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

PURPOSE

The purpose of this study was to estimate fetal O delivery rate across a range of gestational ages (GA). Toward this, a calibration equation for T-based oximetry was derived.

METHODS

Umbilical cord blood of varying hematocrit (Hct) and oxygen saturation (HbO) levels was prepared and T measured using a T-prepared balanced steady-state free precession (T-bSSFP) sequence at 1.5T. The relationship between blood R=1/T, HbO and Hct was established based on the model R = (1 − Hct)R + Hct R + k · Hct · (1 − Hct) · (1 − HbO). Experimental R, HbO and Hct levels were fit to the model yielding values of k, R and R (R of plasma and erythrocytes). Umbilical vein (UV) T measured was then converted to HbO yielding, together with blood flow rate (BFR), fetal O delivery rate in 22 pregnancies (GA 30±3 weeks).

RESULTS

Constants derived from the fit (R = 0.94) were: k=83.1s, R = 1.1s, R = 12.9s. R and k were found to be larger than those obtained for adult blood, likely due to differences in dominant hemoglobin type. Data suggest that the use of adult blood calibration could entail errors up 10% in fetal blood HbO. Average UV BFR (89.5±17.2 mL/min/kg), HbO (84±7%,) and fetal O delivery rate (15.1±3.8 mL O/min/kg) were independent of GA. Fetal O delivery rate agreed well with results obtained with invasive methods at term.

CONCLUSION

The present work describes strategies for measuring UV BFR and HbO and estimates fetal O delivery rate noninvasively with quantitative MRI during the second and third trimesters of pregnancy.