Quantitative ferumoxytol-enhanced MRI in pregnancy: A feasibility study in the nonhuman primate

Magnetic resonance imaging of placental intralobule structure and function in a preclinical nonhuman primate model†

Although the central role of adequate blood flow and oxygen delivery is known, the lack of optimized imaging modalities to study placental structure has impeded our understanding of its vascular function. Magnetic resonance imaging is increasingly being applied in this field, but gaps in knowledge remain, and further methodological developments are needed. In particular, the ability to distinguish maternal from fetal placental perfusion and the understanding of how individual placental lobules are functioning are lacking. The potential clinical benefits of developing noninvasive tools for the in vivo assessment of blood flow and oxygenation, two key determinants of placental function, are tremendous. Here, we summarize a number of structural and functional magnetic resonance imaging techniques that have been developed and applied in animal models and studies of human pregnancy over the past decade. We discuss the potential applications and limitations of these approaches. Their combination provides a novel source of contrast to allow analysis of placental structure and function at the level of the lobule. We outline the physiological mechanisms of placental T2 and T2* decay and devise a model of how tissue composition affects the observed relaxation properties. We apply this modeling to longitudinal magnetic resonance imaging data obtained from a preclinical pregnant nonhuman primate model to provide initial proof-of-concept data for this methodology, which quantifies oxygen transfer and placental structure across and between lobules. This method has the potential to improve our understanding and clinical management of placental insufficiency once validation in a larger nonhuman primate cohort is complete.

The human placenta project: Funded studies, imaging technologies, and future directions

The placenta plays a critical role in fetal development. It serves as a multi-functional organ that protects and nurtures the fetus during pregnancy. However, despite its importance, the intricacies of placental structure and function in normal and diseased states have remained largely unexplored. Thus, in 2014, the National Institute of Child Health and Human Development launched the Human Placenta Project (HPP). As of May 2023, the HPP has awarded over $101 million in research funds, resulting in 41 funded studies and 459 publications. We conducted a comprehensive review of these studies and publications to identify areas of funded research, advances in those areas, limitations of current research, and continued areas of need. This paper will specifically review the funded studies by the HPP, followed by an in-depth discussion on advances and gaps within placental-focused imaging. We highlight the progress within magnetic reasonance imaging and ultrasound, including development of tools for the assessment of placental function and structure.

MR Angiography of Pulmonary Vasculature

Pulmonary MR angiography (MRA) is a useful alternative to computed tomographic angiography (CTA) for the study of the pulmonary vasculature. For pulmonary hypertension and partial anomalous pulmonary venous return, a cardiac MR imaging and the pulmonary MRA are useful for flow quantification and planning treatment. For the diagnosis of pulmonary embolism (PE), MRA-PE has been shown to have non-inferior outcomes at 6 months when compared with CTA-PE. Over the last 15 years, pulmonary MRA has become a routine and reliable examination for the workup of pulmonary hypertension and the primary diagnosis of PE at the University of Wisconsin.

QSM Throughout the Body

Magnetic materials in tissue, such as iron, calcium, or collagen, can be studied using quantitative susceptibility mapping (QSM). To date, QSM has been overwhelmingly applied in the brain, but is increasingly utilized outside the brain. QSM relies on the effect of tissue magnetic susceptibility sources on the MR signal phase obtained with gradient echo sequence. However, in the body, the chemical shift of fat present within the region of interest contributes to the MR signal phase as well. Therefore, correcting for the chemical shift effect by means of water-fat separation is essential for body QSM. By employing techniques to compensate for cardiac and respiratory motion artifacts, body QSM has been applied to study liver iron and fibrosis, heart chamber blood and placenta oxygenation, myocardial hemorrhage, atherosclerotic plaque, cartilage, bone, prostate, breast calcification, and kidney stone.

Pulmonary MRA During Pregnancy: Early Experience With Ferumoxytol

BACKGROUND

Ferumoxytol, an intravenous iron supplement, is commonly used to treat anemia in pregnancy. Ferumoxytol-enhanced magnetic resonance angiography (Fe-MRA) is a viable off-label alternative to gadolinium-enhanced MRA for assessment of pulmonary embolism (PE) in pregnancy.

PURPOSE

To describe our clinical experience with Fe-MRA in pregnant women with suspected PE.

STUDY TYPE

Retrospective, observational, cohort.

POPULATION

A total of 98 Fe-MRA exams (consecutive sample) performed in 94 pregnant women.

FIELD STRENGTH/SEQUENCE

A 1.5 T and 3.0 T, 3D T1-weighted MRA.

ASSESSMENT

After IRB approval including a waiver of informed consent, electronic health records were reviewed retrospectively for all Fe-MRA exams performed at our institution in pregnant between January, 2017 and March, 2022. The Fe-MRA protocol included 3D-MRA for assessment of pulmonary arteries, and T1-weighted imaging for ancillary findings. Fe-MRA exam duration was measured from image time stamps. Fe-MRA exams were reviewed by three cardiovascular imagers using a 4-point Likert scale for image quality and confidence for PE diagnosis (score 4 = best, 1 = worst), and tabulation of ancillary findings.

