 Invasive X-ray coronary angiography remains the reference standard for the evaluation of coronary artery stenoses. Recently, coronary artery computed tomography angiography CorCTA has been introduced as a non-invasive method for the evaluation of coronary artery stenoses and has been shown to be highly accurate in stenosis detection when compared to invasive X-ray angiography. While invasive X-ray angiography and CorCTA evaluate morphological features of coronary arterial plaques, fractional flow reserve is an invasive measure of the hemodynamic significance of a stenosis obtained in the catheterization laboratory by measuring changes in intracoronary arterial pressure before and after maximal vasodilation induced by intracoronary adenosine. An FFR value less than 0.75 has been shown to predict ischemia in vascular beds distal to the stenosis by radionuclide perfusion modalities and has been shown to be associated with worse outcomes. Therefore, FFR is considered to be an invasive hemodynamic reference standard for the evaluation of the hemodynamic significance of coronary arterial stenoses. While intravascular ultrasound IVUS can provide additional morphological information in intermediate stenoses, it can not provide further functional information and cannot be used in more significant stenoses as the IVUS probe cannot be advanced through stenotic lesions. 