Diffusion and perfusion quantified by Magnetic Resonance Imaging are markers of human placenta development in normal pregnancy

Diffusion-derived vessel density (DDVD) computed from a simple diffusion MRI protocol as a biomarker of placental blood circulation in patients with placenta accreta spectrum disorders: A proof-of-concept study

OBJECTIVES

Increasing trend of PAS (placenta accreta spectrum disorders) incidence is a major health concern as PAS is associated with high maternal morbidity and mortality during cesarean section. Prenatal identification of PAS is crucial for delivery planning and patients management. This study aims to explore whether diffusion-derived vessel density (DDVD) computed from a simple diffusion MRI protocol differs in PAS from normal placenta.

METHODS

We enrolled 86 patients with PAS disorders and 40 pregnant women without PAS disorders. Each patient underwent intravoxel incoherent motion (IVIM) MRI sequence with 11 b-values. Placenta diffusion-derived vessel density (DDVD-b0b50) was the signal difference between b = 0 and b = 50 s/mm2 images. DDVD(b0b50) A/N was calculated as [accreta lesion DDVD(b0b50)]/ [normal placenta DDVD(b0b50)]. The correlation between DDVD and gestational age was explored using Spearman rank correlation. Differences of DDVD(b0b50) A/N in patients with normal placentas and with PAS, and in patients with different subtypes of PAS were explored.

RESULTS

DDVD was negatively correlated with gestational age (p = 0.023, r = -0.359) in patients with normal placentas. DDVD(b0b50) A/N was significantly higher in patients with PAS (median:1.16, mean: 1.261) than normal placenta (median:1.02, mean: 1.032, p < 0.001) and especially higher in patients with placenta increta (median:1.14, mean: 1.278) and percreta (median: 1.20, mean: 1.396, p < 0.001).

CONCLUSION

As a higher DDVD indicates higher physiological volume of micro-vessels in PAS, this study suggests DDVD can be a potential biomarker to evaluate the placenta perfusion.

The predictive value of conventional magnetic resonance imaging combined with intravoxel incoherent motion parameters for evaluating maternal and neonatal clinical outcomes in patients with placenta accreta spectrum disorders

INTRODUCTION

We aimed to identify factors predictive of adverse maternal and neonatal outcomes in patients with placenta accreta spectrum (PAS) disorders using magnetic resonance imaging (MRI) and intravoxel incoherent motion (IVIM) parameters.

METHOD

Fifty-six normal singleton pregnancies at 33-39 weeks of gestation underwent MRI examination at 1.5 T. The IVIM parameters were obtained from the placenta. The correlation between the f value and postpartum hemorrhage (PPH) and between the f value and transfused units of red blood cells (RBCs) was estimated by linear regression. The correlation between various influencing factors (clinical risk factors, MRI features, and IVIM parameters) and poor outcomes was investigated using univariate and multivariate analyses.

RESULT

The interobserver agreement ranged from fair to excellent (k = 0.30-0.88). Multivariate analyses showed that previous cesarean sections, low signal intensity bands on T2WI and the D value were independent risk factors for adverse outcomes. The combination of three risk factors demonstrated the highest AUC of 0.903, with a sensitivity and specificity of 73.10 % and 96.90 %, respectively. Last, f was positively correlated with PPH and units of RBCs transfused.

DISCUSSION

Preoperative MRI features and IVIM parameters may be used to predict poor outcomes in patients with invasive placental disorders like PAS.

Two-Compartment Perfusion MR IVIM Model to Investigate Normal and Pathological Placental Tissue

BACKGROUND

Perfusion and diffusion coexist in the placenta and can be altered by pathologies. The two-perfusion model, where f1 and, f2 are the perfusion-fraction of the fastest and slowest perfusion compartment, respectively, and D is the diffusion coefficient, may help differentiate between normal and impaired placentas.

PURPOSE

Investigate the potential of the two-perfusion IVIM model in differentiating between normal and abnormal placentas.

STUDY-TYPE

Retrospective, case-control.

POPULATION

43 normal pregnancy, 9 fetal-growth-restriction (FGR), 6 small-for-gestational-age (SGA), 4 accreta, 1 increta and 2 percreta placentas.

FIELD STRENGTH/SEQUENCE

Diffusion-weighted-echo planar imaging sequence at 1.5 T.

ASSESSMENT

Voxel-wise signal-correction and fitting-controls were used to avoid overfitting obtaining that two-perfusion model fitted the observed data better than the IVIM model (Akaike weight: 0.94). The two-perfusion parametric-maps were quantified from ROIs in the fetal and maternal placenta and in the accretion zone of accreta placentas. The diffusion coefficient D was evaluated using a b ≥ 200 sec/mm2 -mono-exponential decay fit. IVIM metrics were quantified to fix f1  + f2  = fIVIM .

