Duplex Sonography of the Aorta and Mesenteric Vessels - SD

Introduction to Duplex Sonography of the Aorta and Mesenteric Vessels

My name is Dr. Ed Bluth, and I'm going to be talking about duplex sonography of the aorta and mesenteric vessels.

As most of us know, the aorta tapers from its cranial to its caudal extent, and by convention, an aortic aneurysm measures three centimeters.

Most abdominal aortas are usually less than 2.3 centimeters in man and 1.3 centimeters in women.

Patient Preparation for Aortic Ultrasound

In order to perform a ultrasound evaluation of the aorta, the patient needs to be NPO between eight and 10 hours, and the patient should be advised not to smoke or to chew gum to avoid abdominal midline gas.

There's no requirement for a bowel preparation.

And of course, the ultrasound examination should precede any contrast study to avoid reflections of sound and inability to penetrate the abdomen.

Technique for Measuring the Aorta

Now, let's talk a little about technique.

It's very important to realize that measurements of the aorta should be both in the AP and the transverse plane.

As we can see in this image, we take pictures transversely from the proximal abdominal aorta in a transverse sagittal projection and continue distally to the bifurcation of the aorta into the right and left iliac vessels.

As we see in that transverse and in this sagittal image, measurements should also be made perpendicular to the long axis of the vessel.

This is particularly important in patients who have tortuous aorta.

Here in this image, that I drew, you can see that we have a tortuous aorta and the appropriate measurement to measure the transverse dimension of the aorta is A to B, not C to D.

This is one of the benefits of ultrasound, the fact that we can align the image in the proper sequence in order to determine an appropriate transverse image.

Very important Also, it's we need to measure the outer to outer layer of the aorta in order to get an appropriate and accurate measurement.

So you could see in this image we include thrombus if it's present, and we measure the outer to outer margin of the aorta, A to B, C to D appropriately.

Here in this image, we can see that we're measuring outer to outer wall of the aorta ap, as well as transverse images and measuring and coming up with largest measurement of these dimensions.

It's important to also evaluate the relationship to the renal renal arteries and evaluate the iliac arteries and possible popliteal arteries for associated aneurysms.

Defining and Measuring Aneurysms

By definition, as we stated before, the aorta is aneurysmal at three centimeters.

The iliac arteries are normal up to 1.5 centimeters and above that are aneurysmal, and the popliteal artery is normal up to 10 millimeters and enlarged and aneurysmal greater than 10.

Again, it's very important to measure appropriately.

You measure outer to outer margin, both the AP and transverse dimension and report the largest measurement.

This includes both the popliteal, the iliacs, or the aorta.

The measurements are very important because that's explains why an aneurysm ruptures.

La plaster's law states that aneurysm rupture is di directly related to the aneurysm size because the tension of the wall of the aorta is a product of the radius of the artery and the blood pressure.

So as the size increases, the potential for rupture increases as well.

Most abdominal aortic aneurysms are infrarenal, most are symptomatic as well.

30% are asymptomatic.

The symptoms generally occur.

Our patients present with back pain, abdominal pain and leg pain most occur in men over the age of 65, and it's reported that there's a rupture rate of 25% when the aneurysm is greater than five centimeters.

The incidence of abdominal aortic aneurysms is approximately 5% in men over the age of 65.

And very important that when a rupture occurs acutely, only 10 to 15% of patients survive.

In contrast to when we can identify a asymptomatic or even symptomatic abdominal aortic aneurysm before it ruptures, when surgical or interventional procedures can be done to improve survival overall few patients die from aneurysm rupture.

Most will die from another cardiovascular cause.

So most patients will live with their abdominal aortic aneurysm, but these need to be appropriately identified and most people recommend surgery or interventional procedures.

When the aneurysm reaches a size greater than 5.5 centimeters, what are the risk factors for abdominal aortic aneurysms?

Well, they are men of the white race with hypertension, smoking, and chronic obstructive pulmonary disease.

There also is an association between other vascular, uh, abnormalities, including peripheral arteries and abdominal aortic aneurysms and particular first order relatives, particularly brothers, fathers, and sons.

Complications of Aortic Aneurysms

The complications of aortic aneurysms are several.

