LAUSR

Preeclampsia is a placental related complication of pregnancy, which affects 3% to 5% of all pregnancies. The pathophysiology of preeclampsia is not fully understood, and the prediction and treatment of this serious condition remain an obstetric challenge. Evidence suggests that at least 2 subtypes of preeclampsia exist: early and late-onset preeclampsia. Early-onset preeclampsia, in particular, is considered a result of abnormal placentation due to defect trophoblast invasion and insufficient transformation of the spiral arteries, which leads to reduced uteroplacental blood flow resulting in placental stress and dysfunction. Placental stress triggers maternal endothelial dysfunction, systemic inflammation, and the clinical syndrome of preeclampsia. The specific mediators of this relation remain unknown; however, placental serum markers, such as PLGF (placental growth factor), may play a particular role in this complex process. Because of the underlying cause of placental dysfunction in preeclampsia, this condition is very often associated with intrauterine fetal growth restriction (IUGR). The combination of preeclampsia and IUGR is most frequent in early-onset preeclampsia, in which the placental dysfunction tends to be more severe. Preeclampsia is closely related to placental dysfunction; therefore, antenatal assessment of placental function is of utmost importance to understand this complex condition. As placental dysfunction precedes the clinical syndrome of preeclampsia, the antenatal identification of placental dysfunction may lead attention to high-risk patients that are more likely to develop preeclampsia later in pregnancy. Current clinical methods to assess placental function focus on fetal wellbeing such as ultrasound estimates of fetal size and Doppler flow measurements of fetal and umbilical circulation, which are indirect estimates of placental dysfunction. New methods to estimate placental function directly during pregnancy is highly needed. Increasing evidence suggests that placental magnetic resonance imaging (MRI) provides information not only on placental structure but also on placental function. A wide range of quantitative MRI measurements is available, and each of these measurements is sensitive to specific aspects of placental function, including hypoxia and morphological changes. In the human placenta, MRI transversal relaxation times such as T2 and T2* and the longitudinal relaxation time T1 are the most well described. MRI relaxation times are tissue-specific constants, which describe the time for the observed MRI signal to decay. The relaxation time depends on multiple biological and physiological features of the tissue, as it is determined by the molecular environment of the protons within the tissue. In the human placenta, the T2* relaxation depends on morphological features such as the villous structure, the blood volume fractions, the deposition of fibrin or presence of infarctions, and fibrosis. Moreover, the T2* relaxation depends on placental oxygen saturation, as it is sensitive to the amount of the paramagnetic deoxyhemoglobin molecules in the tissue. Invasive animal studies have demonstrated that changes in tissue oxygenation are reflected by changes in the T2* weighted signal intensity, also known as the Blood Oxygen Level Dependent response. In the human placenta, several studies have demonstrated a low placental T2* value in pregnancies complicated by IUGR. The low T2* value is considered a result of both tissue hypoxia and morphological changes of the dysfunctional placenta. However, in pregnancies complicated by preeclampsia, the placental T2* has only been assessed in a few small pilot studies, and the correlation with placental serum markers, including PLGF, still remains to be elucidated. In this issue of Hypertension, Ho et al explores the use of placental MRI in combination with maternal serum markers (PLGF) to bring new insights into the pathophysiology of preeclampsia and to inform prognosis and clinical management of this important clinical syndrome. In a study of 14 women with preeclampsia and 48 gestational matched healthy controls, they define a preeclamptic placental phenotype by magnetic resonance imaging. In the T2 weighted MR image, visual analysis shows a striking difference between the normal and the preeclamptic placenta. The preeclamptic placenta is characterized by varied lobule size and increased granularity when compared with the normal placental. In the T2* map, the preeclamptic placenta is markedly darker, and the mean T2* value of the entire placenta is significantly reduced when compared with the normal placenta. The study by Ho et al also explores the exciting aspects of combining PLGF and placental MRI to evaluate placental function, and they present a strong positive correlation between the level of PLGF in maternal serum and the mean placental T2* value. The opinions expressed in this article are not necessarily those of the editors or of the American Heart Association. From the Department of Obstetrics and Gynecology, Aalborg University Hospital, Denmark (A.S., M.S.); and Department of Clinical Medicine, Aalborg University, Denmark (A.S., M.S.). Correspondence to Anne Sørensen, Aalborg University Hospital, Reberbansgade 15, 9000 Aalborg, Denmark. Email [email protected] Preeclamptic Placenta New Insights Using Placental Magnetic Resonance Imaging