Magnetic resonance imaging of the fetal brain

Evaluation of cerebral fissure depths measured by prenatal ultrasonography in healthy fetuses at 20-24 weeks gestational age

AIM

We aimed to establish normal reference ranges for insula, sylvian fissure (SF), parieto-occipital fissure (POF), and calcarine fissure (CF) measured by prenatal ultrasonography (USG) between 20-24 weeks of gestation in healthy fetuses.

METHOD

A total of 186 fetuses in the second trimester were evaluated by transabdominal USG. All measurements were obtained by a single clinician. The study was divided into four subgroups (Group A: 20-20 weeks six days, Group B: 21-21 weeks six days, Group C: 22-22 weeks six days, Group D: 23-23 weeks six days).

RESULTS

Eight fetuses (4.23 %) between 20 and 21 weeks of gestation could not be included in the study because the sulcus borders could not be clearly evaluated. Measurements were obtained in all fetuses over 21 weeks of gestation. Reference ranges were obtained for insula, SF, POF, and CF in all fetuses and subgroups. At 20 and 23 weeks and six days gestation, mean insula depth was 14.96 ± 1.62 mm (min 11.0 mm - max 18.9 mm), mean SF depth was 6.96 ± 1.35 mm (min 3.6 mm - max 10.0 mm), mean POF depth was 2.05 ± 0.66 mm (min 1.1 mm - max 5.6 mm) and mean CF depth was 2.42 ± 0.68 mm (min 1.5 mm - 6.1 mm). There was a correlation between the cerebellum and cisterna magna and all fissure depths.

CONCLUSION

Our nomograms of healthy fetuses may be helpful in the early detection of cortical maturation abnormalities.

In vivo T2 measurements of the fetal brain using single-shot fast spin echo sequences

PURPOSE

We propose a quantitative framework for motion-corrected T2 fetal brain measurements in vivo and validate the single-shot fast spin echo (SS-FSE) sequence to perform these measurements.

METHODS

Stacks of two-dimensional SS-FSE slices are acquired with different echo times (TE) and motion-corrected with slice-to-volume reconstruction (SVR). The quantitative T2 maps are obtained by a fit to a dictionary of simulated signals. The sequence is selected using simulated experiments on a numerical phantom and validated on a physical phantom scanned on a 1.5T system. In vivo quantitative T2 maps are obtained for five fetuses with gestational ages (GA) 21-35 weeks on the same 1.5T system.

RESULTS

The simulated experiments suggested that a TE of 400 ms combined with the clinically utilized TEs of 80 and 180 ms were most suitable for T2 measurements in the fetal brain. The validation on the physical phantom confirmed that the SS-FSE T2 measurements match the gold standard multi-echo spin echo measurements. We measured average T2s of around 200 and 280 ms in the fetal brain grey and white matter, respectively. This was slightly higher than fetal T2* and the neonatal T2 obtained from previous studies.

CONCLUSION

The motion-corrected SS-FSE acquisitions with varying TEs offer a promising practical framework for quantitative T2 measurements of the moving fetus.

microRNA Biology on Brain Development and Neuroimaging Approach

Proper brain development requires the precise coordination and orchestration of various molecular and cellular processes and dysregulation of these processes can lead to neurological diseases. In the past decades, post-transcriptional regulation of gene expression has been shown to contribute to various aspects of brain development and function in the central nervous system. MicroRNAs (miRNAs), short non-coding RNAs, are emerging as crucial players in post-transcriptional gene regulation in a variety of tissues, such as the nervous system. In recent years, miRNAs have been implicated in multiple aspects of brain development, including neurogenesis, migration, axon and dendrite formation, and synaptogenesis. Moreover, altered expression and dysregulation of miRNAs have been linked to neurodevelopmental and psychiatric disorders. Magnetic resonance imaging (MRI) is a powerful imaging technology to obtain high-quality, detailed structural and functional information from the brains of human and animal models in a non-invasive manner. Because the spatial expression patterns of miRNAs in the brain, unlike those of DNA and RNA, remain largely unknown, a whole-brain imaging approach using MRI may be useful in revealing biological and pathological information about the brain affected by miRNAs. In this review, we highlight recent advancements in the research of miRNA-mediated modulation of neuronal processes that are important for brain development and their involvement in disease pathogenesis. Also, we overview each MRI technique, and its technological considerations, and discuss the applications of MRI techniques in miRNA research. This review aims to link miRNA biological study with MRI analytical technology and deepen our understanding of how miRNAs impact brain development and pathology of neurological diseases.

