FETAL NEUROIMAGING

Radial and tangential neuronal migration pathways in the human fetal brain: anatomically distinct patterns of diffusion MRI coherence

Corticogenesis is underpinned by a complex process of subcortical neuroproliferation, followed by highly orchestrated cellular migration. A greater appreciation of the processes involved in human fetal corticogenesis is vital to gaining an understanding of how developmental disturbances originating in gestation could establish a variety of complex neuropathology manifesting in childhood, or even in adult life. Magnetic resonance imaging modalities offer a unique insight into anatomical structure, and increasingly infer information regarding underlying microstructure in the human brain. In this study we applied a combination of high-resolution structural and diffusion-weighted magnetic resonance imaging to a unique cohort of three post-mortem fetal brain specimens, aged between 19 and 22 post-conceptual weeks. Specifically, we sought to assess patterns of diffusion coherence associated with subcortical neuroproliferative structures: the pallial ventricular/subventricular zone and subpallial ganglionic eminence. Two distinct three-dimensional patterns of diffusion coherence were evident: a clear radial pattern originating in ventricular/subventricular zone, and a tangentio-radial patterns originating in ganglionic eminence. These patterns appeared to regress in a caudo-rostral and lateral-ventral to medial-dorsal direction across the short period of fetal development under study. Our findings demonstrate for the first time distinct patterns of diffusion coherence associated with known anatomical proliferative structures. The radial pattern associated with dorsopallial ventricular/subventricular zone and the tangentio-radial pattern associated with subpallial ganglionic eminence are consistent with reports of radial-glial mediated neuronal migration pathways identified during human corticogenesis, supported by our prior studies of comparative fetal diffusion MRI and histology. The ability to assess such pathways in the fetal brain using MR imaging offers a unique insight into three-dimensional trajectories beyond those visualized using traditional histological techniques. Our results suggest that ex-vivo fetal MRI is a potentially useful modality in understanding normal human development and various disease processes whose etiology may originate in aberrant fetal neuronal migration.

Imaging diagnosis of congenital brain anomalies and injuries

Fetal brain is rapidly developing and changing its appearance week by week during pregnancy. The brain is the most important organ but it is quite hard to observe detailed structure of this organ by conventional transabdominal sonography. Transvaginal high-resolution ultrasound and three-dimensional (3D) ultrasound has been a great diagnostic tool for evaluation of three-dimensional structure of fetal central nervous system (CNS). This method has contributed to the prenatal assessment of congenital CNS anomalies, intracranial vascular anomalies and acquired brain damage in utero. It is possible to observe the whole brain structure by magnetic resonance imaging in the post half of pregnancy but transvaginal high-resolution 3D ultrasound is certainly powerful modality as well for understanding brain anatomy. Longitudinally and carefully evaluation of neurological short- or long-term prognosis should be required according to precise prenatal diagnosis, for proper counseling and management based on precise evidence.

Assessment of cerebellar vermis biometry at 18-32 weeks of gestation by three-dimensional ultrasound examination

OBJECTIVE

The purpose of this study was to construct reference limits for cerebellar vermis (CV) dimensions measured on images reconstructed from three-dimensional (3D) ultrasonography and to evaluate these measurements reproducibility.

METHODS

3D ultrasound volumes were acquired transabdominally from an axial view of the fetal head in 342 fetuses cross-sectionally studied between 18 to 32 weeks of gestation. Offline analysis of fetal brain midsagittal plane was used to evaluate length and area of CV. The agreement between two-dimensional (2D) and 3D measurements as well as the interobserver variability in 3D measurements were assessed by interclass correlation coefficients (ICC).

RESULTS

Adequate visualization of the midsagittal plane was obtained in 96.7% of the fetuses. CV length (r = 0.89, p < 0.0001) and CV area (r = 0.93, p < 0.0001) showed a significant linear growth with gestation. A good agreement was found between measurements from either 2D or 3D ultrasound views (CV length ICC 0.943, CV area ICC 0.940) as well as between measured obtained by different observers (CV length ICC 0.965, CV area ICC 0.905).

CONCLUSIONS

Measurements of the CV can be obtained from the midsagittal plane of fetal brain reconstructed from 3D volumes acquired transabdominally. The constructed nomograms may facilitate the diagnosis of cerebellar abnormalities.

An algorithm based on OmniView technology to reconstruct sagittal and coronal planes of the fetal brain from volume datasets acquired by three-dimensional ultrasound

OBJECTIVE

To describe a novel algorithm, based on the new display technology 'OmniView', developed to visualize diagnostic sagittal and coronal planes of the fetal brain from volumes obtained by three-dimensional (3D) ultrasonography.