STATISTICAL-TESTS

ANOVA with Dunn-Sidák's post-hoc correction and Cohen's d test were used to compare parameters between groups. Spearman's coefficient was evaluated to study the correlation between variables. A P-value<0.05 indicated a statistically significant difference.

RESULTS

There was a significant difference in f1 between FGR and SGA, and significant differences in f2 and fIVIM between normal and FGR. The percreta + increta group showed the highest f1 values (Cohen's d = -2.66). The f2 between normal and percreta + increta groups showed Cohen's d = 1.12. Conversely, fIVIM had a small effective size (Cohen's d = 0.32). In the accretion zone, a significant correlation was found between f2 and GA (ρ = 0.90) whereas a significant negative correlation was found between fIVIM and D (ρ = -0.37 in fetal and ρ = -0.56 in maternal side) and f2 and D (ρ = -0.38 in fetal and ρ = -0.51 in maternal side) in normal placentas.

CONCLUSION

The two-perfusion model provides complementary information to IVIM parameters that may be useful in identifying placenta impairment.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 1.

Diffusion-sensitized magnetic resonance imaging highlights placental microstructural damage in patients with previous SARS-CoV-2 pregnancy infection

INTRODUCTION

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has been a major global health problem since December 2019. This work aimed to investigate whether pregnant women's mild and moderate SARS-CoV-2 infection was associated with microstructural and vascular changes in the placenta observable in vivo by Intravoxel Incoherent Motion (IVIM) at different gestational ages (GA).

METHODS

This was a retrospective, nested case-control of pregnant women during the SARS-CoV-2 pandemic (COVID-19 group, n = 14) compared to pre-pandemic healthy controls (n = 19). MRI IVIM protocol at 1.5T was constituted of diffusion-weighted (DW) images with TR/TE = 3100/76 ms and 10 b-values (0,10,30,50,75,100,200,400,700,1000s/mm2). Differences between IVIM parameters D (diffusion), and f (fractional perfusion) quantified in the two groups were evaluated using the ANOVA test with Bonferroni correction and linear correlation between IVIM metrics and GA, COVID-19 duration, the delay time between a positive SARS-CoV-2 test and MRI examination (delay-time exam+) was studied by Pearson-test.

RESULTS

D was significantly higher in the COVID-19 placentas compared to that of the age-matched healthy group (p < 0.04 in fetal and p < 0.007 in maternal site). No significant difference between f values was found in the two groups suggesting no-specific microstructural damage with no perfusion alteration (potentially quantified by f) in mild/moderate SARS-Cov-2 placentas. A significant negative correlation was found between D and GA in the COVID-19 placentas whereas no significant correlation was found in the control placentas reflecting a possible accelerated senescence process due to COVID-19.

DISCUSSION

We report impaired microstructural placental development during pregnancy and the absence of perfusion-IVIM parameter changes that may indicate no perfusion changing through microvessels and microvilli in the placentas of pregnancies with mild/moderate SARS-Cov-2 after reaching negativity.

Placental volume in pregnant women with opioid use: prenatal MRI assessment

OBJECTIVE

Opioid use in pregnant women is a growing public health concern and is shown to be associated with lower infant birth weights. Placental volume changes in prior studies correlated with various maternal and fetal conditions. We aimed to identify differences between placental volumes in pregnant women with opioid use, and control pregnant women without drug use.

METHODS

We prospectively recruited 27 healthy pregnant women and 17 pregnant women with opioid use disorder who were on medication-assisted treatment (MAT). All women underwent placenta/fetal MRI at 27-39 weeks gestation on a 3 Tesla MR scanner. Placental volumes were measured in a blinded fashion using a previously validated technique. Multiple linear regression was used to identify associations of placental volume with multiple maternal and fetal clinical factors. The significance threshold was set at p < .05.

RESULTS

Placental volume was significantly associated with gestational age at MRI (p < .0001), fetal sex (p = .027), MAT with smoking (p = .0008), MAT with polysubstance use (p = .01), and maternal BMI (p = .032). Placental volume was not associated with opioid MAT alone in our cohort.

CONCLUSION

For pregnant women on medication-assisted treatment for opioid use disorder, there was no significant difference in placental volume compared to healthy pregnant women. However, concomitant smoking and polysubstance use in the setting of medication-assisted treatment may be detrimental to placental health. To our knowledge, this is the first study assessing placental volume in opioid use on prenatal MRI. These results support the benefit of medication-assisted treatment during pregnancy; however additional studies are needed to further elucidate the impact of opioid use on placental and fetal development and postnatal outcomes.

Non-invasive mapping of human placenta microenvironments throughout pregnancy with diffusion-relaxation MRI

INTRODUCTION

In-vivo measurements of placental structure and function have the potential to improve prediction, diagnosis, and treatment planning for a wide range of pregnancy complications, such as fetal growth restriction and pre-eclampsia, and hence inform clinical decision making, ultimately improving patient outcomes. MRI is emerging as a technique with increased sensitivity to placental structure and function compared to the current clinical standard, ultrasound.