A Sparse Volume Reconstruction Method for Fetal Brain MRI Using Adaptive Kernel Regression

Slice-to-volume reconstruction (SVR) method can deal well with motion artifacts and provide high-quality 3D image data for fetal brain MRI. However, the problem of sparse sampling is not well addressed in the SVR method. In this paper, we mainly focus on the sparse volume reconstruction of fetal brain MRI from multiple stacks corrupted with motion artifacts. Based on the SVR framework, our approach includes the slice-to-volume 2D/3D registration, the point spread function- (PSF-) based volume update, and the adaptive kernel regression-based volume update. The adaptive kernel regression can deal well with the sparse sampling data and enhance the detailed preservation by capturing the local structure through covariance matrix. Experimental results performed on clinical data show that kernel regression results in statistical improvement of image quality for sparse sampling data with the parameter setting of the structure sensitivity 0.4, the steering kernel size of 7 × 7 × 7 and steering smoothing bandwidth of 0.5. The computational performance of the proposed GPU-based method can be over 90 times faster than that on CPU.

Biometric assessments of the posterior fossa by fetal MRI: A systematic review

BACKGROUND

Posterior fossa abnormalities (PFAs) are commonly identified within routine screening and are a frequent indication for fetal magnetic resonance imaging (MRI). Although biometric measurements of the posterior fossa (PF) are established on fetal ultrasound and MRI, qualitative visual assessments are predominantly used to differentiate PFAs.

OBJECTIVES

This systematic review aimed to assess 2-dimensional (2D) biometric measurements currently in use for assessing the PF on fetal MRI to delineate different PFAs.

METHODS

The protocol was registered (PROSPERO ID CRD42019142162). Eligible studies included T2-weighted MRI PF measurements in fetuses with and without PFAs, including measurements of the PF, or other brain areas relevant to PFAs.

RESULTS

59 studies were included - 6859 fetuses had 62 2D PF and related measurements. These included linear, area and angular measurements, representing measures of PF size, cerebellum/vermis, brainstem, and supratentorial measurements. 11 measurements were used in 10 or more studies and at least 1200 fetuses. These dimensions were used to characterise normal for gestational age, diagnose a range of pathologies, and predict outcome.

CONCLUSION

A selection of validated 2D biometric measurements of the PF on fetal MRI may be useful for identification of PFA in different clinical settings. Consistent use of these measures, both clinically and for research, is recommended.

The enigmatic fetal subplate compartment forms an early tangential cortical nexus and provides the framework for construction of cortical connectivity

The most prominent transient compartment of the primate fetal cortex is the deep, cell-sparse, synapse-containing subplate compartment (SPC). The developmental role of the SPC and its extraordinary size in humans remain enigmatic. This paper evaluates evidence on the development and connectivity of the SPC and discusses its role in the pathogenesis of neurodevelopmental disorders. A synthesis of data shows that the subplate becomes a prominent compartment by its expansion from the deep cortical plate (CP), appearing well-delineated on MR scans and forming a tangential nexus across the hemisphere, consisting of an extracellular matrix, randomly distributed postmigratory neurons, multiple branches of thalamic and long corticocortical axons. The SPC generates early spontaneous non-synaptic and synaptic activity and mediates cortical response upon thalamic stimulation. The subplate nexus provides large-scale interareal connectivity possibly underlying fMR resting-state activity, before corticocortical pathways are established. In late fetal phase, when synapses appear within the CP, transient the SPC coexists with permanent circuitry. The histogenetic role of the SPC is to provide interactive milieu and capacity for guidance, sorting, "waiting" and target selection of thalamocortical and corticocortical pathways. The new evolutionary role of the SPC and its remnant white matter neurons is linked to the increasing number of associative pathways in the human neocortex. These roles attributed to the SPC are regulated using a spatiotemporal gene expression during critical periods, when pathogenic factors may disturb vulnerable circuitry of the SPC, causing neurodevelopmental cognitive circuitry disorders.

Prenatal diagnosis of central nervous system abnormalities: Neurosonography versus fetal magnetic resonance imaging

OBJECTIVE

To share our experience in diagnosis of congenital central nervous system (CNS) abnormalities by fetal magnetic resonance imaging (MRI).