METHODS

We developed an algorithm to image standard neurosonographic planes by drawing dissecting lines through the axial transventricular view of 3D volume datasets acquired transabdominally. The algorithm was tested on 106 normal fetuses at 18-24 weeks of gestation and the visualization rates of brain diagnostic planes were evaluated by two independent reviewers. The algorithm was also applied to nine cases with proven brain defects.

RESULTS

The two reviewers, using the algorithm on normal fetuses, found satisfactory images with visualization rates ranging between 71.7% and 96.2% for sagittal planes and between 76.4% and 90.6% for coronal planes. The agreement rate between the two reviewers, as expressed by Cohen's kappa coefficient, was > 0.93 for sagittal planes and > 0.89 for coronal planes. All nine abnormal volumes were identified by a single observer from among a series including normal brains, and eight of these nine cases were diagnosed correctly.

CONCLUSIONS

This novel algorithm can be used to visualize standard sagittal and coronal planes in the fetal brain. This approach may simplify the examination of the fetal brain and reduce dependency of success on operator skill.

Satisfactory rate of post-processing visualization of fetal cerebral axial, sagittal, and coronal planes from three-dimensional volumes acquired in routine second trimester ultrasound practice by sonographers of peripheral centers

OBJECTIVE

The aim of this study was to evaluate the feasibility to visualize central nervous system (CNS) diagnostic planes from three-dimensional (3D) brain volumes obtained in ultrasound facilities with no specific experience in fetal neurosonography.

METHODS

Five sonographers prospectively recorded transabdominal 3D CNS volumes starting from an axial approach on 500 consecutive pregnancies at 19-24 weeks of gestation undergoing routine ultrasound examination. Volumes were sent to the referral center (Department of Obstetrics and Gynecology, Università Roma Tor Vergata, Italy) and two independent reviewers with experience in 3D ultrasound assessed their quality in the display of axial, coronal, and sagittal planes.

RESULTS

CNS volumes were acquired in 491/500 pregnancies (98.2%). The two reviewers acknowledged the presence of satisfactory images with a visualization rate ranging respectively between 95.1% and 97.14% for axial planes, 73.72% and 87.16% for coronal planes, and 78.41% and 94.29% for sagittal planes. The agreement rate between the two reviewers as expressed by Cohen's kappa coefficient was >0.87 for axial planes, >0.89 for coronal planes, and >0.94 for sagittal planes. The presence of a maternal body mass index >30 alters the probability of achieving satisfactory CNS views, while existence of previous maternal lower abdomen surgery does not affect the quality of the reconstructed planes.

CONCLUSIONS

CNS volumes acquired by 3D ultrasonography in peripheral centers showed a quality high enough to allow a detailed fetal neurosonogram.

Collaborative study on 3-dimensional sonography for the prenatal diagnosis of central nervous system defects

OBJECTIVES

Prenatal diagnosis of central nervous system (CNS) anomalies by 2-dimensional sonography is challenging because of difficulties in obtaining complete visualization of the fetal brain during routine examinations, which is necessary for identification of its axial, coronal, and sagittal planes. Three-dimensional (3D) sonography has been introduced as a tool for studying the fetal CNS because of its ability to facilitate examinations of the fetal brain. The objective of this study was to determine inter-center agreement in diagnosing CNS defects by review of 3D volume data sets.

METHODS

This study included 11 centers with expertise in 3D fetal neurosonography. A total of 217 fetuses with and without confirmed CNS defects were scanned after 18 weeks' gestation, and their volume data sets were uploaded onto a centralized file transfer protocol server and later analyzed by all of the centers. Intercenter agreement was determined using a κ statistic for multiple raters.

RESULTS

All volumes were made anonymous and sent to the centers for blinded analysis with the exception of the data sets they had themselves previously uploaded. For identification of fetuses with CNS defects, the sensitivity, specificity, positive and negative predictive values, and false-positive and -negative rates were 93.3%, 96.5%, 96.5%, 93.3%, 3.5%, and 6.7%, respectively. No differences were found in the efficacy of the diagnostic indices according to either the route of acquisition (transabdominal or trans-vaginal) or the gestational age at diagnosis (18-24 or >24 weeks). Intercenter agreement was excellent (κ = 0.92; 95% confidence interval, 0.88-0.97).

CONCLUSIONS

Among centers with technical expertise, remote review of 3D sonographic volumes of the fetal CNS resulted in an accurate and reliable method for diagnosis of fetal brain malformations.