METHODS

We demonstrate and evaluate a combined diffusion-relaxation MRI acquisition and analysis pipeline on a sizable cohort of 78 normal pregnancies with gestational ages ranging from 15 + 5 to 38 + 4 weeks. Our acquisition comprises a combined T2*-diffusion MRI acquisition sequence - which is simultaneously sensitive to oxygenation, microstructure and microcirculation. We analyse our scans with a data-driven unsupervised machine learning technique, InSpect, that parsimoniously identifies distinct components in the data.

RESULTS

We identify and map seven potential placental microenvironments and reveal detailed insights into multiple microstructural and microcirculatory features of the placenta, and assess their trends across gestation.

DISCUSSION

By demonstrating direct observation of micro-scale placental structure and function, and revealing clear trends across pregnancy, our work contributes towards the development of robust imaging biomarkers for pregnancy complications and the ultimate goal of a normative model of placental development.

Fetal MRI: what's new? A short review

Fetal magnetic resonance imaging (fetal MRI) is usually performed as a second-level examination following routine ultrasound examination, generally exploiting morphological and diffusion MRI sequences. The objective of this review is to describe the novelties and new applications of fetal MRI, focusing on three main aspects: the new sequences with their applications, the transition from 1.5-T to 3-T magnetic field, and the new applications of artificial intelligence software. This review was carried out by consulting the MEDLINE references (PubMed) and including only peer-reviewed articles written in English. Among the most important novelties in fetal MRI, we find the intravoxel incoherent motion model which allow to discriminate the diffusion from the perfusion component in fetal and placenta tissues. The transition from 1.5-T to 3-T magnetic field allowed for higher quality images, thanks to the higher signal-to-noise ratio with a trade-off of more frequent artifacts. The application of motion-correction software makes it possible to overcome movement artifacts by obtaining higher quality images and to generate three-dimensional images useful in preoperative planning.Relevance statementThis review shows the latest developments offered by fetal MRI focusing on new sequences, transition from 1.5-T to 3-T magnetic field and the emerging role of AI software that are paving the way for new diagnostic strategies.Key points• Fetal magnetic resonance imaging (MRI) is a second-line imaging after ultrasound.• Diffusion-weighted imaging and intravoxel incoherent motion sequences provide quantitative biomarkers on fetal microstructure and perfusion.• 3-T MRI improves the detection of cerebral malformations.• 3-T MRI is useful for both body and nervous system indications.• Automatic MRI motion tracking overcomes fetal movement artifacts and improve fetal imaging.

Differentiating between normal and fetal growth restriction-complicated placentas: is T2∗ imaging imaging more accurate than conventional diffusion-weighted imaging?

AIM

To compare the performance of T2∗ imaging and apparent diffusion coefficient (ADC) in differentiating normal placentas from those complicated by fetal growth restriction (FGR).

MATERIALS AND METHODS

This prospective study included 28 control and 30 FGR placentas. Gradient-echo magnetic resonance imaging (MRI) at 16 different echo times and diffusion-weighted imaging (b-value of 0 and 800 s/mm²) were performed on all pregnant women using a 3 T MRI system.

RESULTS

Both T2∗ imaging Z-score and ADC were significantly lower in the FGR placentas (ADC, (1.69 ± 0.19) × 10^-3 versus (1.42 ± 0.28) × 10^-3 mm²/s, p<0.001; T2∗ imaging Z-score, -0.004 ± 0.95 versus -2.441 ± 1.48, p<0.001). The area under the curve for T2∗ imaging Z-score and ADC was 0.917 (95% confidence interval [CI] = 0.842-0.991) and 0.788 (95% CI = 0.655-0.887), respectively. The performance of T2∗ imaging in differentiating FGR placentas was significantly better than that of ADC (Z = 2.043, p=0.041).

CONCLUSION

Placental T2∗ imaging was found to be more reliable than ADC in differentiating between normal and FGR placentas.

Diffusion and perfusion MRI parameters in the evaluation of placenta accreta spectrum disorders in patients with placenta previa

OBJECTIVES

To evaluate the placental function by monoexponential, biexponential, and diffusion kurtosis MR imaging (MRI) in patients with placenta previa.

METHODS

A total of 62 patients with placenta accreta spectrum (PAS) disorders and 11 patients with normal placentas were retrospectively enrolled, who underwent conventional diffusion-weighted imaging (DWI), intravoxel incoherent motion (IVIM), and diffusion kurtosis imaging (DKI). The apparent diffusion coefficient (ADC) and exponential ADC (eADC) from standard DWI, mean kurtosis (MK), and diffusion coefficient (MD) from DKI, and pure diffusion coefficient (D), pseudo-diffusion coefficient (D*), and perfusion fraction (f) from IVIM were measured and compared from the volumetric analysis.

