Visual assessment of the placenta in antenatal magnetic resonance imaging across gestation in normal and compromised pregnancies: Observations from a large cohort study

Fetal brain development in fetal growth restriction using MRI: a systematic review

BACKGROUND

This systematic review investigates potential differences in brain development between growth restricted (FGR)-fetuses compared to appropriate for gestational age (AGA) fetuses using MRI.

METHODS

PubMed, Embase, Cochrane Library and Web of Science databases were searched from 1985 to 2023. FGR was defined as an estimated fetal weight (EFW) < p10 and/or an abdominal circumference (AC) < p10, or 20% reduction in EFW or AC using a minimum interval of two weeks. Outcomes included volumetrics, biometrics, apparent diffusion coefficients (ADC), 1H-MRS-metabolites, and oxygenation of the fetal brain. Risk of bias was assessed using Newcastle-Ottawa Scale (NOS). A meta-analysis was conducted on variables when reported in at least three studies, calculating the mean difference (MD) with a 95% confidence interval (CI).

RESULTS

Twenty-nine studies were included after three-phase screening, 13 used the FGR consensus definition according to the Delphi procedure. Total brain volume and cerebellar volume were significantly reduced in FGR fetuses (n = 183; 74) when compared to AGA fetuses (n = 283; 166) with a MD of -30.84 cm3 (p < 0.01) and - 2.24 cm3 (p < 0.01). ADC values in the frontal white matter (FWM), occipital white matter (OWM), temporal white matter (TWM), thalami, centrum semiovale (CSO), basal ganglia, pons and cerebellum, significantly lower in growth restricted fetuses (-0.07 × 10-3 mm2/s (p < 0.01); -0.06 × 10-3 mm2/s (p < 0.01); -0.07 × 10-3 mm2/s (p < 0.01); -0.10 × 10-3 mm2/s (p < 0.01); -0.06 × 10-3 mm2/s (p < 0.01); -0.07 × 10-3 mm2/s (p < 0.01); -0.07 × 10-3 mm2/s (p < 0.01); -0.02 × 10-3 mm2/s (p < 0.01); respectively). 1H-MRS showed reduced levels of N-acetyl aspartate (NAA): Choline (Cho) and NAA: Creatine(CR) levels in the frontal lobe and central brain tissue, whilst contradictive findings concerning Cho: Cr and Inositol(Ino): Cho ratios were found. Two studies investigated the cerebral hemodynamic changes in FGR fetuses showing no difference in fractional moving blood volume, similar venous blood oxygenation in the superior sagittal sinus and no difference in T2* in the fetal brain.

DISCUSSION

MRI provides additional information on fetal brain development in a growth restricted population. Smaller total brain and cerebellar volumes and lower ADC values in the FWM, OWM, TWM, thalami, CSO, basal ganglia, pons and cerebellum have been observed in FGR. These conclusions are drawn on relatively small sample sizes with high heterogeneity resulting from diverse study populations and MRI techniques. Furthermore, how these findings correlate to long-term neurocognitive abnormalities associated with FGR remains to be elucidated. A large cohort study comparing brain maturation, myelination, metabolic and hemodynamic status between brain-sparing FGR fetuses to healthy age-matched controls is needed.

Placental T2* as a measure of placental function across field strength from 0.55T to 3T

Placental MRI is increasingly implemented in clinical obstetrics and research. Functional imaging, especially T2*, has been shown to vary across gestation and in pathology. Translation into the clinical arena has been slow because of time taken to mask the region of interest and owing to differences in T2* results depending on field strength. This paper contributes methodology to remove these barriers by utilising data from 0.55, 1.5 and 3T MRI to provide a fully automated segmentation tool; determining field strength dependency of placental assessment techniques; and deriving normal ranges for T2* by gestational age but independent of field strength. T2* datasets were acquired across field strengths. Automatic quantification including fully automatic masking was achieved and tested in 270 datasets across fields. Normal curves for quantitative placental mean T2*, volume and other derived measurements were obtained in 273 fetal MRI scans and z-scores calculated. The fully automatic segmentation achieved excellent quantification results (Dice scores of 0.807 at 3T, 0.796 at 1.5T and 0.815 at 0.55T.). Similar changes were seen between placental T2* and gestational age across all three field strengths (p < 0.05). Z-scores were generated. This study provides confidence in the translatability of T2* trends across field strengths in fetal imaging.