When rupture occurs, there's massive para aortic and retroperitoneal hematoma causing obstruction from the hematoma of either the right or left ureters.

It could also be compression of the inferior vena cava.

Distal emboli and AV fistulas have been reported as well.

Follow-up and Surveillance of Aneurysms

How do you follow an an abdominal aortic aneurysm when it's identified?

We reported, uh, in the journal of, uh, quality improvement that patients, uh, who have of course, greater than 5.5 centimeters of an aneurysm should have a surgical intervention when it reaches five centimeters.

People should be directed to seek surgical intervention between four and 4.9 centimeters.

Uh, they should consider the option of, uh, intervention and be followed on at three month and six month intervals.

And when the aneurysm is between three and 3.9 centimeters, we recommended two six month ultrasound examinations and then following that yearly evaluations to see if the aneurysm increases in size, what represents enlargement.

We have found that inter observer variation in, in measurement is two millimeters in the AP projection and three millimeter three millimeters in the transverse projection.

So therefore, we've accepted in our laboratory, uh, the fact that a size change has to be greater than four or five millimeters to indicate expansion.

We develop this, uh, based on an aneurysm model, which we constructed and had many observers measure and understood and learned from the inter observer variability of measuring the same aneurysm.

Again, it's very important to all measure the same way, so I just wanna reinforce that you have to measure, uh, perpendicular to the, uh, transverse plane of the aorta, measuring again, the outer to outer margin of the aorta, making sure you're perpendicular to the axis of the vessel.

Most aneurysms are infrarenal and do not involve the renal arteries, and it's very important to appreciate when renal arteries are involved because it's a different type of surgical procedure and different type of endovascular stent that needs to be used when, uh, the renal arteries are involved in the aneurysm.

Frequently, however, the, the aorta buckles and elongates and, and it's frequently very difficult to distinguish a supra renal from a juxta renal abdominal aortic aneurysm.

This is an example here we're seeing a transverse measurement of the abdominal aorta.

It's nearly four centimeters, and we see the relationship between that and the kidneys we're uncertain.

Is this the renal arteries involved in the aneurysm or are they not?

Frequently you're gonna have to turn to a CT scan in order to actually distinguish the origins and understand the relationship between the abdominal aortic aneurysm and the kidneys.

But with ultrasound, we can use techniques by oblating the patient and identifying the retro caval position of the renal artery and its relationship.

Here again, is another large abdominal aortic Anderson nearly measuring four and a half centimeters.

Again, we're uncertain about where the renal artery is, uh, represent, but when we turn on the color, we can see that the right renal artery is coming directly out of the aneurysm itself.

Pitfalls in Identifying Aortic Aneurysms

What are some pitfalls, uh, in the identification of abdominal aortic aneurysms?

Well, there are three that are very important to appreciate.

One is lymphoma.

The second is metastatic adenopathy surrounding the aorta, and the third is a horseshoe kidney with the cortex of the ectopic kidney being anterior to the abdominal aorta and simulating and smal dilatation with thrombus.

Here's an example of a lookalike here.

We're seeing a transverse dimension.

We're seeing a very large mass like density with a lumen in the middle.

It's uncertain when we look at that image whether this represents thrombus or this represents something else.

When we look at that image in the transverse, we could appreciate the fact as well as in the sagittal image that the aorta is displaced from the spine and there is considerable amount of, uh, uniform hypo homogeneous tissue surrounding the a surrounding the aorta.

This would be very unusual for an aneurysm, and the symmetry of this is also unusual, and when we apply color, we can appreciate the fact that there are branches, uh, of the spinal arteries are being stretched around this metastatic testicular carcinoma and adenopathy lymphoma similarly would appear, uh, in a manner, uh, similar to this in another patient.

Here we're seeing homogeneous material anterior and transverse to the abdominal aorta, and we could appreciate the fact that we're, that this, again is a simulation to an aortic aneurysm.

But when we turn on the oblique projections, we can appreciate there is essential echogenic complex on the right and on the left.

And then when we in include color, we can appreciate the fact that there is vascularity in this mass.

And this does not represent simply an aortic aneurysm, but rather represent a typical horseshoe kidney.

Role of Color Flow Doppler in Aortic Assessment

And now we're going to talk about the role of color flow doppler in assessment of the aorta.

