex would be new onset hypertension and particularly hypertension that's refractory to medical management.

Many of these patients come in and they are on three or four medications without control of their blood pressure.

We also can use this for monitoring of known renal artery stenosis.

We're often referred patients who have a bump up in buin and creatinine or with an epigastric or flank bruie.

We are occasionally asked to look at patients with the potential renal vein occlusion or in follow up to a procedure like renal artery bypass, angioplasty stenting, sometimes for screening for renal artery stenosis prior to beginning ACE inhibitor therapy, suspected renal artery aneurysms, suspected AV fistula, particularly after a biopsy or evaluation of renal transplants.

Causes of Renal Vascular Hypertension

Atherosclerosis causes about 90% of renal vascular hypertension.

Most hypertension is essential hypertension.

A small portion of hypertensive patients have renal vascular hypertension and a, again, most of those are caused by atherosclerosis stenosis usually occurs at the origin or orifice of the renal artery.

If it's atherosclerotic and it's almost half the time it's bilateral, there are no specific risk factors other than the normal cardiovascular risk factors that is, there are no specific risk factors for vascular plaque to develop that isn't associated with other plaque in other parts of the body.

Plaque progression will occur about half of the time within one year, but it is the most common correctable form of hypertension.

So it's important that we identify it.

Contraindications and Limitations

There are no known contra indications to this exam.

However, there are some limitations or challenges.

If the patient has had recent abdominal surgery incisions and it is tender, it may be hard to push on the abdomen.

Bowel gas can be a problem, but it is something that is present with any ultrasound study in the abdomen and you just need to work around it and look at other views to work around any bowel gas that may be in your way.

Patient obesity can be a challenge, or if the patient is uncooperative, can't be positioned properly, that may make the study more of a challenge.

Some of the limitations associated with this ultrasound test are the fact that there are about 25% of the time accessory renal arteries, and they may not come off anywhere near the main renal artery.

So we have to really be vigilant in going after and looking for those.

There may be branch vessel disease that you might not identify.

We're not very good at identifying less than 60% stenosis and doppler angles and tortuosity of the renal artery can make a challenge in getting the appropriate Under 60 degree angle spectral wave forms.

Equipment

The equipment that you need to use is going to be high resolution ultrasound that has good spectral and color doppler deep within the abdomen.

We're gonna use low frequency transducers and I usually use a combination of transducers and performing vascular duplex depending on the patient's body habitus and where you have to go, you often may have to use a phased array transducer with a small footprint to get between the ribs.

It often takes more than one transducer to adequately assess the renal arteries.

And another tip might be that the optimum transducer position to image the kidney is not usually the best to access the entire renal artery.

So you'll, if you're used to doing kidney ultrasound, you're gonna need to use different views to lay out the entire renal artery for assessment.

Patient Preparation

Patient preparation is usually keeping them NPO, but I usually let them have as much water as they want that keeps the kidneys hydrated and it makes for better imaging.

Absolutely you want them to take their antihypertensive medications before coming in for their exam.

And there is some consideration made for diabetics if they need to have a light breakfast that usually won't interfere with your test too badly.

Have a light supper the evening before.

I usually encourage patients not to eat anything that they know is gas producing.

Scanning in the morning usually is best no gum chewing or cigarette smoking because that introduces air into the abdomen.

Usually position them with their head elevated in a comfortable position and most importantly allow enough time to do this exam properly.

Exam Components

The exam components are going to include assessment of the aorta from diaphragm to the iliac bifurcation.

We wanna be able to identify if there's any aortic or atherosclerotic disease there because we're going to use the aorta for a renal to aortic ratio.

And you do not wanna use the aorta if it's severely diseased aneurysmal, or if the flow there is less than 40 centimeters per second or more than a hundred centimeters per second.

We're gonna use direct visualization of the entire renal artery.

We wanna go, even though most of the disease is in the proximal portion, we want to look at the entire renal artery.

Other diseases like fibromuscular dysplasia will be in the distal mid to distal portion of the renal artery.