RESULTS

Comparisons between patients with PAS disorders and patients with normal placentas demonstrated that MD mean, D mean, and D* mean values in patients with PAS disorders were significantly higher than those in patients with normal placentas (p < 0.05). Comparisons between patients with accreta, increta, and percreta, and patients with normal placentas showed that the D mean was significantly higher in patients with placenta increta and percreta than in patients with normal placentas (p < 0.05).

CONCLUSION

The accreta lesions in PAS disorders had deceased cellularity and increased blood movement. The alteration of placental cellularity was more prominent in placenta increta and percreta.

Post treatment imaging in patients with local advanced cervical carcinoma

Cervical cancer (CC) is the fourth leading cause of death in women worldwide and despite the introduction of screening programs about 30% of patients presents advanced disease at diagnosis and 30-50% of them relapse in the first 5-years after treatment. According to FIGO staging system 2018, stage IB3-IVA are classified as locally advanced cervical cancer (LACC); its correct therapeutic choice remains still controversial and includes neoadjuvant chemo-radiotherapy, external beam radiotherapy, brachytherapy, hysterectomy or a combination of these modalities. In this review we focus on the most appropriated therapeutic options for LACC and imaging protocols used for its correct follow-up. We explore the imaging findings after radiotherapy and surgery and discuss the role of imaging in evaluating the response rate to treatment, selecting patients for salvage surgery and evaluating recurrence of disease. We also introduce and evaluate the advances of the emerging imaging techniques mainly represented by spectroscopy, PET-MRI, and radiomics which have improved diagnostic accuracy and are approaching to future direction.

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.

Monoexponential, biexponential and diffusion kurtosis MR imaging models: quantitative biomarkers in the diagnosis of placenta accreta spectrum disorders

BACKGROUND

To investigate the diagnostic value of monoexponential, biexponential, and diffusion kurtosis MR imaging (MRI) in differentiating placenta accreta spectrum (PAS) disorders.

METHODS

A total of 65 patients with PAS disorders and 27 patients with normal placentas undergoing conventional DWI, IVIM, and DKI were retrospectively reviewed. The mean, minimum, and maximum parameters including the apparent diffusion coefficient (ADC) and exponential ADC (eADC) from standard DWI, diffusion kurtosis (MK), and mean diffusion coefficient (MD) from DKI and pure diffusion coefficient (D), pseudo-diffusion coefficient (D*), and perfusion fraction (f) from IVIM were measured from the volumetric analysis and compared between patients with PAS disorders and patients with normal placentas. Univariate and multivariated logistic regression analyses were used to evaluate the value of the above parameters for differentiating PAS disorders. Receiver operating characteristics (ROC) curve analyses were used to evaluate the diagnostic efficiency of different diffusion parameters for predicting PAS disorders.

RESULTS

Multivariate analysis demonstrated that only D mean and D max differed significantly among all the studied parameters for differentiating PAS disorders when comparisons between accreta lesions in patients with PAS (AP) and whole placentas in patients with normal placentas (WP-normal) were performed (all p < 0.05). For discriminating PAS disorders, a combined use of these two parameters yielded an AUC of 0.93 with sensitivity, specificity, and accuracy of 83.08, 88.89, and 83.70%, respectively.

CONCLUSION

The diagnostic performance of the parameters from accreta lesions was better than that of the whole placenta. D mean and D max were associated with PAS disorders.

Intravoxel Incoherent Motion (IVIM) MR Quantification in Locally Advanced Cervical Cancer (LACC): Preliminary Study on Assessment of Tumor Aggressiveness and Response to Neoadjuvant Chemotherapy

The aim of this study was to determine whether quantitative parameters obtained from intravoxel incoherent motion (IVIM) model at baseline magnetic resonance imaging (MRI) correlate with histological parameters and response to neoadjuvant chemotherapy in patients with locally advanced cervical cancer (LACC). Methods: Twenty patients with biopsy-proven cervical cancer, staged as LACC on baseline MRI and addressed for neoadjuvant chemotherapy were enrolled. At treatment completion, tumor response was assessed with a follow-up MRI evaluated using the revised response evaluation criteria in solid tumors (RECIST; version 1.1), and patients were considered good responders (GR) if they had complete response or partial remission, and poor responders/non-responders (PR/NR) if they had stable or progressive disease. MRI protocol included conventional diffusion-weighted imaging (DWI; b = 0 and 1000 s/mm²) and IVIM acquisition using eight b-values (range: 0–1500 s/mm²). MR-images were analyzed using a dedicated software to obtain quantitative parameters: diffusion (D), pseudo-diffusion (D*), and perfusion fraction (fp) from the IVIM model; apparent diffusion coefficient (ADC) from conventional DWI. Histologic subtype, grading, and tumor-infiltrating lymphocytes (TILs) were assessed in each LACC. Results: D showed significantly higher values in GR patients (p = 0.001) and in moderate/high TILs (p = 0.018). Fp showed significantly higher values in squamous cell tumors (p = 0.006). Conclusions: D extracted from the IVIM model could represent a promising tool to identify tumor aggressiveness and predict response to therapy.