STUDY DESIGN

This study consisted of 110 pregnancies. Neurosonography (NS) findings were compared with MRI results. Anomalies were categorized into 10 groups: 1) Corpus callosum (CC) and cavum septum pellucidum (CSP) anomalies, 2) Neural tube defects (NTD), 3) Posterior fossa anomalies (PFA), 4) Primary ventriculomegaly (PVM), 5) Microcephaly, 6) Macrocephaly, 7) Periventricular leukomalacia (PVL), 8) Craniosynostosis, 9) Intracranial hemorrhage (ICH) and 10) Lumbosacral teratoma. Demographic features, clinical characteristics and perinatal outcomes of the study subjects were evaluated.

RESULTS

Gestational weeks for NS and for MRI were 25.5 and 26.5 weeks, respectively. Fourteen (12.7%) pregnancies were terminated. PVM (n = 36, 32.7%), CC and CSP anomalies (n = 29, 26.3%), PFA (n = 11, 10%) and NTD (n = 11, 10%) were the most common fetal MRI indications. There were no statistically significant differences between the accuracy of fetal NS and fetal MRI for CC and CSP anomalies, NTDs, PFA and PVM (p = 0.09, 0.43, 0.45 and 0.23, respectively). However, fetal MRI was more accurate for the detection of normal anatomic findings in cases with suspected microcephaly, macrocephaly and craniosynostosis in NS when pooled together (p = 0.007). Furthermore, MRI also seemed to be advantageous in CC & CSP anomalies though it was not validated by statistical measures. No statistically significant difference was found for diagnostic performance of NS and MRI according to gestational week (p = 0.27).

CONCLUSION

Fetal MRI in addition to NS may improve diagnostic accuracy in pregnancies with congenital CNS abnormalities.

Ex vivo fetal brain MRI: Recent advances, challenges, and future directions

During early development, the fetal brain undergoes dynamic morphological changes. These changes result from neurogenic events, such as neuronal proliferation, migration, axonal elongation, retraction, and myelination. The duration and intensity of these events vary across species. Comparative assessments of these neurogenic events give us insight into evolutionary changes and the complexity of human brain development. Recent advances in magnetic resonance imaging (MRI), especially ex vivo MRI, permit characterizing and comparing fetal brain development across species. Comparative ex vivo MRI studies support the detection of species-specific differences that occur during early brain development. In this review, we provide a comprehensive overview of ex vivo MRI studies that characterize early brain development in humans, monkeys, cats, as well as rats/mice. Finally, we discuss the current advantages and limitations of ex vivo fetal brain MRI.

The "vermian-crest angle": does it allow accurate categorisation of fetal upward rotation of cerebellar vermis on intrauterine MRI? A pilot study

AIM

To test a new parameter to assess the position of the fetal cerebellar vermis in the posterior fossa (PF) using intrauterine magnetic resonance imaging (MRI).

MATERIALS AND METHODS

The angle between the cerebellar vermis and the internal occipital crest (vermian-crest angle, VCA) was assessed retrospectively using MRI in fetuses with and without PF anomalies. Spearman's rank test was used to investigate correlation of the VCA with gestational age (GA). Groups were compared using Student's t-test and the one-way analysis of variance (ANOVA) with the Bonferroni adjustment. Box-and-whisker plots were also used.

RESULTS

One hundred and two normal cases were identified. Mean±SD GA at MRI was 26.5±2.8 weeks (range: 22-32 weeks). The VCA was 64.49±11.5° independently of GA (r=0.19; p=0.12). In addition, 30 fetuses at 19-28 weeks were identified with Blake's pouch cyst (BPC; n=5), Dandy-Walker malformation (DWM; n=12), mega cisterna magna (MCM; n=10), and vermian hypoplasia (VH; n=3). The VCA was significantly different in the DWM (p<0.001) and BPC (p<0.001) subgroups, but was not significantly different in cases of VH (p=0.84) and MCM (p=0.95) in comparison with controls.

CONCLUSIONS

A new method to assess vermian position within the PF using intrauterine MRI was assessed. In combination with the other existing parameters, it may be helpful for addressing the categorisation of upward rotation of the fetal cerebellar vermis; however, further studies are necessary to strengthen the present findings.

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.

