Who Should Get Optical Coherence Tomography?

Editor's Note: The following is a related resource to this Hot Topic. Finn AV, Chandrashekhar Y, Narula J. IVUS and OCT: Either or Survivor. JACC Cardiovasc Imaging 2011;4(9):1047-9.

Decades after its development, coronary angiography remains the standard technique for both the diagnosis of coronary artery disease and for guiding endovascular interventions. Over this time, the limitations of angiography have also become clear. Most prominently, coronary angiography produces a two-dimensional representation of the vessel lumen only. As result, lesion severity and plaque burden can be difficult to assess. This can result in unnecessary interventions on functionally insignificant lesions. During percutaneous coronary interventions (PCI), decisions on stent size and expansion are based on “eye-ball” estimates which can result in lesion-stent mismatch or malapposition of stent and vessel lumen. As result, there has been significant interest in the development of techniques that allow for more direct assessment of lesion severity and more accurate guidance during PCI.

Developed in the 1980s, intravascular ultrasound (IVUS) is currently the most commonly utilized methods for endovascular imaging. Via a 6-F and 5-F pull back system, IVUS uses high frequency ultrasound to visualize coronary arteries to a depth of 10mm with a resolution of 150µm.1 As a research tool, IVUS has allowed for better understanding of plaque morphology, vessel wall remodeling and the natural history of atherosclerotic disease.2 Lesion severity can be assessed more accurately by IVUS than angiography but this role has been largely supplanted by fractional flow reserve (FFR) which allows for the more important functional assessment of lesion severity.3 By the most recent guidelines, IVUS can also be utilized to assess allograft vasculopathy and the mechanism of stent restenosis or thrombosis.4 As a clinical tool, the IVUS has an established role in guiding coronary interventions. Pre-intervention, IVUS can accurately determine the lesion length and reference lumen diameter. Post-intervention, PCI complications and inadequate stent expansion can be identified.4 The clinical role for IVUS was established via both prospective and retrospective studies showing potential clinical benefit to patients.1

In comparison to IVUS, optical coherence tomography (OCT) is a novel technology with the first in-human studies performed in 2008. OCT functions as an optical analog to IVUS detecting the back scatter of light to generate an image.5 It uses near-infrared light and measures the magnitude and echo time delay of reflected light. Because blood scatters light, images are obtained from a vessel segment cleared of blood by either saline or contrast flush. Current OCT systems utilizing a rapid automated pull back system can image vessels at rate of 20mm/s minimizing the flush time required during image acquisition. The procedures are performed via a 6F guiding catheter with low complication rates.6 OCT provides an image resolution of 15 µm to a depth of 2-3mm.7 As result, far clearer images of the vessel lumen and wall, superficial coronary plaque components and endovascular stents are generated when compared to IVUS. Unlike with IVUS, OCT cannot measure total plaque burden because of its relatively shallow tissue penetration.

Among intravascular imaging modalities, OCT provides uniquely detailed images of superficial coronary plaque components. Based on histological analysis of autopsy specimens, there are three general plaque types: fibrous, fibro-calcific and lipid rich. All three plaque types can be accurately identified by OCT.8-9 The high resolution of OCT permits identifications of plaque characteristics that predispose to rupture including thin fibrous caps, large lipid cores and accumulation of macrophages. In addition to its role in characterizing atherosclerotic plaques, OCT can in theory provide important information to guide coronary interventions. Prior to PCI, OCT can be utilized to measure the reference vessel diameter, minimal luminal diameter and the length of the target lesion. OCT is potentially more accurate than IVUS at determining lesion severity although it is unlikely to replace FFR.10 Lesion characteristics such as lipid and calcium content can be defined prior to the intervention and these findings can be predictive of post-procedure myocardial infarction.11 Post-intervention, OCT can be used to identify stent malapposition, tissue prolapse and both in-stent and edge dissection with higher sensitivity than IVUS.12-14 OCT is of specific benefit in assessing stent apposition with overlapping stents.15 It is important to note that the clinical significance of malapposition and dissection as visualized by OCT has not been firmly established. In small studies, malapposition of stents appears to correlate with poor endotheliziation of the stent and may be a risk factor for late stent thrombosis.16-17 During long-term follow-up after PCI, OCT has been utilized to assess the degree of late acquired malapposition, strut tissue coverage and neointimal hyperplasia. 

To date, OCT has been predominantly utilized as a research tool to study coronary artery pathophysiology and response to endovascular intervention. Vulnerable plaques that are at high risk of precipitating an acute coronary event have several histologic features that distinguish them from more stable coronary plaques. These include thin fibrous caps (<65µm), large lipid cores (more than 40% of the overall plaque volume), and increased infiltration of macrophages into the plaque cap.18 Because OCT can readily identify several of these features, most importantly fibrous cap thickness, it has become a useful research tool in the study of vulnerable plaques.19-22 OCT studies have shown the morphologic features of plaques that precipitate acute coronary syndromes (ACS) and features that predict plaque progression.23 Other studies have revealed the changes that occur in response to medical therapy.24-26 OCT has also been utilized as research tool to compare various stent platforms and to study novel stent platforms in terms of stent deployment, endotheliziation and in-stent restenosis.27-30 In addition, OCT has proven useful in characterizing the different pathological processes resulting in stent restenosis and stent thrombosis. For example, OCT studies have shown that late in-stent restenosis, unlike early in stent restenosis, results from de novo atheroschlerosis rather than neointimal hyperplasia.31 Because of the ten-fold higher resolution possible with OCT, stent strut coverage and the underlying mechanism of neoatherosclerosis can be better analyzed with the use OCT than with IVUS. As with the other intracoronary details visualized by OCT, the clinical significance of stent strut coverage has not been established.

Unlike with IVUS, there have been no prospective studies of clinical outcomes in patients undergoing OCT. In the absence of such data, there are currently no definite indications for the use of OCT in the clinical setting.32 The potential clinical indications for OCT likely parallel IVUS and include delineation of angiographically uncertain lesions, evaluation for allograft vasculopathy, lesion assessment pre-PCI, and evaluation for complications and adequate stent deployment post-PCI. OCT may provide a significant advantage over IVUS for post-PCI stent evaluation as it generates far clearer images of stent strut apposition, expansion, and complications such as dissections. For patients presenting with in stent restenosis or thrombosis, OCT would likely be more accurate than IVUS at defining the underlying etiology. The use of OCT has clarified our understanding of coronary biology and intervention but outcome date showing the clinical significance of OCT findings are lacking. Unless OCT is to remain a research tool only, prospective trial and registry date showing an impact on clinically meaningful outcomes are needed.


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Clinical Topics: Invasive Cardiovascular Angiography and Intervention

Keywords: Allografts, Stents, Thrombosis, Vascular Diseases

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