To characterize spatial patterns of T₂* in the rhesus macaque placenta, to correlate these patterns with placental perfusion determined using DCE-MRI, and to evaluate the potential for using the BOLD effect to quantify placental perfusion without the use of exogenous contrast reagent.

Magnetic resonance imaging was performed on three pregnant rhesus macaques at gestational day 110. Multi-echo spoiled gradient echo measurements were used to compute maps of T₂*. Spatial maxima in these maps were compared with foci of early enhancement determined by DCE-MRI. We hypothesize that a pattern of high-to-low T₂* observed with increasing spatial distance from the spiral artery source within individual perfusion domains in the placenta represents a high-to-low gradient in oxygen concentration in maternal blood within the intervillous space. From this, we develop a mathematical model relating the spatial distribution of R₂* within a single placental lobule to relevant underlying physiological parameters. This model predicts that the spatial variation of R₂* within individual lobules as a function of distance (ρ) from the spiral artery supplying oxygenated blood from the mother to the fetus can be described by

$R_2^*(\rho)=(R_{20}^* + r_2^* ([Hb]-[Hb_{o,f}])) + r_2^*([Hb_{o,f}] - [Hb_{o,in}]) \exp{-\frac{4 \pi}{3} \frac{PS}{\Phi} v_i \rho^3}$

where R₂₀* is the intrinsic R₂* in the absence of deoxyhemoglobin (Hb_d), r₂* is the R₂* relaxivity of Hb_d, [Hb] is the total maternal hemoglobin concentration, [Hb_o,f] is the effective concentration of oxyhemoglobin in the fetal arterial blood, [Hb_o,in] is the maternal oxyhemoglobin concentration at the spiral artery outlet to the lobule, PS is the permeability-surface area product for oxygen exchange from the intervillous space to the fetal villi, v_i is the volume fraction of intervillous space, and Φ is the total spiral artery blood flow to the lobule (in ml/min). Using [Hb] values obtained from maternal blood draws, [Hb_o,in] from maternal arterial pulse oximetry, and the experimental r₂* value for deoxyhemoglobin, this equation can be fit to measured data to estimate R₂₀*, [Hb_o,f], and v_i PS/Φ for individual lobules within the placenta.

Local maxima in T₂* maps are 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 R₂* within individual placental lobules can be quantitatively analyzed using a simple model to estimate fetal arterial oxyhemoglobin concentration [Hb_o,f] and a parameter, v_i PS/Φ, reflecting oxygen transport to the fetus. Estimated mean values of [Hb_o,f] ranged from 4.25 mM to 4.46 mM, while v_i PS/Φ ranged from 2.80×10⁵ cm^-3 to 1.61×10⁶ cm^-3.Many aspects of placental growth, development, and function are poorly understood as a result of the lack of non-invasive tools for their assessment in vivo. We have developed a novel application of blood oxygen level dependent (BOLD) MRI that allows in-vivo assessment of placental perfusion and fetal oxygen saturation and have compared the results with dynamic contrast-enhanced (DCE) MRI imaging in pregnant rhesus macaques. Maternal spiral arteries identified on contrast-enhanced imaging show strong spatial correlation with foci of extended T₂* observed in the primate placenta. A simple model of oxygen transport accurately describes the spatial dependence of R₂* within placental lobules and enables assessment of placental function and oxygenation without requiring administration of an exogenous contrast reagent.It is possible to non-invasively assess placental perfusion and fetal oxygenation status in vivo using quantitative T2* relaxometry using a novel model describing the spatial distribution of T2* within placental lobules. This non-contrast method may provide a means for identifying abnormal placental development and facilitate early intervention for high-risk pregnancies.