Establishing an in-vivo magnetic resonance imaging (MRI) protocol for measurement of placental vascular function in rats

Placental vascular physiology and function are critical for appropriate fetal growth and health outcomes in later life. Understanding how placental vasculature aligns with placental function, particularly in-vivo, may reveal underlying mechanisms that can inform interventions to improve long term fetal development outcomes. Classically, these research questions are addressed using invasive and terminal techniques such as histology and gene expression. However, in-vivo study of the placenta can permit repeated measures to serially investigate function and track development. Magnetic resonance imaging (MRI) is a noninvasive tool which can be used to study both placental morphology and function, as well as monitor development in-vivo.

This study aimed to establish in our laboratory an efficient and effective protocol of in-vivo MRI scans to assess placental function in rats. There are technical challenges in using MRI to assess placental function of rats in-vivo, e.g., motion, variable feto-placental position, image contrast, etc., which must be considered.

Pregnant Wistar rats were scanned serially at embryonic day (E) 15, 18, and 21 using 9.4 T MRI. Our established in-vivo MRI protocol includes: (i) T2 TurboRARE for fetal and placental anatomical imaging; (ii) an oxygen-challenge paradigm (air vs O2) obtaining 3D T1 and T2* relaxation maps for investigating tissue perfusion and blood oxygenation; (iii) bidirectional arterial spin labelling for feto-placental ordering; and (iv) dynamic contrast enhanced MRI for dynamic evaluation of placental perfusion (E21 only). After the E21 scan session, dams were euthanized for tissue collection and the number and ordering of feto-placental units confirmed.

Our preliminary data is congruent with the literature indicating that the relaxation mapping is sensitive to the oxygen-challenge paradigm. Research is on-going to validate the approach to study a maternal stressor model and how the MRI outcomes align with placental vascular structure.