Assessment of corpus callosum biometric measurements at 18 to 32 weeks' gestation by 3-dimensional sonography

OBJECTIVES

The purposes of this study were to construct reference limits for corpus callosum dimensions measured on images reconstructed from 3-dimensional (3D) sonography and to evaluate the reproducibility of these measurements.

METHODS

Three-dimensional sonographic volumes were acquired transabdominally from an axial view of the head in 361 fetuses cross-sectionally studied at 18 to 32 weeks' gestation. Offline analysis of the fetal brain midsagittal plane was used to evaluate the length and area of the corpus callosum and corpus callosum-cavum septi pellucidi complex. The agreement between 2-dimensional (2D) and 3D measurements as well as the interobserver variability in 3D measurements were assessed by interclass correlation coefficients (ICCs).

RESULTS

Adequate visualization of the midsagittal plane was obtained in 98.1% of the fetuses. A clear distinction between the corpus callosum and cavum septi pellucidi was obtained in 35.7% of the fetuses, whereas in the remaining cases, the corpus callosum-cavum septi pellucidi complex was visualized as a single echogenic structure. The corpus callosum-cavum septi pellucidi complex length (r = 0.806; P < .0001), corpus callosum-cavum septi pellucidi complex area (r = 0.920; P < .0001), and corpus callosum area (r = 0.713; P < .0001) showed a significant linear growth with gestation. A good agreement was found between measurements from both 2D and 3D sonographic views (corpus callosum length ICC, 0.916) as well as between measurements obtained by different observers (corpus callosum-cavum septi pellucidi complex length ICC, 0.936; corpus callosum-cavum septi pellucidi complex area ICC, 0.931).

CONCLUSIONS

Measurements of the corpus callosum and cavum septi pellucidi can be obtained from the midsagittal plane of the fetal brain reconstructed from 3D volumes acquired transabdominally. The constructed nomograms may facilitate the diagnosis of corpus callosum abnormalities.

Morphological evaluation of lateral ventricles of fetuses with ventriculomegaly by three-dimensional ultrasonography and magnetic resonance imaging: correlation with etiology

PURPOSE

To evaluate morphology of lateral ventricles of ventriculomegaly/hydrocephaly fetuses using 3D-sonography by virtual organ computer-aided analysis (VOCAL) technique and magnetic resonance imaging (MRI) and verify morphologic patterns related to etiology.

METHODS

Seventeen fetuses presenting with ventricular enlargement (atria > 10 mm) were evaluated. 3D datasets were acquired from a coronal reference plane and post-processed by the rotational imaging using VOCAL 30°. MRI study was analyzed in the three plans in all sequences. Morphologic aspects such as global shape, anterior, posterior and inferior horn characteristics, wall irregularities and deformities were analyzed and related to etiology factor.

RESULTS

Twenty-nine percent of the cases were secondary to Arnold-Chiari syndrome and presented with global dilation of the three-horns. Cases related to aqueduct stenosis presented with ependymal rupture and wall irregularities in advanced cases. Corpus callosum agenesis cases presented with small ventricular volumes, thin shape, normal or slightly enlarged anterior and inferior horns with dilation restricted to posterior horn. Cases related to trisomy 18 and cytomegalovirus presented irregular ventricular walls associated with anomalous ventricular shapes, suggesting parenchymal destruction.

CONCLUSION

Ventricular morphology evaluation gives important information on etiology of ventricular enlargement, supporting prognosis prediction and decision making process of the affected fetuses and their families.

Fetal Brain Vascularity Visualized by Conventional 2D and 3D Power Doppler Technology

Significant advances have been made in accurate and reliable visualization of the cerebral circulation in normal and abnormal pregnancies. They provided the non-invasive studies of fetal cerebral angiogenesis and further development that filled some of the gaps made by neuroanatomical studies alone. The first breakthrough in the assessment of fetal circulation was development of Doppler system with purpose to obtain velocity waveforms. Continuing technical advances in Doppler ultrasound equipment, especially highly sensitive color flow imagining techniques have made it possible to study smaller anatomical parts of fetal circulation system including cerebral vascularization. Before examination of brain vascularity, anatomical vascular structure and development on the different appearance at each gestational age should be remembered as the basic knowledge. Since the development of the embryo is rapid and significant changes occur during even one week it is important to specify the stage of the embryo or fetus both by age (postmenstrual weeks and days) and by size (crownrump length (CRL) and biparietal diameter (BPD).

Introduction of three-dimensional (3D) sonography and 3D power Doppler techniques have enabled visualization of intracranial vessels. 3D power Doppler assessment particularly adds useful information in the prenatal evaluation of normal brain development, vascular malformation and tumoral vascularity.