Reflected hemodynamic waves influence the pattern of Doppler ultrasound waveforms along the umbilical arteries

The pulsatile pattern of blood motion measured by Doppler ultrasound within the umbilical artery is known to contain useful diagnostic information and is widely used to monitor pregnancies at risk of fetal growth restriction or stillbirth. Animal studies have identified reflected pressure waves traveling counter to the direction of blood flow as an important factor in the shape of these waveforms. In the present study, we establish a method to measure reflected waves in the human umbilical artery and assess their influence on blood velocity pulsation. Ninety-five pregnant women were recruited from a general obstetrics clinic between 26 and 37 wk of gestation and examined by Doppler ultrasound. Blood velocity waveforms were recorded for each umbilical artery at three locations along the umbilical cord. With the use of a computational procedure, a pair of forward and reverse propagating waves was identified to explain the variation in observed Doppler ultrasound waveforms along the cord. Among the data sets that met data quality requirements, waveforms in 93 of the 130 arteries examined agreed with the wave reflection model to within 1.5% and showed reflections ranging in magnitude from 3 to 52% of the forward wave amplitude. Strong reflections were associated with large differences in pulsatility between the fetal and placental ends of the cord. As reflections arise from transitions in the biomechanical properties of blood vessels, these observations provide a plausible mechanism for the link between abnormal waveforms and clinically significant placental pathology and could lead to more precise screening methods for detecting pregnancies complicated by placental disease. NEW & NOTEWORTHY The pulsatile pattern of blood motion measured by Doppler ultrasound within the umbilical artery is known to contain useful diagnostic information and is widely used to monitor pregnancies at risk of fetal growth restriction. We demonstrate based on a study of 95 pregnant women that the shape of these umbilical artery waveforms is explained by the presence of a reflected pressure wave traveling counter to the direction of blood flow.

Fetal hemodynamics and cardiac streaming assessed by 4D flow cardiovascular magnetic resonance in fetal sheep

BACKGROUND

To date it has not been possible to obtain a comprehensive 3D assessment of fetal hemodynamics because of the technical challenges inherent in imaging small cardiac structures, movement of the fetus during data acquisition, and the difficulty of fusing data from multiple cardiac cycles when a cardiac gating signal is absent. Here we propose the combination of volumetric velocity-sensitive cardiovascular magnetic resonance imaging ("4D flow" CMR) and a specialized animal preparation (catheters to monitor fetal heart rate, anesthesia to immobilize mother and fetus) to examine fetal sheep cardiac hemodynamics in utero.

METHODS

Ten pregnant Merino sheep underwent surgery to implant arterial catheters in the target fetuses. Anesthetized ewes underwent 4D flow CMR with acquisition at 3 T for fetal whole-heart coverage with 1.2-1.5 mm spatial resolution and 45-62 ms temporal resolution. Flow was measured in the heart and major vessels, and particle traces were used to visualize circulatory patterns in fetal cardiovascular shunts. Conservation of mass was used to test internal 4D flow consistency, and comparison to standard 2D phase contrast (PC) CMR was performed for validation.

RESULTS

Streaming of blood from the ductus venosus through the foramen ovale was visualized. Flow waveforms in the major thoracic vessels and shunts displayed normal arterial and venous patterns. Combined ventricular output (CVO) was 546 mL/min per kg, and the distribution of flows (%CVO) were comparable to values obtained using other methods. Internal 4D flow consistency across 23 measurement locations was established with differences of 14.2 ± 12.1%. Compared with 2D PC CMR, 4D flow showed a strong correlation (R2 = 0.85) but underestimated flow (bias = - 21.88 mL/min per kg, p < 0.05).

CONCLUSIONS

The combination of fetal surgical preparation and 4D flow CMR enables characterization and quantification of complex flow patterns in utero. Visualized streaming of blood through normal physiological shunts confirms the complex mechanism of substrate delivery to the fetal heart and brain. Besides offering insight into normal physiology, this technology has the potential to qualitatively characterize complex flow patterns in congenital heart disease phenotypes in a large animal model, which can support the development of new interventions to improve outcomes in this population.