Human placental microperfusion and microstructural assessment by intra-voxel incoherent motion MRI for discriminating intrauterine growth restriction: a pilot study

OBJECTIVES

To evaluate the potential of Intravoxel Incoherent Motion (IVIM) Imaging in the quantification of placental micro-perfusion and microstructural features to identify and discriminate different forms of intrauterine growth restriction (IUGR) and normal fetuses pregnancies.

METHODS

Small for gestational age SGA (n = 8), fetal growth restriction FGR (n = 10), and normal (n = 49) pregnancies were included in the study. Placental Magnetic Resonance Imaging (MRI) was performed at 1.5 T using a diffusion-weighted sequence with 10 b-values. IVIM fractional perfusion (fp), diffusion (D), and pseudodiffusion (D*) were evaluated on the fetal and maternal placental sides. Correlations between IVIM parameters, Gestational Age (GA), Birth Weight (BW), and the presence or absence of prenatal fetoplacental Doppler abnormalities at the US were investigated in SGA, FGR, and normal placentae.

RESULTS

fp and D* of the placental fetal side discriminate between SGA and FGR (p = .021; p = .036, respectively), showing lower values in FGR. SGA showed an intermediate perfusion pattern in terms of fp and D* compared to FGR and normal controls. In the intrauterine growth restriction group (SGA + FGR), a significant positive correlation was found between fp and BW (p < .002) in the fetal placenta and a significant negative correlation was found between D and GA in both the fetal (p < .0009) and maternal (p < .006) placentas.

CONCLUSIONS

Perfusion IVIM parameters fp and D* may be useful to discriminate different micro-vascularization patterns in IUGR being helpful to detect microvascular subtle impairment even in fetuses without any sign of US Doppler impairment in utero. Moreover, fp may predict fetuses' body weight in intrauterine growth restriction pregnancies. The diffusion IVIM parameter D may reflect more rapid microstructural rearrangement of the placenta due to aging processes in the IUGR group than in normal controls.

Assessment of placental abruption with diffusion-weighted imaging

AIM

To investigate whether placental abruption without fetal distress could be assessed by apparent diffusion coefficient (ADC) values in magnetic resonance imaging (MRI).

METHODS

We conducted a retrospective case-control study at a single center. ADC values at the lesions of placental abruption in the abruption group (n = 8) were compared to those in the control group (n = 32). In the abruption group, ADC values at the sites of abruption were also compared to those at the nonabruption sites within the same placenta.

RESULTS

The ADC values in the placental area above the abruption site in the abruption group showed lower values than those in the control group when the slice containing the umbilical cord insertion site was set as the reference, and those values were compared in each corresponding slice. Compared with average ADC values, those above the abruption site in the abruption group were also significantly lower than those in the control group (p < 0.001). Furthermore, ADC values at the area above abruption were lower than those at the nonabruption area of all planes in the abruption group.

CONCLUSIONS

ADC values at the lesions above the placental abruption site were reduced compared to those in the normal placenta and those in the nonabruption area. Thus, it would be helpful to understand the pathophysiology of placental abruption in expectant management, although further investigations would be needed.

Placental functional assessment and its relationship to adverse pregnancy outcome: comparison of intravoxel incoherent motion (IVIM) MRI, T2-relaxation time, and umbilical artery Doppler ultrasound

BACKGROUND

Early identification of placental insufficiency can lead to appropriate treatment selections and can improve neonates' outcomes. Possible contributions of magnetic resonance imaging (MRI) have been suggested.

PURPOSE

To evaluate the prognostic capabilities of placental intravoxel incoherent motion (IVIM) parameters and T2-relaxation time, and their correlation with fetal growth and adverse outcomes, comparing umbilical artery (UmA) pulsatility index (PI).

MATERIAL AND METHODS

A total of 68 singleton pregnancies at 24-40 weeks of gestation underwent placental MRI and were reviewed retrospectively. UmA-PI was measured using Doppler ultrasound by obstetricians. IVIM parameters (Dfast, Dslow, and f) were calculated with a Bayesian model fitting. First, the associations between gestational age (GA) with placental IVIM parameters, T2-relaxation time, and placental thickness (PT) were evaluated. Second, IVIM parameters, T2 value (Z-score), PT (Z-score), and UmA-PI (Z-score) were compared between ( 1) those delivering small for gestational age (SGA) and appropriate for gestational age (AGA) neonates, ( 2) emergency cesarean section (ECS), and non-ECS, and ( 3) preterm birth and full-term birth.