Fetal neurology: Principles and practice with a life-course perspective

Clinical service, educational, and research components of a fetal/neonatal neurology program are anchored by the disciplines of developmental origins of health and disease and life-course science as programmatic principles. Prenatal participation provides perspectives on maternal, fetal, and placental contributions to health or disease for fetal and subsequent neonatal neurology consultations. This program also provides an early-life diagnostic perspective for neurologic specialties concerned with brain health and disease throughout childhood and adulthood. Animal models and birth cohort studies have demonstrated how the science of epigenetics helps to understand gene-environment interactions to better predict brain health or disease. Fetal neurology consultations provide important diagnostic contributions during critical or sensitive periods of brain development when future neurotherapeutic interventions will maximize adaptive neuroplasticity. Age-specific normative neuroinformatics databases that employ computer-based strategies to integrate clinical/demographic, neuroimaging, neurophysiologic, and genetic datasets will more accurately identify either symptomatic patients or those at risk for brain disorders who would benefit from preventive, rescue, or reparative treatment choices throughout the life span.

Optimizing maternal fat suppression with constrained image-based shimming in fetal MR

PURPOSE

Echo planar imaging (EPI) is the primary sequence for functional and diffusion MRI. In fetal applications, the large field of view needed to encode the maternal abdomen leads to prolonged EPI readouts, which may be further extended due to safety considerations that limit gradient performance. The resulting images become very sensitive to water-fat shift and susceptibility artefacts. The purpose of this study was to reduce artefacts and increase stability of EPI in fetal brain imaging, balancing local field homogeneity across the fetal brain with longer range variations to ensure compatibility with fat suppression of the maternal abdomen.

METHODS

Spectral Pre-saturation with Inversion-Recovery (SPIR) fat suppression was optimized by investigating SPIR pulse frequency offsets. Subsequently, fetal brain EPI data were acquired using image-based (IB) shimming on 6 pregnant women by (1) minimizing B0 field variations within the fetal brain (localized IB shimming) and (2) with added constraint to limit B0 variation in maternal fat (fat constrained IB shimming).

RESULTS

The optimal offset for the SPIR pulse at 3 Tesla was 550 Hz. Both shimming approaches had similar performances in terms of B0 homogeneity within the brain, but constrained IB shimming enabled higher fat suppression efficiency.

CONCLUSION

Optimized SPIR in combination with constrained IB shimming can improve maternal fat suppression while minimizing EPI distortions in the fetal brain.

Quantitative and Qualitative Analysis of Transient Fetal Compartments during Prenatal Human Brain Development

The cerebral wall of the human fetal brain is composed of transient cellular compartments, which show characteristic spatiotemporal relationships with intensity of major neurogenic events (cell proliferation, migration, axonal growth, dendritic differentiation, synaptogenesis, cell death, and myelination). The aim of the present study was to obtain new quantitative data describing volume, surface area, and thickness of transient compartments in the human fetal cerebrum. Forty-four postmortem fetal brains aged 13–40 postconceptional weeks (PCW) were included in this study. High-resolution T1 weighted MR images were acquired on 19 fetal brain hemispheres. MR images were processed using in-house software (MNI-ACE toolbox). Delineation of fetal compartments was performed semi-automatically by co-registration of MRI with histological sections of the same brains, or with the age-matched brains from Zagreb Neuroembryological Collection. Growth trajectories of transient fetal compartments were reconstructed. The composition of telencephalic wall was quantitatively assessed. Between 13 and 25 PCW, when the intensity of neuronal proliferation decreases drastically, the relative volume of proliferative (ventricular and subventricular) compartments showed pronounced decline. In contrast, synapse- and extracellular matrix-rich subplate compartment continued to grow during the first two trimesters, occupying up to 45% of telencephalon and reaching its maximum volume and thickness around 30 PCW. This developmental maximum coincides with a period of intensive growth of long cortico-cortical fibers, which enter and wait in subplate before approaching the cortical plate. Although we did not find significant age related changes in mean thickness of the cortical plate, the volume, gyrification index, and surface area of the cortical plate continued to exponentially grow during the last phases of prenatal development. This cortical expansion coincides developmentally with the transformation of embryonic cortical columns, dendritic differentiation, and ingrowth of axons. These results provide a quantitative description of transient human fetal brain compartments observable with MRI. Moreover, they will improve understanding of structural-functional relationships during brain development, will enable correlation between in vitro/in vivo imaging and fine structural histological studies, and will serve as a reference for study of perinatal brain injuries.

Added value of fetal MRI in fetuses with suspected brain abnormalities on neurosonography: a systematic review and meta-analysis

PURPOSE

To evaluate the additional diagnostic value of fetal Magnetic Resonance Imaging (MRI) in fetuses with suspected brain abnormalities identified with advanced neurosonography (NS).