We're going to use B mode, spectral and color doppler.

All of the tools within the arsenal that our equipment can give us to identify where the blood flow is.

And then there is indirect analysis that can be done.

Shouldn't be done independent of the main renal artery assessment, but can be some adjunctive pieces of diagnostic information used in the parenchyma for acceleration time and systolic rise time and resistive index, and we'll talk about those separately.

Views and Transducers

There are a number of views and transducers that can be used.

AP view usually is where I start.

The intercostal view can be used when you don't have good ap imaging.

My favorite view would be the subcostal view.

The image here on the screen is of the banana peel view, as some folks call it.

And this is not a view that I have particular success with, but many people do, where you can see both renal arteries in the longitudinal view coming off of the aorta.

You can put patients into the lateral decubitus and move their abdomen out of the way if you need to define certain portions that are not readily available in the AP view.

Sometimes you can actually come in posteriorly using the liver and the spleen is a window is helpful.

Anytime that you go through something that's a tissue that is homogeneous that can help just provide a window into the kidney, as I said before, it's probably helpful to use a number of different transducers depending on the patient's body habitus and what access points what windows you're going to use.

Normal Renal Vasculature

So if we talk about normal renal vasculature, the renal arteries arise from the aorta.

Anter laterally usually about one to two centimeters inferior to the superior mesenteric artery.

And the right renal artery passes posterior to the inferior vena cava.

It's a little longer than the left renal artery.

The right renal vein is very short and has a short course from the renal hilum to the IVC or inferior vena cava.

The left renal artery is much longer than the right and it passes between the SMA and the aorta and the renal vein exits anteriorly at the hilum posteriorly and the artery is between the two.

Accessory renal arteries are usually inferior to the main renal artery, but they actually can come off of one of the poles of the kidney and go back to the aorta at almost any point.

So it's we'll talk about ways to look for those.

The landmarks that you're gonna use are those that are displayed here.

You're gonna use the left renal vein, the superior mesenteric artery, and again, the liver and the spleen to help identify.

And here you see a fair amount of portion of the right renal artery displayed just below where the SMA comes off.

The renal artery divides into four or five inner lobar segments, the inter lobar arteries arch over the pyramids, and they come to form the arcuate arteries, which give rise to the inter lobar arteries.

Those further divide into the afferent arterials that go into the glomerulus that does the work of the kidney, all of the filtering.

And then when that is done, blood leaves the glomerulus through the aernt arterials.

Accessory Renal Arteries

I said that multiple renal arteries are present in about 25% of patients, and that does present a challenge for us.

So you can check the symmetry throughout the entire kidney if you're in transverse and normally the right renal artery comes off at about 10 or 11 o'clock and the left renal artery comes off at about four o'clock.

If you see renal arteries that come off at a different location than this, then be suspicious for duplicated renal arteries, because usually that's the presentation.

When you do find multiple renal arteries, you need to assess them in their entirety.

And there is always the chance that you're going to miss one of them if there are multiple renal arteries.

Another tip in finding them is to evaluate the hilar regions and poles of the kidney and look for vessels that are coming off there and track all the way back to the aorta.

Anatomic Variants

There are a number of anatomic variants that may occur that you'll encounter in doing vascular study.

Horseshoe kidney is fairly rare, less than 1% of the time, 90% of those are fused at the lower pole and anterior to the aorta.

Generally at the fourth to fifth vertebrae, early branching will occur per outside the hilum of the kidney about 15% of times.

So don't think that that's a duplicated artery, it's just that there is an early branching before the hilum, retro aortic left renal vein is present about 3% of the time in a circum aortic left renal vein about 9% of the time.

Protocol

So let's talk about the protocol.

We're going to use a subxiphoid approach generally to begin looking for the areas of any aortic disease.

We're going to look in the senal juxta renal and infrarenal segments of the aorta and sample the aortic velocity about the level of the SMA.

And that's the one we're gonna use for the RAR or renal to aortic ratio.

