Placental Perfusion and Permeability: Simultaneous Assessment with Dual-Echo Contrast-enhanced MR Imaging in Mice

Repeatability of tumor perfusion kinetics from dynamic contrast-enhanced MRI in glioblastoma

Background

Dynamic contrast-enhanced MRI (DCE-MRI) parameters have been shown to be biomarkers for treatment response in glioblastoma (GBM). However, variations in analysis and measurement methodology complicate determination of biological changes measured via DCE. The aim of this study is to quantify DCE-MRI variations attributable to analysis methodology and image quality in GBM patients.

Methods

The Extended Tofts model (eTM) and Leaky Tracer Kinetic Model (LTKM), with manually and automatically segmented vascular input functions (VIFs), were used to calculate perfusion kinetic parameters from 29 GBM patients with double-baseline DCE-MRI data. DCE-MRI images were acquired 2-5 days apart with no change in treatment. Repeatability of kinetic parameters was quantified with Bland-Altman and percent repeatability coefficient (%RC) analysis.

Results

The perfusion parameter with the least RC was the plasma volume fraction (v _p ), with a %RC of 53%. The extra-cellular extra-vascular volume fraction (v _e ) %RC was 82% and 81%, for extended Tofts-Kety Model (eTM) and LTKM respectively. The %RC of the volume transfer rate constant (K ^trans ) was 72% for the eTM, and 82% for the LTKM, respectively. Using an automatic VIF resulted in smaller %RCs for all model parameters, as compared to manual VIF.

Conclusions

As much as 72% change in K ^trans (eTM, autoVIF) can be attributable to non-biological changes in the 2-5 days between double-baseline imaging. Poor K ^trans repeatability may result from inferior temporal resolution and short image acquisition time. This variation suggests DCE-MRI repeatability studies should be performed institutionally, using an automatic VIF method and following quantitative imaging biomarkers alliance guidelines.

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.

Human placental perfusion measured using dynamic contrast enhancement MRI

Objectives

To evaluate the feasibility of dynamic contrast enhanced magnetic resonance imaging (DCE MRI) and measure values of in vivo placental perfusion in women.

Methods

This study was part of the Placentimage trial (NCT01092949). Gadolinium-chelate (Gd) enhanced dynamic MRI was performed two days before termination of pregnancies at 16 to 34 weeks gestational age (GA). Quantitative analysis was performed using one-compartment intravascular modeling. DCE perfusion parameters were analyzed across GA and were compared in IUGR and AGA fetuses.

Results

134 patients were enrolled. After quality control check, 62 DCE MRI were analyzed including 48 and 14 pregnancies with normal and abnormal karyotypes, respectively. Mean placental blood flow was 129±61 mL/min/100ml in cases with normal karyotypes. Fetuses affected by IUGR (n = 13) showed significantly lower total placental blood flow values than AGA fetuses (n = 35) (F _total = 122±88 mL/min versus 259±34 mL/min, p = 0.002). DCE perfusion parameters showed a linear correlation with GA.

Conclusions

Measuring placental perfusion in vivo is possible using DCE MRI. Although this study has many limitations it gives us the first DCE MRI values that provide a potential standard for future research into placental perfusion methods and suggests that placental functional parameters are altered in IUGR pregnancies.

On the use of multicompartment models of diffusion and relaxation for placental imaging

Multi-compartment models of diffusion and relaxation are ubiquitous in magnetic resonance research especially applied to neuroimaging applications. These models are increasingly making their way into the world of placental imaging. This review provides a framework for their motivation and implementation and describes some of the outstanding questions that need to be answered before they can be routinely adopted.

Deuterium Magnetic Resonance Imaging and the Discrimination of Fetoplacental Metabolism in Normal and L-NAME-Induced Preeclamptic Mice

Recent magnetic resonance studies in healthy and cancerous organs have concluded that deuterated metabolites possess highly desirable properties for mapping non-invasively and, as they happen, characterizing glycolysis and other biochemical processes in animals and humans. A promising avenue of this deuterium metabolic imaging (DMI) approach involves looking at the fate of externally administered ²H_6,6′-glucose, as it is taken up and metabolized into different products as a function of time. This study employs deuterium magnetic resonance to follow the metabolism of wildtype and preeclamptic pregnant mice models, focusing on maternal and fetoplacental organs over ≈2 h post-injection. ²H_6,6′-glucose uptake was observed in the placenta and in specific downstream organs such as the fetal heart and liver. Main metabolic products included ²H_3,3′-lactate and ²H-water, which were produced in individual fetoplacental organs with distinct time traces. Glucose uptake in the organs of most preeclamptic animals appeared more elevated than in the control mice (p = 0.02); also higher was the production of ²H-water arising from this glucose. However, the most notable differences arose in the ²H_3,3′-lactate concentration, which was ca. two-fold more abundant in the placenta (p = 0.005) and in the fetal (p = 0.01) organs of preeclamptic-like animals, than in control mice. This is consistent with literature reports about hypoxic conditions arising in preeclamptic and growth-restricted pregnancies, which could lead to an enhancement in anaerobic glycolysis. Overall, the present measurements suggest that DMI, a minimally invasive approach, may offer new ways of studying and characterizing health and disease in mammalian pregnancies, including humans.

Magnetic resonance imaging of placentome development in the pregnant Ewe

INTRODUCTION

Novel imaging measurements of placental development are difficult to validate due to the invasive nature of gold-standard procedures. Animal studies have been important in validation of magnetic resonance imaging (MRI) measurements in invasive preclinical studies, as they allow for controlled experiments and analysis of multiple time-points during pregnancy. This study characterises the longitudinal diffusion and perfusion properties of sheep placentomes using MRI, measurements that are required for future validation studies.

METHODS

Pregnant ewes were anaesthetised for a MRI session on a 3T scanner. Placental MRI was used to classify placentomes morphologically into three types based on their shape and size at two gestational ages. To validate classification accuracy, placentome type derived from MRI data were compared with placentome categorisation results after delivery. Diffusion-Weighted MRI and T2-relaxometry were used to measure a broad range of biophysical properties of the placentomes.

RESULTS

MRI morphological classification results showed consistent gestational age changes in placentome shape, as supported by post-delivery gold standard data. The mean apparent diffusion coefficient was significantly higher at 110 days gestation than at late gestation (~140 days; term, 150 days). Mean T2 was higher at mid gestation (152.2 ± 58.1 ms) compared to late gestation (127.8 ms ± 52.0). Significantly higher perfusion fraction was measured in late gestation placentomes that also had a significantly higher fractional anisotropy when compared to the earlier gestational age.

