Sonography of the fetal posterior fossa: false appearance of mega-cisterna magna and Dandy-Walker variant

Three-dimensional volumetric study with VOCAL in normal and abnormal posterior fossa fetuses

Purpose: The aim of this study was to compare volumetric parameters in the abnormal and normal posterior fossa using the Virtual Organ Computer-aided AnaLysis (VOCAL™) technique to determine whether fetuses with an abnormal posterior fossa have different volumes.Methods: A prospective study was conducted on 17 fetuses with an abnormal posterior fossa including, Dandy Walker malformation (DWM) (n = 6), vermian hypoplasia (VH) (n = 3), mega cisterna magna (MCM) (n = 8), and 99 healthy control fetuses from 20 to 34 weeks' gestation. Measurement of the fetal cisterna magna and cerebellar volume was performed in the standard transcerebellar plane through the VOCAL™ method. To establish the correlation of volumes with gestational age, polynomial regression analysis was performed. For comparison between groups, univariate ANCOVA was performed using gestational age as a covariate. The reliability was analyzed by the intraclass correlation coefficient (ICC).Results: Cerebellar volume and cisterna magna volume were correlated with gestational age. Posterior fossa volume was significantly larger in DWM (p < .0001) and MCM (p < .0001) in comparison to the control group. In VH group, cisterna magna volume does not seem to expand (p = .298). Cerebellar volume does not seem to change in subgroups when the influence of gestational age is discarded (p = .09). The ratio of cerebellar volume to the cisterna magna volume decreases significantly in abnormal fetuses (p < .0001). Good intraobserver and interobserver reliabilities were found for both cerebellum and cisterna magna measurements.Conclusions: Volume analysis may have a role in discrimination of different posterior fossa pathologies.

Quantitative Evaluation of the Fetal Cerebellar Vermis Using the Median View on Two-Dimensional Ultrasound

Background

Evaluation of the cerebellum and vermis is one of the integral parts of the fetal cranial anomaly screening.

Objectives

The aim of this study was to create a nomogram for fetal vermis measurements between 17 and 30 gestational weeks.

Patients and Methods

This prospective study was conducted on 171 volunteer pregnant women between March 2013 and December 2014. Measurements of the fetal cerebellar vermis diameters in the sagittal plane were performed by two-dimensional transabdominal ultrasonography.

Results

Optimal median planes were obtained in 117 of the cases. Vermian diameters as a function of gestational age were expressed by regression equations and the correlation coefficients were found to be highly statistically significant (P < 0.001). The normal mean (± standard deviation) for each gestational week was also defined.

Conclusion

This study presents the normal range of the two-dimensional fetal vermian measurements between 17 and 30 gestational weeks. In the absence of a three-dimensional ultrasonography, two-dimensional ultrasonography could also be used confidently with more time and effort.

Reference ranges of fetal cisterna magna length and area measurements by 3-dimensional ultrasonography using the multiplanar mode

This study aimed to establish reference values for the length and area of the fetal cisterna magna using the multiplanar mode of 3-dimensional ultrasonography. A cross-sectional study including 224 normal pregnant women between 17 weeks 0 days and 29 weeks 6 days of gestation was carried out. The area and length of the fetal cisterna magna were measured in the axial plane at the level of the cerebellar transverse diameter. Reliability was determined by intraclass correlation coefficient. The mean length and area of the fetal cisterna magna ranged from 0.50 ± 0.10 to 0.79 ± 0.18 cm and 0.95 ± 0.18 to 3.09 ± 0.62 cm(2), respectively. Intraobserver reliability for the length and area (intraclass coefficients: 0.86 and 0.91, respectively) and interobserver reliability (intraclass coefficients: 0.64 and 0.82, respectively) were good. Three-dimensional ultrasonography using the multiplanar mode is a reliable method for the determination of reference values for the length and area of the fetal cisterna magna.

Fetal CNS: a systematic approach

Prenatal sonography is routinely used to evaluate fetal biometry and anatomy between 16 and 20 weeks. Ventriculomegaly is easily seen on these routine views and is commonly associated with numerous intracranial anomalies. Although ventriculomegaly can be isolated, it should always prompt a detailed search to evaluate for an underlying cause. Using a systematic approach to evaluate the intracranial structures can help the clinician arrive at a correct diagnosis for many abnormalities of central nervous system.

The fundamentals of fetal MR imaging: Part 1

Congenital malformations detected in any fetal system using ultrasound may be further evaluated with magnetic resonance imaging (MRI) to improve counseling, to plan deliveries appropriately, and sometimes to enable fetal interventions. In this first half of a 2-part review, the history and safety factors regarding fetal MRI, as well as the practical aspects of image acquisition, are discussed. In addition, as central nervous system anomalies are most commonly and best evaluated using fetal MRI, challenging central nervous system anomalies, such as fetal ventriculomegaly, posterior anomalies, and neural tube defects, detected using prenatal ultrasound are also reviewed with a focus on the fundamental implications of these diagnoses.

Prenatal diagnosis and outcome of fetal posterior fossa fluid collections

OBJECTIVE

To evaluate the accuracy of fetal imaging in differentiating between diagnoses involving posterior fossa fluid collections and to investigate the postnatal outcome of affected infants.

METHODS

This was a retrospective study of fetuses with posterior fossa fluid collections, carried out between 2001 and 2010 in two referral centers for prenatal diagnosis. All fetuses underwent multiplanar neurosonography. Parents were also offered fetal magnetic resonance imaging (MRI) and karyotyping. Prenatal diagnosis was compared with autopsy or postnatal MRI findings and detailed follow-up was attempted by consultation of medical records and interview with parents and pediatricians.