RESULTS

Low birth weight was observed in 15/68 cases (22%). GA was significantly associated only with T2-relaxation time and PT. SGA was significantly associated with T2 value (Z-score), f, and UmA-PI (Z-score). In the ECS groups, T2 value (Z-score), f, and Dfast were significantly lower than those in non-ECS groups. All IVIM parameters and T2 values (Z-score) showed significantly lower scores in the preterm birth group.

CONCLUSION

Placental f and T2 value (Z-score) had significant associations with low birth weight and clinical adverse outcomes and could be potential imaging biomarkers of placental insufficiency.

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.

Normative placental structure in pregnancy using quantitative Magnetic Resonance Imaging

INTRODUCTION

To characterize normative morphometric, textural and microstructural placental development by applying advanced and quantitative magnetic resonance imaging (qMRI) techniques to the in-vivo placenta.

METHODS

We enrolled 195 women with uncomplicated, healthy singleton pregnancies in a prospective observational study. Women underwent MRI between 16- and 40-weeks' gestation. Morphometric and textural metrics of placental growth were calculated from T2-weighted (T2W) images, while measures of microstructural development were calculated from diffusion-weighted images (DWI). Normative tables and reference curves were constructed for each measured index across gestation and according to fetal sex.

RESULTS

Data from 269 MRI studies from 169 pregnant women were included in the analyses. During the study period, placentas undergo significant increases in morphometric measures of volume, thickness, and elongation. Placental texture reveals increasing variability with advancing gestation as measured by grey level non uniformity, run length non uniformity and long run high grey level emphasis. Placental microstructure did not vary with gestational age. Placental elongation was the only metric that differed significantly between male and female fetuses.

DISCUSSION

We report quantitative metrics of placental morphometry, texture and microstructure in a large cohort of healthy controls during the second and third trimesters of pregnancy. These measures can serve as normative references of in-vivo placental development to better understand placental function in high-risk conditions and allow for the early detection of placental mal-development.

IntraVoxel Incoherent Motion (IVIM) MRI of fetal lung and kidney: Can the perfusion fraction be a marker of normal pulmonary and renal maturation?

PURPOSE

To investigate the use of IntraVoxel Incoherent Motion (IVIM) MRI in the study of microstructural tissue changes occurring in fetal lung and kidney during gestation.

METHODS

34 normal pregnancies were enrolled. Patients were divided into two groups based on gestational age (GA): group A (21-29 weeks) and group B (30-39 weeks). MR examinations were performed at 1.5T, with a standard fetal MR protocol including a Diffusion-Weighted Echo-Planar Imaging sequence with 10 different b-values (0, 10, 30, 50, 75, 100, 200, 400, 700, 1000s/mm²). For each fetus, two bilateral ROIs were manually placed in lung and renal parenchyma. Mean values of perfusion fraction f, pseudo-diffusion coefficient D* and diffusion coefficient D were obtained. The correlation between IVIM parameters and GA was investigated.

RESULTS

In renal ROIs a positive correlation between f_kidney and GA (p < 0.005) was found; similarly f_lung showed a statistically significant correlation with GA (p < 0.001). F mean values were significantly higher in group B compared to group A in both renal (p = 0.0002) and lung (p = 0.018) ROIs. No correlation was found in D and D* as a function of GA.

CONCLUSIONS

The IVIM perfusion fraction f may be considered as a potential marker of pulmonary and renal maturation in relation to hemodynamic changes described in intrauterine life. Our results highlight that IVIM model is useful as an additional prenatal diagnostic tool to study lung and renal development.

The effects of maternal position, in late gestation pregnancy, on placental blood flow and oxygenation: an MRI study

KEY POINTS

Maternal supine sleep position in late pregnancy is associated with an increased risk of stillbirth. Maternal supine position in late pregnancy reduces maternal cardiac output and uterine blood flow. Using MRI, this study shows that compared to the left lateral position, maternal supine position in late pregnancy is associated with reduced utero-placental blood flow and oxygen transfer across the placenta with an average 6.2% reduction in oxygen delivery to the fetus and an average 11% reduction in fetal umbilical venous blood flow.

ABSTRACT

Maternal sleep position in late gestation is associated with an increased risk of stillbirth, though the pathophysiological reasons for this are unclear. Studies using magnetic resonance imaging (MRI) have shown that compared with lateral positions, lying supine causes a reduction in cardiac output, reduced abdominal aortic blood flow and reduced vena caval flow which is only partially compensated for by increased flow in the azygos venous system. Using functional MRI techniques, including an acquisition termed diffusion-relaxation combined imaging of the placenta (DECIDE), which combines diffusion weighted imaging and T2 relaxometry, blood flow and oxygen transfer were estimated in the maternal, fetal and placental compartments when subjects were scanned both supine and in left lateral positions. In late gestation pregnancy, lying supine caused a 23.7% (P < 0.0001) reduction in total internal iliac arterial blood flow to the uterus. In addition, lying in the supine position caused a 6.2% (P = 0.038) reduction in oxygen movement across the placenta. The reductions in oxygen transfer to the fetus, termed delivery flux, of 11.2% (P = 0.0597) and in fetal oxygen saturation of 4.4% (P = 0.0793) did not reach statistical significance. It is concluded that even in healthy late gestation pregnancy, maternal position significantly affects oxygen transfer across the placenta and may in part provide an explanation for late stillbirth in vulnerable fetuses.