STATISTICAL TESTS

Continuous data are presented as mean ± standard deviation. The overall image quality and confidence score is given as the mean of three readers.

RESULTS

The 98 Fe-MRA exams were performed in 94 pregnant women (age 30 ± 6, range 19-48 years, gestational week 23 ± 10, range 3-38 weeks), with four undergoing two Fe-MRA exams during their pregnancy. Median Fe-MRA exam durration was 8 minutes (interquantile range 6 minutes). Overall image quality score was 3.3 ± 0.9. Confidence score for diagnosing PE was 3.5 ± 0.8. One subject was positive for PE (1/94, 1%); 42 of the 94 (45%) subjects Fe-MRA had ancillary findings including hydronephrosis or pneumonia.

CONCLUSION

Ferumoxytol enhanced MRA is a radiation- and gadolinium-free alternative for diagnosis of PE during pregancy.

EVIDENCE LEVEL

4 TECHNICAL EFFICACY: Stage 5.

Ferumoxytol-enhanced MR demonstration of changes to internal placental structure in placenta accreta spectrum: Preliminary findings

INTRODUCTION

The goal of this pilot study is to determine if ferumoxytol-enhanced MR might provide a new approach to the diagnosis of placenta accreta spectrum (PAS), and if so, to identify signs of PAS.

METHODS

Ten pregnant women were referred for MRI evaluation for PAS. MR studies consisted of pre-contrast SSFSE, SSFP, DWI, and ferumoxytol-enhanced sequences. Post-contrast images were rendered as MIP and MinIP images to separately display the maternal and fetal circulations respectively. Two readers examined the images for architectural changes to placentone (fetal cotyledon) that might distinguish PAS cases from normal. Attention was given to the size and morphology of the placentone, villous tree, and vascularity. In addition, the images were examined for evidence of fibrin/fibrinoid, intervillous thrombus, basal and chorionic plate bulges. Interobserver agreement was characterized with kappa coefficients and levels of confidence for feature identification was recorded on a 10-point scale.

RESULTS

At delivery, there were five normal placentas and five with PAS (one accreta, two increta, two percreta). The ten changes of placental architecture in PAS included: focal/regional expansion of placentone(s); lateral displacement and compression of the villous tree; disruption of a regular pattern of normal placentones; bulging of the basal plate; bulging of the chorionic plate; transplacental stem villi; linear/nodular bands at basal plate; non-tapering villous branches; intervillous hemorrhage; and dilated subplacental vessels. All these changes were more common in PAS; the first five achieved statistical significance in this small sample. The interobserver agreement and confidence for the identification of these features was good to excellent except for dilated subplacental vessels.

DISCUSSION

Ferumoxytol-enhanced MR imaging appears to depict derangements of the internal architecture of placentas with PAS, thereby suggesting a promising new strategy to diagnose PAS.

Ferumoxytol-enhanced magnetic resonance imaging with volume rendering: A new approach for the depiction of internal placental structure in vivo

INTRODUCTION

Placental function is vitally important, but placental assessment is limited by current imaging methods in vivo. The goal of this study is to determine if ferumoxytol-enhanced MR studies might be used to depict placental structure during pregnancy.

METHODS

Ten pregnant women were referred for MRI evaluation of abnormal placentation. The study group was composed five of these patients whose placentas were normal at pathology. MR studies consisted of pre-contrast SSFSE (single-shot fast spin-echo), SSFP (steady-state free procession), diffusion, and ferumoxytol-enhanced acquisitions. The post-contrast sequences were compared to pre-contrast SSFSE, SSFP, and diffusion acquisitions for features of correspondence. MR images were also compared to histopathology for anatomic landmarks including the three-ring pattern of the functional vascular exchange unit (the placentone) created by this central cavity surrounded by a ring of clustered villi, and an outer ring of dispersed villi corresponding to the maternal venous outflow channel. The measured sizes of these features on MR were compared to reported sizes.

RESULTS

Post-ferumoxytol images showed enhancement of the maternal blood within the placenta, notably the intervillous space and the myometrial vessels. The unenhanced fetal vessels were most visible on the MinIP (minimum intensity projection) images; the enhanced maternal vessels were most visible on the MIP (maximum intensity projection) images. Composite MIP/MinIP images show the relation of maternal and fetal circulations. The signal intensities replicate the relative contributions from enhanced maternal blood and unenhanced chorionic villi.

DISCUSSION

Ferumoxytol-enhanced MR imaging can depict the internal anatomy of the placenta in vivo of clarity and detail. This could represent a new diagnostic approach to placental disorders.