Fast gas chromatography-atmospheric pressure (photo)ionization mass spectrometry of polybrominated diphenylether flame retardants

Gas chromatography (GC) and mass spectrometry (MS) are powerful, complementary techniques for the analysis of environmental toxicants. Currently, most GC-MS instruments employ electron ionization under vacuum, but the concept of coupling GC to atmospheric pressure ionization (API) is attracting revitalized interest. API conditions are inherently compatible with a wide range of ionization techniques as well high carrier gas flows that enable fast GC separations. This study reports on the application of atmospheric pressure chemical ionization (APCI) and a custom-built photoionization (APPI) source for the GC-MS analysis of polybrominated diphenyl ethers (PBDEs), a ubiquitous class of flame retardants. Photoionization of PBDEs resulted in the abundant formation of molecular ions M^•+ with very little fragmentation. Some photo-oxidation was observed, which differentiated critical BDE isomers. Formation of protonated molecules [M+H]⁺ did not occur in GC-APPI because the ionization energy of H₂O (clusters) exceeds the energy of the ionizing photons. Avoiding mixed-mode ionization is a major advantage of APPI over APCI, which requires careful control of the source conditions. A fast GC-API-MS method was developed using helium and nitrogen carrier gases that provides good separation of critical isomers (BDE-49/71) and elution of BDE 209 in less than 7 min (with He) and 15 min (with N₂). It will be shown that the GC-APPI and GC-APCI methods match the sensitivity and improve upon the selectivity and throughput of established methods for the analysis of PBDEs using standard reference materials (NIST SRM 1944 and SRM 2585) and selected environmental samples.

Placental vascular abnormalities in the mouse alter umbilical artery wave reflections

Current methods to detect placental vascular pathologies that monitor Doppler ultrasound changes in umbilical artery (UA) pulsatility have only moderate diagnostic utility, particularly in late gestation. In fetal mice, we recently demonstrated that reflected pressure waves propagate counter to the direction of flow in the UA and proposed the measurement of these reflections as a means to detect abnormalities in the placental circulation. In the present study, we used this approach in combination with microcomputed tomography to investigate the relationship between altered placental vascular architecture and changes in UA wave reflection metrics. Fetuses were assessed at embryonic day (E) 15.5 and E17.5 in control C57BL6/J mice and dams treated with combination antiretroviral therapy (cART), a known model of fetal growth restriction. Whereas the reflection coefficient was not different between groups at E15.5, it was 27% higher at E17.5 in cART-treated mice compared with control mice. This increase in reflection coefficient corresponded to a 36% increase in the total number of vessel segments, a measure of overall architectural complexity. Interestingly, there was no difference in UA pulsatility index between groups, suggesting that the wave reflections convey information about vascular architecture that is not captured by conventional ultrasound metrics. The wave reflection parameters were found to be associated with the morphology of the fetoplacental arterial tree, with the area ratio between the UA and first branch points correlating with the reflection coefficient. This study highlights the potential for wave reflection to aid in the noninvasive clinical assessment of placental vascular pathology. NEW & NOTEWORTHY We used a novel ultrasound methodology based on detecting pulse pressure waves that propagate along the umbilical artery to investigate the relationship between changes in wave reflection metrics and altered placental vascular architecture visualized by microcomputed tomography. Using pregnant mice treated with combination antiretroviral therapy, a model of fetal growth restriction, we demonstrated that reflections in the umbilical artery are sensitive to placental vascular abnormalities and associated with the geometry of the fetoplacental tree.

Altered myelin maturation in four year old children born very preterm

Children born very preterm (VPT; <32 weeks gestational age [GA]) are at greater risk for a range of cognitive deficits that typically manifest at school age. Here we examine the hypothesis that these children have altered myelin maturational that can be detected by myelin sensitive MRI measures prior to school age. We included 33 four-year old children born VPT (mean GA; 28.7 weeks) and 23 four-year old full term (FT) children and completed magnetization transfer (MT), T1-weighted (T1-w) and T2-weighted (T1-w) magnetic resonance imaging as well as developmental assessments. Both MT ratio (MTR) and T1-w/T2-w ratio images were calculated, and group differences were probed using tract-based spatial statistics (TBSS) in white matter, and region of interest (ROI) analysis in white, subcortical gray and cortical gray matter. The relations between MTR and T1-w/T2-w ratio, as well as with developmental assessments, were investigated in all three brain divisions. In children born VPT, TBSS and ROI analysis revealed that both MTR and T1-w/T2-w ratio were significantly reduced in white matter compared to children born FT. ROI analysis showed reductions in T1-w/T2-w ratio in VPT children compared to FT children in the thalamus, putamen and amygdala, as well as in the occipital and temporal lobes. Across the VPT and FT children, T1-w/T2-w ratio and MTR were highly correlated across white, subcortical gray and cortical gray matter. Both measures correlated positively with developmental assessments in individual white matter tracts and cortical and subcortical ROIs, suggesting that higher MTR and T1-w/T2-w ratio is related to better cognitive performance. Together these findings are consistent with delayed myelination in VPT born children.

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Very preterm children have widespread decreased MTR in white matter.

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T1-w/T2-w ratio measures showed consistent white matter alterations.

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T1-w/T2-w ratio was also reduced in subcortical, occipital and temporal regions.

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MTR and T1-w/T2-w were correlated throughout the brain.

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MTR and T1-w/T2-w correlated with developmental assessments.

