Siauve Nathalie, Chalouhi Gihad E, Deloison Benjamin, Alison Marianne, Clement Olivier, Ville Yves, Salomon Laurent J
INSERM, U970, Sorbonne Paris Cite, Paris Cardiovascular Research Center-PARCC, Paris, France; EA FETUS and LUMIERE Unit, Université Paris-Descartes, Paris, France; Hôpital Européen Georges Pompidou, Paris, France.
INSERM, U970, Sorbonne Paris Cite, Paris Cardiovascular Research Center-PARCC, Paris, France; EA FETUS and LUMIERE Unit, Université Paris-Descartes, Paris, France; Assistance Publique-Hôpitaux de Paris, Hôpital Necker-Enfants Malades, Paris, France.
Am J Obstet Gynecol. 2015 Oct;213(4 Suppl):S103-14. doi: 10.1016/j.ajog.2015.06.045.
Abnormal placentation is responsible for most failures in pregnancy; however, an understanding of placental functions remains largely concealed from noninvasive, in vivo investigations. Magnetic resonance imaging (MRI) is safe in pregnancy for magnetic fields of up to 3 Tesla and is being used increasingly to improve the accuracy of prenatal imaging. Functional MRI (fMRI) of the placenta has not yet been validated in a clinical setting, and most data are derived from animal studies. FMRI could be used to further explore placental functions that are related to vascularization, oxygenation, and metabolism in human pregnancies by the use of various enhancement processes. Dynamic contrast-enhanced MRI is best able to quantify placental perfusion, permeability, and blood volume fractions. However, the transplacental passage of Gadolinium-based contrast agents represents a significant safety concern for this procedure in humans. There are alternative contrast agents that may be safer in pregnancy or that do not cross the placenta. Arterial spin labeling MRI relies on magnetically labeled water to quantify the blood flows within the placenta. A disadvantage of this technique is a poorer signal-to-noise ratio. Based on arterial spin labeling, placental perfusion in normal pregnancy is 176 ± 91 mL × min(-1) × 100 g(-1) and decreases in cases with intrauterine growth restriction. Blood oxygen level-dependent and oxygen-enhanced MRIs do not assess perfusion but measure the response of the placenta to changes in oxygen levels with the use of hemoglobin as an endogenous contrast agent. Diffusion-weighted imaging and intravoxel incoherent motion MRI do not require exogenous contrast agents, instead they use the movement of water molecules within tissues. The apparent diffusion coefficient and perfusion fraction are significantly lower in placentas of growth-restricted fetuses when compared with normal pregnancies. Magnetic resonance spectroscopy has the ability to extract information regarding metabolites from the placenta noninvasively and in vivo. There are marked differences in all 3 metabolites N-acetyl aspartate/choline levels, inositol/choline ratio between small, and adequately grown fetuses. Current research is focused on the ability of each fMRI technique to make a timely diagnosis of abnormal placentation that would allow for appropriate planning of follow-up examinations and optimal scheduling of delivery. These research programs will benefit from the use of well-defined sequences, standardized imaging protocols, and robust computational methods.
胎盘形成异常是导致大多数妊娠失败的原因；然而，对于胎盘功能的了解在很大程度上仍未被非侵入性的体内研究揭示。磁共振成像（MRI）在孕期对于高达3特斯拉的磁场是安全的，并且越来越多地被用于提高产前成像的准确性。胎盘的功能磁共振成像（fMRI）尚未在临床环境中得到验证，大多数数据来自动物研究。fMRI可通过使用各种增强过程来进一步探索与人类妊娠中血管生成、氧合和代谢相关的胎盘功能。动态对比增强MRI最能够量化胎盘灌注、通透性和血容量分数。然而，钆基造影剂经胎盘通过对人体的这个检查程序来说是一个重大的安全问题。有一些替代造影剂在孕期可能更安全或不会穿过胎盘。动脉自旋标记MRI依靠磁标记水来量化胎盘内的血流。该技术的一个缺点是信噪比更低。基于动脉自旋标记，正常妊娠时胎盘灌注为176±91 mL×min⁻¹×100 g⁻¹，在宫内生长受限的情况下会降低。血氧水平依赖和氧增强MRI不评估灌注，而是以血红蛋白作为内源性造影剂来测量胎盘对氧水平变化的反应。扩散加权成像和体素内不相干运动MRI不需要外源性造影剂，而是利用水分子在组织内的运动。与正常妊娠相比，生长受限胎儿的胎盘表观扩散系数和灌注分数显著更低。磁共振波谱能够在体内非侵入性地从胎盘中提取有关代谢物的信息。在小胎儿和发育正常的胎儿之间所有三种代谢物N - 乙酰天门冬氨酸/胆碱水平、肌醇/胆碱比值都有显著差异。当前的研究集中于每种fMRI技术及时诊断胎盘形成异常的能力，这将有助于进行适当的后续检查规划和优化分娩时机。这些研究项目将受益于使用明确的序列、标准化的成像方案和强大的计算方法。
