MRI quantification of human fetal O 2 delivery rate in the second and third trimesters of pregnancy.

PURPOSE: The purpose of this study was to estimate fetal O2 delivery rate in vivo across a range of gestational ages. Toward this, a calibration equation for T2 -based oximetry was derived.

METHODS: Umbilical cord blood of varying hematocrit (Hct) and oxygen saturation (HbO2 ) levels was prepared and T2 measured using a T2 -prepared balanced steady-state free-precession sequence at 1.5 T. The relationship between blood R2  = 1/T2 , HbO2 and Hct was established based on the model R2=(1-Hct)R2,plasma+Hct R2,RBC+k·Hct·(1-Hct)·(1-HbO2)2. Experimental R2 , HbO2 , and Hct levels were fit to the model-yielding values of k, R2,plasma, and R2,RBC (R2 of plasma and erythrocytes). Umbilical vein T2 measured in vivo was then converted to HbO2 , yielding-together with blood flow rate-the fetal O2 delivery rate in 22 pregnancies (gestational age 30 ± 3 weeks).

RESULTS: Constants derived from the fit (R2  = 0.94) were k = 83.1 s-1 , R2,plasma=1.1  s-1, and R2,RBC=12.9  s-1. The R2,RBC and k were found to be larger than those obtained for adult blood, likely the result of differences in dominant hemoglobin type. Data suggest that the use of adult blood calibration could entail errors up 10% in fetal blood HbO2 . The average umbilical vein blood flow rate (89.5 ± 17.2 mL/min/kg), HbO2 (84 ± 7%,), and fetal O2 delivery rate (15.1 ± 3.8 mL O2 /min/kg) were independent of gestational age. The fetal O2 delivery rate agreed well with the results obtained with invasive methods at term.

CONCLUSION: The present work describes strategies for measuring umbilical vein blood flow rate and HbO2 in vivo and estimates fetal O2 delivery rate noninvasively with quantitative MRI during the second and third trimesters of pregnancy. Magn Reson Med 80:1148-1157, 2018. © 2018 International Society for Magnetic Resonance in Medicine.