DISCUSSION

We report baseline measurements of techniques common in placental MRI for the sheep placenta. These measurements are essential to support future validation measurements of placental MRI techniques.

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.

Exploring early human brain development with structural and physiological neuroimaging

Early brain development, from the embryonic period to infancy, is characterized by rapid structural and functional changes. These changes can be studied using structural and physiological neuroimaging methods. In order to optimally acquire and accurately interpret this data, concepts from adult neuroimaging cannot be directly transferred. Instead, one must have a basic understanding of fetal and neonatal structural and physiological brain development, and the important modulators of this process. Here, we first review the major developmental milestones of transient cerebral structures and structural connectivity (axonal connectivity) followed by a summary of the contributions from ex vivo and in vivo MRI. Next, we discuss the basic biology of neuronal circuitry development (synaptic connectivity, i.e. ensemble of direct chemical and electrical connections between neurons), physiology of neurovascular coupling, baseline metabolic needs of the fetus and the infant, and functional connectivity (defined as statistical dependence of low-frequency spontaneous fluctuations seen with functional magnetic resonance imaging (fMRI)). The complementary roles of magnetic resonance imaging (MRI), electroencephalography (EEG), magnetoencephalography (MEG), and near-infrared spectroscopy (NIRS) are discussed. We include a section on modulators of brain development where we focus on the placenta and emerging placental MRI approaches. In each section we discuss key technical limitations of the imaging modalities and some of the limitations arising due to the biology of the system. Although neuroimaging approaches have contributed significantly to our understanding of early brain development, there is much yet to be done and a dire need for technical innovations and scientific discoveries to realize the future potential of early fetal and infant interventions to avert long term disease.

Non-invasive assessment of placental perfusion in vivo using arterial spin labeling (ASL) MRI: A preclinical study in rats

INTRODUCTION

Non-invasive assessment of placental perfusion is of great interest to characterize placental function in clinical practice. This article proposes a strictly non-invasive MRI technique using ASL to quantify placental blood flow in vivo. The aim of this study was to develop a fMRI tool to quantify placental blood flow (PBF) in rat, by using arterial spin labeling (ASL) MRI at 4.7 T.

MATERIALS AND METHODS

MRI was performed with a dedicated magnet for small animals, in pregnant rats on day 20 of the 22-day gestation period. A Look-Locker flow-sensitive alternating inversion recovery gradient echo sequence was developed as ASL technique (TE: 1.55 ms; TR: 3.5 ms, TI: 56 ms, deltaTI: 56 ms, FA: 20°, Matrix: 128 × 128, 8 segments, 4 Nex). Labeling was performed with global and slice-selective inversions, and T1 map was obtained for each mode of inversion. PBF was then derived from a compartmental model of the variation of T1 between global and slice-selective inversions.

RESULTS

The full protocol was completed and ASL image post-processing was successful in 18 rats. Forty-seven placentas were analyzed, with a mean PBF of 147 ± 70 ml/min/100 g of placenta, consistent with published values of placental perfusion using invasive techniques.

CONCLUSION

ASL MRI is feasible for the quantification of PBF in rats at 4.7 T. This technique, which requires no administration of contrast media, could have implications for non-invasive longitudinal and in vivo animal studies and may be useful for the management of human pregnancies.

Causes and Treatment of Acute Ischemic Stroke During Pregnancy

PURPOSE OF REVIEW

Treatment recommendations for pregnancy associated ischemic stroke are scarce. This may be due to the fact that, in general, obstetricians tend not to make recommendations for stroke patients and neurologists are not commonly involved in the care of pregnant women. Herein, we review the multiple etiologies of ischemic stroke during pregnancy, considerations for diagnostic testing, and acute treatment and prevention options, including associated risks specific to the pregnant and puerperal state.

RECENT FINDINGS

Intravenous tissue plasminogen activator (tPA) and endovascular thrombectomy have been used successfully to treat pregnant women with acute ischemic stroke. Recent national guidelines recommend considering tPA use during pregnancy for moderate and severe strokes if the potential benefits offset the risks of uterine hemorrhage. Pregnancy-associated ischemic stroke is rare, but can be devastating, and recanalization therapy should not be systematically withheld. Women who are at risk for stroke should be followed carefully, and providers caring for pregnant women should be educated regarding stroke signs and symptoms. Many of the standard post stroke diagnostic modalities may be used safely in pregnancy, and primary and secondary stroke prevention therapy must be tailored to avoid fetal toxicity.

Assessment of Placental Perfusion in the Preeclampsia L-NAME Rat Model with High-Field Dynamic Contrast-Enhanced MRI

PURPOSE

To evaluate placental function and perfusion in a rat model of preeclampsia infused with L-nitro-arginine methyl ester (L-NAME) by dynamic contrast-enhanced (DCE) MRI using gadolinium chelates.

METHODS

Pregnant female Sprague-Dawley rats were fitted on embryonic day 16 (E16) with subcutaneous osmotic minipumps loaded to deliver, continuously, L-NAME (50 mg/day per rat; case group) or saline solution (control group). DCE MRI was performed on E19 using gadolinium chelates and a 4.7-T MRI apparatus for small animals. Quantitative analysis was performed using an image software program: placental blood flow (perfusion in mL/min/100 mL of placenta) and fractional volume of the maternal vascular placental compartment (ratio between the placental blood volume and the placental volume, Vb in %) were calculated by compartmental analysis.

RESULTS

A total of 176 placentas (27 rats) were analyzed by DCE MRI (97 cases and 79 controls). The model was effective, inducing intrauterine growth retardation, as there was a significant difference between the two groups for placental weight (p < 0.01), fetal weight (p = 0.019), and fetal length (p < 0.01). There was no significant difference in placental perfusion between the L-NAME and control groups (140.1 ± 74.1 vs. 148.9 ± 97.4, respectively; p = 0.496). There was a significant difference between the L-NAME and control groups for Vb (53 ± 12.9 vs. 46.7 ± 9%, respectively; p < 0.01).

CONCLUSION

In the L-NAME preeclampsia model, placental perfusion is normal and the fractional blood volume is increased, suggesting that preeclampsia is not always expressed as a result of decreased placental perfusion. This highlights the usefulness of MRI for investigating the physiopathology of preeclampsia.

DCE MRI reveals early decreased and later increased placenta perfusion after a stress challenge during pregnancy in a mouse model

OBJECTIVES

Stress during pregnancy is known to have negative effects on fetal outcome. The purpose of this exploratory study was to examine placental perfusion alterations after stress challenge during pregnancy in a mouse model.

