Static and dynamic responses to hyperoxia of normal placenta across gestation with T2*‐weighted sequences

ABSTRACT OBJECTIVES T2*‐weighted sequences have been identified as non‐invasive tools to study the placental oxygenation in‐vivo. This study aims to investigate both static and dynamic responses to hyperoxia of the normal placenta across gestation. METHODS We conducted a single‐center prospective study including 52 uncomplicated pregnancies. Two T2*‐weighted sequences were performed: T2*‐relaxometry was performed before and after maternal hyperoxia. The histogram distribution of T2* values was assessed by fitting a gamma distribution as T2*~Γ(αβ) . A dynamic acquisition (BOLD protocol) was also performed before and during oxygen supply, until placental oxygen saturation. The signal change over time was modeled using a sigmoid function, used to determine the intensity of enhancement ( ∆BOLD,% ), a temporal variation coefficient ( λ ,min ‐1 , controlling the slope of the curve), and the maximal steepness (Vmax, ∆BOLD .min ‐1 ) of placental enhancement. RESULTS The histogram analysis of the T2* values in normoxia showed a whole‐placenta variation, with a decreasing linear trend in the mean T2* value (R= ‐0.83, 95% CI [‐0.9, ‐0.71], p<0.001) along with a more peaked and narrower distribution of T2* values across gestation. After maternal hyperoxia, the mean T2* ratios (mean T2* hyperoxia / mean T2* baseline ) were positively correlated with gestational age, while the other histogram parameters remained stable, suggesting a translation of the histogram towards higher values with a similar aspect. The ∆BOLD showed a non‐linear increase across gestation. Conversely, the λ (min −1 ) parameter, showed an inverted trend across gestation, with a significantly weaker correlation (R = ‐0.33, 95% CI [‐0.58, ‐0.02], p=0.04, R 2 = 0.1). As a combination of ∆BOLD and λ , the changes in Vmax throughout gestation were mainly influenced by the changes in ∆BOLD and resulted in a positive non‐linear correlation with gestational age. CONCLUSION Our results suggest that the decrease in the T2* placental signal over gestation does not reflect a dysfunction. The BOLD effect, representative of a free‐diffusion model of oxygenation, highlights the growing differences in oxygen saturation between mother and fetus across gestation ( ∆BOLD ), and placental permeability to oxygen ( λ ). This article is protected by copyright. All rights reserved.