Marlena Jbara, MD reviews current imaging modalities that may be ordered for patients with peripheral vascular disease. Dr Jbara also reviews the pros, cons, and contraindications for each imaging modality and its role in assessing vascular patients. She reviews the current guidelines for ordering vascular studies and formulates an algorithm when deciding to order a vascular study and what criterion to consider when ordering.
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Marlena Jbara: Imaging of Peripheral Arterial Disease - How to Choose the Right Imaging Procedure. My name is Marlena Jbara and for the next 25 to 30 minutes, I'll be discussing different forms of imaging that we can assess for diseases affecting patients related to smoking, diabetes and peripheral vascular disease.
Disclosures. I or a related party, have no financial relationships to disclose.
Objectives. What's in the imaging toolbox available to clinicians to sort out what's the best treatment for the rising number of patients afflicted with peripheral vascular disease? What we'll do in this next time together is review the pros, cons and contraindications for imaging test such as duplex ultrasound, CAT scan angiography, MR angiography, digital subtraction angiography. We'll look at how to choose the right imaging modality, review the current guidelines and provide an algorithm for the imaging of peripheral vascular disease.
So what's in the toolbox? We'll be looking at duplex ultrasound, CT and MR angiography, digital subtraction angiography, going forward. Beginning with our first tool in the toolbox, the duplex Doppler.
Doppler ultrasound is a noninvasive method to evaluate blood vessels using sound waves by using a spinning crystal inside of a transducer. Sound is produced, which echoes through the body tissues and reflects back to the transducer, which is then recorded to provide both anatomic and hemodynamic information. Given distance and velocity curves, one can tell the speed of flowing blood, and even the caliber of a vessel to determine the degree of stenosis and look at rates of the potential intervention.
Color Doppler also has the power to localize regions of abnormal flow. Therefore, giving us a glimpse of hyperemia and areas of cellulitis, and even potential areas surrounding blood vessels of abscess. And pulse-Doppler evaluates the peak systolic velocity and the waveform, which gives tremendous inflammation regarding the size, caliber and viability of a blood vessel.
In terms of the duplex Doppler and the anatomy of a waveform, this is one of the most important curves that we can study. So on the Y axis, we have velocity. And on the X axis, this is over time. And what we're measuring is the flow or the signal intensity through a blood vessel as resonated by the spinning crystal and heard through reverberations of sound through the soft tissue.
This is the amplitude of the wave, with coursing through the blood vessel, and we can measure a peak systolic velocity. And in diastolic velocity, we can see how broad the spectrum is determining if it's stenotic or phasic, and we can even see the spectral width. Normal waveforms demonstrate a forward flow insistently, a reversal flow in early diastole and then another forward flow in late diastole. And we can see, as we're talking about this forward flow, the slight reverse flow, and then another forward flow in late diastole. And here, we can see color Doppler rendering of whether or not the blood flow in red is going towards the transducer or blue, away from the transducer. And in this case, there is likely volume averaging through that area where a vein is being partially averaged with an artery.
In this case of a superficial femoral artery stent stenosis, we can examine it nicely with Doppler evaluation. And what we're seeing is that the proximal superficial femoral artery velocity is measuring 96 centimeters and the peak systolic velocity ratio of 278 over the proximal SFA at 96 centimeters per second or 2.9. And we can then go to a chart where we can see velocity ratios versus percentage stenosis and understand that 2.9 essentially is on the order of between 50% and 75% stenosis in this example. So it's quite a powerful tool not only determining anatomic but actual hemodynamic information that can give you percentage of stenosis, which is ultimately what it's going to decide whether or not and what type of surgery one might need.
Color Doppler ultrasound, the pros and cons. The pros of this modality is it's right there. You can wheel it into the patient's bedside and it requires absolutely no radiation. The sensitivity is best and greater than 50% of stenosis. In the iliac, 89%, and popliteal, 68%. And the sensitivity to detect occlusions are great in the [ATM PTs 00:06:44] over 90%, but less well so in the peroneal arteries where you see about 82%.