MATERIAL AND METHODS

Seven Tesla MRI was performed on pregnant mice at embrionic day (ED) 14.5 and 16.5. Twenty dams were exposed to an established acoustic stress challenge model while twenty non-exposed dams served as controls. Placental perfusion was analyzed in dynamic contrast-enhanced (DCE) MRI using the steepest slope model. The two functional placental compartments, the highly vascularized labyrinth and the endocrine junctional zone, were assessed separately.

RESULTS

Statistical analysis revealed decreased perfusion levels in the stress group at ED 14.5 compared to controls in both placenta compartments. On ED 16.5, the perfusion level increased significantly in the stress group while placenta perfusion in controls remained similar or even slightly decreased leading to an overall increased perfusion in the stress group on ED 16.5 compared to controls.

CONCLUSION

MR imaging allows noninvasive placenta perfusion assessment in this fetal stress mimicking animal model. In this exploratory study, we demonstrated that stress challenge during pregnancy leads to an initial reduction followed by an increase of placenta perfusion.

Placental physiology monitored by hyperpolarized dynamic 13C magnetic resonance

Placental functions, including transport and metabolism, play essential roles in pregnancy. This study assesses such processes in vivo, from a hyperpolarized MRI perspective. Hyperpolarized urea, bicarbonate, and pyruvate were administered to near-term pregnant rats, and all metabolites displayed distinctive behaviors. Little evidence of placental barrier crossing was observed for bicarbonate, at least within the timescales allowed by ¹³C relaxation. By contrast, urea was observed to cross the placental barrier, with signatures visible from certain fetal organs including the liver. This was further evidenced by the slower decay times observed for urea in placentas vis-à-vis other maternal compartments and validated by mass spectrometric analyses. A clear placental localization, as well as concurrent generation of hyperpolarized lactate, could also be detected for [1-¹³C]pyruvate. These metabolites also exhibited longer lifetimes in the placentas than in maternal arteries, consistent with a metabolic activity occurring past the trophoblastic interface. When extended to a model involving the administration of a preeclampsia-causing chemical, hyperpolarized MR revealed changes in urea's transport, as well as decreases in placental glycolysis vs. the naïve animals. These distinct behaviors highlight the potential of hyperpolarized MR for the early, minimally invasive detection of aberrant placental metabolism.

Assessment of human placental perfusion by intravoxel incoherent motion MR imaging

PURPOSE

To provide functional information on the human placenta, including perfusion, and diffusion, with no contrast agent injection, and to study correlations between intravoxel incoherent motion (IVIM) placental parameters and fetal growth.

MATERIALS AND METHODS

MRI was performed in women undergoing legal termination of pregnancy at 17-34 weeks, including a 4-b-value and 11-b-value DW sequences. The apparent diffusion coefficient (ADC), the restricted diffusion coefficient (D), the pseudoperfusion coefficient (D*), and the perfusion fraction (f) were calculated. Their relationships with gestational age, Z-scores for fetal and placental weight were evaluated by means of regression analysis. Logistic regression analysis was used to assess the ability of IVIM parameters to predict/detect intrauterine growth retardation (SGA).

RESULTS

Fifty-five pregnant women, including nine cases of SGA (16%), were included in the study. The ADC (n = 55) showed a quadratic correlation with gestational age (p < .001) and a linear correlation with the fetal weight Z-score (p = .02). Mean ADC values were significantly different between normally growing and SGA fetuses (2.37 ± 0.25 versus 2.29 ± 0.33 10^-3.mm².s^-1, p=.048). The perfusion fraction f (n = 23) showed a quadratic correlation with gestational age (p = .017) and a linear correlation with the fetal weight Z - score (p = .008). Mean f values differed significantly between normally growing and SGA fetuses (42.55 ± 9.30% versus 27.94 ± 8.76%, p = .002). The receiver operating characteristics (ROC) curve for f to predict SGA was produced (area under the ROC curve = 0.9).

CONCLUSIONS

The observed association between f and fetal weight suggests that fMRI could be suitable for studying placental insufficiency and for identifying risk of SGA.

Gadolinium-based contrast agents: A comprehensive risk assessment

Gadolinium-based contrast agents (GBCAs) have been used in magnetic resonance imaging (MRI) since the 1980s and are now administered in up to 35% of all MRI examinations. While GBCAs were initially felt to carry minimal risk, the subsequent identification of GBCAs as the key etiologic factor in the development of nephrogenic systemic fibrosis (NSF) has raised concerns about the broader health impacts of gadolinium exposure. Clinicians, radiologists, and patients should be aware of the most up-to-date data pertaining to the risks of GBCA administration. Specific issues covered in this review article include immediate adverse reactions; pregnancy and lactation; and gadolinium deposition and toxicity, with a special focus on NSF. Practice recommendations based on the presented data, as well as current professional society guidelines, are provided for each section.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 5 J. MAGN. RESON. IMAGING 2017;46:338-353.

Non-invasive evaluation of placental blood flow: lessons from animal models

In human obstetrics, placental vascularisation impairment is frequent as well as linked to severe pathological events (preeclampsia and intrauterine growth restriction), and there is a need for reliable methods allowing non-invasive evaluation of placental blood flow. Uteroplacental vascularisation is complex, and animal models are essential for the technical development and safety assessment of these imaging tools for human clinical use; however, these techniques can also be applied in the veterinary context. This paper reviews how ultrasound-based imaging methods such as 2D and 3D Doppler can provide valuable insight for the exploration of placental blood flow both in humans and animals and how new approaches such as the use of ultrasound contrast agents or ultrafast Doppler may allow to discriminate between maternal (non-pulsatile) and foetal (pulsatile) blood flow in the placenta. Finally, functional magnetic resonance imaging could also be used to evaluate placental blood flow, as indicated by studies in animal models, but its safety in human pregnancy still requires to be confirmed.

A longitudinal study of placental perfusion using dynamic contrast enhanced magnetic resonance imaging in murine pregnancy

INTRODUCTION

To evaluate changes in placental perfusion with advancing gestation in normal murine pregnancy using dynamic contrast enhanced magnetic resonance imaging (DCE-MRI).

METHODS

Seven timed-pregnant CD-1 mice underwent DCE-MRI scanning longitudinally on gestational days (GD) 13, 15 and 17. Placentas were segmented into high (HPZ) and low perfusion zones (LPZ) using tissue similarity mapping. Blood perfusion of the respective regions and the whole placenta was quantified using the steepest slope method. The diameter of the maternal central canal (CC) was also measured.