Magnetic resonance imaging of the monkey fetal brain in utero

Non-human primates (NHPs) are the closest living relatives of the human and play a critical role in investigating the effects of maternal viral infection and consumption of medicines, drugs, and alcohol on fetal development. With the advance of contemporary fast MRI techniques with parallel imaging, fetal MRI is becoming a robust tool increasingly used in clinical practice and preclinical studies to examine congenital abnormalities including placental dysfunction, congenital heart disease (CHD), and brain abnormalities non-invasively. Because NHPs are usually scanned under anesthesia, the motion artifact is reduced substantially, allowing multi-parameter MRI techniques to be used intensively to examine the fetal development in a single scanning session or longitudinal studies. In this paper, the MRI techniques for scanning monkey fetal brains in utero in biomedical research are summarized. Also, a fast imaging protocol including T2-weighted imaging, diffusion MRI, resting-state functional MRI (rsfMRI) to examine rhesus monkey fetal brains in utero on a clinical 3T scanner is introduced.

Emerging placental biomarkers of health and disease through advanced magnetic resonance imaging (MRI)

Placental dysfunction is a major cause of fetal demise, fetal growth restriction, and preterm birth, as well as significant maternal morbidity and mortality. Infant survivors of placental dysfunction are at elevatedrisk for lifelong neuropsychiatric morbidity. However, despite the significant consequences of placental disease, there are no clinical tools to directly and non-invasively assess and measure placental function in pregnancy. In this work, we will review advanced MRI techniques applied to the study of the in vivo human placenta in order to better detail placental structure, architecture, and function. We will discuss the potential of these measures to serve as optimal biomarkers of placental dysfunction and review the evidence of these tools in the discrimination of health and disease in pregnancy. Efforts to advance our understanding of in vivo placental development are necessary if we are to optimize healthy pregnancy outcomes and prevent brain injury in successive generations. Current management of many high-risk pregnancies cannot address placental maldevelopment or injury, given the standard tools available to clinicians. Once accurate biomarkers of placental development and function are constructed, the subsequent steps will be to introduce maternal and fetal therapeutics targeting at optimizing placental function. Applying these biomarkers in future studies will allow for real-time assessments of safety and efficacy of novel interventions aimed at improving maternal-fetal well-being.

Dynamic contrast enhanced magnetic resonance imaging: A review of its application in the assessment of placental function

It is important to develop a better understanding of placental insufficiency given its role in common maternofetal complications such as preeclampsia and fetal growth restriction. Functional magnetic resonance imaging offers unprecedented techniques for exploring the placenta under both normal and pathological physiological conditions. Dynamic contrast-enhanced magnetic resonance imaging (DCE MRI) is an established and very robust method to investigate the microcirculatory parameters of an organ and more specifically its perfusion. It is currently a gold standard in the physiological and circulatory evaluation of an organ. Its application to the human placenta could enable to access many microcirculatory parameters relevant to the placental function such as organ blood flow, fractional blood volume, and permeability surface area, by the acquisition of serial images, before, during, and after administration of an intravenous contrast agent. Widely used in animal models with gadolinium-based contrast agents, its application to the human placenta could be possible if the safety of contrast agents in pregnancy is established or they are confirmed to not cross the placenta.

Magnetic Resonance Safety: Pregnancy and Lactation

MRI is a powerful diagnostic tool with excellent soft tissue contrast that uses nonionizing radiation. These advantages make MRI an appealing modality for imaging the pregnant patient; however, specific risks inherent to the magnetic resonance environment must be considered. MRI may be performed without and/or with intravenous contrast, which adds further fetal considerations. The risks of MRI with and without intravenous contrast are reviewed as they pertain to the pregnant or lactating patient and to the fetus and nursing infant. Relevant issues for gadolinium-based contrast agents and ultrasmall paramagnetic iron oxide particles are reviewed.

Feasibility of QSM in the human placenta

PURPOSE

Quantitative susceptibility mapping (QSM) is an emerging tool for the precise characterization of human tissue, including regional oxygenation. A critical function of the human placenta is oxygen transfer to the developing fetus, which remains difficult to study in utero. The purpose of this study is to investigate the feasibility of performing QSM in the human placenta in utero.

METHODS

In healthy pregnant women, 3D gradient echo data of the placenta were acquired with prospective respiratory gating at 1.5 Tesla and 3 Tesla. A brief period (6-7 min) of maternal hyperoxia was induced to increase placental oxygenation in a subset of women scanned at 3 Tesla, and data were acquired before and during oxygen administration. Susceptibility and T 2 ∗ / R 2 ∗ maps were reconstructed from gradient echo data, and mean and SD of these measures within the whole placenta were calculated.

RESULTS

A total of 54 women were studied at a mean gestational age of 30.7 ± 4.2 (range: 24 5/7-38 4/7) weeks. Susceptibility and T 2 ∗ maps demonstrated lobular contrast reflecting regional oxygenation difference at both field strengths. SD of susceptibilities, mean R 2 ∗ , and SD of R 2 ∗ of the placenta showed a linear relationship with gestational age (P < .01 for all). These measures were also responsive to maternal hyperoxia, and there was an increasing response with advancing gestational age (P < .01 for all).

CONCLUSION

This study demonstrates the feasibility of performing placental QSM in pregnant women and supports the potential for placental QSM to provide noninvasive in vivo assessment of placental oxygenation.