We do not use the A RAR when aortic velocities are less than 40 or greater than a hundred centimeters, or if there's significant atherosclerotic disease within the aorta or aneurysmal disease because that's gonna affect the velocity, normal velocity.

Aortic Waveform Morphology

Waveform morphology in the supra renal aorta has more diastolic flow, and the reason for that is that it is feeding a lower resistant vascular bed.

The celiac will come off there and feed the liver and the spleen, both of which are low resistant vascular beds.

And the renal arteries, of course, are feeding the kidneys once you get past the renal arteries.

The infrarenal aorta is a much higher resistant signal, as you can see, and that's because it's only feeding high resistant peripheral arterial flow.

Aortic Velocity Sampling

When we're doing the proximal aortic velocity, we kind of need to walk through the renal artery proximal, mid and distal, and we're going to need to walk into the origin of the renal artery from the aorta.

So you'll start with that lower res, lower resistant signal in the aorta to make sure you don't miss any oral stenosis.

We'll document samples in the aorta at the oste of the renal artery, proximal mid distal in the renal artery, always at less than 60 degrees angle of incidents and excluding any stenotic disease that it can occur up, prolong any segment of that main renal artery.

And then if there are segmental branches, particularly if they are easily assessed outside the hilum of the kidney, you'll check some of those segmental branches, particularly if you have any reason to suspect that there may be a stenosis there.

Renal Parenchymal Assessment

Once we have completed assessment of the entire renal artery, we're gonna go to the parenchyma and we're going to look at the parenchymal signals in at least the upper and distal poles of the kidney.

They should be symmetrical, they should be symmetrical in the waveform morphology as where well as symmetrical in about the velocity and systolic rise time and acceleration time.

We're going to do renal parenchymal resistive index indices and realtime spectral doppler.

Document the patency of the renal vein somewhere along the course of the renal vein.

Measure kidney pull to pull length and then document any incidental findings found along the course of the study such as mass hydronephrosis, hydroureter or stones.

Use B mode color, color power, angio harmonic imaging, any of the tools that you have with your equipment to identify the flow.

If you're gonna use color power, you're not gonna be able to identify direction of flow, but certainly you'll be able to tell that it's a well perfused kidney.

Identify the branches and then you can change into regular color to identify the arteries that you need to assess The color.

And b mode settings need to be adjusted for proper sensitivity.

You'll need different sensitivity settings when looking at the main renal artery.

And then of course, lower sensitivity settings.

When you get out into the small vessels in the parenchyma, you'll wanna adjust the size and angle of your color box, adjust the wall filters.

They will need to be low when you get out the parenchyma, the gains should be a little bit higher.

PRF should be lower, and you can actually increase the color persistence on your machine to allow the color to hang around just a little bit longer so that you can place your doppler within the small vessels within the parenchyma of the kidney.

And that's particularly helpful when there's a low flow state going on in the kidney.

Use color power doppler sometimes adjust the doppler sweep speed to a very fast sweep speed, and that'll give you a larger waveform to measure your systolic rise time or acceleration time.

Just a bit of a tip.

Accessing the Renal Artery

Some of the my favorite ways of accessing the renal artery.

If you have the patient turn onto their left side, and then take the transducer and angle that against the rib cage so that you're almost in an oblique position, paralleling the rib cage and then angle up.

That will give you usually the view that you see below this demonstration, which is the aorta in transverse.

And unless there's some sort of tortuosity in the renal artery, you can follow the renal artery all the way out to the high level of the kidney and get all of that in one plane of view.

If you can't get it all in one plane of view, at least get it into larger segments instead of small pieces of the renal artery.

That'll give you better assessment of the entire renal artery.

On the left, you can go subcostal under the rib cage and angle up.

Of course, the left renal artery is much shorter, so it's easier to lay the entire left renal artery out in one point of view.

Another technical tip is don't be afraid to push.

Patients usually are fine with you pushing on their abdomen.

Here's an example of the transducer.

Basically just lying on the surface of the skin and you can barely see the left renal artery there.

And with a slight push in, the renal artery is much more visible and available for assessment.

