Intracoronary imaging techniques have been developed to provide complementary, more accurate measurements and details to coronary angiography in patients undergoing complex PCI. Compared with IVUS, OCT is superior in the assessment of unstable plaque features, including the presence of thrombus, rupture plaques, thin-cap fibroatheroma (TCFA) and pools of superficial macrophages [41, 42]. In addition, OCT provides superior details on the immediate results of stent implantation (vessel injury, tissue prolapse, stent malapposition and stent geometry distortion). Finally, OCT at the follow-up time is more accurate in assessing the remaining thrombus and the amount and type of tissue growing into the stent, discriminating between covered and uncovered struts, neointimal formation and newly developing atherosclerosis [42–44]. Based on these unique characteristics OCT can critically contribute to a better mechanistic understanding of the pathophysiology of ST and to the identification of surrogate markers, anticipating the risk.

The multi-factorial nature of ST and the high risk related to this event favors a fully informative, accurate assessment of the culprit lesion and vessel at time of acute presentation, through high resolution intravascular imaging modalities. Similarly to any acute MI, ST requires immediate thrombus removal and quick restoration of an effective TIMI flow as the goals of primary interventions. After vessel recanalization, OCT may be used for guiding a more effective thrombus removal, via mechanical and/or additional pharmacological treatments. Further, OCT may help to recognize the stent-related factors (if any) responsible for ST (malapposition, under expansion, incomplete coverage, lipid laden neointima) and to select, based upon the imaging details, the most effective treatment to avoid recurrences. Indeed, while stent under-sizing, under-expansion and other suboptimal PCI results are usually observed in the acute/subacute ST cases, the main mechanisms involved in both late/very late ST are usually linked to the presence of uncovered stent struts and/or neoatherosclerosis.

The planning for imaging in a patient arriving at the hospital with possible ST should start before the patient’s arrival at the Cath Lab. If the patient was treated with stent(s) in the same hospital all clinical and procedural information (co-morbidities, clinical presentation at the index procedure, stent type and size, eventually complications at the stent implant) has to be made available to the operator before primary PCI. In patients arriving with ST and TIMI 0–1 flow grade or large filling defect, thrombus aspiration is at the frontline of treatment for reestablishing the flow and allowing subsequent histopathology analysis of the aspirated thrombotic material. In this case thrombus aspiration has to be performed until coronary flow is reestablished and in any case before the OCT imaging pullback. It is relevant to the safety of the patient and equally relevant for having high quality OCT imaging, not to proceed with the OCT scan when the vessel is occluded with no distal flow or in the presence of an extremely high thrombus burden anticipated at the angiography views. Major arrhythmias and images unreadable due to the excess of remaining thrombus are the main reasons for adopting a dedicated strategy. In the presence of a large remaining thrombus the imaging catheter can be occlusive. One possible trick to anticipating if an OCT pullback can be performed with enough quality is a quick check in angiography with a few cc of contrast, maintaining the aspiration catheter at the culprit site. If the contrast passes freely the probability that it will not be occlusive with the image catheter is high and you may proceed with the OCT pullback. A combined use of IVUS and OCT when possible is recommended, since both imaging techniques may provide complementary data on mechanisms of ST. In these cases it seems wise to start with IVUS, since no contrast flushing and no selective coronary intubation is required. IVUS may add unique information on vessel remodeling that cannot be provided by OCT. It is important to remember that OCT imaging may increase the overall amount of procedural contrast. In the presence of hemodynamic instability, frequently observed during ST, the total amount of contrast needs to be strictly monitored. When flow is reestablished the operator should take time to analyze the imaging data, understand the main causative mechanism and the possible contributing factors and to tailor the treatment strategy. Since the time interval between the baseline assessment and the final post-intervention pullbacks can be wide, the OCT catheter should be maintained wet in saline solution to avoid fibrin deposition on the imaging element and contrast medium crystallization.