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which remains vasoconstricted in lambs because of the low oxygen saturation of the blood supplied to the pulmonary arteries Of the left ventricular output, the majority passes into the head and upper limbs, returning to the right atrium via the superior vena cava (SVC), while a small proportion passes across the aortic isthmus to join the ductus arteriosus flow in the descending aorta Rudolph estimated that descending aortic flow accounted for approximately 40% of the combined ventricular output in the human fetus, with 25% being returned to the placenta via the umbilical veins Foramen ovale flow was calculated to comprise 20% of the combined ventricular output, with 3% being supplied to the coronary circulation Similar estimates of the distribution of the normal fetal circulation have subsequently been reported based on ultrasound measurements of blood flow made in human fetuses.3–10 However, the measurement of fetal vessel flow by ultrasound is challenging due to difficulties in accounting for variations in flow velocity across the vessel lumen, problems with making accurate vessel area measurements, and limitations in obtaining an adequate angle of insonation These limitations have resulted in limited data regarding the redistribution of flow that is expected to result from CHD in the fetus Instead, investigators have focused on other Doppler ultrasound measures, such as peak velocity or pulsatility index, to assess the impact of obstructions to flow or changes in downstream vascular resistance to help interpret fetal cardiovascular physiology in human fetuses with CHD Similarly, changes in fetal oxygenation have been inferred from Doppler assessments of changes in placental and cerebral vascular resistance, and so such data also have major limitations Magnetic Resonance Imaging Techniques for Assessing Fetal Circulatory Physiology While technical challenges arising from artifacts resulting from fetal motion and difficulties in obtaining adequate signal from small blood vessels remain, noninvasive MRI techniques for use in human pregnancies have recently been developed The data from these studies resemble the invasive flow and oximetry measurements that Rudolph first reported in fetal sheep 50 years ago This approach is currently only feasible in late-gestation fetuses and consists of cine phase contrast vessel flow measurements and oximetry based on magnetic resonance (MR) relaxometry Cine Phase Contrast Magnetic Resonance Imaging Flow Quantification As the electrocardiographic signal usually used for temporal calibration of cardiac MRI is not easily available from the fetus, alternative methods for achieving high-resolution flow measurements have been developed Metric optimized gating is a technique that uses a postprocessing solution to achieve cardiac triggering.11 To summarize, the MRI data are acquired using an artificial QRS complex, which has been programmed into the imaging acquisition, as shown in Fig 7.1 The data are then reconstructed through a range of candidate heart rates, with the heart rate that was present during that particular acquisition identified through the lack of artifact in the resulting images Examples of cine phase contrast acquisitions obtained in the ascending aorta at 1.5 and 3.0 T using metric optimized gating are shown in Fig 7.2, which demonstrates the improvement in signal-to-noise ratio with higher magnetic field strength The imaging plane is obtained in the short axis of the vessel, allowing for “through plane” quantification of flow The accuracy of the flow measurement depends on achieving a true short-axis orientation, which can be achieved by obtaining two orthogonal long-axis views using anatomic survey images through the thorax and upper abdomen Following metric optimization of the phase contrast acquisition, a region of interest is placed around the vessel using standard postprocessing software in order to quantify flow, which can be indexed to fetal volume (or weight) based on segmentation of the fetal envelope When acquiring phase contrast flow imaging it is important to have adequate spatial resolution, which entails providing at least 8 voxels across the vessel cross-section, and adequate temporal resolution—in the range of 50 ms or better