So don't be afraid to push other technical considerations.

Doppler Angles

Obviously appropriate angles is crucial and we need to be 60 degrees or less corrected very to be parallel to the vessel walls.

This is very important.

We know that the smaller the angle, the lower the estimate of velocity, but if it's not parallel to flow, then the math calculation that is gonna be done by the equipment for you is going to be completely off.

And you can see here where there's not good angle correction to parallel the flow in the image to the left, you're calculating an erroneous velocity calculation of 191 centimeters per second.

When that angle is corrected to the correct angle of incidents parallel to the flow at 22 degrees within that vessel, you have a much different velocity, but it is more correct.

Frequently the renal arteries can be angulated.

You wanna be careful not to give the patient disease when it's in fact just tortuous.

So be careful not to overestimate disease in tortuous vessels.

One of the clues is that if you don't have post stenotic turbulence after you find a focal increase in velocity, you probably don't have a stenosis.

It may be due simply to tortuosity normal renal doppler velocity wave forms have a very low resistant flow characteristic with forward flow throughout.

Diastole should have a rapid systolic upstroke.

There may be an early compliance peak that can be higher or lower than the actual systolic peak.

We should see rapid deceleration to that constant forward diastolic flow.

And the renal vein flow is normal.

Normally phasic, when we do see renal artery stenosis, there are some clinical features that are associated with functionally significant renal artery stenosis of greater than 60 degrees, and that may include smaller kidneys.

A discrepancy in a kidney size greater than one and a half centimeters as long as it's not congenital, is usually consistent with renal artery stenosis.

In the smaller supplying the smaller kidney, certainly we'll begin to see hypertension and some patients may present with flash pulmonary edema or unexplained recurrent CHF or congestive heart failure.

Diagnosis of Renal Artery Stenosis

We need to rely on multiple parameters for the diagnosis of renal artery stenosis.

As long as the renal artery ratio can be used, the renal to aortic ratio can be used.

That is a good ratio that takes out some of the velocity variations.

We're gonna look for increased peak systolic velocity greater than 180 or 200 centimeters per second.

We're gonna be looking to document that post stenotic turbulence to be sure we have a stenosis present, and that should be within a centimeter or so distal to the stenosis.

The pole to pole kidney length should be symmetrical.

Acceleration time or systolic rise time can be measured as an adjunct.

Additional piece of information to confirm stenosis and of course, look at your B mode, look at the echogenicity of the kidney as well.

Resistive Index

We're looking now talking about parenchymal artery flow.

Let's talk just a little bit about resistive indices.

Resistive indices really have nothing to do with renal artery stenosis, so why do we talk about it?

Well, resistive indices when they're elevated are an indication of medical renal disease within the kidney that has nothing to do with renal artery flow, but it has to do with medical renal disease within the parum of the kidney.

But this can affect some of our other measurements, so we need to know when it's present.

We calculate the renal artery resistant index, which measures in diastolic to peak systolic ratio, and it's usually ab considered abnormal when it's over 0.65 to 0.70.

How is this measured?

The peak systolic velocity then is you subtract the end diastolic from the peak and then divide that by the peak.

And that's how it resistive index is calculated.

Most people use resistive index to calculate the resistivity within the kidney.

There is also another method of calculating resistivity within the kidney, and that's called the diastolic systolic ratio.

It's measuring basically the same thing, how the resistance in the kidney, it is, but it's just measured with the diastolic velocity divided by the peak systolic velocity.

When you use this measurement instead of the resistive index, you're looking for a measurement to be normal of less than 0.2.

So here are examples of a normal resistive index and you see the normal amount of diastolic flow within that kidney.

And this is a resistive index in the normal range of 0.53.

As you lose more of the diastolic flow, you see to the right, the picture is abnormal with a very high resistant index of 0.86.

Echogenicity of the Kidney

Let's talk about echogenicity of the kidney.

And normally the renal cortex is homogeneous and slightly less echogenic than the liver.