RESULTS

An increase in perfusion was observed between GD13 and GD17 in the overall placenta (p = 0.04) and in the HPZ (p = 0.02). Although perfusion in the LPZ showed a slight increasing trend, it was not significant (p = 0.07). Perfusion, in units of ml/min/100 ml, in the overall placenta and the HPZ was respectively 61.2 ± 31.2 and 106.2 ± 56.3 at GD13 (n = 19 placentas); 90.3 ± 43.7 and 139 ± 55.4 at GD15 (n = 20); and 104.9 ± 76.1 and 172.2 ± 85.6 at GD17 (n = 14). The size of the CC increased with advancing gestation (p < 0.05).

DISCUSSION

Using longitudinal DCE-MRI, the gestational age-dependent perfusion change in the normal murine placenta and in its regional compartments was quantified. In mid and late gestations, placental constituent regions differ significantly in their perfusion rates. The CC diameter also showed increase with advancing gestation, which may be playing an important role toward the gestational age-dependent increase in placental perfusion.

A liposomal Gd contrast agent does not cross the mouse placental barrier

The trans-placental permeability of liposomal Gadolinium (Gd) nanoparticle contrast agents was evaluated in a pregnant mouse model. Pregnant Balb/c mice at 16.5 (±1) days of gestation were imaged using a 3D Spoiled Gradient Echo method at 9.4 T using two contrast agents: a clinically approved Gd chelate, Multihance^® (gadobenate dimeglumine), and a novel experimental liposomal Gd agent. A Dynamic Contrast Enhancement (DCE) protocol was used to capture the dynamics of contrast entry and distribution in the placenta, and clearance from circulation. A blinded clinical radiologist evaluated both sets of images. A reference region model was used to measure the placental flow and physiological parameters; volume transfer constant (K^trans), efflux rate constant (K^ep). The Gd content of excised placentae and fetuses was measured, using inductively coupled plasma mass spectrometry (ICP-MS). MRI images of pregnant mice and ICP-MS analyses of placental and fetal tissue demonstrated undetectably low transplacental permeation of the liposomal Gd agent, while the clinical agent (Multihance) avidly permeated the placental barrier. Image interpretation and diagnostic quality was equivalent between the two contrast agents. Additional testing to determine both maternal and fetal safety of liposomal Gd is suggested.

Functional imaging of the human placenta with magnetic resonance

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.

Effect of T2* correction on contrast kinetic model analysis using a reference tissue arterial input function at 7 T

OBJECTIVES

We aimed to investigate the effect of T2* correction on estimation of kinetic parameters from T1-weighted dynamic contrast enhanced (DCE) MRI data when a reference-tissue arterial input function (AIF) is used.

MATERIALS AND METHODS

DCE-MRI data were acquired from seven mice with 4T1 mouse mammary tumors using a double gradient echo sequence at 7 T. The AIF was estimated from a region of interest in the muscle. The extended Tofts model was used to estimate pharmacokinetic parameters in the enhancing part of the tumor, with and without T2* correction of the lesion and AIF. The parameters estimated with T2* correction of both the AIF and lesion time-intensity curve were assumed to be the reference standard.

RESULTS

For the whole population, there was significant difference (p < 0.05) in transfer constant (K(trans)) between T2* corrected and not corrected methods, but not in interstitial volume fraction (ve). Individually, no significant differences were found in K(trans) and ve of four and six tumors, respectively, between the T2* corrected and not corrected methods. In contrast, K(trans) was significantly underestimated, if the T2* correction was not used, in other tumors for which the median K(trans) was larger than 0.4 min(-1).

CONCLUSION

T2* effect on tumors with high K(trans) may not be negligible in kinetic model analysis, even if AIF is estimated from reference tissue where the concentration of contrast agent is relatively low.

Functional MRI of the placenta--From rodents to humans

The placenta performs a wide range of physiological functions; insufficiencies in these functions may result in a variety of severe prenatal and postnatal syndromes with long-term negative impacts on human adult health. Recent advances in magnetic resonance imaging (MRI) studies of placental function, in both animal models and humans, have contributed significantly to our understanding of placental structure, blood flow, oxygenation status, and metabolic profile, and have provided important insights into pregnancy complications.

Placental perfusion in uterine ischemia model as evaluated by dynamic contrast enhanced MRI

BACKGROUND

To validate DCE MRI method of placental perfusion estimation and to demonstrate application of the method in a rabbit model of fetal antenatal hypoxia-ischemia.

METHODS

Placental perfusion was estimated by dynamic contrast imaging with bolus injection of Gd-DTPA in 3 Tesla GE magnet in a rabbit model of placental ischemia-reperfusion in rabbit dams at embryonic day 25 gestation age. Placental perfusion was measured using steepest slope method on DCE MRI before and after intermittent 40 min uterine ischemia. Antioxidants (n = 2 dams, 9 placentas imaged) or vehicle (n = 5 dams, 23 placenta imaged) were given systemically in a separate group of dams during reperfusion-reoxygenation. Placental perfusion was also measured in two dams from the antioxidant group (10 placentas) and two dams from the control group (12 placentas) by fluorescent microspheres method.

RESULTS

While placental perfusion estimates between fluorescent microspheres and DCE MRI were significantly correlated (R(2)  = 0.85; P < 0.01), there was approximately 33% systematic underestimation by the latter technique. DCE MRI showed a significant decrease in maternal placental perfusion in reperfusion-reoxygenation phase in the saline, 0.44 ± 0.06 mL/min/g (P = 0.012, t-test), but not in the antioxidant group, 0.62 ± 0.06 mL/min/g, relative to pre-occlusion values (0.77 ± 0.07 and 0.84 ± 0.12 mL/min/g, correspondingly).

CONCLUSION

Underestimation of true perfusion in placenta by steepest slope DCE MRI is significant and the error appears to be systematic.

Vitamin C supplementation ameliorates the adverse effects of nicotine on placental hemodynamics and histology in nonhuman primates

OBJECTIVE

We previously demonstrated that prenatal nicotine exposure decreases neonatal pulmonary function in nonhuman primates, and maternal vitamin C supplementation attenuates these deleterious effects. However, the effect of nicotine on placental perfusion and development is not fully understood. This study utilizes noninvasive imaging techniques and histological analysis in a nonhuman primate model to test the hypothesis that prenatal nicotine exposure adversely effects placental hemodynamics and development but is ameliorated by vitamin C.

