Until today it was not possible to reliably assess the function of a new bypass, i.e., to measure myocardial perfusion. Measuring intramyocardial blood flow directly after revascularisation would greatly improve the assessment of graft function. With transthoracic contrast echocardiography, myocardial perfusion can be reliably assessed, as it has been shown in numerous studies. In the first part of the study the investigators will clarify methodological aspects of contrast echocardiography. In the second part they will analyze the validity of contrast TEE in the operating room.