Feasibility of quantification of the distribution of blood flow in the normal human fetal circulation using CMR: a cross-sectional study

Background: We present the first phase contrast (PC) cardiovascular magnetic resonance (CMR) measurements of the distribution of blood flow in twelve late gestation human fetuses. These were obtained using a retrospective gating technique known as metric optimised gating (MOG). Methods: A validation experiment was performed in five adult volunteers where conventional cardiac gating was compared with MOG. Linear regression and Bland Altman plots were used to compare MOG with the gold standard of conventional gating. Measurements using MOG were then made in twelve normal fetuses at a median gestational age of 37 weeks (range 30-39 weeks). Flow was measured in the major fetal vessels and indexed to the fetal weight. Results: There was good correlation between the conventional gated and MOG measurements in the adult validation experiment (R=0.96). Mean flows in ml/min/kg with standard deviations in the major fetal vessels were as follows: combined ventricular output (CVO) 540 +/- 101, main pulmonary artery (MPA) 327 +/- 68, ascending aorta (AAo) 198 +/- 38, superior vena cava (SVC) 147 +/- 46, ductus arteriosus (DA) 220 +/- 39, pulmonary blood flow (PBF) 106 +/- 59, descending aorta (DAo) 273 +/- 85, umbilical vein (UV) 160 +/- 62, foramen ovale (FO) 107 +/- 54. Results expressed as mean percentages of the CVO with standard deviations were as follows: MPA 60 +/- 4, AAo37 +/- 4, SVC 28 +/- 7, DA 41 +/- 8, PBF 19 +/- 10, DAo50 +/- 12, UV 30 +/- 9, FO 21 +/- 12. Conclusion: This study demonstrates how PC CMR with MOG is a feasible technique for measuring the distribution of the normal human fetal circulation in late pregnancy. Our preliminary results are in keeping with findings from previous experimental work in fetal lambs.