STUDY DESIGN

Time-mated macaques (n = 27) were divided into 4 treatment groups: control (n = 5), nicotine only (n = 4), vitamin C only (n = 9), and nicotine plus vitamin C (n = 9). Nicotine animals received 2 mg/kg per day of nicotine bitartrate (approximately 0.7 mg/kg per day free nicotine levels in pregnant human smokers) from days 26 to 160 (term, 168 days). Vitamin C groups received ascorbic acid at 50, 100, or 250 mg/kg per day with or without nicotine. All underwent placental dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) at 135-140 days and Doppler ultrasound at 155 days to measure uterine artery and umbilical vein velocimetry and diameter to calculate uterine artery volume blood flow and placental volume blood flow. Animals were delivered by cesarean delivery at 160 days. A novel DCE-MRI protocol was utilized to calculate placental perfusion from maternal spiral arteries. Placental tissue was processed for histopathology.

RESULTS

Placental volume blood flow was significantly reduced in nicotine-only animals compared with controls and nicotine plus vitamin C groups (P = .03). Maternal placental blood flow was not different between experimental groups by DCE-MRI, ranging from 0.75 to 1.94 mL/mL per minute (P = .93). Placental histology showed increased numbers of villous cytotrophoblast cell islands (P < .05) and increased syncytiotrophoblast sprouting (P < .001) in nicotine-only animals, which was mitigated by vitamin C.

CONCLUSION

Prenatal nicotine exposure significantly decreased fetal blood supply via reduced placental volume blood flow, which corresponded with placental histological findings previously associated with cigarette smoking. Vitamin C supplementation mitigated the harmful effects of prenatal nicotine exposure on placental hemodynamics and development, suggesting that its use may limit some of the adverse effects associated with smoking during pregnancy.

Using dynamic contrast-enhanced MRI to quantitatively characterize maternal vascular organization in the primate placenta

PURPOSE

The maternal microvasculature of the primate placenta is organized into 10-20 perfusion domains that are functionally optimized to facilitate nutrient exchange to support fetal growth. This study describes a dynamic contrast-enhanced magnetic resonance imaging method for identifying vascular domains and quantifying maternal blood flow in them.

METHODS

A rhesus macaque on the 133rd day of pregnancy (G133, term = 165 days) underwent Doppler ultrasound procedures, dynamic contrast-enhanced magnetic resonance imaging and Cesarean-section delivery. Serial T1 -weighted images acquired throughout intravenous injection of a contrast reagent bolus were analyzed to obtain contrast reagent arrival time maps of the placenta.

RESULTS

Watershed segmentation of the arrival time map identified 16 perfusion domains. The number and location of these domains corresponded to anatomical cotyledonary units observed following delivery. Analysis of the contrast reagent wave front through each perfusion domain enabled determination of volumetric flow, which ranged from 9.03 to 44.9 mL/s (25.2 ± 10.3 mL/s). These estimates are supported by Doppler ultrasound results.

CONCLUSIONS

The dynamic contrast-enhanced magnetic resonance imaging analysis described here provides quantitative estimates of the number of maternal perfusion domains in a primate placenta and estimates flow within each domain. Anticipated extensions of this technique are to the study placental function in non-human primate models of obstetric complications.

Differentiation of breast cancer from fibroadenoma with dual-echo dynamic contrast-enhanced MRI

Dynamic contrast-enhanced magnetic resonance imaging (DCE MRI) of the breast is a routinely used imaging method which is highly sensitive for detecting breast malignancy. Specificity, though, remains suboptimal. Dynamic susceptibility contrast magnetic resonance imaging (DSC MRI), an alternative dynamic contrast imaging technique, evaluates perfusion-related parameters unique from DCE MRI. Previous work has shown that the combination of DSC MRI with DCE MRI can improve diagnostic specificity, though an additional administration of intravenous contrast is required. Dual-echo MRI can measure both T₁W DCE MRI and T₂*W DSC MRI parameters with a single contrast bolus, but has not been previously implemented in breast imaging. We have developed a dual-echo gradient-echo sequence to perform such simultaneous measurements in the breast, and use it to calculate the semi-quantitative T₁W and T₂*W related parameters such as peak enhancement ratio, time of maximal enhancement, regional blood flow, and regional blood volume in 20 malignant lesions and 10 benign fibroadenomas in 38 patients. Imaging parameters were compared to surgical or biopsy obtained tissue samples. Receiver operating characteristic (ROC) curves and area under the ROC curves were calculated for each parameter and combination of parameters. The time of maximal enhancement derived from DCE MRI had a 90% sensitivity and 69% specificity for predicting malignancy. When combined with DSC MRI derived regional blood flow and volume parameters, sensitivity remained unchanged at 90% but specificity increased to 80%. In conclusion, we show that dual-echo MRI with a single administration of contrast agent can simultaneously measure both T₁W and T₂*W related perfusion and kinetic parameters in the breast and the combination of DCE MRI and DSC MRI parameters improves the diagnostic performance of breast MRI to differentiate breast cancer from benign fibroadenomas.

Real-time placental perfusion on contrast-enhanced ultrasound and parametric imaging analysis in rats at different gestation time and different portions of placenta

Objectives

To quantitatively analyze placental perfusion in a rat model at different gestation time and different portions of placenta by real-time contrast-enhanced ultrasound (CEUS) and parametric imaging analysis.

Materials and Methods

Sixty pregnant rats at different gestation time (15 dys,17 days and 20 days) were injected intravenously with microbubbles (5×10⁵ microbubbles /ml, 1.0 ml/kg), and cadence contrast pulse sequencing (transmission frequency of 7 MHz, mechanical index 0.18) was performed. Dynamic enhancement changes in placenta at different gestation time and different portions of placenta were measured and enhancement parameters analyzed with software. Correlation between enhancement parameters and average area densities of placenta vascular compartment was compared.

Results

The pattern and real-time sequence of enhancement in uterus and placenta were clearly depicted by CEUS. The time-to-peak enhancement was earlier in central portion than that in peripheral portion (12.30±6.33s vs 36.26±10.65 s, p = 0.005), and peak intensity of enhancement is much higher in central portion than that in peripheral portion (30.20±2.85 dB vs 20.95±6.25 dB, p = 0.000). The peak intensity of enhancement at day 15 (27.70±4.47 dB) was lower than that at day 17 (30.20±2.85 dB, p = 0.042) and at day 20 (31.85±4.41 dB, p = 0.015) of gestation. Significant correlation between average area densities of vascular compartment and the peak intensity of enhancement was identified in placenta at different gestation time (p<0.05). The average area densities of vascular compartment was higher in central portion than that in peripheral portion and has significant correlation with peak intensity of enhancement of the two potions (p<0.01).

Conclusion

CEUS is feasible to depict real-time sequence and quantitative parameters of perfusion in different portion of placenta at different gestational time in a rat model.

