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thrombosis [71, 78–80]. However, biological and mechanical factors (including levels of circulating endothelial progenitor cells or regional shear stress) can also have a role in neointimal healing and differences in percentage coverage cannot always entirely explain clinically overt stent thrombosis [80].

      Tissue protrusion

      In OCT, plaque protrusion is characterized by a smooth surface and no signal attenuation, and thrombus protrusion by irregular surface and significant signal attenuation. OCT can provide better images and clearer visualization of tissue protrusion compared with IVUS. Tissue protrusion is more frequently in the culprit lesions of acute coronary syndromes, as unstable lesions contain soft lipid tissue and thrombi. In a multicenter registry, including 780 patients, 50% ACS, irregular protrusion was more common in patients treated for MI and was an independent predictor of target lesion revascularization. In fact, tissue prolapse has a worse clinical impact in ACS as suggested in CLI‐OPCI substudies [54].

      Vascular injury: dissections

      OCT is a very sensitive tool in detecting micro‐dissections and subclinical dissections [81]. Dissections occur more frequently when the plaque at the edge of the stent is fibro‐calcific or lipid‐rich than when is fibrous [82]. Distal stent edge dissections (>200 μm) in CLI OPCI II Study revealed by OCT emerged as an independent predictor of MACE; minimal dissection, at the edge or instent, were not associated with adverse effects [54].

      Guidance of complex lesion treatment: bifurcations, calcified, CTO, long and distal, ostial lesions

      Bifurcations are coronary lesions with high rates of acute and late stent failure. Knowing the reference diameter of the vessel distal and proximal to the side branch is critical in the correct sizing of stents and post‐dilatation balloons. In both simple (one stent) and especially complex (two‐stents) strategies of bifurcation stenting, OCT showed that the rate of malapposed struts is significantly higher at the side branch ostium than in the vessel side opposite to the ostium [83]. In a series of 45 lesions, OCT showed that the persistence of malapposition was as high as 43%, despite consistent use of kissing balloon dilatation and proximal optimization technique [84]. The overall rate of malapposed struts was significantly higher in the lesions treated with angiography‐guided PCI than in those undergoing OCT‐guided PCI [84].

The position where the guidewire re‐crosses the stent struts into the side branch (i.e. proximal, mid, or distal cell) has been demonstrated to be one of the most important factors driving full strut apposition and larger lumen in the side branch ostium after balloon dilatation. Crossing into the side branch through a proximal cell provides no scaffolding of the side branch ostium and leaves many struts unapposed near the carina, reducing the strut‐free side branch ostial area. It has been suggested to use OCT to confirm re‐crossing the wire through the most distal cell of the main vessel stent in order to efficiently widen the opening at the side branch ostium [85]. The feasibility and effectiveness of OCT‐guided stent re‐crossing compared with angiography guidance have been evaluated in 52 patients [86]. The OCT‐guided group showed a significantly lower number of malapposed stent struts, especially in the quadrants toward the side branch ostium (9 vs 42%, p <0.0001).

      Three‐dimensional (3D) reconstruction of OCT images is also potentially useful to better understand wire positioning and lumen expansion in bifurcation stenting [87–91]. The clinical application of high quality off‐line 3D‐OCT to optimize side branch opening by identifying the configuration of overhanging struts in front of the side branch ostium according to the presence of the link between hoops at the carina and the appropriate distal cell for the re‐crossing position has been evaluated in 22 patients [92]. This study showed that 3D‐ OCT confirmation of the re‐crossing into the jailed side branch is feasible during PCI and helps to achieve distal rewiring and favorable stent positioning against the side branch ostium, leading to reduction in incomplete strut apposition and potentially better clinical outcomes [76]. Finally, OCT has also been used to assess the procedural success of dedicated side branch stents compared with conventional strategies [93].

Calcified lesions still represent a major challenge for PCI resulting in difficulties to cross lesions, to advance and expand balloons and stents and to identify a plaque free landing zone for safe stent placement, impeding an optimal result (see Figure 9.6). Angiography is limited in its ability to determine the extent and type of calcium present in a lesion. Characterization of the calcific burden is crucial in determining if treatment with an ablative device is needed to adequately achieve full stent optimization. Furthermore, intravascular imaging helps to identify the correct landing zones so as to minimize the risk of edge dissections. OCT clearly defines the calcium morphology into three distinct categories: deep, superficial or nodular. Stent under‐expansion in heavily calcified lesions is associated with a high rate of target vessel failure and future ischemic events.

Figure 9.6a OCT cross‐section demonstrates a protruding nodule with an MLA of 12.95mm². The OCT cross‐section is co‐registered with the angiogram in the upper, left corner (red arrow in the left image).

Figure 9.6b OCT of the proximal LAD revealed a lesion with an MLA of 3.55mm², despite a mild angiographic appearance.

Figure 9.6c OCT proximal to the bifurcation demonstrates a calcified lesion with >270° arc of superficial calcium with a protruding nodule. Calcium thickness measures 0.72mm.

Figure 9.6d OCT at the bifurcation of the LAD and 1^st diagonal branch. It is important to note the location of bifurcations on the co‐registered angiogram to assist in precise stent implantation.

Figure 9.6e OCT following stent implantation demonstrating optimal stent expansion. In the longitudinal profile of Panel B, stent rendering has been activated. Final angiogram is shown on the right panel.

      Dedicated devices to approach coronary calcific lesions include rotational and orbital atherectomy (RA‐OA), but because of their complexity, these methods are used in a small minority of the patients in need [94]. Cutting and scoring balloons offer possible alternatives but their deliverability and effectiveness are suboptimal and they achieved inferior results in a recent randomized trial against RA. Intravascular lithotripsy (IVL) has the potential to overcome some of the limitations of the aforementioned tools. Characterization of the longitudinal and circumferential calcium distribution with OCT plays a pivotal role in the identification of calcified lesion requiring dedicated treatment devices. OCT analysis is highly accurate in assessing calcium including thickness, a clear advantage over IVUS where shadowing helps to detect calcium at first glance but precludes assessment of other features. Calcium thickness is an important determinant of calcium fracture with PCI [95]. Superficial calcium can be stratified based on the calcium‐volume index (CVI) score, integrating the depth, length, and arc of calcium.

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