METHODS

A systematic literature search was performed for studies reporting on a comparison between diagnosis with NS and MRI, in fetuses suspected for brain abnormalities. Abnormalities detected on NS were compared with those detected on MRI as well as with postnatal imaging findings to assess the added value of fetal MRI.

RESULTS

We included 27 articles, reporting on 1184 cases in which NS and MRI diagnosis were compared. In 65% of cases [773/1184] fetal NS and fetal MRI diagnosis agreed completely. In 23% [312/1184], MRI showed additional or different pathology. In 8% [99/1184], MRI rejected the NS diagnosis with normal brain as conclusion. For 454 cases a comparison with postnatal imaging could be made. Compared to the postnatal diagnosis, fetal MRI diagnosis agreed completely in 80% [364/454] and fetal NS in 54% [243/454] (difference 27%, 95% CI 21-33%). Additional abnormalities were found on postnatal imaging in 36% [164/454] after NS and in 14% [61/454] after fetal MRI.

CONCLUSIONS

This meta-analysis shows that fetal MRI in addition to NS improves diagnostic accuracy in detecting brain abnormalities.

T2* relaxometry of fetal brain at 1.5 Tesla using a motion tolerant method

PURPOSE

The aim of this study was to determine T2* values for the fetal brain in utero and to compare them with previously reported values in preterm and term neonates. Knowledge of T2* may be useful for assessing brain development, brain abnormalities, and for optimizing functional imaging studies.

METHODS

Maternal respiration and unpredictable fetal motion mean that conventional multishot acquisition techniques used in adult T2* relaxometry studies are not practical. Single shot multiecho echo planar imaging was used as a rapid method for measuring fetal T2* by effectively freezing intra-slice motion.

RESULTS

T2* determined from a sample of 24 subjects correlated negatively with gestational age with mean values of 220 ms (±45) for frontal white matter, 159 ms (±32) for thalamic gray matter, and 236 ms (±45) for occipital white matter.

CONCLUSION

Fetal T2* values are higher than those previously reported for preterm neonates and decline with a consistent trend across gestational age. The data suggest that longer than usual echo times or direct T2* measurement should be considered when performing fetal fMRI to reach optimal BOLD sensitivity.

Fetal MRI: A Technical Update with Educational Aspirations

Fetal magnetic resonance imaging (MRI) examinations have become well-established procedures at many institutions and can serve as useful adjuncts to ultrasound (US) exams when diagnostic doubts remain after US. Due to fetal motion, however, fetal MRI exams are challenging and require the MR scanner to be used in a somewhat different mode than that employed for more routine clinical studies. Herein we review the techniques most commonly used, and those that are available, for fetal MRI with an emphasis on the physics of the techniques and how to deploy them to improve success rates for fetal MRI exams. By far the most common technique employed is single-shot T2-weighted imaging due to its excellent tissue contrast and relative immunity to fetal motion. Despite the significant challenges involved, however, many of the other techniques commonly employed in conventional neuro- and body MRI such as T1 and T2*-weighted imaging, diffusion and perfusion weighted imaging, as well as spectroscopic methods remain of interest for fetal MR applications. An effort to understand the strengths and limitations of these basic methods within the context of fetal MRI is made in order to optimize their use and facilitate implementation of technical improvements for the further development of fetal MR imaging, both in acquisition and post-processing strategies.

Multidimensional analysis of fetal posterior fossa in health and disease

Fetal magnetic resonance imaging (MRI) is now routinely used to further investigate cerebellar malformations detected with ultrasound. However, the lack of 2D and 3D biometrics in the current literature hinders the detailed characterisation and classification of cerebellar anomalies. The main objectives of this fetal neuroimaging study were to provide normal posterior fossa growth trajectories during the second and third trimesters of pregnancy via semi-automatic segmentation of reconstructed fetal brain MR images and to assess common cerebellar malformations in comparison with the reference data. Using a 1.5-T MRI scanner, 143 MR images were obtained from 79 normal control and 53 fetuses with posterior fossa abnormalities that were grouped according to the severity of diagnosis on visual MRI inspections. All quantifications were performed on volumetric datasets, and supplemental outcome information was collected from the surviving infants. Normal growth trajectories of total brain, cerebellar, vermis, pons and fourth ventricle volumes showed significant correlations with 2D measurements and increased in second-order polynomial trends across gestation (Pearson r, p < 0.05). Comparison of normal controls to five abnormal cerebellum subgroups depicted significant alterations in volumes that could not be detected exclusively with 2D analysis (MANCOVA, p < 0.05). There were 15 terminations of pregnancy, 8 neonatal deaths, and a spectrum of genetic and neurodevelopmental outcomes in the assessed 24 children with cerebellar abnormalities. The given posterior fossa biometrics enhance the delineation of normal and abnormal cerebellar phenotypes on fetal MRI and confirm the advantages of utilizing advanced neuroimaging tools in clinical fetal research.