Magnetic resonance imaging detects placental hypoxia and acidosis in mouse models of perturbed pregnancies

Endothelial dysfunction as a result of dysregulation of anti-angiogenic molecules secreted by the placenta leads to the maternal hypertensive response characteristic of the pregnancy complication of preeclampsia. Structural abnormalities in the placenta have been proposed to result in altered placental perfusion, placental oxidative stress, cellular damage and inflammation and the release of anti-angiogenic compounds into the maternal circulation. The exact link between these factors is unclear. Here we show, using Magnetic Resonance Imaging as a tool to examine placental changes in mouse models of perturbed pregnancies, that T₂ contrast between distinct regions of the placenta is abolished at complete loss of blood flow. Alterations in T₂ (spin-spin or transverse) relaxation times are explained as a consequence of hypoxia and acidosis within the tissue. Similar changes are observed in perturbed pregnancies, indicating that acidosis as well as hypoxia may be a feature of pregnancy complications such as preeclampsia and may play a prominent role in the signalling pathways that lead to the increased secretion of anti-angiogenic compounds.

In vivo MRI assessment of placental and foetal oxygenation changes in a rat model of growth restriction using blood oxygen level-dependent (BOLD) magnetic resonance imaging

OBJECTIVES

To evaluate whether changes in BOLD signal intensities following hyperoxygenation are related to intrauterine growth restriction (IUGR) in a rat model.

METHODS

IUGR was induced in pregnant rats by ligating the left vascular uterine pedicle at day 16 of gestation. BOLD MR imaging using a balanced steady-state free-precession (balanced-SSFP) sequence on a 1.5-T system was performed on day 19. Signal intensities (SI) before and after maternal hyperoxygenation were compared in the maternal liver and in control and growth-restricted foetoplacental units (FPUs).

RESULTS

Maternal hyperoxygenation resulted in a significant increase in SI in all regions of interest (P < 0.05) in the 18 rats. In the control group, the SI (mean ± SD) increased by 21 % ± 15 in placentas (n = 74) and 13 % ± 8.5 in foetuses (n = 53). In the IUGR group, the increase was significantly lower: 6.5 % ± 4 in placentas (n = 36) and 7 %± 5.5 in foetuses (n = 34) (P < 0.05).

CONCLUSION

BOLD MRI allows non-invasive assessment of the foetoplacental response to maternal hyperoxygenation in the rat and demonstrates its alteration in an IUGR model. This imaging method may provide a useful adjunct for the early diagnosis, evaluation, and management of human IUGR.

KEY POINTS

• Intra-uterine growth restriction is an important cause of perinatal morbidity and mortality. • Blood oxygen level-dependent MRI non-invasively assesses foetoplacental response to maternal hyperoxygenation. • In the rat, foetoplacental response to maternal hyperoxygenation is altered in IUGR. • Functional MRI may help to assess human IUGR.

MRI of the placenta - a short review

While ultrasound is still the gold standard method of placental investigation, magnetic resonance imaging (MRI) has certain benefits. In advanced gestational age, obese women, and posterior placental location, MRI is advantageous due to the larger field of view and its multiplanar capabilities. Some pathologies are seen more clearly in MRI, such as infarctions and placental invasive disorders. The future development is towards functional placental MRI. Placental MRI has become an important complementary method for evaluation of placental anatomy and pathologies contributing to fetal problems such as intrauterine growth restriction.

Motor deficits are triggered by reperfusion-reoxygenation injury as diagnosed by MRI and by a mechanism involving oxidants

The early antecedents of cerebral palsy (CP) are unknown but are suspected to be due to hypoxia-ischemia (H-I). In our rabbit model of CP, the MRI biomarker, apparent diffusion coefficient (ADC) on diffusion-weighted imaging, predicted which fetuses will develop postnatal hypertonia. Surviving H-I fetuses experience reperfusion-reoxygenation but a subpopulation manifested a continued decline of ADC during early reperfusion-reoxygenation, which possibly represented greater brain injury (RepReOx). We hypothesized that oxidative stress in reperfusion-reoxygenation is a critical trigger for postnatal hypertonia. We investigated whether RepReOx predicted postnatal neurobehavior, indicated oxidative stress, and whether targeting antioxidants at RepReOx ameliorated motor deficits, which included testing of a new superoxide dismutase mimic (MnTnHex-2-PyP). Rabbit dams, 79% gestation (E25), were subjected to 40 min uterine ischemia. Fetal brain ADC was followed during H-I, immediate reperfusion-reoxygenation, and 4-72 h after H-I. Endpoints were postnatal neurological outcome at E32, ADC at end of H-I, ADC nadir during H-I and reperfusion-reoxygenation, and area under ADC curve during the first 20 min of reperfusion-reoxygenation. Antioxidants targeting RepReOx were administered before and/or after uterine ischemia. The new MRI-ADC biomarker for RepReOx improved prediction of postnatal hypertonia. Greater superoxide production, mitochondrial injury, and oligodendroglial loss occurred in fetal brains exhibiting RepReOx than in those without. The antioxidants, MnTnHex-2-PyP and Ascorbate and Trolox combination, significantly decreased postnatal motor deficits and extent of RepReOx. The etiological link between early injury and later motor deficits can thus be investigated by MRI, and allows us to distinguish between critical oxidative stress that causes motor deficits and noncritical oxidative stress that does not.

Is administration of gadolinium-based contrast media to pregnant women and small children justified?

The use of gadolinium-based contrast media in pregnant or lactating women has been discouraged at many radiology departments due to the lack of knowledge of the risks for the fetus and the unwillingness to expose neonates to unnecessary drugs. In the present review the current literature and present guidelines regarding the use of gadolinium-based contrast media have been reviewed to validate the justification for their administration to pregnant or lactating women and small children.

Functional phenotyping of the maternal albumin turnover in the mouse placenta by dynamic contrast-enhanced MRI

Purpose

The purpose of this study was to develop a tool for functional phenotyping of the maternal circulation in the mouse placenta.

Procedures

In utero macromolecular dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was performed on embryonic day 10.5 (E10.5), E13.5, and E18.5. Fluorescence analysis was also used for validation of the results.

Results

The initial rate of contrast enhancement revealed an increased maternal blood volume fraction as the pregnancy progressed. Serial imaging of E10.5 and E13.5 placentas revealed a loss of contrast enhancement due to phagocytic uptake. A key application of macromolecular DCE-MRI would be to follow mouse pregnancies during fetal and placental manipulation including embryo transfer, tetraploid complementation, and fetal resorptions. We were able to resolve strain differences in ICR outbred mice carrying both ICR and C57Bl/6J embryos and to differentiate in utero resorptions from functional placentas.