Placental magnetic resonance imaging in chronic hypertension: A case-control study

INTRODUCTION

We aimed to explore the use of magnetic resonance imaging (MRI) in vivo as a tool to elucidate the placental phenotype in women with chronic hypertension.

METHODS

In case-control study, women with chronic hypertension and those with uncomplicated pregnancies were imaged using either a 3T Achieva or 1.5T Ingenia scanner. T2-weighted images, diffusion weighted and T1/T2* relaxometry data was acquired. Placental T2*, T1 and apparent diffusion coefficient (ADC) maps were calculated.

RESULTS

129 women (43 with chronic hypertension and 86 uncomplicated pregnancies) were imaged at a median of 27.7 weeks' gestation (interquartile range (IQR) 23.9-32.1) and 28.9 (IQR 26.1-32.9) respectively. Visual analysis of T2-weighted imaging demonstrated placentae to be either appropriate for gestation or to have advanced lobulation in women with chronic hypertension, resulting in a greater range of placental mean T2* values for a given gestation, compared to gestation-matched controls. Both skew and kurtosis (derived from histograms of T2* values across the whole placenta) increased with advancing gestational age at imaging in healthy pregnancies; women with chronic hypertension had values overlapping those in the control group range. Upon visual assessment, the mean ADC declined in the third trimester, with a corresponding decline in placental mean T2* values and showed an overlap of values between women with chronic hypertension and the control group.

DISCUSSION

A combined placental MR examination including T2 weighted imaging, T2*, T1 mapping and diffusion imaging demonstrates varying placental phenotypes in a cohort of women with chronic hypertension, showing overlap with the control group.

Functional diagnosis of placenta accreta by intravoxel incoherent motion model diffusion-weighted imaging

OBJECTIVES

To investigate the diagnostic value of intravoxel incoherent motion (IVIM) DWI for placenta accreta by comparing diffusion and perfusion characteristics of placentas with accreta lesions (APs) with those of normal placentas (NPs).

METHODS

Twenty-five pregnant women with AP and 24 with NP underwent 3-T magnetic resonance examinations with IVIM-DWI. The perfusion percentage (f), pseudo-diffusion coefficient (D*), and diffusion coefficient (D) values were calculated from different ROIs: the entire-plane of the AP (AP-ROI) and NP (NP-ROI) and the implanted (IR-ROI) and non-implanted region (NIR-ROI) of the AP. The AP-ROIs and NP-ROIs were compared using covariance analysis; the IR-ROIs and NIR-ROIs were compared using the Wilcoxon signed-rank test. ROC curves were produced to evaluate the parameters for predicting placenta accreta.

RESULTS

The f and D* values for the AP-ROIs ([45.0 ± 7.63]%, [11.64 ± 2.15]mm²/s) were significantly higher than those for the NP-ROIs ([31.85 ± 5.96]%, [9.04 ± 3.13]mm²/s) (both p < 0.05); the IR-ROIs (54.8%, 14.03 mm²/s) were also significantly higher than the NIR-ROIs (37.4%, 11.4 mm²/s) (both p < 0.05). No significant differences were found between the D values of the AP-ROIs and NP-ROIs (p > 0.05) or of the IR-ROIs and NIR-ROIs (p > 0.05). The areas under the curve for f and D* of the ROC curves were 0.93 and 0.79, respectively.

CONCLUSIONS

These results suggest that the IVIM parameters f and D* can be used to quantitatively evaluate the higher perfusion of AP when compared with NP. Furthermore, IVIM may be a useful functional diagnostic technique to predict placenta accreta.

KEY POINTS

• Intravoxel incoherent motion (IVIM) may be a useful diagnostic technique to quantitatively estimate the perfusion of the placenta. • The perfusion percentage (f) and pseudo-diffusion coefficient (D*) values differed significantly between placentas with accreta lesions and normal placentas. • ROC curves showed that perfusion percentage (f) and pseudo-diffusion coefficient (D*) values could accurately predict placenta accreta.

