Functional imaging of the nonhuman primate Placenta with endogenous blood oxygen level–dependent contrast

Purpose: To characterize spatial patterns of T^* ₂ in the placenta of the rhesus macaque (Macaca mulatta), to correlate these patterns with placental perfusion determined using dynamic contrast-enhanced MRI (DCE-MRI), and to evaluate the potential for using the blood oxygen level–dependent effect to quantify placental perfusion without the use of exogenous contrast reagent. Methods: MRI was performed on three pregnant rhesus macaques at gestational day 110. Multiecho spoiled gradient echo measurements were used to compute maps of T^* ₂. Spatial maxima in these maps were compared with foci of early enhancement determined by DCE-MRI. Results: Local maxima in T^* ₂ maps were strongly correlated with spiral arteries identified by DCE-MRI, with mean spatial separations ranging from 2.34 to 6.11 mm in the three animals studied. Spatial patterns of T^* ₂ (= 1/ T^* ₂) within individual placental lobules can be quantitatively analyzed using a simple model to estimate fetal arterial oxyhemoglobin concentration [Hb_{o, f}] and a parameter _viPS/ɸ, reflecting oxygen transport to the fetus. Estimated mean values of [Hb_{o, f}] ranged from 4.25 mM to 4.46 mM, whereas _viPS/ɸ ranged from 2.80 × 10⁵ cm⁻³ to 1.61 × 10⁶ cm⁻³. Conclusions: Maternal spiral arteries show strong spatial correlation with foci of extended T^* ₂ observed in the primate placenta. A simple model of oxygen transport accurately describes the spatial dependence of R^* ₂ within placental lobules and enables assessment of placental function and oxygenation without requiring administration of an exogenous contrast reagent. Magn Reson Med 76:1551–1562, 2016.