Conclusions

Our results highlight the importance of the functional in utero analysis of placental vascularization in physiological phenotyping of transgenic mice and suggest MRI, particularly macromolecular DCE-MRI, as a non-invasive tool for the analysis of the placenta.

Magnetic resonance imaging of the placenta identifies placental vascular abnormalities independently of Doppler ultrasound

OBJECTIVE

To evaluate the relationship between placental vascular pathology detected by prenatal magnetic resonance imaging (MRI) and perinatal outcome.

METHODS

This was a retrospective, hospital-based, cross-sectional study in which all fetal MRI examinations of singleton pregnancies with vascular placental pathology (i.e. infarction with/without hemorrhage, subchorionic thrombi/hemorrhages, intervillous thrombi/hemorrhages, or retroplacental hematoma) in the period 2002-2007 were included. The extent of the pathology was expressed as a percentage of the total placental volume. Abnormalities of umbilical artery Doppler ultrasound examinations within 7 days between MRI and ultrasound examination were noted. Death in utero or postnatally was the primary outcome. Gestational age at MRI and at birth and the occurrence of intrauterine growth restriction (IUGR) were noted. Logistic regression analysis was performed to assess the impact of gestational age at MRI, extent of the vascular lesion and presence of pathological Doppler ultrasound measurements on the prediction of mortality.

RESULTS

Fifty-nine structurally normal singleton pregnancies with placental vascular abnormalities were included in the analysis. Mortality rate was 36%; among the survivors, 87% were born before 37 + 0 gestational weeks and 50% suffered from IUGR. In 55% of the pregnancies pathological umbilical artery Doppler findings were identified, of which 27% were non-survivors. Mortality was predicted by earlier gestational age at fetal MRI for placental pathology (P < 0.05) and increasing extent of the vascular lesion (P < 0.05), but not by the presence of pathological Doppler ultrasound data. Accuracy of the prediction was 82%, sensitivity was 67% and specificity 89%.

CONCLUSION

MRI-detected vascular placental pathologies may help to identify pregnancies at risk for adverse outcome and fetal death independently of umbilical artery Doppler status.

Dynamic contrast-enhanced magnetic resonance imaging: definitive imaging of placental function?

The placenta constitutes a complex circulatory interface between the mother and fetus, but the relationship between the maternal and fetal circulation is still very difficult to study in vivo. There is growing evidence that magnetic resonance imaging (MRI) is useful and safe during pregnancy, and MRI is increasingly used for fetal and placental anatomical imaging. MRI functional imaging is now a modern obstetric tool and has the potential to provide new insights into the physiology of the human placenta. Placental perfusion has been studied during the first pass of an MR contrast agent, by arterial spin labeling, diffusion imaging, T1 and T2 relaxation time measurement using echo-planar imaging, and by a combination of magnetization transfer with established stereological methods. The BOLD (blood oxygen level-dependent) effect offers new perspectives for functional MRI evaluation of the placenta.

Maternofetal pharmacokinetics of a gadolinium chelate contrast agent in mice

PURPOSE

To determine the maternofetal pharmacokinetics of gadoterate meglumine in mice during the first 48 hours following maternal intravenous injection of a high dose of 0.5 mmol of gadolinium per kilogram.

MATERIALS AND METHODS

All the studies complied with French law and the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. Balb/C mice (n = 23) at 16 days of gestation were examined for 48 hours after maternal intravenous administration of 0.5 mmol gadolinium per kilogram of gadoterate meglumine. Gadolinium concentration in the placentas, fetuses, and amniotic fluid was determined by using mass spectrometry, and the total placental and fetal gadolinium content was calculated. Gadoterate meglumine half-life in the different compartments was estimated with one- and two-compartment models. Kruskal-Wallis and Wilcoxon signed-rank tests were used to compare the pharmacokinetic profiles.

RESULTS

Gadoterate meglumine passed the placental barrier, entering the fetuses and amniotic fluid before being redistributed back to the mother. The placental gadolinium concentration showed two-compartmental decay, with a first half-life of distribution of 47 minutes and a second half-life of elimination of 107 hours. The half-lives in the fetuses and amniotic fluid were, respectively, 4 and 5 hours and followed a monocompartmental model after the initial peak. The maximal gadolinium fetal concentration (31.8 nmol/g) was observed 30 minutes after injection, which corresponded to a total fetal content of 0.077% of the injected dose.

CONCLUSION

In mice, gadoterate meglumine, an extracellular nonspecific gadolinium chelate contrast medium, passed the placenta before being redistributed back to the mother, resulting in undetectable fetal concentrations after 48 hours.

Placental MRI in intrauterine fetal growth restriction

OBJECTIVE

Our objectives were to determine if MR imaging of the placenta could demonstrate a specific placental phenotype in small for gestational age fetuses with increasing severity of fetal growth restriction, and if MRI findings at the time of scan could be used to predict fetal or neonatal mortality.

METHOD

We included singleton growth restricted fetuses with increasing severity of fetal growth restriction secondary to placental insufficiency. 20 growth restricted fetuses and 28 normal fetuses were scanned once during pregnancy at varying gestations. MRI scans were performed on a 1.5T system using ssFSE sequences through the uterus. Data was collected on the severity of fetal growth restriction and pregnancy outcome, including clinical neonatal details, perinatal mortality, and birthweight and centile. Placental volume, maximal placental thickness, the placental thickness to volume ratio, the placenta to amniotic fluid signal intensity ratio, and the presence of abnormal signal intensity consistent with placental pathology were noted. In a subset of patients, histopathological diagnosis was compared with the MRI appearance of the placenta.

RESULTS

There was a significant increase in the placental volume affected by pathology in growth restricted fetuses (p < 0.001). The placental appearance was also thickened and globular, with an increase in the placental thickness to volume ratio (p < 0.001). Although placental volume increased with increasing gestation, it remained reduced in the growth restricted fetuses (p = 0.003). There was a significant correlation between the severity of fetal growth restriction and the placental volume affected by pathology, the placental thickness to volume ratio, and the placental volume. ROC analysis showed that fetal or neonatal death was predicted by the percentage of abnormal signal intensity consistent with placental pathology (p = 0.002). The presence of a thickened, globular placenta and a maximal placental thickness to volume ratio above the 95% confidence limit for gestation was significantly associated with an increased incidence of fetal or neonatal mortality (relative risk = 1.615, p = 0.001 and relative risk = 7, p < 0.001).

CONCLUSIONS

The MRI appearance of the placenta provides an indication of the severity and underlying disease process in fetal growth restriction. In units where MRI imaging of the growth restricted fetus occurs, we suggest that the assessment of the placenta should also occur as it may contribute to management decisions in cases at the threshold of viability. It may have a role to play in monitoring disease severity, and the effect of future interventions designed to improve placental function.

