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Purpose-To characterize the MRI relaxation properties of human umbilical cord blood at 1.5 Tesla.Methods-Relaxometry measurements were performed on cord blood specimens (N=88, derived from 6 caesarean deliveries) spanning a broad range of hematocrits (Hct=0.19-0.76) and oxygen saturations (sO 2 =4-100%), to characterize the dependence of T 1 and T 2 on these blood properties.Adult blood data (N = 31 specimens, derived from two volunteers) were similarly studied to validate our experimental methods by comparison with existing literature. Using biophysical models previously developed for adult blood, new model parameters were estimated, which relate Hct and sO 2 to the observed cord blood relaxation times.Results-Fitted biophysical models explained more than 90% of the variation in T 1 and T 2 . In general, T 2 relaxation times of cord blood were longer (by up to 35%) than those of adult blood, while T 1 relaxation times were slightly shorter (by up to 10%).Conclusion-The models and fitted parameters presented here can be used for calibration of future MRI investigations of fetal and neonatal blood physiology. This study is an important step in facilitating accurate, non-invasive assessments of fetal blood oxygen content, a valuable diagnostic parameter in the identification and treatment of fetal hypoxia.

Phase-contrast cine MRI (PC-MRI) is the gold-standard non-invasive technique for measuring vessel blood flow and has previously been applied in the human fetal circulation. We aimed to assess the feasibility of using PC-MRI to define the distribution of the fetal circulation in sheep. Fetuses were catheterized at 119-120 days gestation (term, 150 days) and underwent MRI at 123 days gestation under isoflurane anesthesia, ventilated at a FiO of 1.0. PC-MRI was performed using a fetal arterial blood pressure catheter signal for cardiac triggering. Blood flows were measured in the major fetal vessels, including the main pulmonary artery, ascending and descending aorta, superior vena cava, ductus arteriosus, left and right pulmonary arteries, umbilical vein, ductus venosus, and common carotid artery; and were indexed to estimated fetal weight. The combined ventricular output, pulmonary blood flow and flow across the foramen ovale were calculated from vessel flows. Intra-observer, inter-observer agreement and reproducibility were assessed. Blood flow measurements were successfully obtained in 61 out of 74 vessels (82.4%) interrogated in 9 fetuses. There was good intra-observer (R=0.998, P<0.0001; ICC=0.997) and inter-observer agreement (R=0.996, P<0.0001; ICC=0.996). Repeated MRI measurements showed good reproducibility (R=0.989, P=0.0002; ICC=0.990). We conclude that PC-MRI using fetal catheters for gating triggers is feasible in the major vessels of late gestation fetal sheep. This approach may provide a useful new tool for assessing the circulatory characteristics of fetal sheep models of human disease, including fetal growth restriction and congenital heart disease.

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