Neurogliovascular dysfunction in a model of repeated traumatic brain injury

Traumatic brain injury (TBI) research has focused on moderate to severe injuries as their outcomes are significantly worse than those of a mild TBI (mTBI). However, recent epidemiological evidence has indicated that a series of even mild TBIs greatly increases the risk of neurodegenerative and psychiatric disorders. Neuropathological studies of repeated TBI have identified changes in neuronal ionic concentrations, axonal injury, and cytoskeletal damage as important determinants of later life neurological and mood compromise; yet, there is a paucity of data on the contribution of neurogliovascular dysfunction to the progression of repeated TBI and alterations of brain function in the intervening period.

Methods: Here, we established a mouse model of repeated TBI induced via three electromagnetically actuated impacts delivered to the intact skull at three-day intervals and determined the long-term deficits in neurogliovascular functioning in Thy1-ChR2 mice. Two weeks post the third impact, cerebral blood flow and cerebrovascular reactivity were measured with arterial spin labelling magnetic resonance imaging. Neuronal function was investigated through bilateral intracranial electrophysiological responses to optogenetic photostimulation. Vascular density of the site of impacts was measured with in vivo two photon fluorescence microscopy. Pathological analysis of neuronal survival and astrogliosis was performed via NeuN and GFAP immunofluorescence.

Results: Cerebral blood flow and cerebrovascular reactivity were decreased by 50±16% and 70±20%, respectively, in the TBI cohort relative to sham-treated animals. Concomitantly, electrophysiological recordings revealed a 97±1% attenuation in peri-contusional neuronal reactivity relative to sham. Peri-contusional vascular volume was increased by 33±2% relative to sham-treated mice. Pathological analysis of the peri-contusional cortex demonstrated astrogliosis, but no changes in neuronal survival.

Conclusion: This work provides the first in-situ characterization of the long-term deficits of the neurogliovascular unit following repeated TBI. The findings will help guide the development of diagnostic markers as well as therapeutics targeting neurogliovascular dysfunction.

Effects of placental growth factor deficiency on behavior, neuroanatomy, and cerebrovasculature of mice

Preeclampsia, a hypertensive syndrome occurring in 3-5% of human pregnancies, has lifelong health consequences for fetuses. Cognitive ability throughout life is altered, and adult stroke risk is increased. One potential etiological factor for altered brain development is low concentrations of proangiogenic placental growth factor (PGF). Impaired PGF production may promote an antiangiogenic fetal environment during neural and cerebrovascular development. We previously reported delayed vascularization of the hindbrain, altered retinal vascular organization, and less connectivity in the circle of Willis in Pgf^-/- mice. We hypothesized Pgf^-/- mice would have impaired cognition and altered brain neuroanatomy in addition to compromised cerebrovasculature. Cognitive behavior was assessed in adult Pgf^-/- and Pgf^+/+ mice by four paradigms followed by postmortem high-resolution MRI of neuroanatomy. X-ray microcomputed tomography imaging investigated the three-dimensional cerebrovascular geometry in another cohort. Pgf^-/- mice exhibited poorer spatial memory, less depressive-like behavior, and superior recognition of novel objects. Significantly smaller volumes of 10 structures were detected in the Pgf^-/- compared with Pgf^+/+ brain. Pgf^-/- brain had more total blood vessel segments in the small-diameter range. Lack of PGF altered cognitive functions, brain neuroanatomy, and cerebrovasculature in mice. Pgf^-/- mice may be a preclinical model for the offspring effects of low-PGF preeclampsia gestation.

Involvement of the Amygdala in Memory and Psychosocial Functioning in Pediatric-Onset Multiple Sclerosis

Youth with multiple sclerosis (MS) often experience cognitive impairment and psychosocial disturbances. We describe the relationship between memory function, psychosocial skills, and brain volume in 32 patients with pediatric-onset MS and 30 controls. Amygdala volume was significantly lower in patients compared with controls. In general, poorer memory was associated with reduced functional communication skills and reduced amygdala volume. Greater amygdala volume in patients correlated with parent-reported functional communication and social skills. Adjusting for whole-brain volume, right amygdala volume was positively associated with visual memory; left amygdala volume was a stronger predictor of parent-reported social skills.

