Ten-fifteen percent of acute coronary syndromes without ST-segment elevation (NSTE-ACS) are caused by intermediate lesions without signs of unstable plaque. In this subset of patients, fractional flow reserve (FFR) has some drawbacks and may not be always able to predict outcome, especially when negative (above 0.80). In this particular nique of patients, advanced imaging techniques are suggested by International guidelines. However, it is actually unknown how these techniques may impact treatment strategies. With the present study, the investigators want to characterize the mechanism of disease in this particular subset of patients through multimodality imaging (intravascular ultrasound-near infrared spectroscopy (IVUS-NIRS), optical coherence tomography (OCT)) in order to understand the proper treatment.