The Use of OCT Compared with IVUS

What are the advantages of optical coherence tomography (OCT) over intravascular ultrasound (IVUS)?

Both systems offer an anatomic assessment of the vasculature, but OCT is a leap forward in assessing coronary vessels from an anatomic standpoint. It has much better resolution, with 10x the axial and lateral resolution of IVUS. OCT has a much faster rotational and pullback speed, and data acquisition only takes 2.5 seconds. This means we can keep the workflow going, obtain the necessary images, and interpret them with confidence. Our lab frequently works with both OCT and IVUS. I tell our fellows that IVUS can be compared to black-and-white TV, while OCT is like high-definition color. It’s a totally different picture. It’s like going from a chest x-ray to chest CT. You can see so much more and get so much more information from an OCT image versus IVUS that, for me, it’s really a no-brainer. I rarely use IVUS at the present time.

Yet we are still not sure what to do with all the information OCT provides.

That is absolutely true. Many of the physicians and technologists who see OCT images get a little nervous at first, because they aren’t sure what they are looking at. People need to learn how to interpret the images. The next question is, “What do I do with it, now that I found it?” My answer is, we will learn.  We have never turned down new technology because we don’t know what to do with more information. When all we had was IVUS, everyone said, if we could only see more clearly, image faster, and interpret the images with more confidence, that would be great!  Well, that’s OCT. We will learn to interpret and deal with the information we find, but to ignore it, or to wish we couldn’t see more, has never, ever been what we have hoped for, in practice. So, yes, we will see thrombus for the first time, we will see edge dissections, and we will ask, how many do we treat? How many do we leave alone? This will take more research and more experience, but we will have answers shortly. To not embrace the technology because we are not exactly sure what do with what we find is not going to happen.

What is the learning curve for OCT use?

Any physician who uses IVUS at the present time has no problem placing the OCT Dragonfly catheter (St. Jude Medical). It’s very small, 2.7 French at the tip, and is very flexible. It is easy to move around curves and significant angulation. Placing the device is not a problem for any interventional cardiologist. It takes one or two tutorials for staff to figure out how to measure vessels and how the system works. The system itself is very user friendly and easy to set up. It’s very quick to acquire images. The only learning curve is with image interpretation. Interventionalists have to learn how to interpret edge dissection, stent malapposition, vessel sizing, and identify different types of plaque. Any physician who wants to have OCT in the lab will be provided with an expert in OCT to come down and work with them for a day or two, and give them some material to study about image interpretation. There is also a very small set of artifacts, which rarely take place, but that the physician needs to be aware of. OCT can be learned very quickly.

Additional contrast is required to obtain an OCT image. Are there any issues with renally compromised patients?

The additional contrast, roughly 10 to 12 ccs per run, is a non-issue for the vast majority of patients. However, there are a few places where we do choose IVUS. One would be in a patient with very severely compromised renal function, where we need to do PCI and are trying to minimize contrast usage. If multiple OCT runs are required for vessel sizing or to assess stent expansion, perhaps 3 or 4 runs, then we will use IVUS in these patients. We also use IVUS to assess the true ostium in aortal ostial lesions, where we have to disengage the guide from the aorta and can’t get good contrast flow down the vessel. OCT images can be obtained in this scenario, but they are not as crisp or clear as they would be normally, so we use IVUS instead. Still, I would say that more than 95% of lesions can be safely assessed by OCT.

If you are seeing vulnerable plaque with OCT, what have you decided to do with that information?