Magnetic resonance imaging of hypoxic injury to the murine placenta

We assessed the use of magnetic resonance imaging (MRI) to define placental hypoxic injury associated with fetal growth restriction. On embryonic day 18.5 (E18.5) we utilized dynamic contrast-enhanced (DCE)-MRI on a 4.7-tesla small animal scanner to examine the uptake and distribution of gadolinium-based contrast agent. Quantitative DCE parameter analysis was performed for the placenta and fetal kidneys of three groups of pregnant C57BL/6 mice: 1) mice that were exposed to Fi(O(2)) = 12% between E15.5 and E18.5, 2) mice in normoxia with food restriction similar to the intake of hypoxic mice between E15.5 and E18.5, and 3) mice in normoxia that were fed ad libitum. After imaging, we assessed fetoplacental weight, placental histology, and gene expression. We found that dams exposed to hypoxia exhibited fetal growth restriction (weight reduction by 28% and 14%, respectively, P < 0.05) with an increased placental-to-fetal ratio. By using MRI-based assessment of placental contrast agent kinetics, referenced to maternal paraspinous muscle, we found decreased placental clearance of contrast media in hypoxic mice, compared with either control group (61%, P < 0.05). This was accompanied by diminished contrast accumulation in the hypoxic fetal kidneys (23%, P < 0.05), reflecting reduced transplacental gadolinium transport. These changes were associated with increased expression of placental Phlda2 and Gcm1 transcripts. Exposure to hypoxia near the end of mouse pregnancy reduces placental perfusion and clearance of contrast. MRI-based DCE imaging provides a novel tool for dynamic, in vivo assessment of placental function.

In vivo virtual histology of mouse embryogenesis by ultrasound biomicroscopy and magnetic resonance imaging

Feasibility of magnetic resonance imaging (MRI) and ultrasound biomicroscopy (UBM) for sequential in vivo study of mouse embryo development between Days 6.5 and 13.5 of pregnancy was assessed in a first experiment. A second trial, based on the results of the first, determined the accuracy of UBM for imaging morphogenesis from implantation to the late embryo stage (Days 4.5 to 15.5). MRI allowed imaging of the entire uterus and all gestational sacs and embryos inside whilst the small scanning range of UBM precluded accurate counting of fetuses; however, its high resolution identified the decidual reaction at implantation sites from Day 4.5. At later stages, it was possible to assess key morphogenetic processes such as differentiation of the placenta, the cephalic region, the thoracic and abdominal organs, the skeletal system and the limbs, and dynamic structures such as the cardiovascular system. Thus, both techniques are reliable for in utero imaging of mouse embryo development. MRI may be more appropriate for studying embryo lethality and intrauterine growth retardation, because the entire uterus can be viewed. UBM may be more suitable for studies of cellular components of organs and tissues and assessment of haemodynamic changes in the circulatory system.

Pharmacokinetic modeling of perfluorooctanoic acid during gestation and lactation in the mouse

Perfluorooctanoic acid (PFOA) is a processing aid for the polymerization of commercially valuable fluoropolymers. Its widespread environmental distribution, presence in human blood, and adverse effects in animal toxicity studies have triggered attention to its potential adverse effects to humans. PFOA is not metabolized and exhibits dramatically different serum/plasma half-lives across species. Estimated half-lives for humans, monkeys, mice, and female rats are 3-5 years, 20-30 days, 12-20 days, and 2-4h, respectively. Developmental toxicity is one of the most sensitive adverse effects associated with PFOA exposure in rodents, but its interpretation for risk assessment is currently hampered by the lack of understanding of the inter-species pharmacokinetics of PFOA. To address this uncertainty, a biologically supported dynamic model was developed whereby a two-compartment system linked via placental blood flow described gestation and milk production linked a lactating dam to a growing pup litter compartment. Postnatal serum levels of PFOA for 129S1/SvImJ mice at doses of 1mg/kg or less were reasonably simulated while prenatal and postnatal measurements for CD-1 mice at doses of 1mg/kg or greater were simulated via the addition of a biologically based saturable renal resorption description. Our results suggest that at low doses a linear model may suffice for describing the pharmacokinetics of PFOA while a more complex model may be needed at higher doses. Although mice may appear more sensitive based on administered dose of PFOA, the internal dose metrics estimated in this analysis indicate that they may be equal or less sensitive than rats.

Simultaneous quantification of perfusion and permeability in the prostate using dynamic contrast-enhanced magnetic resonance imaging with an inversion-prepared dual-contrast sequence

The aim of the present study was to quantify both perfusion and extravasation in the prostate to discriminate tumor from healthy tissue, which might be achieved by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) using a nonspecific low-molecular-weight contrast medium (CM). To determine extravasation as well as tissue perfusion an inversion-prepared dual-contrast sequence employing a parallel acquisition technique (PAT) was designed for interleaved acquisition of T(1)-weighted images for extravasation measurement and T(2)*-weighted images for determination of the highly concentrated bolus with a sufficiently high temporal and spatial resolution at an acceptable signal-to-noise ratio. Thirteen patients with proven prostate cancer were examined with the sequence using a combined body-array prostate coil. Before pharmacokinetic evaluation the images were intensity-corrected and, if required, motion-corrected. The pharmacokinetic model used to calculate perfusion, permeability, blood volume, interstitial volume, transit time, and vessel size index included two compartments and a correction of delay and dispersion of the arterial input function. The information provided by the dual-contrast sequence allowed application of a more elaborate model for evaluation and enabled quantification of all parameters. Peripheral prostate tumors were found to differ from peripheral healthy prostate tissue in perfusion (1.38 mL/(min cm(3)) vs. 0.23 mL/(min cm(3)), p=0.004), mean transit time (2.88 vs. 4.88 s, p=0.039), and blood volume (1.9 vs. 0.7%, p=0.019). A inversion-prepared dual-contrast sequence acquiring T(1)- and T*(2)-weighted images with sufficient temporal resolution and signal-to-noise ratio was successfully applied in patients with prostate cancer to quantify all pharmacokinetic parameters of inflow and extravasation of a low-molecular-weight inert tracer.

Has the Time Arrived to Image Placental Perfusion?

Taillieu et al have shown that it is possible to noninvasively measure the placental blood flow, fractional volume of the maternal vascular placental compartment, and rate of transfer of contrast material between the maternal and fetal circulation in gravid mice through the use of dynamic contrast-enhanced MR imaging. Clearly, much work lies ahead before functional evaluation of the placenta becomes a clinical reality, but contrast-enhanced imaging shows promise for functional evaluation of placental disease.