The contraindications or the downside is that in heavily calcified vessels, they block the sound. So it really can't -- it's challenging to assess the disease on these images and there may be diffused multi segment, long segments of involvement. Of course, the cons is that it's operator-dependent. So unless you're doing the study, you have to rely that someone else has adequately assessed and used the correct standards. In terms of soft tissue edema in cellulitis, these factors can increase the soft tissue's thickness and density, and decrease sound penetration.
Imaging goals. Is Doppler enough? Physiologically, we can detect the presence of peripheral arterial disease and we can even quantify the degree of stenosis to an extent. And anatomically, we provide a comprehensive evaluation involving the distribution, degree and length of poststenotic dilatation.
And it's a roadmap to plan interventions. But is it enough? With that said, it isn't enough. In those areas of multi long segment stenosis, we run risk of not seeing the disease fully. And therefore, we move on to the second tool in our toolbox, peripheral MR angiography and this is where we take a multi-station MRI. Essentially, we begin at the aortobiiliac lower aorta. I want to begin with a precontrast, three-dimensional volume acquisition, initially taking a precontrast mask of all three stations, which takes a lot of time and the patient has to be very still.
And then, we do the same after we inject intravenous gadolinium. The pros of this is that there's no ionizing radiation. The superior pedal artery evaluation over CT is superior. There's no calcium blooming. However, the cons or the downside is if the GFR or your glomerular filtration rate is less than 30, you may not be able to be someone who can receive gadolinium since there is a risk of nephrogenic systemic fibrosis, and that is gadolinium-induced in patients who have renal disease. The cons, in addition, by the time you're moving down the stations given the length of the examination, you often run the risk of venous contamination. Again, it could be a very long exam, up to 30 minutes, and you can't always image all patients. Some patients are claustrophobic and simply cannot tolerate.
Then, you have patients you can use a double bolus, which is a technique to decrease venous contamination so you might inject an image, the tibial station first, and then go back into the lower aortobiiliac and superficial femoral artery regions, through the level of the tibial peroneal trunk.
MRA is the test of choice in diabetics. In CT, this example to the left demonstrating a CAT scan, you can see the calcium that's studding the tibialis anterior artery, and the CT gives you this heavy blooming artifact in these heavily calcified vessels. Where if you look on the right, in the MRI, that calcium bloom just isn't there. What you can see are areas of stenosis. So oftentimes, surgeons may want a combination of both studies to assess how much calcium is in an area that's looking very stenotic.
In this example of an MRA, what we're seeing in the example all the way to the left is that on the right, you have a distal superficial femoral artery to popliteal occlusion. You have some hypertrophy of the proximal profunda vessels. And on the central area, you're seeing the lower station runoff where you're seeing the reconstitution of the tibioperoneal trunk and peroneal artery. You can also see in this example a distal SFA nitinol stent with no metal artifacts. You're able to image your stents pretty well with MRA.
Moving on to CT angiography, this has been the workhorse of vascular angiography, and the pros is that the high spatial resolution make it ideal. You can really assess the size of a blood vessel. You can assess the surrounding soft tissues, excellent spatial resolution, and it's a study that you can do quickly. It can be done rapidly. Within five minutes, the patient can be on and off the table. It's reproducible and accessible, and it gives you the ability to detect greater than 50-degree, 50% stenosis.
And then, it has a pretty high sensitivity and specificities. Sensitivity-wise, it's about 92% to 95%, and specificity, about 93% to 96%. In terms of the cons, it's inconsistent in its pedal imaging given the calcification we looked at before the calcification bloom. Of course there is a degree of radiation involved and accumulative dose maybe somewhat significant over the course of imaging these very sick patients overtime. Some patients can't receive the contrast and they may have unpredictable anaphylactic allergy, and it's nephrotoxic iodinated contrast. So those are the contraindications in general.
In this example of a CTA in a patient with peripheral vascular disease, we can see the excellent resolution at the lower aorta right iliac with long segment right common iliac artery occlusion, and reconstitution of internal and external arched arteries.
