Identification of calcium can affect an interventional cardiologist’s decision making. In fact, in the presence of circumferential calcification, direct stenting should be avoided and strategy can span from a careful pre-dilatation to test expansion, to cutting balloons for very short calcific rings or to pre-treatment with a rotational atherectomy [11, 12].

Both IVUS and OCT can identify calcium with accuracy [13]. However, IVUS is unable to measure its thickness due to a shadowing effect [14]. Infrared light penetrates calcium better, enabling measurement of superficial calcific components with a thickness less than 1.0–1.3 mm [15, 16]. Only the less common deep intra-plaque calcium deposits cannot be addressed with OCT.

As a relatively new concept stent positioning should be performed by aiming for complete coverage of the lipid pool [17], avoiding the placement of stent edges over lipid formations, that can potentially lead to stent thrombosis and plaque embolization [18]. OCT identifies lipid pools with high accuracy and its pre-intervention use can also be encouraged to reduce the risk of embolization of plaque components.

Hazy and intermediate lesions on angiogram are considered a grey-zone for interventional cardiologists [19]. OCT allows detailed analysis of plaque morphology and lumen area, being therefore very instrumental in guiding interventions and helping decide whether to treat coronary narrowings. Unlike other intravascular imaging modalities, OCT has the unique ability to identify thrombi and measure the amount of thrombus. The “OCT defined thrombus score” is a semi-quantitative assessment of a thrombus [20, 21], that can be used in cases of angiographically hazy lesions. These kinds of lesions often appear as ruptured plaques with large amounts of thrombus attached to the site of disruption [19].

Coronary angiograms of patients with acute coronary syndrome may show one or more non-significant lesions, exhibiting only haziness on angiogram. This can pose a problem for the interventional cardiologist. In fact, misinterpretation of culprit lesions may cause ischemic events in the short and mid-term follow-up. Under these circumstances the decision to proceed with treatment rests more on morphologic observations than on the absolute measurement of lumen area.

The fact that lumen contours are easily obtained enables the on-line application of an automated algorithm, that facilitates operator decision making by displaying the reconstructed segment in a longitudinal view.

A number of IVUS studies attempted to characterize the appearance of vulnerable plaque. Recently, the PROSPECT trial, based on a signal radiofrequency analysis of the IVUS backscatter, for the first time showed that intravascular imaging modalities can identify plaque with a higher risk of events at 3 years [22]. OCT, due to its high accuracy in the detection of superficial plaque components, can potentially identify plaque at risk of rupture. In fact, OCT can address many features that have been related to plaque vulnerability such as the presence of thin fibrous cap, extension of lipid pools [5], micro-vessels [23] and inflammatory cells [24]. However dedicated soft-wares, based on the acoustic properties of the OCT signal, are needed to improve identification of the local signs of inflammation [24].

To better understand the change of plaque composition in response to specific treatment, serial studies should be carried out with the specific goal of identifying the same location at different time points. For this task, dedicated three-dimensional soft-wares can be adopted. The recently developed “carpet view software” which unfolds the vessel segment, reconstructing it as an open structure, will likely improve the off-line serial comparisons of target plaques and enable the matching of different imaging modalities [25]. Many experts have suggested that the identification of a vulnerable plaque requires the combination of OCT and other imaging modalities such as IVUS or near infrared spectroscopy to better characterize the plaque and obtain information on deep lesion components [26, 27].

The multicenter CLI-OPCI study [34], addressed the role of OCT guidance. The registry compared the clinical outcome of 335 patients with OCT guided intervention to those from a control group by means of a propensity score adjustment. OCT guidance was found to improve the 1-year composite event of cardiac death or non-fatal myocardial infarction after PCI in a real-world population. The study also addressed the issue of how to treat OCT findings indicative of suboptimal stent deployment. In 34.7 % of the stented segments, it was determined, based on the OCT results, that further intervention with either balloon dilation (22.3 %) or additional stenting (12.4 %) was needed. The final conclusion of the study was that explicit quantitative OCT thresholds are required in order to improve clinical outcome of patients undergoing PCI.

OCT studies, in patients with acute coronary syndrome, have shown that in-stent tissue protrusion due to the presence of residual thrombus, is a common finding [29]. Recent data revealed that residual intra-stent thrombus is related to peri-procedural myocardial infarction if left untreated [29]. Preliminary data showed that additional OCT-driven in-stent balloon dilatation can significantly reduce the in-stent thrombus area percentage without worsening the microcirculatory indexes [35].

As another crucial application, OCT can clarify mechanisms of restenosis and thrombosis early or late after the index procedure, guiding repeat revascularization and thus minimizing the risk of additional adverse events.

Assessment of stent under-expansion by OCT can be obtained by comparing the minimal stent area with the reference lumen area. Additionally, a threshold of absolute minimum lumen cross-sectional area within the stent could be applied, with an area of at least 5.0–5.5 mm² previously advocated as the target minimum stent area to prevent failure [36].

A recent study from our group, addressed the incidence of suboptimal OCT results in 21 consecutive patients exhibiting sub-acute thrombosis. The patients were matched 1:2 with a control group of 42 patients from the Rome Heart Research core-lab database [37]. OCT showed that minimum lumen area and minimum stent area measurements were significantly smaller in the stent thrombosis group together with a higher frequency of stent under-expansion, edge dissection and reference lumen narrowing.

Stent follow-up, delayed healing and poor endothelialization are common findings in the pathological specimens of vessels treated with drug eluting stent (DES) [38], and recent post-mortem studies demonstrated that late-stent thrombosis was strongly associated with the ratio of uncovered/total stent struts [39].

Several experimental and clinical studies have demonstrated the high correlation between OCT and histological measurements of tissue coverage of stent struts [40, 41], although OCT is unable to identify the endothelium [5]. In a sub-analysis of the ODESSA trial, 8 % of the stented segments with no detectable neo-intima by IVUS were found to have neo-intimal coverage by OCT [42]. Follow-up OCT data revealed that most drug eluting stents, including those with a biodegradable polymer, were covered with thin neointima, but few revealed complete coverage [42–44]. Incomplete stent apposition without neo-intimal hyperplasia was significantly associated with the presence of an OCT-detected thrombus at the follow-up, and may constitute a potent substrate for late-stent thrombosis [45]; however, so far a subclinical thrombus has not been related to the risk of major adverse cardiac events [46]. Furthermore, incomplete stent apposition and the absence of tissue coverage detected by OCT are more frequently found in the setting of acute coronary syndromes, particularly after DES deployment [47].

Optical coherence tomography can be applied for the treatment of late in-stent restenosis because the strong reflective power of the stent struts allows their detection through thick layers of hyperplasia, allowing optimal sizing of high-pressure balloons to correct under-expansion and facilitating the use of cutting balloons [48].

OCT can image neo-intimal tissue distinguishing among several types on the base of OCT signal attenuation and backscattering. The most common type is the homogeneous form that is characterized by uniform optical properties without focal variations in the backscattering pattern [49–51].