High-resolution diffusion MRI studies of development in pregnant mice visualized by novel spatiotemporal encoding schemes

This study introduces an MRI approach to map diffusion of water in vivo with high resolution under challenging conditions; the approach's potential is then used in diffusivity characterizations of embryos and fetoplacental units in pregnant mice, as well as of newborn mice in their initial postnatal period. The method relies on performing self-referenced spatiotemporal encoded MRI acquisitions, which can achieve the motional and susceptibility immunities needed to target challenging regions such as a mouse's abdominal cavity in a single shot. When suitably combined with zooming-in and novel interleaving procedures, these scans can overcome the inhomogeneity and sensitivity challenges arising upon targeting ≈100 μm in-plane resolutions, and thereby enable longitudinal development studies of abdominal organs that have hitherto eluded in vivo diffusion-weighted imaging. This is employed here to follow processes related to embryonic implantation and placentation, including the final stages of mouse gastrulation, the development of white matter in fetal brains, the maturation of fetal spines, and the evolution of the different layers making up mouse hemochorial placentas. The protocol's ability to extract diffusivity information in challenging regions as a function of embryonic mouse development is thus demonstrated, and its usefulness as a tool for visualizing pregnancy-related developmental changes in rodents is discussed.

In Utero Diffusion MRI: Challenges, Advances, and Applications

In utero diffusion magnetic resonance imaging (MRI) provides unique opportunities to noninvasively study the microstructure of tissue during fetal development. A wide range of developmental processes, such as the growth of white matter tracts in the brain, the maturation of placental villous trees, or the fibers in the fetal heart remain to be studied and understood in detail. Advances in fetal interventions and surgery furthermore increase the need for ever more precise antenatal diagnosis from fetal MRI. However, the specific properties of the in utero environment, such as fetal and maternal motion, increased field-of-view, tissue interfaces and safety considerations, are significant challenges for most MRI techniques, and particularly for diffusion. Recent years have seen major improvements, driven by the development of bespoke techniques adapted to these specific challenges in both acquisition and processing. Fetal diffusion MRI, an emerging research tool, is now adding valuable novel information for both research and clinical questions. This paper will highlight specific challenges, outline strategies to target them, and discuss two main applications: fetal brain connectomics and placental maturation.

Perfusion of the placenta assessed using arterial spin labeling and ferumoxytol dynamic contrast enhanced magnetic resonance imaging in the rhesus macaque

PURPOSE

To investigate the correspondence between arterial spin labeling (ASL) flow-sensitive alternating inversion recovery (FAIR) and ferumoxytol DCE MRI for the assessment of placental intervillous perfusion.

METHODS

Ten pregnant macaques in late second trimester were imaged at 3 T using a 2D ASL FAIR, with and without outer-volume saturation pulses used to control the bolus width, and a 3D ferumoxytol DCE-MRI acquisition. The ASL tagged/control pairs were averaged, subtracted, and normalized to create perfusion ratio maps. Contrast arrival time and uptake slope were estimated by fitting the DCE data to a sigmoid function. Macaques (N = 4) received interleukin-1β to induce inflammation and disrupt perfusion.

RESULTS

The FAIR tag modification with outer-volume saturation reduced the median ASL ratio percentage compared with conventional FAIR (0.64% ± 1.42% versus 0.71% ± 2.00%; P < .05). Extended ferumoxytol arrival times (34 ± 25 seconds) were observed across the placenta. No significant DCE signal change was measured in fetal tissue ( - 0.6% ± 3.0%; P = .52) or amniotic fluid (1.9% ± 8.8%; P = .59). High ASL ratio was significantly correlated with early arrival time and high uptake slope (P < .05), but ASL signal was not above noise in late-DCE-enhancing regions. No significant differences were observed in perfusion measurements between the interleukin-1β and controls (P > .05).

CONCLUSION

The ASL-FAIR and ferumoxytol DCE-MRI methods are feasible to detect early blood delivery to the macaque placenta. Outer volume saturation reduced the high macrovascular ASL signal. Interleukin-1β exposure did not alter placental intervillous perfusion. An endogenous-labeling perfusion technique is limited due to extended transit times for flow within the placenta beyond the immediate vicinity of the maternal spiral arteries.

Texture analysis of magnetic resonance images of the human placenta throughout gestation: A feasibility study

As fetal gestational age increases, other modalities such as ultrasound have demonstrated increased levels of heterogeneity in the normal placenta. In this study, we introduce and apply ROI-based texture analysis to a retrospective fetal MRI database to characterize the second-order statistics of placenta and to evaluate the relationship between heterogeneity and gestational age. Positive correlations were observed for several Haralick texture metrics derived from fetal-brain specific T2-weighted and gravid uterus T1-weighted and T2-weighted images, confirming a quantitative increase in placental heterogeneity with gestational age. Our study shows the importance of identifying baseline MR textural changes at certain gestational ages from which placental diseased states may be compared. Specifically, when evaluating for placental invasion or insufficiency, findings should be evaluated in the context of the normal placental aging process, which occurs throughout gestation.