Methods All scans were performed with a Philips 3-Tesla MR scanner. Infants were fed and allowed to fall into natural sleep without the use of sedation or anesthesia. Infants were laid in a custom made cradle, with ear protection and routine monitoring. A Flex-M surface receiver coil was placed on the chest wall. No respiratory compensation techniques were used. Acquisition parameters for phase contrast (PC) and balanced fast field echo (bFFE) imaging were optimised. PC assessments of flow were performed immediately distal to the aortic and pulmonary valves, as well as in the superior vena cava (SVC) and descending aorta (DAo). External validation of phase contrast measures was performed with a flow phantom. bFFE sequences were applied for 2 chamber, 4 chamber and short axis views. A short axis stack of 5-7 slices covered the left ventricle from base to apex. Left (LVO) and right (RVO) ventricular output were assessed using CMR Tools processing software. Echocardiographic measurements of LVO were performed within 24 hours of MR imaging.