Placental magnetic resonance imaging T2∗ measurements in normal pregnancies and in those complicated by fetal growth restriction

Abstract

Objectives The magnetic resonance imaging (MRI) variable transverse relaxation time (T2∗) depends on multiple factors, one important one being the presence of deoxyhemoglobin. We aimed to describe placental T2∗ measurements in normal pregnancies and in those with fetal growth restriction (FGR). Methods We included 24 normal pregnancies at 24-40 weeks' gestation and four FGR cases with an estimated fetal weight below the 1st centile. Prior to MRI, an ultrasound examination, including Doppler flow measurements, was performed. The T2∗ value was calculated using a gradient echo MRI sequence with readout at 16 different echo times. In normal pregnancies, repeat T2∗ measurements were performed and interobserver reproducibility was assessed in order to estimate the reproducibility of the method. Placental histological examination was performed in the FGR cases. Results The method was robust regarding the technical and interobserver reproducibility. However, some slice-to-slice variation existed owing to the heterogeneous nature of the normal placenta. We therefore based T2∗ estimations on the average of two slices from each placenta. In normal pregnancies, the placental T2∗ value decreased significantly with increasing gestational age, with mean ± SD values of 120 ± 17 ms at 24 weeks' gestation, 84 ± 16 ms at 32 weeks and 47 ± 17 ms at 40 weeks. Three FGR cases had abnormal Doppler flow, histological signs of maternal hypoperfusion and a reduced T2∗ value (Z-score < -3.5). In the fourth FGR case, Doppler flow, placental histology and T2∗ value (Z-score, -0.34) were normal. Conclusions The established reference values for placental T2∗ may be clinically useful, as T2∗ values were significantly lower in FGR cases with histological signs of maternal hypoperfusion.