In studying the aorta when applying color flow to it, you can appreciate that the normal abdominal aorta appears to have typical laminar flow, smooth flow.

On occasion, there is retrograde flow within the abdominal aorta, and this does not represent, uh, retrograde flow.

It really represents a natural phenomenon related to the elasticity of the, uh, aorta itself.

Aneurysms, if you care to study them with color doppler imaging, you'll appreciate the fact that there were two distinct flow patterns.

One, a smooth laminar pattern and a second turbulent irregular pattern.

These are two examples of two five centimeter abdominal aortic aneurysms.

One has smooth laminar flow and the second has turbulent irregular flow in its proximal quadrant.

We were very interested to see if this could represent a specific abnormality, and together with a bioengineering group at Tulane University, we studied, uh, these by developing aortic aneurysm models, a little digression, a little bio-engineering.

In order to understand laminar and turbulent flow, you have to appreciate what a Reynolds number is.

And by definition, turbulent flow with a is a Reynolds number of greater than 3000 and a laminar flow has a Reynolds number less than 2000.

Now, how do you derive a Reynolds number?

It relates to viscosity times diameter of the tube, times density of the tube divided by viscosity.

The variable related to aortic aneurysms is diameter of the tube, and that's what we identified with our bioengineering studies.

We looked at aneurysm models and found that as the endothelial damage occurs and the aneurysm begins to enlarge the diameter of the tube of course increases and a rentals number increases causing turbulent flow, which continues to cause damage on the vessel wall.

The body's response is to produce thrombus, therefore causing a reduction in the residual lumen of the aneurysm causing the Reynolds number to decrease and return to laminar flow.

So what was the bottom line of our discussion and analysis of bioengineering was when we looked at an aneurysm, we began to appreciate that an five centimeter aneurysm, which contains laminar flow as we see here, has a different risk factor for rupture than probably an aneurysm here, which measures five centimeters and has turbulent flow.

Those that have turbulent flow are continuing to damage the wall and are the ones that are at risk for rupture while the ones who have laminar flow probably are at, can be simply followed for continued development, uh, and the risk of ultimately developing turbulent flow as well.

So in summary, what could we learn from this?

We could learn that, uh, turbulent flow aneurysm may be at higher risk for aneurysm, but most people are not yet ready to evaluate, uh, and intervene on patients who have simply turbulent flow.

But I think this is a future direction, which as we determine who or who should not have and luminal therapy, probably the identification of, uh, turbulent flow may help in that, uh, differentiation.

Assessing Endoluminal Grafts with Color Flow Doppler

One other useful component of color flow doppler is to look at, uh, endoluminal grafts themselves and to look for endoluminal graft leakage.

And here you can see an endoluminal graft, uh, and when we applied color, we can see flow is around it and flow is into the external to the graft itself, demonstrating that there is exactly an end luminal leak.

Mesenteric Vessels: Anatomy and Flow Patterns

Let's move on to discuss aortic aneurysm branches.

And there are three major branches that we look for when we are looking at the mesenteric vessels.

These include the celiac, the SMA and the IMA.

The celiac artery has a low resistance pattern since it supplies the liver in the spleen and we need to have continuous flow.

The SMA in the fasting state has a high resistance, and when the patient eats it transfers over to a low resistance flow.

The IMA also has a high resistance flow as well.

So how do these patterns appear?

Here we're seeing the aorta in a sagittal projection.

We're seeing the celiac artery and the SMA, and when we interrogate the celiac artery, we're seeing a low resistance plat pattern with considerable diastolic flow.

In contrast to that is this image in which we're seeing the SMA and when we interrogate that vessel, we see a high resistance flow pattern In this fasting patient, the IMA as stated also has a higher resistance pattern as we see in this image.

Diagnosing Mesenteric Ischemia

What is important about these vessels?

What are we looking for and why bother to study them?

Well, we're looking to identify a patient who has mesenteric ischemia, chronic arterial insufficiency of these mesenteric vessels.

These are patients who usually have pain after eating, and they're required because of this pain to have very small frequent meals.

Frequently the cachectic elderly patients with diffuse atherosclerosis and progressive weight loss.

Interestingly enough, some patients who have this type of syndrome won't, will be totally asymptomatic, uh, because of collateral vessels resupplying, uh, their abdominal major structures anyway.