Structural covariance of brain region volumes is associated with both structural connectivity and transcriptomic similarity

An organizational pattern seen in the brain, termed structural covariance, is the statistical association of pairs of brain regions in their anatomical properties. These associations, measured across a population as covariances or correlations usually in cortical thickness or volume, are thought to reflect genetic and environmental underpinnings. Here, we examine the biological basis of structural volume covariance in the mouse brain. We first examined large scale associations between brain region volumes using an atlas-based approach that parcellated the entire mouse brain into 318 regions over which correlations in volume were assessed, for volumes obtained from 153 mouse brain images via high-resolution MRI. We then used a seed-based approach and determined, for 108 different seed regions across the brain and using mouse gene expression and connectivity data from the Allen Institute for Brain Science, the variation in structural covariance data that could be explained by distance to seed, transcriptomic similarity to seed, and connectivity to seed. We found that overall, correlations in structure volumes hierarchically clustered into distinct anatomical systems, similar to findings from other studies and similar to other types of networks in the brain, including structural connectivity and transcriptomic similarity networks. Across seeds, this structural covariance was significantly explained by distance (17% of the variation, up to a maximum of 49% for structural covariance to the visceral area of the cortex), transcriptomic similarity (13% of the variation, up to maximum of 28% for structural covariance to the primary visual area) and connectivity (15% of the variation, up to a maximum of 36% for structural covariance to the intermediate reticular nucleus in the medulla) of covarying structures. Together, distance, connectivity, and transcriptomic similarity explained 37% of structural covariance, up to a maximum of 63% for structural covariance to the visceral area. Additionally, this pattern of explained variation differed spatially across the brain, with transcriptomic similarity playing a larger role in the cortex than subcortex, while connectivity explains structural covariance best in parts of the cortex, midbrain, and hindbrain. These results suggest that both gene expression and connectivity underlie structural volume covariance, albeit to different extents depending on brain region, and this relationship is modulated by distance.

Longitudinal Study of White Matter Development and Outcomes in Children Born Very Preterm

Identifying trajectories of early white matter development is important for understanding atypical brain development and impaired functional outcomes in children born very preterm (<32 weeks gestational age [GA]). In this study, 161 diffusion images were acquired in children born very preterm (median GA: 29 weeks) shortly following birth (75), term-equivalent (39), 2 years (18), and 4 years of age (29). Diffusion tensors were computed to obtain measures of fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD), which were aligned and averaged. A paediatric atlas was applied to obtain diffusion metrics within 12 white matter tracts. Developmental trajectories across time points demonstrated age-related changes which plateaued between term-equivalent and 2 years of age in the majority of posterior tracts and between 2 and 4 years of age in anterior tracts. Between preterm and term-equivalent scans, FA rates of change were slower in anterior than posterior tracts. Partial least squares analyses revealed associations between slower MD and RD rates of change within the external and internal capsule with lower intelligence quotients and language scores at 4 years of age. These results uniquely demonstrate early white matter development and its linkage to cognitive functions.

Altered white matter development in children born very preterm

Children born very preterm (VPT) at less than 32 weeks' gestational age (GA) are prone to disrupted white matter maturation and impaired cognitive development. The aims of the present study were to identify differences in white matter microstructure and connectivity of children born VPT compared to term-born children, as well as relations between white matter measures with cognitive outcomes and early brain injury. Diffusion images and T1-weighted anatomical MR images were acquired along with developmental assessments in 31 VPT children (mean GA: 28.76 weeks) and 28 term-born children at 4 years of age. FSL's tract-based spatial statistics was used to create a cohort-specific template and mean fractional anisotropy (FA) skeleton that was applied to each child's DTI data. Whole brain deterministic tractography was performed and graph theoretical measures of connectivity were calculated based on the number of streamlines between cortical and subcortical nodes derived from the Desikan-Killiany atlas. Between-group analyses included FSL Randomise for voxel-wise statistics and permutation testing for connectivity analyses. Within-group analyses between FA values and graph measures with IQ, language and visual-motor scores as well as history of white matter injury (WMI) and germinal matrix/intraventricular haemorrhage (GMH/IVH) were performed. In the children born VPT, FA values within major white matter tracts were reduced compared to term-born children. Reduced measures of local strength, clustering coefficient, local and global efficiency were present in the children born VPT within nodes in the lateral frontal, middle and superior temporal, cingulate, precuneus and lateral occipital regions. Within-group analyses revealed associations in term-born children between FA, Verbal IQ, Performance IQ and Full scale IQ within regions of the superior longitudinal fasciculus, inferior fronto-occipital fasciculus, forceps minor and forceps major. No associations with outcome were found in the VPT group. Global efficiency was reduced in the children born VPT with a history of WMI and GMH/IVH. These findings are evidence for under-developed and less connected white matter in children born VPT, contributing to our understanding of white matter development within this population.

