The cerebral perfusion of infants with congenital heart defects is a critical determinant during cardiac surgery. The majority of these interventions is performed during deep or moderate hypothermia (25-32°C) with reduced blood flow of the heart-lung-machine (HLM). Assuming a symmetrical anatomy of the cerebral arteries (C. Willisii) a single vessel (T. brachiocephalicus) is then supplying the cerebral blood flow. Using contrast-enhanced ultrasound the cerebral blood flow will be assessed during cardiac surgery interventions.