Returning to the mesenteric vessels, the celiac SM A and IMA can be seen when you examine a patient in nearly 70% of patients, so it is worthwhile to evaluate them.

What are the criteria that are accepted for abnormality?

Well, for celiac arteries, normal is accepted up to 200 centimeters per second.

The SMA greater than 275 centimeters per second represents an abnormal velocity, and most important of all is the relationship of the major vessel to aortic ratio.

All these vessels, the celiac, SMA and IMA should have a greater than three to one ratio if there is more than 60% stenosis.

And this of course, is very important because many of our patients who are sent to us have hypertension and would have elevated velocities, which we can only correct by performing a vessel to aortic ratio.

So what exactly does a celiac, uh, stenosis look like?

Here we're visualizing at the celiac artery some turbulent flow in the vessel itself.

It tells us where to interrogate, and here we could, uh, identify a celiac artery with peak velocity of 278 centimeters per second with an aorta of 50 centimeters per second, giving us more than a three to one ratio.

We try to differentiate celiac stenosis from patients who have median arcuate ligament syndrome.

And median arcuate ligament syndrome involves the compression of the celiac artery by the diaphragm and other ligaments, and that leads to changing velocities with inspiration and expiration.

So in summary, what we see is when we have patients take a d take a deep inspiration and cause an expiration, we'll see changing velocities in the celiac artery.

If someone truly has stenosis, the velocities will always be elevated.

Here is an example.

Here again, we're seeing the aorta, celiac artery, and when we interrogate that on expiration, we see the peak velocity is 361 centimeters per second.

And on inspiration, we see the velocity is 128.

So this is not a stenosis.

This merely represents median ular ligament syndrome.

As you could see in this color image alone, there is considerable movement of the celiac artery and healthy individuals here on inspiration.

It's a vertical.

And here on expiration, you can see the considerable movement and see why you would expect to see an elevated velocity in this particular, uh, individual.

We found in our experience that not only do you need to do an inspiration and expiration image if the patient has elevated velocities that don't change, but you also need to do an erect uh, image to make sure that the velocities don't change In the erect position here in this individual, we could see that in inspiration, the peak velocity is 513 centimeters per second.

In expiration, it's nearly 600 centimeters per second.

And in when we stand, the patient erect it.

The velocity is reduced to 163 centimeters per second, demonstrating that this is not a mesenteric, this is not a celiac stenosis, but this actually is median arcuate ligament syndrome.

This is important to realize because particularly in these CTIC and elderly individuals, it's hard for them to take a deep inspiratory and exploratory change, and therefore, by putting them in the erect position, we can easily demonstrate there was a velocity change.

And this does not represent a significant stenosis.

Mesenteric stenosis never changes.

Here an inspiration, the velocity is 337 centimeters per second in expiration to 310, and in the erect position it's 302 centimeters per second.

And in all these times, with the aortic velocity of 75 centimeters per second, measured at the level of these vessels, that the ratio is four to one, meaning that this is definitely a greater than 60% stenosis.

And here in this angiogram that corresponded to this patient, we can see both an SMA and celiac stenosis.

Conclusion

So in conclusion, regarding the aorta and the mesenteric vessels, ultrasound is an excellent method to study the aorta and mesenteric arteries.

It's important to remember to measure the aorta perpendicular to the long axis of the vessel and remember to measure outer to outer margin, including the thrombus.

And remember, not like the leading edge of a bi ter, but outer to outer dimension of the aneurysm.

By definition, three centimeters represent an aortic aneurysm.

1.5 represents an iliac.

An aneurysm 1.0 represents a popliteal artery aneurysm.

25% of aortic aneurysms rupture when the size gets greater than five centimeters.

And a size change greater than four millimeters is indicative of expansion.

Below that is only inter observer variability.

I've demonstrated to you the turbulent flow may be a significant risk factor for aortic aneurysm ruptures.

Remember that aortic aneurysms have two different flow patterns that can either be laminar and smooth or turbulent.

And as I stated, turbulent flow may be a risk factor, risk factor for aneurysm rupture.

And remember also that for celiac arteries, they have to remain elevated in both the inspiration expiration and erect position.

If a true stenosis continues to exist.