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Marc D. Feldman, MD, FACC

Marc D. Feldman, MD, FACC is Associate Professor of Medicine, Director of Interventional Research and the Cardiac Catheterization Laboratories at University of Texas Health Science Center, San Antonio.

Besides his clinical practice, Dr. Feldman works in a number of research areas, including nanotechnology, drug-eluting stent hypersensitivity and optical coherence tomography (OCT). In 2005, along with Thomas Milner, Ph.D., professor of biomedical engineering at UT Austin, Dr. Feldman co-founded CardioSpectra to develop OCT technology for clinical applications in the heart. The company was acquired by Volcano Corporation in December 2007.

The coronary OCT device is not yet approved for use in the U.S., but has just been used for the first time in The Netherlands. The OCT images illustrating this interview were provided from the "First in Man Volcano OCT" performed at ThoraxCenter in Rotterdam by Prof. Patrick Serruys and Dr. Evelyn Regar.



Marc D. Feldman, MD, FACC
Marc D. Feldman, MD, FACC

OCT image inside proximal Right Coronary Artery
OCT image inside the proximal portion of a Right Coronary Artery, courtesy of the First in Man Volcano OCT, performed at ThoraxCenter
in Rotterdam by Prof. Patrick Serruys
and Dr. Evelyn Regar

Q: What is OCT – Optical Coherence Tomography?
Dr. Feldman: It's like ultrasound -- only ultrasound is using sound, and OCT is using light. And because light is so much faster than sound, it has a much better resolution. So you can see details in the blood vessel with light or OCT that you cannot see with sound. That's the main reason it's considered an advance.

Q: Tell me a little about how you developed the device.
Dr. Feldman: Actually OCT was invented at MIT in the early '90s. It's been used in many different areas, but most of it has been basic research. Our group at the University of Texas was one of the first to develop a cardiovascular catheter to use in the heart. I worked with Thomas Milner, who's a professor of engineering, and a lot of the graduate students. It actually took us ten years and we developed about seven patents: on devices, catheters, techniques, and ways to clear the blood. In 2005 the University of Texas helped us spin out a company, CardioSpectra, based on those patents. We found local investors and the State of Texas gave us money as well. Then, after three years, Volcano acquired our technology.

Q: So how does this work? Is it an over-the-wire catheter?
Dr. Feldman: Yes. It’s just like intravascular ultrasound (IVUS). It's over-the-wire. It spins just like intravascular ultrasound. What makes it more difficult though is that, where sound can see and reflect through blood, light cannot. Light is scattered by the red cells, and so we have to clear the blood.

One approach, taken by the Harvard/MIT group, is a device where you have to blow up a balloon and occlude the vessel, and then flush it out with saline. People don't like that because the balloon itself may cause some injury. So we're focusing on a faster imaging technique or physics, so that we don't have to occlude the vessel. We can just flush with saline or contrast or both together.

Q: How do you see this device being utilized during procedures? You’ve said that OCT can actually see whether or not a stent has been adequately covered by endothelial cells.
Dr. Feldman: The device is not FDA-approved still, so it cannot be used yet. But in the future, it could be used to look at patients who've got drug-eluting stents, where patients are at risk for the stents clotting off, and causing heart attacks. Although it’s not common, people feel that those patients who don't have endothelialization of their drug-eluting stents are those at risk for acute stent thrombosis. IVUS can't see that, but OCT can. So one application will be in trying to identify those patients who are at risk for acute stent thrombosis.

The other application, which is more futuristic, is trying to predict heart attacks. We know at autopsy the patient characteristic that the pathologist finds: a very thin fibrous cap with a very big lipid core beneath. We know those features can result in rupture of the fibrous cap which ends in heart attacks. We can actually see the thin fibrous cap with OCT. So we’re hoping we can predict future heart attacks by looking at the plaques when patients have heart catheterizations.

     OCT image just proximal to a stent in Right Coronary Artery
OCT image just proximal to a stent in a Right
Coronary Artery, courtesy of the First in Man Volcano OCT performed at ThoraxCenter
in Rotterdam by Prof. Patrick Serruys
and Dr. Evelyn Regar

Q: What you’re describing is what’s often called “vulnerable plaque”?
Dr. Feldman: Yes. There's a lot of discussion about vulnerable plaque, but this would be the first tool to try to predict or locate vulnerable plaques.

Q: My understanding is that OCT won’t actually be competing with IVUS, because they complement each other. Don’t they measure different types of things?
Dr. Feldman: Although light has better resolution, it cannot see more than 2mm -- as opposed to sound frequencies, which can see much deeper into vessel wall. So yes, in fact, the two techniques complement each other.

What's fortunate for light is that a lot of the vulnerable plaques are very superficial, within those 2mm, as well as the stents that you’re looking at that are at risk for acute thrombosis. So OCT is very useful for seeing the superficial features that will predict heart attacks, and looking for stent features that may cause them to clot off. But its weakness is that it can't see very deep into the vessel wall, like 5 or 6mm, whereas ultrasound can. So they really complement each other.