Red blood cell antibody-induced anemia causes differential degrees of tissue hypoxia in kidney and brain

Moderate anemia is associated with increased mortality and morbidity, including acute kidney injury (AKI), in surgical patients. A red blood cell (RBC)-specific antibody model was utilized to determine whether moderate subacute anemia could result in tissue hypoxia as a potential mechanism of injury. Cardiovascular and hypoxic cellular responses were measured in transgenic mice capable of expressing hypoxia-inducible factor-1α (HIF-1α)/luciferase activity in vivo. Antibody-mediated anemia was associated with mild intravascular hemolysis (6 h) and splenic RBC sequestration ( day 4), resulting in a nadir hemoglobin concentration of 89 ± 13 g/l on day 4. At this time point, renal tissue oxygen tension (P_tO₂) was decreased in anemic mice relative to controls (13.1 ± 4.3 vs. 20.8 ± 3.7 mmHg, P < 0.001). Renal tissue hypoxia was associated with an increase in HIF/luciferase expression in vivo ( P = 0.04) and a 20-fold relative increase in renal erythropoietin mRNA transcription ( P < 0.001) but no increase in renal blood flow ( P = 0.67). By contrast, brain P_tO₂ was maintained in anemic mice relative to controls (22.7 ± 5.2 vs. 23.4 ± 9.8 mmHg, P = 0.59) in part because of an increase in internal carotid artery blood flow (80%, P < 0.001) and preserved cerebrovascular reactivity. Despite these adaptive changes, an increase in brain HIF-dependent mRNA levels was observed (erythropoietin: P < 0.001; heme oxygenase-1: P = 0.01), providing evidence for subtle cerebral tissue hypoxia in anemic mice. These data demonstrate that moderate subacute anemia causes significant renal tissue hypoxia, whereas adaptive cerebrovascular responses limit the degree of cerebral tissue hypoxia. Further studies are required to assess whether hypoxia is a mechanism for acute kidney injury associated with anemia.

Modelling and interpretation of magnetization transfer imaging in the brain

Magnetization transfer contrast has yielded insight into brain tissue microstructure changes across the lifespan and in a range of disorders. This progress has been aided by the development of quantitative magnetization transfer imaging techniques able to extract intrinsic properties of the tissue that are independent of the specifics of the data acquisition. While the tissue properties extracted by these techniques do not map directly onto specific cellular structures or pathological processes, a growing body of work from animal models and histopathological correlations aids the in vivo interpretation of magnetization transfer properties of tissue. This review examines the biophysical models that have been developed to describe magnetization transfer contrast in tissue as well as the experimental evidence for the biological interpretation of magnetization transfer data in health and disease.

Magnetic resonance thermometry of flowing blood

Blood temperature is a key determinant of tissue temperature and can be altered under normal physiological states, such as exercise, in diseases such as stroke or iatrogenically in therapies which modulate tissue temperature, such as therapeutic hypothermia. Currently available methods for the measurement of arterial and venous temperatures are invasive and, for small animal models, are impractical. Here, we present a methodology for the measurement of intravascular and tissue temperature by magnetic resonance imaging (MRI) using the lanthanide agent TmDOTMA^- (DOTMA, tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid; Tm, thulium). The approach makes use of phase-sensitive imaging measurements, combined with spectrally selective excitation, to monitor the temperature-dependent shift in the resonance of proton nuclei associated with water and with methyl groups of TmDOTMA^- . Measurements were first made in a flow phantom modelling diastolic blood flow in the mouse aorta or inferior vena cava (IVC) and imaged using 7-T preclinical MRI with a custom-built surface coil. Flowing and static fluid temperatures agreed to within 0.12°C for these experiments. Proof-of-concept experiments were also performed on three healthy adult mice, demonstrating temperature measurements in the aorta, IVC and kidney following a bolus injection of contrast agent. A small (0.7-1°C), but statistically significant, higher kidney temperature compared with the aorta (p = 0.002-0.007) and IVC (p = 0.003-0.03) was shown in all animals. These findings demonstrate the feasibility of the technique for in vivo applications and illustrate how the technique could be used to explore the relationship between blood and tissue temperature for a wide range of applications.

A mouse model of antepartum stillbirth

BACKGROUND

Many stillbirths of normally formed fetuses in the third trimester could be prevented via delivery if reliable means to anticipate this outcome existed. However, because the etiology of these stillbirths is often unexplained and although the underlying mechanism is presumed to be hypoxia from placental insufficiency, the placentas often appear normal on histopathological examination. Gestational age is a risk factor for antepartum stillbirth, with a rapid rise in stillbirth rates after 40 weeks' gestation. We speculate that a common mechanism may explain antepartum stillbirth in both the late-term and postterm periods. Mice also show increasing rates of stillbirth when pregnancy is artificially prolonged. The model therefore affords an opportunity to characterize events that precede stillbirth.

OBJECTIVE

The objective of the study was to prolong gestation in mice and monitor fetal and placental growth and cardiovascular changes.

