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Fetal MRI

Fetal Growth Restriction by Phase Contrast MR

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Assessment of late-onset fetal growth restriction by phase contrast MR

Redistribution of fetal blood flow in late-onset fetal growth restriction is measured by phase contrast MR showing brain-sparing physiology, and is compared with normal fetal blood flow measured by MR and Doppler ultrasound.

Summary

During the last few weeks of pregnancy, there is rapid growth in the fetal cerebral cortex and this places an increased demand for nutrition on a placenta that is nearing the end of its life. Late-onset fetal growth restriction (FGR) affects up to 10% of all pregnancies and is increasingly being recognized as an important cause of developmental delay in children. The diagnosis is usually based on the demonstration of a reduced pulsatility index in the middle cerebral artery by Doppler ultrasound, with an increased umbilical artery pulsatility index also indicative of FGR. However, diagnosis remains challenging and characteristic Doppler changes may be relatively late manifestations of FGR. Recently, metric optimized gating has enabled fetal flow measurement by phase contrast MR, allowing for measurement of the redistribution of blood flow in late-onset fetal growth restriction.

The fetal circulation adapts to reduced oxygen and nutrient supply by redistributing blood flow to different organs through modification of the resistance of various vascular beds and changes in shunting patterns. Phase contrast MR results are in keeping with the concept of “brain sparing physiology” where the fetus responds to placental insufficiency with an increase in cerebral perfusion demonstrated with a rise in superior vena caval flow. The increase in cerebral blood flow may be achieved by an increase in left ventricular output resulting in increased ascending aorta flow, as in FGR subject 5, or through increased right ventricular output with reduced pulmonary blood flow and increased arterial ductal flow supplying retrograde flow to the aortic arch, as in FGR subjects 1-4, compared to normal fetal flows.

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In utero measures of fetal blood flow in normal and late-onset FGR subjects. Significance differences are indicated. PC MR-phase contrast magnetic resonance; CVO-combined ventricular output; MPA-main pulmonary artery; AAo-ascending aorta; SVC-superior vena cava; DA-ductus arteriosus; DAo-descending aorta; PBF-pulmonary blood flow; UV-umbilical vein; FO-foramen ovale.