The Use of Optical Coherence Tomography (OCT)

Can you share some of your long history with OCT and describe this technology? Optical coherence tomography (OCT) technology was first developed several years ago. We participated during its early development stages at the Washington Hospital Center. Initially, obtaining OCT images, which show the inner lining of the vessel, involved occluding the artery with a balloon and flushing it with saline to remove blood from the vessel. First-generation OCT technology was developed and eventually sold in Europe, Asia, and South America, leading to a great deal of published research. The use of OCT sparked enough interest that the company (Lightlab Imaging, now owned by St. Jude Medical, Minnetonka, MN) developed new, second-generation technology that allows OCT to be used without occlusion and flushing of the artery. This led to its recent approval by the U.S. FDA and clinical availability. The second-generation device has been a huge step forward and will allow OCT use to further expand. Images are acquired during a single injection of contrast through a normal guiding catheter. The imaging is done in about 3 seconds, and the entire coronary artery is perfectly well imaged. A normal injection of contrast, 12-16 ccs, is usually enough. Analysis of the images is done frame by frame. With the aid of the software, a longitudinal display of the vessel is available, so side branches and different levels of plaque formation are visible, as well as stents. There is also a short axis view, which allows you to see a cross-section of the different layers, starting with the intima. To give you an idea of the quality of the images, consider that intravascular ultrasound (IVUS) has a resolution of 100-120 microns, which is very good. OCT has a resolution of 10 microns, meaning it is 10 times more precise. With such resolution, it is possible to see the inner layers of the vessel wall in much greater detail, as well as stent struts that could never be visualized before. Intima thickness can be measured. The vaso vasorum can be visualized and experts can even see microphage clumps in the inner layers of the vessel. The possibilities are tremendous. Most of the recent research has been done in the area of stents, because, like never before, we can see whether the strut is or not well-apposed against the vessel wall, or whether it is buried into the vessel wall. It is possible to see how much intimal tissue has grown over the strut and measure it in microns. One can see, with exquisite definition, the presence of thrombus, whether it is attached to struts, or to ulcerated plaques. The enthusiasm for this technology is tremendous, because in medicine, high-quality imaging leads to better diagnosis and treatment. We don’t know exactly where this knowledge will take us. We have just acquired enough knowledge to eventually develop some practical tools and direct patients to particular treatments. We are acquiring data in many different subsets. In patients with unstable angina, for example, we are looking at plaque rupture, lipid cores in the vessel wall, connections between the plaque rupture and the lipid core, etc. OCT is much more sensitive and specific to diagnose plaque rupture than prior imaging techniques. OCT has much better image quality and provides much better information, with a tremendous option for growth in our understanding and ability to treat patients with coronary disease. Can you share more about the process of obtaining images? With the current-generation OCT catheter, it takes 3 seconds to obtain the images. It is a very fast procedure. It is very easy and simple to set up. It requires minimum training. It is very safe for the patient. We have proven in the past that there is no risk in doing intracoronary ultrasound in expert hands. For OCT, it’s even safer, because the OCT technique is so simple and fast. It has no deleterious effect on the patient. I tell my patients there is no additional risk incurred by adding OCT to an invasive procedure. What we hear from the company is that in Europe, its use is expanding rapidly and in some cases, replacing the use of IVUS. The challenge now seems to be learning about how best to use the information that OCT provides. Yes, a great deal of clinical research will be required to determine the clinical implications of the morphologic findings we see on OCT. There have been several recent publications in Europe showing that stent struts still uncovered by tissue are more likely to have thrombus. However, most publications show no correlation between stent malapposition, strut thrombus, and clinical events, which is surprising. While OCT has tremendous precision in imaging the inner layers of the vessel wall, it does not have enough penetration to see the media or the adventitia. IVUS gives you a better image of the entire vessel wall, from adventitia to intima, but with less definition than OCT. You don’t expect that OCT will replace IVUS? At this stage, the way we see it, you should really do both in order to truly understand the pathology or lack of pathology in the vessel. Both devices give information of great value. Of course, we wish we had technology that would give us both types of information at the same time, with the same catheter, so the patient would need just one imaging technique. You think that might be a future possibility? We’re looking forward to that, yes, to future generations that might have more capabilities. For now, we are learning to treat patients with stents, just with OCT, and continue to do IVUS to evaluate the contributions of both techniques, and ensure optimal treatment of the patient. Are there any formal clinical trials that are ongoing? Many, many trials have been conducted outside of the U.S. (in Europe, Asia, and South America), on specific clinical syndromes like stable or unstable angina, acute myocardial infarction, stent restenosis or thrombosis, and so on. What about research into plaque characterization? Plaque characterization has important implications: can it help predict clinical outcome? “Vulnerable” plaques have been of great interest for the past 15 years, as they seem to be associated with acute ischemic syndromes. OCT has the unique capability of measuring the fibrous cap of the plaque. We know that areas of the fibrous cap that are thinned out are much more prone to rupture, leading to thrombosis. OCT will also tell us what the effect of medical treatment is on the plaque components, including the fibrous cap. OCT preliminary studies have shown that effective treatment of hyperlipidemia increases the thickness of the fibrous cap. So OCT may be useful in measuring treatment efficacy for medical therapy? Yes, that’s a very exciting new field of research. Let me mention, too, that you can see macrophage clumps with OCT, which has important clinical relevance, because it allows you to see zones of inflammation. It is inflammation within the plaque that brings a number of reactions that lead to plaque rupture. Another benefit of OCT is its ability to visualize vaso vasorum in the plaque. Increased neovascularization of the vessel wall may be related to advanced atherosclerotic process. Stenosis severity can also be evaluated very precisely with OCT. We have very good data correlating the minimal lumen area with clinical ischemia. In fact, with IVUS, we had a magic number of 4 mm2 as a cut-off line, above which you supposedly don’t need intervention. Recent fractional flow reserve (FFR) data shows that the 4 mm2 is not such a simple, easy cut-off number. Depending on the vessel size, the length of the lesion, the location of the lesion and the amount of myocardium perfused by this vessel, an area Any final thoughts? I’d like to conclude by saying that I hope many interventionalists will be enthusiastic about OCT and the ability to precisely image the vessel wall, the plaque and the effects of percutaneous treatment. By seeing better, we will understand better, and make progress in our ability to better treat the vessels in patients with coronary disease. Dr. Pichard can be contacted at apichard@medstar.net Disclosure: Dr. Pichard reports he has been a speaker on fractional flow reserve for St. Jude Medical.

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