Registration of 3D fetal neurosonography and MRI

We propose a method for registration of 3D fetal brain ultrasound with a reconstructed magnetic resonance fetal brain volume. This method, for the first time, allows the alignment of models of the fetal brain built from magnetic resonance images with 3D fetal brain ultrasound, opening possibilities to develop new, prior information based image analysis methods for 3D fetal neurosonography. The reconstructed magnetic resonance volume is first segmented using a probabilistic atlas and a pseudo ultrasound image volume is simulated from the segmentation. This pseudo ultrasound image is then affinely aligned with clinical ultrasound fetal brain volumes using a robust block-matching approach that can deal with intensity artefacts and missing features in the ultrasound images. A qualitative and quantitative evaluation demonstrates good performance of the method for our application, in comparison with other tested approaches. The intensity average of 27 ultrasound images co-aligned with the pseudo ultrasound template shows good correlation with anatomy of the fetal brain as seen in the reconstructed magnetic resonance image.

Structural congenital brain disease in congenital heart disease: results from a fetal MRI program

OBJECTIVE

To identify the type and incidence of fetal brain pathology in fetuses with a prenatal diagnosis of congenital heart disease (CHD).

PATIENTS AND METHODS

67 pregnant women underwent a fetal MR-examinations between 20 and 38 gestational weeks. MR was done on a 1.5 T superconducting system. The type of cardiac malformation was defined by fetal echocardiography. Fetuses with a chromosomal abnormality or an extracardiac anomaly were excluded.

RESULTS

Fetal MRI scans in the final study cohort (53 fetuses) yielded normal results in 32 fetuses and a brain abnormality in 21 fetuses. Congenital brain disease (CBD) was found in 39% of the final study cohort of fetuses with CHD. MRI findings were classified into malformations, acquired lesions and widening of the ventricles and/or outer CSF spaces (malformations: 7 fetuses, acquired lesions: 5 fetuses, changes in CSF spaces: 9 fetuses). Asymmetry of the ventricles was the most common finding in the CSF group.

CONCLUSIONS

Our data suggest that fetal MRI can be used to characterize structural CBD in CHD. Advanced MRI techniques such as diffusion tensor imaging and proton spectroscopy are tools that, in the future, will certainly shed light on the spectrum of structural and functional CBDs that are associated with CHD.

Magnetic resonance methods in fetal neurology

Fetal magnetic resonance imaging (MRI) has become an established clinical adjunct for the in-vivo evaluation of human brain development. Normal fetal brain maturation can be studied with MRI from the 18th week of gestation to term and relies primarily on T2-weighted sequences. Recently diffusion-weighted sequences have gained importance in the structural assessment of the fetal brain. Diffusion-weighted imaging provides quantitative information about water motion and tissue microstructure and has applications for both developmental and destructive brain processes. Advanced magnetic resonance techniques, such as spectroscopy, might be used to demonstrate metabolites that are involved in brain maturation, though their development is still in the early stages. Using fetal MRI in addition to prenatal ultrasound, morphological, metabolic, and functional assessment of the fetus can be achieved. The latter is not only based on observation of fetal movements as an indirect sign of activity of the fetal brain but also on direct visualization of fetal brain activity, adding a new component to fetal neurology. This article provides an overview of the MRI methods used for fetal neurologic evaluation, focusing on normal and abnormal early brain development.