We have good cortico medullary definition then between the cortex and the medulla.

When this medical renal disease increases, for whatever reason, there's a diffuse increase in the echogenicity of the cortex and it may be more difficult to separate the cortico medullary definition.

So here's an example where there's normal ri and no significant increased resistivity in the kidney.

And you see good, normal echogenicity within the kidney and the cortex is usually a little less echogenic than the adjacent liver.

The picture on the right, you can see that it is much more echogenic and much more difficult to separate where the cortex and the medulla may be in a patient with a poorly functioning or non-functioning kidney.

That kidney becomes very echogenic and as you can see, more echogenic than the adjacent liver her.

Indirect Testing: Systolic Rise Time and Acceleration Time

Now I'd said we were gonna talk about some of the indirect testing that can be adjunctive to the evaluation of the main renal artery systolic rise time or also called acceleration.

Time is the interval from the onset of systole in one cardiac cycle to the initial peak, and it's normally less than a hundred milliseconds.

It actually is normally less than about 60 milliseconds, and over a hundred milliseconds is usually consistent with significant stenosis or occlusion.

These are very small wave forms and very subject to error.

So you really need to speed the sweep speed up to elongate the waveform and decrease the scale to enlarge the waveforms that you have a larger field to do this very small measurement with.

Now, there is one caveat.

If the patient has a significantly elevated ri, this will affect the accuracy of the systolic rise time or acceleration time measurements.

And you may not get a delay when you ordinarily would because the kidney's just too stiff with the ri being elevated to produce the normal systolic rise time delay.

So you would not wanna use that in a patient who has markedly elevated resistive index normal.

Systolic rise time or acceleration time, as you see here of 60 milliseconds, is measured from in not in diastole but beginning asystole to that first compliance peak.

And you can see how that's measured when that is quite delayed is sometimes called tardis and parvus where is a very delayed rise to peak.

So again, concomitant medical renal disease will influence these calculations.

The systolic rise time cannot differentiate stenosis from occlusion, so that's a limitation of that measurement.

It can't localize where the lesion is.

As can direct assessment of the renal artery, and it doesn't give you any velocity information for serial follow up in the presence of multiple renal arteries.

It's may not be as helpful.

And if you don't have a really good waveform, if you don't pick up a good center stream inter lobar RQ I or intra lobar vessel, you may miss the first compliance peak and not do a correct measurement.

So those are all limitations.

Now talking about that compliance peak, they're sometimes difficult to see.

So here is where this was actually measured, which is the true peak, not the first compliance peak.

Here is a better waveform to measure and here is where that first compliance peak is.

So you will want to measure from beginning systole to that first compliance peak, not the actual systolic peak.

Post-Intervention Assessment

So we'll talk briefly now about what happens after some kind of intervention to renal artery.

For the presence of renal artery stenosis, that can be surgical revascularization with either a bypass graft or an endarterectomy of the renal artery.

Or more often today we see percutaneous angioplasty with or without a stent present.

Careful documentation of follow-up studies so that we can find out if there is residual versus recurrent stenosis.

Changes in the aortic condition or the velocities can affect this renal artery.

Veloc should be taken from at least two views when following up after any revascularization.

And sometimes it's really hard to get a good angle because vessel angulation will change after a stent has been placed in there and it may be harder to get an under 60 degree angle.

There's a lot of research going on right now to determine what is a du a correct duplex velocity following PTA and stent, particularly in stents when there's a decreased compliance of the vessel, we need to walk the entire stent through the entire stent.

With spectral doppler, you can have a stenosis occur anywhere within the course of the stent.

The distal stent is the most common site of stenosis, but again, you can have it anywhere along the course of the stent.

So we need to look at the entire stented portion.

Rigid stent structures can cause elevated velocities without any re-stenosis.

So what you're looking for is any focal increase within the stent, not necessarily just pan stent increase velocities that might be caused due to loss of compliance.

Frequently stents are placed within the renal artery and extruded into the aorta.

The picture to the left, you can see in the aorta, the aortic the renal artery stent that is extending out into that.

