Separating oxygen transfer and contraction effects in placental BOLD MRI: volume-signal coupling in uncomplicated pregnancies and fetal growth restriction.

PURPOSE: Blood oxygen level-dependent (BOLD) MRI enables non-invasive assessment of placental function under maternal hyperoxia. Transient uterine contractions alter placental volume and BOLD signal, complicating the interpretation. Beyond this confounding effect, contraction-induced changes may reflect intrinsic placental biomechanical properties.

OBJECTIVE

To separate contraction-from oxygenation-related BOLD components and investigate whether the volume-signal coupling could reflect of placental biomechanical integrity.

METHODS

Among 125 pregnancies, 19 exhibited a single contraction during acquisition. Contractions were identified based on dynamic changes in placental shape, volume, and signal. Placental volume was modeled with an inverted gamma distribution, and the BOLD signal as the product of a sigmoid component (oxygenation response) and an inverted gamma term (contraction effect). We first validated the modeling framework using Bayesian fitting metrics. Contraction-related changes were then described using model-derived parameters. Finally, we derived a multivariate quadratic model describing volume-signal coupling in uncomplicated pregnancies and tested it fetal growth restriction (FGR) cases.

RESULTS

The framework achieved robust fitting of both volume and BOLD signal dynamics. In uncomplicated pregnancies, contraction-related volume and signal responses were tightly coupled, with a consistent delay between volume and signal onset (median Δt = 54.5 s [33.6-79.4], p < 0.001) and significant amplitude differences (p = 0.016). FGR cases deviated systematically from the reference coupling and showed higher multivariate prediction errors (p = 0.003) suggesting altered mechanical-functional behavior.

CONCLUSION

This proof-of-concept framework enables assessment of oxygen transfer despite contractions and suggests contraction-signal coupling as a potential marker of placental structure and function.