Optimization of stent deployment during percutaneous coronary interventions (PCI) is one of the most important factors in achieving favorable immediate and long-term outcomes. Intravascular ultrasound (IVUS) imaging studies demonstrated that the strongest predictor of early stent thrombosis and restenosis is the absolute degree of stent expansion characterized by the minimal stent area (MSA) after PCI [1, 2]. Stent underexpansion is poorly recognized by angiography. By achieving greater stent expansion, IVUS-guided PCI has been associated with significant reduction in the risk of major adverse coronary events (MACE) compared to angiography-guided PCI [3].

Although optical coherence tomography (OCT) has higher resolution compared to IVUS, the limited penetration depth by OCT does not allow visualization of the external elastic membrane (EEM) and assessment of the vessel size, an important measurement used in IVUS-guided trials. Whether OCT-guidance can result in a similar degree of stent expansion compared to IVUS-guided PCI has been under investigation by a series of ILUMIEN trials. In a post hoc analysis of two prospective imaging trials (ILUMIEN II), IVUS and OCT guidance demonstrated a comparable degree of stent expansion [4]. Post-PCI MSA with OCT guidance was non-inferior to that of IVUS-guidance in the first prospective randomized trial, ILUMIEN III. Procedural and 30 day MACE were also similar between the groups [5]. The goal of the next randomized study, ILUMIEN IV , is to determine whether similar clinical outcomes can be obtained after stenting guided by these imaging modalities.

In addition to stent underexpansion, OCT can detect several features of suboptimal stent deployment including edge dissection, malapposition, intrastent plaque protrusion, and reference lumen area narrowing. In-stent minimum lumen area less than 4.5 mm², distal stent edge dissection more than 200 μm, and the reference lumen area less than 4.5 mm² at either distal or proximal stent edge were independent predictors of MACE in a retrospective analysis with 1 year follow-up [6]. In contrast, percent stent expansion less than 70%, stent malapposition more than 200 μm, large plaque protrusion and proximal edge dissection more than 200 μm, were not associated with worse clinical outcomes. Multiple reports emphasize the importance of adequate high pressure postdilatation to obtain optimal stent expansion in order to reduce the risks of complications [7]. It is important to select a technique that allows for avoidance of unwanted and potentially dangerous complications caused by vessel overstretching. Postdilatation with noncompliant balloons has been shown to improve MSA without causing deeper injury of the vessel wall with intima-media rupture [7].

Stent thrombosis (ST) is an infrequent but highly morbid complication of PCI with mortality rates of 10–20% during the first year [8, 9]. PCI for ST is associated with a low rate of reperfusion and high rates of recurrent ST, myocardial infarction (MI), and death [10–12]. Unfavorable long-term outcomes with a high mortality and ST recurrence rate of 26% were observed after a first definite ST [9]. Premature discontinuation of antiplatelet therapy, renal failure, bifurcation lesion PCI, diabetes, and low ejection fraction were identified as predictors of future thrombotic events [13]. Procedure-related factors including stent underexpansion and residual reference segment stenosis have been shown to play a major role in early ST, which occurs within 1 month of stent implantation [14, 15]. Impaired re-endothelialization, incomplete stent apposition or fracture, hypersensitivity to the drug-eluting stent (DES) polymer, chronic inflammation with outward remodeling, and neoatherosclerotic plaque rupture play important roles in late (1–12 month after stent implantation) and very late ST (more than 12 month after stenting) [16, 17]. Increased risk of ST is also associated with inadequate antiplatelet therapy and/or resistance to antiplatelet treatment [18, 19]. OCT imaging represents a unique tool for ST diagnosis and evaluation. It can help understand whether a procedure-related mechanism is involved in ST and select the most appropriate treatment strategy. Stent underexpansion and strut malapposition are the most common procedural factors leading to ST. In addition, the length of the segment with uncovered struts by OCT was an independent predictor of late stent thrombosis after DES implantation [17]. Therefore, efforts should be made to ensure optimal stent expansion and apposition after stenting. OCT can help identify and correct suboptimal stent implantation, which can prevent ST recurrence in future.

Although introduction of DES dramatically reduced the occurrence of aggressive neointimal proliferation, treatment of in-stent restenosis (ISR) after stent implantation remains a major clinical problem [20]. Treatment of patients with ISR lesions includes conventional coronary balloon angioplasty (BA), cutting and scoring balloon therapy, stenting, atherectomy, and vascular brachytherapy. Intrastent plaque morphology assessment with OCT imaging can positively impact outcomes by optimizing PCI in complex lesions. OCT imaging can provide unique insights into the underlying substrates of ISR by detailed visualization of neointimal tissue and select the best treatment strategy for the patient and to make the decision whether another stent implantation is necessary. While normal neointimal tissue in DES mainly consists of smooth muscle cells and extracellular matrix, recent pathological and imaging studies provided convincing evidence for the presence of atherosclerosis within stent neointima, or neoatherosclerosis [21–24]. Neoatherosclerosis is characterized by neointimal formation of lipid-rich plaque and calcification and may represent an important mechanism of DES failure. OCT imaging can provide unique insights into the etiology of in-stent restenosis and help select the most appropriate treatment strategy. Rotational atherectomy has been successfully used in calcified neoatherosclerosis causing undilatable in-stent restenosis [25, 26]. In addition, OCT imaging before stenting can assess the risks for periprocedural myocardial infarction. The unstable features of neoatherosclerosis including thin-cap fibroatheroma and plaque rupture were the most common findings in patients with periprocedural myocardial infarction [27].