Preeclampsia Associated Differences in the Placenta, Fetal Brain, and Maternal Heart Can Be Demonstrated Antenatally: An Observational Cohort Study Using MRI

BACKGROUND

Preeclampsia is a multiorgan disease of pregnancy that has short- and long-term implications for the woman and fetus, whose immediate impact is poorly understood. We present a novel multiorgan approach to magnetic resonance imaging (MRI) investigation of preeclampsia, with the acquisition of maternal cardiac, placental, and fetal brain anatomic and functional imaging.

METHODS

An observational study was performed recruiting 3 groups of pregnant women: those with preeclampsia, chronic hypertension, or no medical complications. All women underwent a cardiac MRI, and pregnant women underwent a placental-fetal MRI. Cardiac analysis for structural, morphological, and flow data were undertaken; placenta and fetal brain volumetric and T2* (which describes relative tissue oxygenation) data were obtained. All results were corrected for gestational age. A nonpregnant cohort was identified for inclusion in the statistical shape analysis.

RESULTS

Seventy-eight MRIs were obtained during pregnancy. Cardiac MRI analysis demonstrated higher left ventricular mass in preeclampsia with 3-dimensional modeling revealing additional specific characteristics of eccentricity and outflow track remodeling. Pregnancies affected by preeclampsia demonstrated lower placental and fetal brain T2*. Within the preeclampsia group, 23% placental T2* results were consistent with controls, these were the only cases with normal placental histopathology. Fetal brain T2* results were consistent with normal controls in 31% of cases.

CONCLUSIONS

We present the first holistic assessment of the immediate implications of preeclampsia on maternal heart, placenta, and fetal brain. As well as having potential clinical implications for the risk stratification and management of women with preeclampsia, this gives an insight into the disease mechanism.

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.

Fetal growth restriction and stillbirth: Biomarkers for identifying at risk fetuses

Fetal growth restriction (FGR) is a major cause of stillbirth, prematurity and impaired neurodevelopment. Its etiology is multifactorial, but many cases are related to impaired placental development and dysfunction, with reduced nutrient and oxygen supply. The fetus has a remarkable ability to respond to hypoxic challenges and mounts protective adaptations to match growth to reduced nutrient availability. However, with progressive placental dysfunction, chronic hypoxia may progress to a level where fetus can no longer adapt, or there may be superimposed acute hypoxic events. Improving detection and effective monitoring of progression is critical for the management of complicated pregnancies to balance the risk of worsening fetal oxygen deprivation in utero, against the consequences of iatrogenic preterm birth. Current surveillance modalities include frequent fetal Doppler ultrasound, and fetal heart rate monitoring. However, nearly half of FGR cases are not detected in utero, and conventional surveillance does not prevent a high proportion of stillbirths. We review diagnostic challenges and limitations in current screening and monitoring practices and discuss potential ways to better identify FGR, and, critically, to identify the “tipping point” when a chronically hypoxic fetus is at risk of progressive acidosis and stillbirth.

Magnetic resonance imaging (MRI) of normal human placentae

INTRODUCTION

The MRI appearances of the human placenta in the absence of maternal or fetal pathology have not been extensively studied, with only a few studies reporting findings in the uncomplicated pregnancy. The purpose of this study is to review the placental MRI appearances in low-risk pregnancies in a prospective study.

METHODS

A prospective observational study of placental MRI in low-risk pregnancies was initially planned, however recruitment was terminated early due to the COVID19 pandemic. The protocol was subsequently modified to compare the placental appearances in the enrolled cohort with pregnancies having had MRI for non-placental pathologies. The data from the two groups were then pooled to assess the range of normal placental appearances.

RESULTS

Eighty-three pregnancies were prospectively assessed with MRI at a median gestation of 29 weeks (range 14-39) from a mixed group of prospective cases (n = 28) and retrospectively recruited obstetric MRI (n = 55). Placental thickness in the third trimester ranged from 18 to 35 mm. T2 heterogeneity was seen in 75% (25/33) at second trimester and by the third trimester 50% (25/50) were moderately or markedly heterogenous. T2 dark bands (>6 mm) were seen in 9% (3/33) and 20% (10/50) of second and third trimester pregnancies, respectively. Undetectable myometrium or loss of the subplacental myometrial plane was present in 15% (5/33) of second and 38% (19/50) of third trimester placentae.

CONCLUSION

This qualitative study of normal placental MRI appearances expands the current knowledge base by confirming they vary, evolve with gestation, and can overlap with signs of placenta accreta spectrum.