Interventional Cardiology. Группа авторов

and the arc is >180^°, ablative therapy using rotational atherectomy, orbital atherectomy, intravascular lithotripsy, or excimer laser atherectomy therapies should be considered. While OCT may miss some deep calcium due to limited penetration, this may not be a clinically relevant limitation since superficial calcium is the true obstacle to expansion. Nodular calcium is distinctly difficult to treat for a myriad of reasons. These lesions often do not yield to traditional balloon techniques, as it is common for the balloon to only expand on the non‐calcified side of the wall leaving the nodule untreated. The other component is that they often behave like a thin cap fibroatheroma and become unstable with platelet aggregation causing further complications. OCT can detect more reliably the presence of calcium fractures after IVL or atherectomy because of its greater resolution. Occasionally, there will be instances where the OCT catheter is unable to cross a calcified lesion and pre‐treatment is necessary. In these circumstances it is imperative to understand that prior to deploying a stent, OCT should be performed after lesion preparation to accurately assess fracture and vessel dimensions, ensuring full stent expansion. In our experience the application of a standardized algorithm with intravascular imaging guidance, mainly OCT, of IVL facilitated second generation DES expansion delivers excellent immediate lumen expansion and patient outcome [96].

      For chronic total occlusions, there is a relative contraindication to inject contrast at high speed and pressure before wire crossing or if the wire has followed a possible subintimal position because of the risk of inducing a dangerous distal dissection. IVUS is definitely preferable to confirm the intraluminal position of the wire distally and in the occlusion body and decide the length of the stented segment and its size. After stenting, however, OCT can also be used to detect distal dissections, double channels, distal plaques and, especially, stent expansion and strut apposition.

      Long lesions are often at risk of suboptimal expansion, side branch compromise, slow flow and high restenosis rates when a full metal jacket approach is used. OCT can identify relatively healthy segment where a stent can be spared and exclude nasty deep dissections at the edges after stent implantation. The long tip of the prior generation OCT catheters can create difficulties during imaging of very distal segments of the arteries, and has been shortened by 5 mm for the new Dragonfly OpStar catheter for improved deliverability.

      In aorto‐ostial lesions a difficulty is created by the need to clear blood, requiring intubation of the catheter that may shield the wall and preclude assessment of the ostium. Occasionally, rapid injections with a large guiding catheter immediately outside the ostium can obtain acceptable images.

      Assessment at follow‐up

      The possibility to detect thin layers of tissue coverage and neointima formation in DES over time is an interesting application of OCT to investigate the underlying mechanisms implicated in stent failure, such as stent thrombosis, in‐stent restenosis, and neoatherosclerosis. Delayed neointimal healing has been considered a possible underlying substrate of fatal stent thrombosis [78,97]. The percentage of uncovered stent struts represents the best morphometric predictor of late DES thrombosis and the risk increases with the percentage of uncovered stent struts per section [78].

An important caveat is the inability of OCT to detect reendothelialization when the layer is below its axial resolution and to differentiate between neointima and other pathologic components such as fibrin or thrombus. The latter becomes an issue at very early phases after stenting, when the prevalence of struts covered by fibrin is high. Thus, DES may appear completely covered already 1–3 days after implantation, but with fibrin instead of a mature neointima, with smooth muscle cells and matrix covered by endothelium. The low discriminative power of OCT results in false coverage rates of 45–76%in the first weeks after DES implantation [98]. The analysis of optical density might aid discriminating between neointima and fibrin [99, 100]. The greatest interest, however, is to assess intimal coverage at late follow‐up, when the prevalence of fibrin‐covered struts is low and the practical impact of this limitation is minimal. Significant differences exist in stent strut coverage and apposition between various DES at 3–12 months post implantation and this could explain the different clinical results obtained with second generation compared with first generation DES [100]. Strut thickness and biocompatibility of the polymer that could trigger inflammatory reactions have been compared, offering a possible surrogate for MACE, so low with modern stents to make studies with clinical endpoints in need of a too large population to be feasible.

      In‐stent restenosis and neoatherosclerosis

      OCT offers data concerning the underlying pathophysiology that contributes to in‐stent restenosis (ISR), such as stent underexpansion, strut fracture, strut distribution, neointimal hyperplasia and neoatherosclerosis [101]. Unlike with IVUS that may miss poorly echogenic neointima OCT can perfectly delineate the lumen shape and characteristics of neointimal tissue. Yet, because of reduced tissue penetration of OCT, plaque behind the stent struts is poorly visualized [102]. OCT accurately measures the percentage of neointimal volume obstruction and has become a standard in trials assessing the efficacy and safety of novel stents [103]. Various ISR tissue patterns have been defined based on optical homogeneity (homogenous, heterogeneous, and layered), restenotic tissue backscatter (high, low), visibility of microvessels, lumen shape (regular, irregular), and the presence of intraluminal components [104]. Bare metal stents are associated with more homogenous patterns with low echogenicity components around struts representing a counterpart of the giant cell reaction [105,106]. The typical pseudoaneurysms described by Raeber et al around first generation paclitaxel and sirolimus eluting stents with IVUS are rare with modern thin‐strut DES. On the contrary, second generation DES are not spared by a pathological phenomenon such as neoatherosclerosis, well studied by OCT. Emerging data claim the relevance of late de novo neoatherosclerosis in mimicking ISR or thrombosis [107,108].

      In ISR, OCT can also be used to precisely follow the irregular lumen contour after cutting balloon and to guide cutting balloon sizing. In particular, OCT can confirm if cutting balloons have scored the plaque up to the stent at multiple points, which greatly facilitates extrusion and lumen expansion. This is possible because metal struts are powerful enough light reflectors to be visualized through very thick plaques. Through an OCT‐guided cutting balloon strategy, intimal hyperplasia was reduced from 69% to 25% in the stented segment with the minimal lumen area allowing better preparation for drug‐eluting balloon dilatation [68].

      Optimal stent expansion can frequently be compromised when there is a calcium burden that has not been adequately treated or fractured prior to stent deployment. OCT is able to accurately assess the MSA which is not achievable with angiography alone. Treatment of these lesions remains challenging and may require frequent non‐compliant balloon inflations with escalating size, excimer laser coronary atherectomy (ELCA) or IVL. Unfortunately, it is found that despite best efforts with these therapies the result on final OCT shows a less than ideal MSA. It is further confirmation that intravascular imaging pre‐treatment for de novo lesions remains instrumental in avoiding such scenarios.

      Neo‐intimal hyperplasia and neoatherosclerosis can be comprised of mixed morphology with fibrotic, calcific or lipid components. OCT is significant in guiding the direct therapy of these lesions. If it is determined that there is only one layer of stent and it is fibrotic or lipidic, it is recommended to pre‐treat with a balloon and implant another stent. If two or more layers are visualized or if plaque morphology is calcific in nature, treatment options may include ELCA, rotational atherectomy, non‐compliant balloon or vascular brachytherapy.

      Imaging with OCT confirms the underlying mechanisms for ISR and allows for precise tailoring of appropriate therapy for each type of ISR.

      Bioabsorbable vascular scaffolds

      Bioresorbable vascular scaffold represented a revolutionary and ground‐breaking concept in interventional cardiology with early registries suggesting superiority of BVS over DES in restoring vasomotion and remodelling. OCT has been used since the first implants of BVS to study the vessel wall response [110,111] and the timing of the resorption process [112]. OCT performed in two groups of patients respectively at 6 and 24 months (B1, n=45) and 12 and 36 months (B2, n=56) in ABSORB cohort