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The Role of Micro-CT in Peripheral Artery Disease (PAD)

Author Sara Stewart covers The Role of Micro-CT in Peripheral Artery Disease (PAD) on BackTable VI

Sara Stewart • Jun 30, 2024 • 38 hits

Micro-CT operates under the same principles as traditional CT but produces significantly higher-resolution images. This advanced imaging modality has proven useful in researching peripheral artery disease treatments, specifically with laser atherectomy.

In this article, Dr. John Rundback provides an overview of micro-CT and its role in analyzing medial calcium patterns in peripheral artery disease. Dr. Rundback recently authored a cadaveric study that used micro-CT to assess the effectiveness of the Auryon laser atherectomy in disrupting calcium buildup. The study identified various medial calcium patterns and showed promising results on the Auryon laser’s ability to disrupt certain calcium patterns, thereby enhancing our understanding of treatment mechanisms at a microscopic level.

This article features transcripts for the BackTable Podcast. We’ve provided the highlight reel here, and you can listen to the full podcast below.

The BackTable Brief

• Micro-CT offers detailed, non-destructive imaging at a resolution down to 1 micron, significantly surpassing traditional CT's 1-3 millimeter resolution by rotating the sample instead of the X-ray source.

• Micro-CT is particularly valuable for assessing calcium disruption in peripheral artery disease treatments, including treatment with laser atherectomy.

• Dr. Rundback’s study identified different calcium patterns on micro-CT (speckled, shingle, sheet, and plate) and found that the Auryon laser atherectomy was successful in disrupting calcium in the continuous plate pattern.

The Role of Micro-CT in Peripheral Artery Disease (PAD)

Table of Contents

(1) What is Micro-CT?

(2) Observing Laser Atherectomy Effects with Micro-CT

(3) Micro-CT Medial Calcium Pattern Analysis

What is Micro-CT?

Unlike traditional CT, which typically offers 1-3 millimeter resolution, micro-CT provides high-resolution imaging down to 1 micron by rotating the sample instead of the X-ray source. This allows for a detailed, non-destructive analysis of tissues. Micro-CT can be particularly useful for evaluating calcium disruption in peripheral artery disease treatment, especially when using laser atherectomy devices like the Auryon atherectomy.

[Dr. Sabeen Dhand]
That's great. Sometimes some cases need to be done in the hospital, like some of these complex cases. That's great that you have both options there. So, yes. Okay. Micro-CT. I mean, when I heard this topic, I heard micro-CT, I was thinking, is this some microscopic CT scanner? What is it?

[Dr. John Rundback]
You know, that's what I thought too originally. You're not alone in that. This really started, the genesis of this is that we do these interventions all the time, Sabeen, right? We're kind of told mechanisms of action, and we think we understand it, but we don't really understand these things on a fundamental level, what's really happening at that interface between our devices and the tissue, and what the result of that interaction is. So working in this particular case with AngioDynamics, we wanted to get a better understanding of the mechanism of action of their atherectomy device, the Auryon atherectomy.

We talked about how to do that, and came across some of the material which had been published by Shockwave, looking at disruption of calcium that was using micro-CT. Really, a lot of the credit here now goes out to Aloke Finn, who we reached out to at Cardiovascular Path, down in Gaithersburg, Maryland. We reached out to them.

Micro-CT is not a miniaturized CT scanner. As a matter of fact, it's sort of the opposite in the sense that obviously in a CT scanner, the subject is stationary and the image or the fluorosource or the X-ray source rotates in the gantry, and obviously, various technologies around that. In micro-CT, the X-ray source is stationary, but the object is rotating. So that's how you get the three-dimensional perspective. The difference is that, unlike a CAT scanner, which you get maybe 3 millimeter, if you really want to get thin cuts, 1 millimeter resolution, the resolution from micro-CT, which is a non-destructive imaging method, is in the range of 3 to 5 microns, with the Nikon device we use, and in some cases, as low as 1 micron. It's really a microscopic evaluation of the tissues.

Listen to the Full Podcast

MicroCT for PAD: What You Need to Know with Dr. John Rundback on the BackTable VI Podcast)
Ep 353 MicroCT for PAD: What You Need to Know with Dr. John Rundback
00:00 / 01:04

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Observing Laser Atherectomy Effects with Micro-CT

To study the effects of laser atherectomy on calcified arteries, cadaver arteries were placed in a gel matrix to mimic physiological conditions, and human blood was circulated through the arteries to simulate real-life interventions. By comparing different treatment methods, including balloon angioplasty and atherectomy, the goal was to observe the specific impact on medial calcium at a microscopic level with the help of micro-CT. As Dr. Rundback explains, micro-CT revealed detailed images of calcium disruption, enhancing the understanding of how these treatments affect arterial calcification.

[Dr. Sabeen Dhand]
Okay. It's funny you talk about we don't know the process behind things, because it's true. I see a 2D black line, it's narrow. I put a balloon atherectomy, it gets bigger. We have IVUS and things like that, but yes, we don't know what's going on in that 1 micron, 3 micron level. The subjects that you're putting on this rotating table, is it like a cadaveric specimen after treatment, and it's a small, little piece of tissue, or what are you scanning?

