This editorial refers to 'Optical coherence tomography imaging during percutaneous coronary intervention impacts physician decision-making: ILUMIEN I study' † , by W. Wijns et al., on page 3346. The field of intracoronary imaging emerged to overcome the drawbacks of coronary angiography. The visual evaluation of lesion severity of coronary stenosis by angiography remains insufficient for accurate diagnosis of the vessel indices and its estimate of functional significance. 1 Intracoronary

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... was introduced to optimize the outcome of percutaneous coronary intervention (PCI), aiming to reduce short-and long-term cardiovascular events. Intracoronary imaging systems, such as intravascular ultrasound (IVUS) and optical coherence tomography (OCT), offer an inside look into the artery, which enables precision in evaluation of the vessel dimension, plaque composition, and degree of narrowing, and allows optimization of management by selecting the appropriate method of intervention, such as pre-dilatation with balloon vs. direct stenting, use of atheroablative devices, 2 stent sizing, and optimization of stent deployment and lesion coverage. IVUS-guided PCI has been in use for over two decades and has demonstrated a major contribution in regard to the importance of stent expansion and apposition that resulted in reduction of stent thrombosis. Overall, numerous studies showed that intracoronary imaging is safe and feasible for guidance of coronary interventions. 3,4 However, the lack of adequately powered randomized clinical trials to support improvement in outcome, difficulties in image interpretation, and cost limited its utilization to guide routine PCI. Nevertheless, a few meta-analyses suggest that IVUS-guided stent placement could improve clinical outcome 5 by reducing stent thrombosis, restenosis, and repeat revascularization. 6 Other indications such as assessment of intermediate lesions by IVUS showed only modest correlation to fractional flow reserve (FFR). 7 Although intracoronary imaging to detect plaque morphology may impact the physician decisionmaking process, it has not proven to improve clinical outcomes. 8, 9 In the last decade, the focus has shifted to physiological assessment of intermediate lesions using FFR, which has proven to be effective in deciding whether intermediate lesions should be treated by PCI or deferred in three randomized clinical trials. 10 -13 The professional societies differentiated the guidelines' class level for the physiological (FFR) and anatomical (invasive imaging IVUS) assessments. FFR has a higher class level than IVUS for the evaluation of intermediate coronary lesions with a class of recommendation IIa, level of evidence A in the American guidelines and recommendation Ia, level of evidence A in the European guidelines. FFR-guided multivessel PCI has a similar indication to IVUS, recommendation II(a), level of evidence B. For IVUS, the American College of Cardiology Foundation/American Heart Association/Society for Cardiac Angiography and Interventions PCI guidelines from 2011 14 gave a class IIa, level of evidence B for the diagnostic assessment with IVUS in the case of: (i) angiographically indeterminante left main coronary artery disease (LMCAD); (ii) progressive allograft vasculopathy after transplantation; and (iii) in-stent restenosis. However, in the case of stent thrombosis, the class of recommendation with IVUS is class IIb (level of evidence C). Moreover, routine lesion assessment is not recommended when revascularization with PCI or coronary artery bypass grafting is not being contemplated (level of evidence C). Concerning the European Society of Cardiology/European Association for Cardio-Thoracic Surgery guidelines on myocardial revascularization from 2014, 15 the use of IVUS is class IIa: (i) in selected patients to optimize stent implantation (level of evidence B); (ii) for assessment of lesion severity and optimization of treatment of unprotected left main lesions (level of evidence B); and (iii) to assess mechanisms of stent failure (level of evidence C). Optical coherence tomography entered the stage as an upgrade of intracoronary imaging, providing a resolution that is greater than IVUS: 10 -15 mm axial resolution and 20 -25 mm lateral and out-of-plane resolutions, with faster acquisition of seconds vs. minutes by IVUS. Although the image is spectacular, and the ease of use and interpretation has been improved, the appropriate role of OCT in routine clinical decision-making has not been established because The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology. †