The first thing we have to determine is whether we think we can truly identify vulnerable plaque. That would be the Holy Grail. We are certainly headed in that direction and there have been some studies out of Massachusetts General on this topic, but we are not there yet. In a patient where vessel luminal size was normal and OCT showed a thin-cap fibroatheroma (<65 microns) overlying lipid plaque, but the lumen was not narrowed or compromised, we would continue with medical therapy and not perform PCI. That being said, with OCT, we can clearly see small areas of dissection, intimal tears, true plaque rupture and/or the origin of plaque rupture, usually at the shoulder of the plaque, maybe with some overlying thrombus, all of which might not have been picked up in a patient that had normal coronary vessels on angiography. Perhaps the patient came in with chest pain, positive cardiac markers and a non-STEMI, and we are not sure where the lesion is. If we find the culprit vessel and clearly see that shoulder, edge or plaque rupture with overlying thrombus formation on OCT, that would be an area we would probably try to secure or tack up with stent.

What about OCT use for stent sizing or vessel sizing?

The precision we get with OCT is amazing. The resolution of OCT is far better than IVUS for determining the vessel’s luminal diameter and cross-sectional area. When we place a stent and look to see if the stent is well apposed or not, there is another order of magnitude in security with OCT versus IVUS. Stent struts can still be assessed with IVUS. It is still a viable imaging modality, but OCT takes out a lot of the guesswork. It clearly shows if stents are well apposed or whether there is late stent malapposition. Vessel sizing by OCT means that when we bring in that next size post dilation balloon — even though when I look at the coronary angiogram I think this is a 3.5 vessel, and I need to bring in a 4, 4.5 balloon to post-dilate — I have confidence that it is actually the required size to make sure that the stent is well-apposed. There have been many times when we thought we had post-dilated aggressively, then went back in and looked with OCT, and saw the stent was under-expanded. At this point, you need to be very aggressive with your post-dilation balloon. People always worry about dissection or vessel rupture. When we can clearly see the vessel size, we feel much more confidant when doing post dilation.

The ILUMIEN system (St. Jude Medical) offers OCT and fractional flow reserve (FFR) measurement in the same system. Has using the system impacted your FFR use?

The ILUMIEN system is the first integrated diagnostic technology that combines OCT and FFR in one platform. We can get both a physiologic and anatomic assessment all in the same system. We have to change out wires, but for the physicians and staff it’s easy to use. It lets us do both FFR and OCT, but requires learning only one piece of equipment. Being able to do it wirelessly is also an advantage. We have always had FFR available in our lab, and are comfortable using it. Now staff is comfortable using both FFR and OCT.

As knowledge about the coronary vasculature and what we should stent and not stent changes, the clinical utility of FFR has increased. We now use FFR much more frequently to determine what should be done for the patient. We have had multiple cases where patients have one or two lesions, and we plan to stent, but decide to first measure FFR in the third vessel, because it looks like a moderate lesion. If the FFR shows the lesion is significant, now we are looking at three-vessel disease, and need to change our approach to that patient, who may be better off with bypass surgery.

In addition, in the near future, it is likely we are going to have to prove an anatomic or physiologic need for placing a stent in a moderate lesion (40-70% narrowing). Both OCT and FFR, despite how often they are used now, will need to be used a great deal more frequently in the future. A lot of press has been given to physicians who may have placed more stents than were necessary. The ILUMIEN system provides one platform to assess both anatomy and physiology to help make proper decisions for patients.

Any final thoughts?

We do use IVUS if there are true contrast limitations or to assess aorto-ostial lesions. I still think IVUS is the better choice in these patients. For all others, OCT provides very fast anatomic assessment with high resolution. OCT allows us to determine vessel sizing, stent under-expansion, dissection, thrombus, and gives us a good look at intermediate lesions, which on coronary angiography sometimes are hard to determine. (Interventionalists like to say that a lesion looks “hazy”. What is hazy?) An anatomic assessment with OCT can show you plaque rupture or a small dissection. It’s easier to see calcium. It’s easier to see in-stent restenosis. This technology will be important for our upcoming bioabsorbable stent trials and there is expanding data in using OCT outside of the coronary tree. It is an exciting new imaging modality whose clinical utility is just beginning to be appreciated.

Dr. Southard can be contacted at jeffrey.southard@ucdmc.ucdavis.edu.