[Dr. John Rundback]
Exactly. Micro-CT has been used in industry for many years. It's a very rapid technique. It allows a tremendous evaluation of just the internal structure of anything. It's used to look at baffles and microscopic structure of electronic equipment. In this case, yes, it was screened cadaver arteries. The arteries were screened based upon cardiovascular risk factors in the individuals while they were alive.

Those who had substantial risk factors suggesting atherosclerotic burden were then obtained, the limbs were obtained, and they were just subject to regular x-ray, those limbs, isolated limbs to see if there was indeed calcification, because that was our target, both for femoral popliteal, but particularly in this study for infrapopliteal or tibial calcification. Once we had cadaver limbs, and that's a challenging screening process in its own, to get limbs, subject them to x-ray, divide these into sections so we can have an idea where we can and can't apply our technology or do our experiment. Once we have that, then the arteries were actually isolated. They were dissected free from the limb. Now you had these calcified arterial segments laid out on the table.

[Dr. Sabeen Dhand]
Reminding me of anatomy lab in med school, dissecting these arteries out.

[Dr. John Rundback]
Exactly, exactly, but even then, they're sort of still attached to the tissue. These were just pieces of spaghetti, in essence. Separate arteries, obviously branching and everything else that arteries do, which we would then lay out. What we did is we then, because you want tissue support when you do interventions to kind of simulate real life, basically it's a gel matrix inside a water bottle, which is slid open. You lay the arteries inside this gel matrix.

So you have tissue support around the artery. You cannulate both ends. You have a pulsatile pump, and now you have flow through these arteries. In this particular work, since we thought that the medium inside the arteries might affect the results, we actually had human blood, whole blood, which we were circulating through the arteries.

[Dr. Sabeen Dhand]
Whoa. Okay. Now I get it. So you put basically this artery in a gel. It's like the connective tissue, and then it gives you some compliance when you start putting fluid through the artery so it plumps up, right? You're kind of recreating a little, let's say a cuff basically on the table. Okay. You're circulating human blood through that and you're doing interventions in that ex vivo experiment. Is that true?

[Dr. John Rundback]
Exactly. Under fluoroscopy with an OEC camera.

[Dr. Sabeen Dhand]
Oh, wow. Then what kind of interventions were you testing? Was it just anything? Balloon angioplasty, POBA, DCB, IVL, everything, or specifically you were looking at the atherectomy?

[Dr. John Rundback]
Yes. In this particular case, we were very interested in the impact of this atherectomy on medial calcium. Obviously when you're treating infrapopliteal work, which is what we like to use in particular the smaller laser, and this happened to be the Auryon platform, but this may apply to other technologies as well. It'll be interesting to see the differential impact of other technologies on medial calcium now compared to this experiment, but in this particular case, which was sponsored by AngioDynamics, we threaded this down into the tibial vessel and the goal was to see the impact of atherectomy on medial calcium.

We basically went ahead, did our angiogram, identified our segments, had different treatment groups and algorithms set up. Some were plain old balloon angioplasty, as a control. Completely untreated areas actually served as a better control. For instance, if there was disruption of calcium, was it just from our handling the specimen? Did we, in laying it down–

[Dr. Sabeen Dhand]
Did you crack it?

[Dr. John Rundback]
Crack it, right?

[Dr. Sabeen Dhand]
Yes, exactly.

[Dr. John Rundback]
Then obviously there's the treatment segments and then we subjected those treatment segments to different energies and algorithms, 50 millijoules alone, 60 millijoules alone,
50 and 60 millijoules, 50 millijoules with angioplasty to see where we had impact on calcium. Again, the goal was to reproduce the images that shockwave had had. You're probably also familiar with some of the work that CSI has done where they've looked at orbital atherectomy utilizing both OCT, I believe, and definitely IVUS. We have this change in this arc of calcium, but that's not as visually spectacular. When you do IVUS, okay, something is changing, it's reverberating a little bit differently, but you don't get those same pictures of disruption of calcium.

[Dr. Sabeen Dhand]
The resolution is just-- you have intimal calcification on an IVUS image. Everything else is shadowed out essentially, right? Before we go into the effects of the devices, when you first started looking at this, whether it's the ex vivo specimen or the micro-CT images, what did you learn on these arteries? Was there any big aha moments you said, "Oh, this is showing me something completely different pre-intervention?"

Micro-CT Medial Calcium Pattern Analysis

Various calcium patterns are visible on micro-CT: speckled, shingle, sheet, and plate. The plate pattern, which is circumferential and uninterrupted, presents challenges for balloon angioplasty due to its resistance to vessel expansion. However, when using the Auryon laser atherectomy device on plate pattern calcium, the study found that the laser provided sufficient energy to disrupt the calcium. In contrast, this effect was not observed in non-contiguous calcium patterns, as the vessels in these cases were already compliant.

[Dr. Sabeen Dhand]
All right. We all see these pretty pictures. If we look, and we'll include it on our podcast notes of what a micro-CT image looks like so people can see it. After you performed different procedures, including the Auryon laser, what did you find? Were there breaks in calcium? Did it restore the compliance like you're mentioning? Did you do anything bringing it back to as you described a white stop sign when the whole vessel was just calcium? Did you look at that too? Tell us your results.