Motion-compensation techniques in neonatal and fetal MR imaging

SUMMARY

Fetal and neonatal MR imaging is increasingly used as a complementary diagnostic tool to sonography. MR imaging is an ideal technique for imaging fetuses and neonates because of the absence of ionizing radiation, the superior contrast of soft tissues compared with sonography, the availability of different contrast options, and the increased FOV. Motion in the normally mobile fetus and the unsettled, sleeping, or sedated neonate during a long acquisition will decrease image quality in the form of motion artifacts, hamper image interpretation, and often necessitate a repeat MR imaging to establish a diagnosis. This article reviews current techniques of motion compensation in fetal and neonatal MR imaging, including the following: 1) motion-prevention strategies (such as adequate patient preparation, patient coaching, and sedation, when required), 2) motion-artifacts minimization methods (such as fast imaging protocols, data undersampling, and motion-resistant sequences), and 3) motion-detection/correction schemes (such as navigators and self-navigated sequences, external motion-tracking devices, and postprocessing approaches) and their application in fetal and neonatal brain MR imaging. Additionally some background on the repertoire of motion of the fetal and neonatal patient and the resulting artifacts will be presented, as well as insights into future developments and emerging techniques of motion compensation.

An interdisciplinary fetal/neonatal neurology program

A fetal/neonatal neurology program should encompass interdisciplinary service, educational and research objectives, merging curricula concerning maternal, placental, fetal and neonatal contributions to brain health and disease. This approach is anchored by research in early life programming that demonstrates that prenatal and postnatal factors influence long-term neurologic health. This concept also supports the design of neuroprotective interventions during critical periods of brain development when brain circuitries more optimally adapt to maturational challenges. Preventive, rescue and repair protocols will transform pediatric medical practices, to promote improved childhood outcomes. Inclusion of life-course science and research will identify medical and socioeconomic factors that favorably or adversely affect quality of life into adulthood. Greater awareness of the convergence of developmental origins of brain health and disease and developmental aging theories will influence public health policies, to encourage financial support for programs that will improve the quality of life for the child and adult.

A review of fetal volumetry: the need for standardization and definitions in measurement methodology

Volume charts of fetal organs and structures vary considerably among studies. This review identified 42 studies reporting normal volumes, namely for fetal brain (n = 3), cerebellum (n = 4), liver (n = 6), femur (n = 2), lungs (n = 15), kidneys (n = 3) and first-trimester embryo (n = 9). The differences among median volumes were expressed both in percentage form and as standard deviation scores. Wide discrepancies in reported normal volumes make it extremely difficult to diagnose pathological organ growth reliably. Given its magnitude, this variation is likely to be due to inconsistencies in volumetric methodology, rather than population differences. Complicating factors include the absence of clearly defined anatomical landmarks for measurement; inadequate assessment and reporting of method repeatability; the inherent difficulty in validating fetal measurements in vivo against a reference standard; and a multitude of mutually incompatible three-dimensional (3D) imaging formats and software measuring tools. An attempt to standardize these factors would improve intra- and inter-researcher agreement concerning reported volumetric measures, would allow generalization of reference data across different populations and different ultrasound systems, and would allow quality assurance in 3D fetal biometry. Failure to ensure a quality control process may hamper the wide use of 3D ultrasound.

Comparison of measurements of the transverse diameter and perimeter of the fetal thymus obtained by magnetic resonance and ultrasound imaging

PURPOSE

To compare measurements of the fetal thymus obtained by magnetic resonance imaging (MRI) and ultrasound (US).

MATERIALS AND METHODS

Written informed consent was obtained from the patients that participated in this Institutional Review Board-approved observational study. The study population consisted of 17 pregnant women carrying fetuses between 21 and 34 weeks of gestation with suspected abnormalities. The transverse diameter and perimeter of the thymus were measured in these fetuses at the level of an axial view of the thorax that includes the pulmonary, aorta, and superior vena cava. The degree of agreement between MRI and US measurements was determined using Lin's concordance correlation coefficient and Bland-Altman analysis.

RESULTS

The mean (standard deviation, SD) gestational age at the time of the prenatal evaluation was 28.4 weeks (3.6). The thymus was measured by MRI and US in all cases. Comparison of the measurements from these two imaging modalities demonstrated a relatively good reproducibility with no evidence of systematic error.

CONCLUSION

MRI and US measurements of the fetal thymus during the second half of pregnancy are comparable. This finding suggests that MRI can become a useful adjuvant to US for assessment of the fetal thymus.

Fetal akinesia and associated abnormalities on prenatal MRI

OBJECTIVE

In view of the increasing role of magnetic resonance imaging (MRI) as an adjunct to prenatal ultrasonography (US), this study sought to demonstrate the visualization of fetal akinesia and associated abnormalities on MRI.

METHODS

This retrospective study included six fetuses with akinesia and associated abnormalities, depicted on fetal MRI after suspicious prenatal US. The whole fetus was assessed for musculoskeletal abnormalities and associated pathological conditions elsewhere. Fetal outcome data were compared with prenatal imaging. US and MRI findings were also compared.

