What is Optical Coherence Tomography (OCT)?

Optical Coherence Tomography (OCT) is a novel technology with the first in-human studies. It directly assesses the severity of coronary blockage for angioplasty.

Functions of OCT

The standard technique for diagnosis of coronary artery disease and endovascular interventions has always been Coronary Angiography.  But over a period of time, its limitations had also become clear. Since it produces a two-dimensional representation of the vessel lumen only, the severity of lesion and plaque burden becomes difficult to assess. This results in unnecessary interventions on functionally insignificant lesions.

However, in Percutaneous Coronary Interventions (PCI), decisions on stent size and expansion are based on “eye ball” estimates that can result in a lesion-stent mismatch or malapposition of stent and vessel lumen. This has aroused significant interest in the development of techniques, thereby allowing direct assessment of lesion severity.

How is OCT useful?

• OCT functions as an optical analog to IVUS detecting the back scatter of light to generate an image.
• It uses the infrared light and measures the magnitude and echo time delay of reflected light. As blood scatters light, images are obtained from a vessel segment cleared of blood by either saline or contrast flush.
• OCT provides an image resolution of 15 µm to a depth of 2-3mm. As a result, clear images of the vessel lumen and wall, superficial coronary plaque components and endovascular stents are generated when compared to IVUS.
• Current OCT systems utilizing a rapid automated pull back system can image vessels at rate of 36mm/s, thereby minimizing the flush time required during image acquisition. The procedures are performed via a 6F guiding catheter with low complication rates.
• It is useful in characterizing different pathological processes, resulting in stent restenosis and stent thrombosis.

Note:  Unlike with IVUS, OCT cannot measure total plaque burden because of its relatively shallow tissue penetration.

How OCT benefits patients?

• It provides detailed images of superficial coronary plaque components. As per the histological analysis of autopsy specimens, there are 3 general plaque types: Fibrous, Fibro-calcific and Lipid Rich. All these plaque types can be accurately identified by OCT.
• The high resolution of OCT identifies plaque characteristics that predispose to rupture, including the thin fibrous caps, large lipid cores, and accumulation of macrophages.
• OCT can provide critical information to guide coronary interventions, in addition to characterizing atherosclerotic plaques.
• OCT can be utilized to measure the reference vessel diameter, minimal luminal diameter, and length of the target lesion.
• OCT can be used to identify stent malapposition, tissue prolapse and both in-stent and edge dissection with higher sensitivity than IVUS.
• It is of specific benefit in assessing stent apposition with overlapping stents.
• It is utilized as a research tool to study coronary artery pathophysiology as well as response to endovascular interventions. In addition, it is also used to compare various stent platforms in terms of deployment, endotheliziation and in-stent restenosis.

How can you distinguish the vulnerable plaques?

These plaques are at a high risk of precipitating an acute coronary event that has several histologic features like thin fibrous caps (<65µm), large lipid cores and increased infiltration of macrophages into the plaque cap, which distinguishes them from stable coronary plaques.

As the ten-fold higher resolution is possible with OCT, stent strut coverage and underlying mechanism of neoatherosclerosis can be better analyzed with the use of OCT than IVUS.

What are the Clinical Indications of OCT?

• Delineation of angiographically uncertain lesions
• Evaluation for allograft vasculopathy
• Lesion assessment pre-PCI
• Stent deployment post-PCI

OCT is preferred over IVUS for post-PCI stent evaluation as it generates clear images of stent strut apposition, expansion, and complications such as dissections.

What are the applications of OCT?

The application of OCT in daily clinical practice can help in improving the outcomes for the patients and provide an Optimal Percutaneous coronary Intervention. OCT can help guide physicians on specific clinical situations and can help optimize physician treatment strategies.

Some of the day to day practical applications can be as below:

• In-stent Restenosis
• Assessment of Calcified Lesions
• Thrombosis
• Stent Deployment and Malapposition
• Stent Deployment and Edge Dissection
• Bifurcation Lesion Assessment and Percutaneous Coronary Intervention
• Bioabsorbable Scaffold – OCT is the only imaging modality that can visualize a BVS.
• OCT in ACS

As the complexity of lesions treated and the number of devices used, continue to increase, applications for OCT are certain to expand. OCT offers several distinct advantages over IVUS and other imaging modalities. In particular, the increased resolution of OCT sets the stage for its use in imaging a wealth of scenarios. We have found that OCT is particularly helpful for situations where fine tissue resolution is required – such as imaging edge dissections, tissue prolapse with stents and thrombus.

How it can guide to improve outcome – Physician’s Opinion

Dr. Naveen Bhamri says optical coherence tomography is the most advanced coronary imaging modality currently available for clinical use. This technique helps to understand the disease process and improve PCI outcomes. It helps in resolving angiographic ambiguities and optimizing stent deployment. It is also used with complex lesions such as calcification, bifurcation, and long lesions. While using it with calcified lesions, it helps in identifying lesions requiring rotational atherectomy; and with complex lesions, it helps in identifying and resolving suboptimal stent deployment.