[Dr. John Rundback]
Well, first of all, I learned that there's more than one type of medial calcium. As we get into this, medial calcium has many different patterns. There's a speckled pattern, which as you can imagine, just little islands. There is a shingle pattern, which looks like shingles on a roof, kind of overlapping. There's sort of a sheet pattern where there's non-congruous, incomplete curvilinear sheets, and then there's a plate pattern.

The plate pattern is really what we worry about. That's the worst form of Mönckeberg's medial calcification, because the plate generally is circumferential, uninterrupted, continuous calcium. That's the one that's preventing any pulsatility or expansion of the vessel. That's the one that's physically limiting us. When you go in there and you do your balloon, the balloon won't open. That's plate calcium.

[Dr. Sabeen Dhand]
Fluoroscopically, is that when we see medial calcinosis? Is that the tram tracking, or is there any way to see that plate-like calcification on a non-micro-CT image?

[Dr. John Rundback]
When you have very dense circumferential calcium, that's often plate, but not always plate calcium. The truth is both the sheet calcium and the shingle calcium can look fairly dense, but since it's not contiguous and you can't tell that in any one plane, there is still pulsatility. It is still not contiguous, so the vessel is still able to expand and be compliant. In those cases, generally the balloon will expand. It's when you have this solid kind of plate of calcium circumferentially. I'm not sure you can tell that entirely just by fluoro.

[Dr. Sabeen Dhand]
About these different patterns of medial calcification, what did you find with your results then after that?

[Dr. John Rundback]
When we do atherectomy, and again, we're talking about the Auryon here, but in general, if you think about it, we're giving a lot of energy into the artery, and obviously with laser in particular, there are various modes of tissue interaction. As we all know, you can have a photochemical effect, you can have a photothermal effect, and then you have a photomechanical effect. Each of these work differently, but even the fact that you have a photothermal effect, obviously you're imparting a lot of energy. When you're doing something like orbital atherectomy, this thing is rotating around.

Extirpated atherectomy is a little bit different because it's cutting, it's not delivering energy, but any of these things that are delivering modes of energy, that energy is not necessarily longitudinally transmitted. Previously, the idea is that there's probably radial transmission of that energy into the vessel wall, and that's been called pulsatile waves.

[Dr. Sabeen Dhand]
Okay. Outward.

[Dr. John Rundback]
Right, outward. CSI picked up on this as a mechanism of action whereby they're not just going ahead and sanding intimal calcium, but now they're having an impact on deeper calcium, which is why you can expand balloons at a lower inflation force than you would if you did not do orbital atherectomy. They already had this idea of pulsatile waves, and the belief was that laser is probably doing the same thing. It's a different outward radial force, it's not an orbiting solid crown, but there is tremendous energy being delivered. In the Auryon laser in particular, it's got an extremely short bandwidth, so it's a 10 nanosecond bandwidth.

Think about a jackhammer. You could take something that's sort of moving slowly and try to make your way through the pavement, and you'll make a dent, but with a jackhammer, this short pulse width, it's a very high focal delivery of energy briefly, and as a result, not only is that forward-directed energy to open up any forward-facing plaque, that's outwardly radiating, and that's the idea to get these pulsatile waves, which are disrupting the vessel wall. When we ran the Auryon laser, we found that there are a couple of dependent interactions.

First of all, the 50 millijoules was not as effective as the 60 millijoules, which supports this idea that you need more energy to disrupt the calcium. Secondly, we really only saw the effect on circumferential plate calcium, because otherwise, you have compliant tissue, which would just dissipate that outward radial force.

[Dr. Sabeen Dhand]
It'll absorb it.

[Dr. John Rundback]
Absorb it, right, or it would have give. In the circumferential plate calcium, when you use 60 millijoules, in every single analyzed segment, we saw in our disruption of calcium. What's really amazing is if you look at those micro-CT images, the longitudinal images or the inside-out images like you're looking down the vessel, it looks exactly the same as shockwave.

Podcast Contributors

Dr. John Rundback discusses MicroCT for PAD: What You Need to Know on the BackTable 353 Podcast

Dr. John Rundback

Dr. John Rundback is a practicing Vascular Interventional Radiologist at AIVS LLP in the New York City area.

Dr. Sabeen Dhand discusses MicroCT for PAD: What You Need to Know on the BackTable 353 Podcast

Dr. Sabeen Dhand

Dr. Sabeen Dhand is a practicing interventional radiologist with PIH Health in Los Angeles.

Cite This Podcast

BackTable, LLC (Producer). (2023, August 7). Ep. 353 – MicroCT for PAD: What You Need to Know [Audio podcast]. Retrieved from https://www.backtable.com

Disclaimer: The Materials available on BackTable.com are for informational and educational purposes only and are not a substitute for the professional judgment of a healthcare professional in diagnosing and treating patients. The opinions expressed by participants of the BackTable Podcast belong solely to the participants, and do not necessarily reflect the views of BackTable.

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