We can see excellent rendition on this example on the central screen where you're having long segment occlusion of the left proximal SFA. We can even see this mid SFA on the right here has what I would consider a moderate occlusion over a length of approximately a centimeter. There's some slight subtle poststenotic dilatation so it's moderate to severe. And then, in this example over here on the left, you can begin to see the calcium bloom, the poor resolution of the level of the ankles, the artifacts inherent in the venous contamination.
Moving on to digital subtraction angiography, this is performed by our interventional colleagues and it's the gold standard of arterial imaging. It's invasive and requires an arterial puncture. Fluoroscopically, we obtained a precontrast mask. We obtained that first and then we're going to take a postconstrast angiogram. So we're going to inject iodinated contrast and then take a fluoroscopic cinematic clip. We're going to then subtract that from the precontrast and we're going to be able to determine areas of stenosis.
Given that it is an invasive procedure, there are small but significant risks. There can be contrast-induced nephrotoxicity given the degree of contrast that maybe administered in patient's contrast allergy. Note that these patients are often already renally insufficient and that they've had longstanding peripheral arterial disease. The benefits include spatial resolution. It's the gold standard in X-ray imaging. And in addition, it provides direct access to therapy. The intervention is immediately possible.
In the future, you may see CO2 angiography, which is 20 times more soluble than blood, and displaces blood volume which you can image. And this is an example I have given on the left. So you're standard invasive angiography here on the left where you can see your iodinated contrast and you could see the blood vessels, and you could see mild areas of narrowing here. On this example of CO2, what you're seeing is the relieve the lack of the movement of gas through that region, which turns out to be safe because it's 20 times more soluble than blood, and displaces it, which you can image immediately. So it's exciting in the future -- with future developments in technology to allow improvements over what already was a good test.
So the guidelines for peripheral arterial disease imaging are used for the Society of Vascular Surgery Practice Guidelines and all agree that you will start with the duplex ultrasound. Two important papers, important studies, the TASC II and ESC, looked to choose CTA or contrast-enhanced MRA, based on local availability, cost and experience. For example, in your institution, if no one's performing MRAs, then people who are going to be more skilled at interpreting a CTA, so that maybe the better imaging modality in that particular area.
The American Heart Association and the American College of Cardiology Foundation produced a taskforce on practice guidelines that was put out in 2011 that CTA or MRA can be used to diagnose peripheral arterial disease, and I have included these references at the bottom available for your review.
The American College of Radiology puts out appropriateness criteria and this may be an important moment for me to just stop and put it in a plug for radiology, and even those of you have friends and family, or you're often wondering how it is to image things or how do I know how to do it? Technology is always changing. We're not always in that loop. Well, the American College of Radiology puts a very helpful guidelines which are updated by the leaders in the field.
The purpose of the guidelines are to decrease cost, decrease radiation, and guide for appropriate imaging so that whatever imaging test that you choose would answer the question, the clinical question at hand. In this case where you -- we're going to be looking at the clinical condition of claudication and its suspected vascular ideology. And what's usually appropriate is a rating scale of seven, eight and nine. And as you can see for segmental Doppler pressures and post volume recordings, there's usually appropriate indication, appropriate for screening patients with symptoms and finding suggestive of PVD. Compressibility artifact limits interpretations of pressures but post volume recordings remain interpretable in this setting.
Moving on down the line to an MRA of a lower extremity without and with contrast, that's excellent for dialysis patients and patients with a GFR less than 30. And in that case, you can see that's also usually appropriate.
Now, it's not recommended that everyone initially have both exams, but if the patient has a contraindication to an MRA, either being claustrophobic or having renal insufficiency, and not being able to have a GFR less than 30, then those patients may go on to have a CTA of the lower extremity with contrast and that would be the test of choice, and the patient on dialysis would be at best served with that, and then dialyzed the contrast to follow.