STUDY DESIGN

From embryonic day 15.5 to embryonic day 18.5, pregnant CD-1 mice received daily progesterone injections to prolong pregnancy by an additional 24 hour period (to embryonic day 19.5). To characterize fetal and placental development, experimental assays were performed throughout late gestation (embryonic day 15.5 to embryonic day 19.5), including postnatal day 1 pups as controls. In addition to collecting fetal and placental weights, we monitored fetal blood flow using Doppler ultrasound and examined the fetoplacental arterial vascular geometry using microcomputed tomography. Evidence of hypoxic organ injury in the fetus was assessed using magnetic resonance imaging and pimonidazole immunohistochemistry.

RESULTS

At embryonic day 19.5, mean fetal weights were reduced by 14% compared with control postnatal day 1 pups. Ultrasound biomicroscopy showed that fetal heart rate and umbilical artery flow continued to increase at embryonic day 19.5. Despite this, the embryonic day 19.5 fetuses had significant pimonidazole staining in both brain and liver tissue, indicating fetal hypoxia. Placental weights at embryonic day 19.5 were 21% lower than at term (embryonic day 18.5). Microcomputed tomography showed no change in quantitative morphology of the fetoplacental arterial vasculature between embryonic day 18.5 and embryonic day 19.5.

CONCLUSION

Prolongation of pregnancy renders the murine fetus vulnerable to significant growth restriction and hypoxia because of differential loss of placental mass rather than any compromise in fetoplacental blood flow. Our data are consistent with a hypoxic mechanism of antepartum fetal death in human term and postterm pregnancy and validates the inability of umbilical artery Doppler to safely monitor such fetuses. New tests of placental function are needed to identify the late-term fetus at risk of hypoxia to intervene by delivery to avoid antepartum stillbirth.

Feto- and utero-placental vascular adaptations to chronic maternal hypoxia in the mouse

KEY POINTS

Chronic fetal hypoxia is one of the most common complications of pregnancy and is known to cause fetal growth restriction. The structural adaptations of the placental vasculature responsible for growth restriction with chronic hypoxia are not well elucidated. Using a mouse model of chronic maternal hypoxia in combination with micro-computed tomography and scanning electron microscopy, we found several placental adaptations that were beneficial to fetal growth including capillary expansion, thinning of the interhaemal membrane and increased radial artery diameters, resulting in a large drop in total utero-placental vascular resistance. One of the mechanisms used to achieve the rapid increase in capillaries was intussusceptive angiogenesis, a strategy used in human placental development to form terminal gas-exchanging villi. These results contribute to our understanding of the structural mechanisms of the placental vasculature responsible for fetal growth restriction and provide a baseline for understanding adaptive physiological responses of the placenta to chronic hypoxia.

ABSTRACT

The fetus and the placenta in eutherian mammals have a unique set of compensatory mechanisms to respond to several pregnancy complications including chronic maternal hypoxia. This study examined the structural adaptations of the feto- and utero-placental vasculature in an experimental mouse model of chronic maternal hypoxia (11% O₂ from embryonic day (E) 14.5-E17.5). While placental weights were unaffected by exposure to chronic hypoxia, using micro-computed tomography, we found a 44% decrease in the absolute feto-placental arterial vascular volume and a 30% decrease in total vessel segments in the chronic hypoxia group compared to control group. Scanning electron microscopy imaging showed significant expansion of the capillary network; consequently, the interhaemal membrane was 11% thinner to facilitate maternal-fetal exchange in the chronic hypoxia placentas. One of the mechanisms for the rapid capillary expansion was intussusceptive angiogenesis. Analysis of the utero-placental arterial tree showed significant increases (24%) in the diameter of the radial arteries, resulting in a decrease in the total utero-placental resistance by 2.6-fold in the mice exposed to chronic maternal hypoxia. Together these adaptations acted to preserve placental weight whereas fetal weight was decreased.

Arterio-venous fetoplacental vascular geometry and hemodynamics in the mouse placenta

INTRODUCTION

The fetoplacental vasculature network is essential for the exchange of nutrients, gases and wastes with the maternal circulation and for normal fetal development. The present study quantitatively compares arterial and venous morphological and functional differences in the mouse fetoplacental vascular network.

METHODS

High resolution X-ray micro-computed tomography was used to visualize the 3D geometry of the arterial and venous fetoplacental vasculature in embryonic day 15.5 CD-1 mice (n = 5). Automated image analysis was used to measure the vascular geometry of the approximately 4100 arterial segments and 3200 venous segments per specimen to simulate blood flow through these networks.

RESULTS

Both the arterial and venous trees demonstrated a hierarchical branching structure with 8 or 9 (arterial) or 8 (venous) orders. The venous tree was smaller in volume and overall dimensions than the arterial tree. Venous vessel diameters increased more rapidly than arteries with each successive order, leading to lower overall resistance, although the umbilical vein was notably smaller and of higher resistance than these scaling relationships would predict. Simulation of blood flow for these vascular networks showed that 57% of total resistance resides in the umbilical artery and arterial tree, 17% in the capillary bed, and 26% in the venous tree and umbilical vein.