RESULTS

Akinesia resulting in arthrogryposis was seen in 6/6 fetuses, with abnormal musculature in 5/6 fetuses. Associated brain abnormalities were found in 2/6 fetuses; facial abnormalities in 3/6; lung hypoplasia in 3/6; and polyhydramnios in 2/6. There were 5/6 pregnancies that were terminated and one individual died neonatally. MRI and brain autopsy were concordant in 4/6 cases. MRI and body autopsy were concordant in 1/6 cases and in 5/6 cases, autopsy revealed additional abnormalities. In addition to US, MRI correctly identified central nervous system findings in four cases and lung hypoplasia in three cases.

CONCLUSION

Our MRI results demonstrate fetal akinesia and associated abnormalities, which may have an impact on perinatal management, as an adjunct to prenatal US.

Magnetic resonance imaging of the fetus

Fetal magnetic resonance imaging (MRI) has become established as part of clinical practice in many centres worldwide especially when visualization of the central nervous system pathology is required. In this review we summarize the recent literature and provide an overview of fetal development and the commonly encountered fetal pathologies visualized with MRI and illustrated with numerous MR images. We aim to convey the role of fetal MRI in clinical practice and its value as an additional investigation alongside ultrasound yet emphasize the need for caution when interpreting fetal MR images especially where experience is limited.

Corroboration of normal and abnormal fetal cerebral lamination on postmortem MR imaging with postmortem examination

BACKGROUND AND PURPOSE

The presence of normal fetal cerebral lamination of the germinal matrix, intermediate zone, subplate layer, and cortex can be used as a marker of normal fetal cerebral development. Our aim was to compare postmortem MR imaging assessment of normal and abnormal fetal cerebral lamination on T1- and T2-weighted images with histopathology.

MATERIALS AND METHODS

Fifty-five formalin-fixed brains from postmortem fetuses, ranging from 16 to 30 weeks' gestational age, mean of 23 weeks, underwent T1- and T2- weighted MR imaging and subsequent sectioning and histologic examination. The cerebral lamination was graded as normal or abnormal on T1- and T2-weighted imaging and compared with postmortem findings. The sensitivity, specificity, and positive and negative predictive values of T1 and T2 assessment of cerebral lamination were calculated.

RESULTS

Twenty-six fetuses had abnormal and 29 had normal cerebral lamination on histology. On T1, the overall sensitivity, specificity, and positive and negative predictive values of evaluating cerebral lamination were 96.15%(CI, 78.42%-99.80%), 89.66%(CI, 71.50%-97.29%), 89.29%(CI, 70.63%-97.19%), and 96.29%(CI, 79.11%-99.80%), respectively. On T2, the overall sensitivity, specificity, and positive and negative predictive values of evaluating cerebral lamination were 73.08%(CI, 51.95%-87.65%), 96.55%(CI, 80.37%-99.82%), 95.00%(CI, 73.06%-99.74%), and 80.00%(CI, 62.54%-90.94%), respectively.

CONCLUSIONS

Postmortem MR imaging has high sensitivity, specificity, and positive and negative predictive values in assessing fetal cerebral lamination compared with histology. T1-weighted imaging has a higher sensitivity and negative predictive value, while T2-weighted imaging has a higher specificity and positive predictive value.

Robust super-resolution volume reconstruction from slice acquisitions: application to fetal brain MRI

Fast magnetic resonance imaging slice acquisition techniques such as single shot fast spin echo are routinely used in the presence of uncontrollable motion. These techniques are widely used for fetal magnetic resonance imaging (MRI) and MRI of moving subjects and organs. Although high-quality slices are frequently acquired by these techniques, inter-slice motion leads to severe motion artifacts that are apparent in out-of-plane views. Slice sequential acquisitions do not enable 3-D volume representation. In this study, we have developed a novel technique based on a slice acquisition model, which enables the reconstruction of a volumetric image from multiple-scan slice acquisitions. The super-resolution volume reconstruction is formulated as an inverse problem of finding the underlying structure generating the acquired slices. We have developed a robust M-estimation solution which minimizes a robust error norm function between the model-generated slices and the acquired slices. The accuracy and robustness of this novel technique has been quantitatively assessed through simulations with digital brain phantom images as well as high-resolution newborn images. We also report here successful application of our new technique for the reconstruction of volumetric fetal brain MRI from clinically acquired data.