Placenta microstructure and microcirculation imaging with diffusion MRI

Abstract

PurposeTo assess which microstructural models best explain the diffusion‐weighted MRI signal in the human placenta.MethodsThe placentas of nine healthy pregnant subjects were scanned with a multishell, multidirectional diffusion protocol at 3T. A range of multicompartment biophysical models were fit to the data, and ranked using the Bayesian information criterion.ResultsAnisotropic extensions to the intravoxel incoherent motion model, which consider the effect of coherent orientation in both microvascular structure and tissue microstructure, consistently had the lowest Bayesian information criterion values. Model parameter maps and model selection results were consistent with the physiology of the placenta and surrounding tissue.ConclusionAnisotropic intravoxel incoherent motion models explain the placental diffusion signal better than apparent diffusion coefficient, intravoxel incoherent motion, and diffusion tensor models, in information theoretic terms, when using this protocol. Future work will aim to determine if model‐derived parameters are sensitive to placental pathologies associated with disorders, such as fetal growth restriction and early‐onset pre‐eclampsia. Magn Reson Med 80:756–766, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Placental MRI and its application to fetal intervention

“…Naturally, the product of this perfusion fraction with the pseudodiffusivity is a correlate of blood flow. Although the model fitting is prone to noise, several authors have attempted to make fitting more robust …”

“…Diffusion measurements of this type can be enhanced by including directional sensitisation, and this has been used frequently in other organs to reveal the organisation of the tissue structure, especially the brain . In the placenta, the directional sensitivity might reveal information about the structure of the villous tree and how this changes in pathology such as FGR where insufficient spiral artery remodelling is thought to lead to mechanical damage and immaturity in the fetal villous tree which may reduce the measured diffusion of water.…”

“…The technique is also, in principle, sensitive to water perfusion. There is now some evidence of directionality in flow in the placenta, particularly near to the chorionic plate, and this is likely to be associated with net differences in flow properties between chorionic arteries and veins.…”

“…FGR can be challenging to diagnose as placental function cannot currently be directly measured. Surrogate markers, such as abnormal fetal growth trajectory or abnormal blood flow to the placenta, are used with varying success. At present, there is no treatment for FGR, or the associated condition pre‐eclampsia; however, trials are exploring several new therapeutic avenues, including sildenafil, esomeprazole, metformin, pravastatin, and vascular endothelial growth factor maternal gene therapy .…”

Multi‐modal functional MRI to explore placental function over gestation

“…Recent work has shown that anisotropy is present in both the perfusion (i.e low b-value) and diffusion (high b-value) placental dMRI signal components. 41 We therefore designed a bespoke diffusion weighted single shot EPI sequence. This has 3 gradient directions at b = 5, 10, 25, 50, 100, 200, 400, 600, 1200 and 1600 s/mm 2 , 8 directions at b = 18 s/mm 2 , 7 directions at b = 36 s/mm 2 , and 15 directions at b = 800 s/mm 2 , in addition 6 volumes at b = 0 s/mm 2 were acquired interspersed through the acquisition.…”