DISCUSSION

A detailed examination of the mouse fetoplacental arterial and venous tree revealed features, such as the distribution of resistance and the dimension of the venous tree, that were both morphologically distinct from other vascular beds and that appeared adapted to the specialized requirements of sustaining a fetus.

Early neurovascular dysfunction in a transgenic rat model of Alzheimer's disease

Alzheimer's disease (AD), pathologically characterized by amyloid-β peptide (Aβ) accumulation, neurofibrillary tangle formation, and neurodegeneration, is thought to involve early-onset neurovascular abnormalities. Hitherto studies on AD-associated neurovascular injury have used animal models that exhibit only a subset of AD-like pathologies and demonstrated some Aβ-dependent vascular dysfunction and destabilization of neuronal network. The present work focuses on the early stage of disease progression and uses TgF344-AD rats that recapitulate a broader repertoire of AD-like pathologies to investigate the cerebrovascular and neuronal network functioning using in situ two-photon fluorescence microscopy and laminar array recordings of local field potentials, followed by pathological analyses of vascular wall morphology, tau hyperphosphorylation, and amyloid plaques. Concomitant to widespread amyloid deposition and tau hyperphosphorylation, cerebrovascular reactivity was strongly attenuated in cortical penetrating arterioles and venules of TgF344-AD rats in comparison to those in non-transgenic littermates. Blood flow elevation to hypercapnia was abolished in TgF344-AD rats. Concomitantly, the phase-amplitude coupling of the neuronal network was impaired, evidenced by decreased modulation of theta band phase on gamma band amplitude. These results demonstrate significant neurovascular network dysfunction at an early stage of AD-like pathology. Our study identifies early markers of pathology progression and call for development of combinatorial treatment plans.

Altered cerebral blood flow and cerebrovascular function after voluntary exercise in adult mice

The beneficial effects of physical exercise on brain health are well documented, yet how exercise modulates cerebrovascular function is not well understood. This study used continuous arterial spin labeling magnetic resonance imaging with a hypercapnic challenge to examine changes in cerebral blood flow and vascular function after voluntary exercise in healthy, adult mice. Thirty exercise mice and twenty-one control mice were imaged prior to the start of the exercise regime (at 12 weeks of age) and after 4 weeks of voluntary exercise. After the second in vivo imaging session, we performed high-resolution ex vivo anatomical brain imaging to correlate the structural brain changes with functional measures of flow and vascular reserve. We found that exercise resulted in increases in the normocapnic and hypercapnic blood flow in the hippocampus. Moreover, the change in normocapnic blood flow between pre-exercise and post-exercise was positively correlated to the hippocampal structure volume following exercise. There was no overall effect of voluntary exercise on blood flow in the motor cortex. Surprisingly, the hypercapnic hippocampal blood flow when measured prior to the start of exercise was predictive of subsequent exercise activity. Moreover, exercise was found to normalize this pre-existing difference in hypercapnic blood flow between mice.

Ultrasound detection of altered placental vascular morphology based on hemodynamic pulse wave reflection

Abnormally pulsatile umbilical artery (UA) Doppler ultrasound velocity waveforms are a hallmark of severe or early onset placental-mediated intrauterine growth restriction (IUGR), whereas milder late onset IUGR pregnancies typically have normal UA pulsatility. The diagnostic utility of these waveforms to detect placental pathology is thus limited and hampered by factors outside of the placental circulation, including fetal cardiac output. In view of these limitations, we hypothesized that these Doppler waveforms could be more clearly understood as a reflection phenomenon and that a reflected pulse pressure wave is present in the UA that originates from the placenta and propagates backward along the UA. To investigate this, we developed a new ultrasound approach to isolate that portion of the UA Doppler waveform that arises from a pulse pressure wave propagating backward along the UA. Ultrasound measurements of UA lumen diameter and flow waveforms were used to decompose the observed flow waveform into its forward and reflected components. Evaluation of CD1 and C57BL/6 mice at embryonic day (E)15.5 and E17.5 demonstrated that the reflected waveforms diverged between the strains at E17.5, mirroring known changes in the fractal geometry of fetoplacental arteries at these ages. These experiments demonstrate the feasibility of noninvasively measuring wave reflections that originate from the fetoplacental circulation. The observed reflections were consistent with theoretical predictions based on the area ratio of parent to daughters at bifurcations in fetoplacental arteries suggesting that this approach could be used in the diagnosis of fetoplacental vascular pathology that is prevalent in human IUGR. Given that the proposed measurements represent a subset of those currently used in human fetal surveillance, the adaptation of this technology could extend the diagnostic utility of Doppler ultrasound in the detection of placental vascular pathologies that cause IUGR.NEW & NOTEWORTHY Here, we describe a novel approach to noninvasively detect microvascular changes in the fetoplacental circulation using ultrasound. The technique is based on detecting reflection pulse pressure waves that travel along the umbilical artery. Using a proof-of-principle study, we demonstrate the feasibility of the technique in two strains of experimental mice.

