Synopsis

Understanding placental functions remains largely concealed from non-invasive in vivo investigations, although placental insufficiency is responsible for most failures in pregnancy. Functional MRI offers new perspectives for placental imaging. The vascularisation can be studied with IVIM and the pseudo perfusion coefficient (D*) and the perfusion fraction (f), and with ASL and the blood flow (fASL). DCE MRI also provides a definite assessment of perfusion but requires an exogenous contrast agent. The oxygenation can be studied with BOLD. The placental structure can be studied with DWI and the apparent diffusion coefficient (ADC), and with IVIM and the diffusion coefficient (Dr).

Purpose:

The placenta is the key organ for the fetal development. It is a regulatory organ which has involvement far beyond nutritive and respiratory functions, to also encompass endocrine and immune system regulation. Understanding placental functions remains largely concealed from non-invasive in vivo investigations, although placental insufficiency is responsible for most failures in pregnancy. Magnetic Resonance Imaging (MRI) has been performed in pregnancy for over 30 years, and is mainly a second-line imaging modality after Ultrasonography. Recent developments in MRI offer new perspectives for placental imaging, using functional MRI (fMRI) tools to explore vascularisation, oxygenation and metabolism.

Methods :

FMRI of the placenta adds functional information to anatomical data in vivo and non-invasively. A large panel of the fMRI techniques can be used to further explore placental functions related to vascularisation, oxygenation, metabolism, through various enhancement processes. Available methods will be described. An overview of the theory, practice, and potential applications of each fMRI will be exposed and discussed.

Results :

Diffusion weighting Imaging (DWI) and Intra Voxel Incoherent Motion (IVIM) MRI study the movement of water molecules within tissues and provide information about vascularization and tissular structure. The Apparent Diffusion Coefficient (ADC) is obtained with DWI and is related to tissular structure. Three quantitative parameters are available with IVIM. The pseudo perfusion coefficient (D*) and the perfusion fraction (f) are related to vascularization. The diffusion coefficient (Dr) is related to tissular structure. The Apparent Diffusion Coefficient (ADC) and the perfusion fraction (f) are significantly decreased in growth restriction placentas when compared to normal pregnancies and could be used to detect intra uterine growth retardation (IUGR). Blood Oxygen Level Dependent (BOLD) MRI uses the hemoglobin (Hb) as an endogenous contrast agent. The magnetic properties of Hb depend on its oxygen saturation, and modifications of T2* (ΔT2*), T2 (ΔT2), signal intensity (ΔSI) can be depicted when the oxygenation status of the organ of interest is modified. The variation of T2*, T2 or signal intensity can be used to assess placental oxygenation and response to oxygenation. The placenta has already been studied in physiological conditions during maternal hyperoxia, demonstrating differences between the chorionic and the basal plates of the placenta. A clinical research trial designed to evaluate the use of BOLD MRI in pregnant women as a non-invasive diagnostic tool to detect placental insufficiency and differentiate healthy fetuses from those with IUGR is underway (figure). Arterial Spin Labeling (ASL) MRI relies on magnetically labeled water to quantify the placental blood flow (fASL). This technique was the first fMRI technique used to study the human placenta in 1998 : fASL in normal pregnancy is 176 ± 91 ml.min-1.100 g-1 and decreases in cases with IUGR. Dynamic Contrast Enhanced MRI (DCE-MRI) is best able to quantify placental perfusion (fDCE), permeability and blood volume fractions. The enhancement of the placenta after intravenous injection of a Gadolinium-based contrast agent is compared to the enhancement of the arterial input function using compartimental model. However, the use of Gadolinium-based contrast agents during pregnancy is still limited to situations when the benefits outweigh the potential risks.

Discussion :

FMRI can be used to explore the vascularisation, the oxygenation and the structure of the placenta. The vascularisation can be studied with IVIM and the pseudo perfusion coefficient (D*) and the perfusion fraction (f), and with ASL and the blood flow (fASL). DCE MRI also provides a definite assessment of perfusion but requires the use of an exogenous contrast agent. The oxygenation can be studied with BOLD and the measurement of SI or T2* during two different oxygenation status. The placental structure can be studied with DWI and the apparent diffusion coefficient (ADC), and with IVIM and the diffusion coefficient (Dr). FMRI will be used to investigate normal pregnancy, IUGR, and pre ecclampsia. In normal conditions, basic values of ADC, the perfusion fraction (f) and the placental blood flow (fASL) are available. In IUGR, ADC, perfusion fraction f and T2 decrease. The perfusion fraction (f) can be useful in pre ecclampsia.

Conclusion :

Modern imaging of the placenta will integrate fMRI techniques in the follow up of pregnancy as numerous biomarkers will become available. The objectives of such modern imaging will be to improve the detection of placental insufficiency and the management of high-risk fetuses. However, the respective contribution of each technique remains to be defined. A plateform dedicated to human foeto-placental research (LUMIERE) will be created soon and will allow further research in this area.

Acknowledgements

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