And the reason for that is as many of these plaques can ex extend into the orifice and out into the wall of the aorta from the renal artery, and so they wanna make sure that the stent covers all of the disease that's present.

So here in the longitudinal view, in the middle picture, you can see a stent again that extrudes out into the aortic lumen.

The picture to the right is showing you that there's been a, because that stent is present, there's been a change in the angle of the right renal artery and instead of arching up where which is a more optimum angle for incidents for going through the origin of the aorta.

This one is not at 60 degrees and not accessible at least in this particular view at 60 degrees.

So you'll have to try a different view to walk the Doppler through the stented portion in that part of the right renal artery.

If you do encounter stenosis, we should have post stenotic turbulence present.

Otherwise, again, it may just be increased velocity due to loss of compliance.

The angle of the vessel, as you can see in this picture has changed due to stent placement and close monitoring has to be done to make sure that you in fact have a st a stenosis present within any of these stents.

If renal artery bypass graft or endarterectomy has been done, the anastomotic areas are the most frequent site of any recurrent stenosis, and the protocol for the exam would be pretty much the same as native renal arteries with special attention paid to those anastomotic sites.

Incidental Findings

Incidental findings that might be seen along the course of doing a renal artery duplex include renal cysts, hydronephrosis stones, masses aneurysms, and developmental renal artery stenosis.

Simple cysts should just be measured and noted as an incidental finding.

Polycystic kidneys can just be noted to be there if you run into hydro necrosis or hydroureter.

That should be mentioned as an incidental finding in your report.

Specific Conditions

Fibromuscular Dysplasia

Fibromuscular dysplasia is usually occurs in the mid to distal renal artery.

It's typically found in middle aged females and is described angiographically as a string of beads appearance.

Usually these sequential stenosis, the parenchymal signals, interestingly enough, are usually not affected by FMD and the velocities that are obtained are usually a little lower than similar amounts of stenosis with atherosclerotic disease.

Renal Artery Aneurysms

If you encounter renal artery aneurysms, these should be measured and noted where they occur along the course of the renal artery.

The exact incidence of renal artery aneurysms is not well known.

There are four types. Macro aneurysms, dissecting renal artery aneurysms.

Those that are associated with FMD and those that are associated with arteritis, the etiology of them can be FMD atherosclerotic arteritis trauma and complications of a procedure.

Most renal artery aneurysms are asymptomatic and rupture is rare within the renal artery aneurysms, they can result in embolization or vascular hypertension.

Spontaneous Dissections

Spontaneous dissections most often occur in otherwise normal arteries.

The pathogenesis of this is poorly understood, usually occurs in the mid portion of the renal artery.

And intervention is usually indicated.

If there's an acute occluding dissection or severe renal vascular hypertension associated with it.

The subgroups of these can be spontaneous non IA igenic trauma, it may be catheter based, it can be associated with ischemia, is far more common in men, and about a third of them are bilateral renal arterial venous fistula malformations.

Renal Arteriovenous Fistula

This renal artery ar arteriovenous fistula is an abnormal arterial and venous connection at the capillary level or a direct connection between a main artery and vein.

Acquired lesions after biopsy are much more common than congenital lesions.

And again, biopsy and sometimes penetrating trauma are the main causes of renal AV fs vfs, they usually present with an increased cardiac output and venous return resulting in hypertension, perhaps hematuria.

Abdominal bruery is present about 70% of these patients.

Developmental Stenosis

Developmental stenosis is uncommon, but it causes about 40% of renal vascular hypertension in children.

So it's worth mentioning hyperplastic vessels occur with secondary animal fibroplasia and they usually have an abnormal fusion of the per dorsal aorta.

Conclusion

So in conclusion, renal artery duplex is an accurate and safe exam for the diagnosis of significant renal artery stenosis.

It can be very valuable following a procedure and it can be valuable in following the progression of disease prior to treatment.

The accuracy is directly proportional to the experience of the examiner, and it takes careful preparation and time to perform these studies.

Thank you.