B1 mapping for bias-correction in quantitative T1 imaging of the brain at 3T using standard pulse sequences

PURPOSE

B₁ mapping is important for many quantitative imaging protocols, particularly those that include whole-brain T₁ mapping using the variable flip angle (VFA) technique. However, B₁ mapping sequences are not typically available on many magnetic resonance imaging (MRI) scanners. The aim of this work was to demonstrate that B₁ mapping implemented using standard scanner product pulse sequences can produce B₁ (and VFA T₁ ) maps comparable in quality and acquisition time to advanced techniques.

MATERIALS AND METHODS

Six healthy subjects were scanned at 3.0T. An interleaved multislice spin-echo echo planar imaging double-angle (EPI-DA) B₁ mapping protocol, using a standard product pulse sequence, was compared to two alternative methods (actual flip angle imaging, AFI, and Bloch-Siegert shift, BS). Single-slice spin-echo DA B₁ maps were used as a reference for comparison (Ref. DA). VFA flip angles were scaled using each B₁ map prior to fitting T₁ ; the nominal flip angle case was also compared.

RESULTS

The pooled-subject voxelwise correlation (ρ) for B₁ maps (BS/AFI/EPI-DA) relative to the reference B₁ scan (Ref. DA) were ρ = 0.92/0.95/0.98. VFA T₁ correlations using these maps were ρ = 0.86/0.88/0.96, much better than without B₁ correction (ρ = 0.53). The relative error for each B₁ map (BS/AFI/EPI-DA/Nominal) had 95^th percentiles of 5/4/3/13%.

CONCLUSION

Our findings show that B₁ mapping implemented using product pulse sequences can provide excellent quality B₁ (and VFA T₁ ) maps, comparable to other custom techniques. This fast whole-brain measurement (∼2 min) can serve as an excellent alternative for researchers without access to advanced B₁ pulse sequences.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017;46:1673-1682.

Functional and anatomical evidence of cerebral tissue hypoxia in young sickle cell anemia mice

Cerebral ischemia is a significant source of morbidity in children with sickle cell anemia; however, the mechanism of injury is poorly understood. Increased cerebral blood flow and low hemoglobin levels in children with sickle cell anemia are associated with increased stroke risk, suggesting that anemia-induced tissue hypoxia may be an important factor contributing to subsequent morbidity. To better understand the pathophysiology of brain injury, brain physiology and morphology were characterized in a transgenic mouse model, the Townes sickle cell model. Relative to age-matched controls, sickle cell anemia mice demonstrated: (1) decreased brain tissue pO₂ and increased expression of hypoxia signaling protein in the perivascular regions of the cerebral cortex; (2) elevated basal cerebral blood flow , consistent with adaptation to anemia-induced tissue hypoxia; (3) significant reduction in cerebrovascular blood flow reactivity to a hypercapnic challenge; (4) increased diameter of the carotid artery; and (5) significant volume changes in white and gray matter regions in the brain, as assessed by ex vivo magnetic resonance imaging. Collectively, these findings support the hypothesis that brain tissue hypoxia contributes to adaptive physiological and anatomic changes in Townes sickle cell mice. These findings may help define the pathophysiology for stroke in children with sickle cell anemia.

Effects of voluntary exercise on structure and function of cortical microvasculature

Aerobic activity has been shown highly beneficial to brain health, yet much uncertainty still surrounds the effects of exercise on the functioning of cerebral microvasculature. This study used two-photon fluorescence microscopy to examine cerebral hemodynamic alterations as well as accompanying geometric changes in the cortical microvascular network following five weeks of voluntary exercise in transgenic mice endogenously expressing tdTomato in vascular endothelial cells to allow visualization of microvessels irrespective of their perfusion levels. We found a diminished microvascular response to a hypercapnic challenge (10% FiCO₂) in running mice when compared to that in nonrunning controls despite commensurate increases in transcutaneous CO₂ tension. The flow increase to hypercapnia in runners was 70% lower than that in nonrunners (p = 0.0070) and the runners' arteriolar red blood cell speed changed by only half the amount seen in nonrunners (p = 0.0085). No changes were seen in resting hemodynamics or in the systemic physiological parameters measured. Although a few unperfused new vessels were observed on visual inspection, running did not produce significant morphological differences in the microvascular morphometric parameters, quantified following semiautomated tracking of the microvascular networks. We propose that voluntary running led to increased cortical microvascular efficiency and desensitization to CO₂ elevation.