Moving on to ultrasound of the lower extremity with Doppler, this is useful in patients who have contrast allergies or renal dysfunction. But again, compressibility artifact may limit interpretation and pulse volume recordings may remain interpretable in this setting. Arteriography of the lower extremity is only going to be indicated if intervention is planned. Whenever we take someone to an invasive procedure, we're always outweighing the risk benefit ratio to determine that if a preoperative complication occurs, that there was a significant reason to test that at all by sticking an arterial blood vessel and sending contrast through there. So it's only going to be indicated if intervention is planned.
And then lastly, what may be appropriate in patients with contraindications to iodinated and gadolinium-based contrast agents, maybe to obtain an MRA of the lower extremity without contrast. So to go back to the beginning point regarding ACR appropriateness criteria, in this case of claudication, you can always refer back to this list to guide you in getting the most accurate, cost effective indicated way to image the current disease, and even in diseases that aren't related to foot and ankle.
You can look up any disease. You can look up diverticulitis, appendicitis, rotator cuff tear, and they have -- all the leaders in the field have joined together in pods to determine what's the best use and appropriateness criteria for imaging of that particular area. The take-home algorithm is that if a patient requires noninvasive imaging of the lower extremity for the indication of peripheral arterial disease, you can follow this pathway to determine whether or not and how to treat, and this is produced by the American Heart Association and published at the circulation for cardiovascular imaging.
So patients who have implanted metal devices or claustrophobia, if that's the case, they're going to be imaged with a CTA or a duplex ultrasound. If they don't have implanted metal devices or claustrophobia, and they have renal insufficiency, and in addition they're dialysis-dependent, then again, we're not going to give them gadolinium at risk of running nephrogenic systemic fibrosis. Those patients are going to go on to get a CTA. If the GFR is less than 30 mLs per 1.73 meter squared, then we can obtain an MRA or a duplex ultrasound. If the GFR is 30 to 45, then we will cautiously give them a CTA or a contrast-enhanced MRA with cyclic agent. Alternatively, a non-contrast MRA or duplex ultrasound.
If the patient needs peripheral arterial imaging, doesn't have implanted metal devices, not renally insufficient, not diabetic, but they have native vessel bypass grafts or stents, if it's a native vessel, if it's suspected Fem-Pop disease, then we can do a CTA, MRA or duplex. If it's suspected in the aortal iliac, then CTA or MRA is the preferred indicated imaging method. If suspected runoff disease, then a CTA or MRA. If there's a presence of bypass grafts, if they're infrainguinal, then we can obtain a duplex ultrasound for surveillance postoperatively. If suprainguinal, then CTA or MRA. And if there are stents, then CTA is preferred. If iliac superficial femoral or popliteal artery, we can use duplex ultrasound. However, limited to sensitivity.
Now, if the patient does have diabetes, this is probably the most common booming population we have, and contrast-enhanced MRA is preferable due to the likely heavy calcific peripheral disease, and you can also choose CTA of duplex ultrasound. Of course always duplex ultrasound is indicated and CTA maybe indicated.
So choosing the right test in peripheral arterial disease, take-home points, if you're diagnosed with PAD, you're getting a duplex. If you have renal failure on dialysis, then a contrast-enhanced CTA, then dialyzed. If you're renally insufficient and your GFR is 30 to 45, we'll do a cautious contrast-enhanced CTA or a contrast-enhanced MRA. We can also cautiously watch you through lab monitoring and volume expansion. That's what it means to be cautious. GFR, less than 30, non-contrast enhanced MRA maybe indicated and always a duplex ultrasound.
If you're a diabetic, assume heavily calcified pedal vessels, and a contrast-enhanced MRA would be indicated. Plus renally insufficient, non-contrast MRA. If you have native vessel and you're not diabetic, a contrast-enhanced CTA is for you, and if you have a history of surgery, nitinol stents are very well-imaged through contrast-enhanced MRA. Steel stents, you don't want to use the MR. You want to use the CAT scan with metal reduction protocol. In bypass situations, you want to use contrast-enhanced CTA. And for planned interventions, you use digital subtraction angiography, plus or minus CO2 contrast.
I want to provide references for what we've just discussed and really thank you for your generous time and attention. It's been my pleasure.
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