Q: So to recap -- to be able to actually see the stent and determine if it’s healed and been covered by endothelial cells, there’s been no way to do that until now?
Dr. Feldman: Right. Again because sound does not have the resolution to see that. On the other hand, there are features of vulnerable plaque, like excessive vasa vasorum – an enhancement of blood vessels or the blood vessel itself -- light cannot see that because it’s too far away, but sound (IVUS) will be key to be able to see those features.

Q: So being able to see if the stent struts are covered, how will that affect treatment of the patient?
Dr. Feldman: For instance, how long do you continue Plavix for? The FDA just recommended extending it to longer, 12 months. But for many of us, that's not even long enough. We still see patients 2-3 years out who are having acute stent thrombosis. You never saw that with bare metal stents. The instance is only 1 in 200, right? It's not very common, but does that mean every patient with a drug-eluting stent gets put on Plavix forever? Or can OCT one day say “Aha, that patient has thick endothelial coverage on that drug-eluting stent. Therefore the risk of acute stent thrombosis is very low. Stop the Plavix.”

Or, OCT has shown in Europe, where it's being used more often right now because their regulatory agency is more liberal, they're seeing these necrotic cores behind the drug-eluting stents. So in the vessel walls they’re seeing these necrotic cores that have been seen pathologically. And they’re starting to see those similar features in humans, in patients with drug-eluting stents. Again they're not very common, but OCT can see these types of features that IVUS just cannot.

Q: So, for example, if you discovered a necrotic core that’s behind a stent, what would you do?
Dr. Feldman: Well, one, you'd know that patient was at higher risk. Two, you would never stop the Plavix; the patient would be on Plavix and aspirin life-long. That's all you can do right now. The problem is, if you have that condition and you're at high risk, if you're that 1 in 200 patients, we don't have a good answer right now. But at least it will reassure the other 199 patients that they can stop their Plavix.

Q: I’m sure also that this will be used a lot in research.
Dr. Feldman: For instance, the Endeavor stent was just FDA approved, and there are going to be future drug-eluting stents that are going to be FDA approved, and there's going to be a great desire to study how much they endothelialize. And OCT could do that in humans whereas IVUS just could not. So we've never had a tool before that allows us to quantify the amount of endothelialization.

Q: Are these uses of the device something that’s down the line a bit, or might they occur quite rapidly, once the device is approved?
Dr. Feldman: I think that once it's approved you'll see it appear very quickly, but the market penetration may not be that different than IVUS. You know the number of cases that it’s used on.

The bigger question is: will it predict future heart attacks from vulnerable plaque? That's the bigger question. Now, if that's the case, if it's successful in doing it, then the use of it will increase tremendously because there's no way to predict a future heart attack right now. And so that would expand tremendously its usage once it appears, because anyone having a heart catheterization, most patients have plaques that are really deep in the vessel wall. You can't see them with angiography; you can't see them with IVUS very well. And if you've actually seen one of those vulnerable plaques with OCT, then that would expand its usage.

Q: So are you saying that you can’t really see those vulnerable plaques with IVUS?
Dr. Feldman: No, not really -- the best you can do is that Volcano has a way to look at plaque composition. But they still can't see the thin fibrous cap. So in humans we know that at autopsy those fibrous caps that rupture are usually about 30 microns thick. Sound has a resolution of 100 microns, it can't even see down to that resolution. Light has a resolution of about 10 microns, so OCT can easily see that thin fibrous cap that's been ruptured.

Q: Is there any radiation involved with this?
Dr. Feldman: No, it's optical so it's just light, infrared light. There's no radiation or X-rays.

Q: I’m sure that part of the development will be getting the catheter small enough to get into small narrow vessels.
Dr. Feldman: Our system right now is about 1.1 mm -- a very low profile; it's pretty similar to the current IVUS systems, so we have it down to that level.

Q: Are there trials ongoing or about to start that will be testing this?
Dr. Feldman: No. The goal is to just sort of get into the first patients to get FDA approval right now. So there are no trials that are ongoing at this time. So all these issues that we're talking about right now, predicting heart attacks, for example, they're very futuristic.

Q: Getting FDA approval is basically proving safety?
Dr. Feldman: The goal is to show safety, yes. IVUS is already FDA approved, right? So one approach would be to sort of piggyback it on to IVUS approval and apply for it as a sort of similar device. That would probably make the most sense, because if IVUS is a similar device and light is not toxic, then one would get FDA approval by comparing it to IVUS.

Q: Do you see a time where you’d have perhaps a dual catheter, using both imaging technologies to get full information?
Dr. Feldman: That would make a lot of sense to combine the two.

Q: So, to sum up…
Dr. Feldman: It's a new tool. It's the first uses of infrared light in medicine. It's actually in practice now in ophthalmology. So if you go to ophthalmologists' practices, they're actually using OCT to see not just to see the surface of the retina, but into the retina. And my guess is you'll see it in the next ten years, it will appear in colonoscopy and endoscopy, bronchoscopy. So it'll be piggybacked onto other devices. But those will be easier though. We did one of the harder problems first, because we had the problem of the light scattering from the red cells. If you’re in the eye, or the lung, or the G.I. track, it’s just air or just clear fluid. So the problems we faced with scattering, which are very difficult, just don’t exist in these other organs. So that’s why it will just be a matter of time before it appears in these other organ systems.

This interview was conducted in February 2008 by Burt Cohen of Angioplasty.Org.