Dr. Francesco Prati,
Frequency domain optical coherence tomography (FD-OCT) is an optical analogue of intravascular ultrasound (IVUS), based on infra-red light emission capable of defining the superficial plaque components at a very high resolution, in the range of 20 microns. This excellent definition of the coronary arteries is however obtained at the expense of a reduced penetration that does not exceed 1.3 mm. There is general consensus that FD-OCT is a breakthrough technology that will provide new insights into the pathophysiology of acute coronary syndromes and very likely ameliorate the results of coronary interventions. OCT identifies lipid –necrotic cores, calcified and fibrotic tissue components with high accuracy. Also the ability to reveal presence of coronary plaques with superimposed thrombosis makes it possible to distinguish between different pathogenic mechanisms of plaque rupture, such as erosion vs ulceration. The adoption of post-processing software to be placed in a selected region of interest can facilitate the study of local inflammation by showing and quantifying inflammatory cells such as macrophages. The ability to obtain such information represents a breakthrough in cardiology to better understand the pathophysiology of coronary events. Unfortunately the impossibility to fully penetrate the plaque represents a limit. For instance the signal attenuation induced by clusters of macrophages in the superficial layers can significantly affect the assessment of plaque components. In some circumstances, presence of lipid pools that also lead to a marked attenuation of the signal, can be wrongly diagnosed. A number of IVUS studies attempted to characterize the appearance of vulnerable plaques. FD-OCT, due to its high accuracy in the detection of superficial plaque components, can directly measure the fibrous cap thickness and the extension of a lipid pool. These features together with the detection of local inflammation promise to detect plaques having a high risk of a rupture. There is a general consensus that nowadays the role of FD-OCT is to better understand the mechanism of local thrombosis as the cause of coronary events. It is likely that this precious information will offer in the future the possibly to identify and treat vulnerable plaques.
FD-OCT has the potential to provide incisive guiding information for coronary interventions. To this end, novel criteria of optimal stent positioning with explicit quantitative thresholds have to be applied. Besides enabling the comparison of minimal stent area with the reference area, which is the most-often used IVUS criterion for optimal stent expansion, FD-OCT can identify details such as stent under-expansion, malapposition, uneven stent strut distribution, intra-stent thrombotic formations, and dissections at the edges and inside the stents, with a level of accuracy unmatched by IVUS. There is still some uncertainty regarding the OCT criteria to be applied to address stent positioning. There is also debate regarding whether and how to use FD-OCT pre-intervention. Having said that, preliminary data from the multicenter CLI-OPCI registry have been promising indeed. The study was designed to verify whether the use of OCT can improve the 1-year composite event of cardiac death or non-fatal myocardial infarction after PCI in a real-world population. Data were matched with those obtained from a control group selected by applying propensity score matching. In the OCT-guidance group, the combined incidence of acute myocardial infarction and cardiac death was significantly lower. Complex interventions such as treatment of left main lesions, bifurcations or unfavorable clinical settings, such as acute coronary syndromes or stent restenosis, represent possible indications for FD-OCT guidance. Stent restenosis can be either caused by neointimal growth that progressively reduces the lumen or by formation of atherosclerotic plaque, with this latter event being particularly common for drug eluting stents. The use of OCT can easily distinguish the two mechanisms of stent restenosis and can used to facilitate its inherent treatment.
Clinical value of optical coherence tomography
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