RESULTS

During the study period, 105 fetuses were examined, at a mean gestational age of 24 (range, 17-28) weeks. Sonographic diagnoses (Blake's pouch cyst, n = 32; megacisterna magna, n = 27; Dandy-Walker malformation, n = 26; vermian hypoplasia, n = 17; cerebellar hypoplasia, n = 2; arachnoid cyst, n = 1) were accurate in 88% of the 65 cases in which confirmation was possible. MRI proved more informative than ultrasound in only 1/51 cases. Anatomic anomalies and/or chromosomal aberrations were found in 43% of cases. Blake's pouch cysts and megacisterna magna underwent spontaneous resolution in utero in one third of cases and over 90% of survivors without associated anomalies had normal developmental outcome at 1-5 years. Isolated Dandy-Walker malformation and vermian hypoplasia were associated with normal developmental outcome in only 50% of cases.

CONCLUSION

Prenatal neurosonography and MRI are similarly accurate in the categorization of posterior fossa fluid collections from mid gestation. Blake's pouch cyst and megacisterna magna are risk factors for associated anomalies but when isolated have an excellent prognosis, with a high probability of intrauterine resolution and normal intellectual development in almost all cases. Conversely, Dandy-Walker malformation and vermian hypoplasia, even when they appear isolated antenatally, are associated with an abnormal outcome in half of cases.

How accurately does current fetal imaging identify posterior fossa anomalies?

OBJECTIVE

The first objective of our study was to describe the prevalence and spectrum of posterior fossa anomalies over 5 years in a major fetal care center where the referral diagnosis (by fetal sonography) was investigated by fetal MRI and, if confirmed, by postnatal MRI if possible. The second objective was to assess the accuracy with which fetal MRI predicts postnatal MRI findings in this population.

MATERIALS AND METHODS

We retrospectively identified all cases of suspected fetal posterior fossa anomalies referred to our center from 2002 through 2006. We reviewed maternal, fetal, neonatal, and follow-up records of all cases and fetal and early postnatal imaging studies.

RESULTS

Of the 90 cases of suspected fetal posterior fossa anomalies (by fetal sonography) referred over the study period, 60 (67%) were confirmed by fetal MRI. Of 42 live-born infants, 39 (93%) underwent postnatal MRI. There was complete agreement in fetal and postnatal MRI diagnoses in 23 infants (59%). In 16 cases (41%), fetal and postnatal MRI diagnoses disagreed; postnatal MRI excluded fetal MRI diagnoses in six cases (15%) and revealed additional anomalies in 10 cases (26%).

CONCLUSION

Although a valuable adjunct to fetal sonography in cases of suspected posterior fossa anomaly, current fetal MRI, particularly in early gestation, has limitations in accurately predicting postnatal MRI abnormalities. Advancing the accuracy of MRI for the diagnosis of posterior fossa anomalies will require greater understanding of normal brain development and improved tissue resolution of fetal MRI. During the interim, our findings strongly support the need for postnatal MRI follow-up in cases with suspected posterior fossa anomalies by fetal MRI.

Dandy-Walker malformation complex: correlation between ultrasonographic diagnosis and postmortem neuropathology

OBJECTIVE

This autopsy-based study was designed to evaluate sonographic and neuropathologic findings of fetuses diagnosed prenatally with Dandy-Walker malformation complex.

METHODS

The retrospective study encompassed a series of 44 autopsy cases from 2 tertiary referral centers with a prenatal ultrasound diagnosis of Dandy-Walker malformation complex between 1995 and 2003. Ultrasound and pathology data from the cases and from age-matched controls were reviewed in a blinded manner. An unequivocal diagnosis of Dandy-Walker malformation complex from ultrasonography or pathology archival images required significant hypoplasia or aplasia of the cerebellar vermis.

RESULTS

Neuropathologic examination failed to confirm the prenatal diagnosis of Dandy-Walker malformation complex in 59% (26/44, 95% confidence interval [CI] 44-72) of the cases. After standardized reevaluation of high quality archival sonograms and pathology images, concordance remained poor at 55% (6/11 cases, 95% CI 28-79). Sonographic features that favored concordance included marked enlargement of the cisterna magna (> or = 10 mm), complete aplasia of the vermis, and a trapezoid-shaped gap between the cerebellar hemispheres. This latter finding contrasted with a keyhole-shaped gap in fetuses with no cerebellar neuropathology.

CONCLUSION

Correlation between a prenatal ultrasound diagnosis of Dandy-Walker malformation complex and autopsy neuropathology findings is poor. Unequivocal prenatal sonographic diagnosis of Dandy-Walker malformation complex should be reserved for cases with the classic findings of Dandy-Walker malformation, including enlargement of the cisterna magna, aplasia of the vermis, and a trapezoid-shaped, rather than keyhole-shaped, interhemispheric gap.

LEVEL OF EVIDENCE

III.

The Fetal Cerebellar Vermis

Fetal magnetic resonance provides a new tool in the imaging of the posterior fossa and is proving useful in cases that are difficult to assess sonographically by allowing further assessment of the fourth ventricle, cisterna magna, and vermian growth and development. We describe various criteria with which to evaluate vermian growth, including vermian biometry and the relationship between the superior and inferior lobes. We demonstrate 2 markers of normal vermian development: the primary fissure and fastigial point. We illustrate the tegmento-vermian angle, "closure" of the fourth ventricle, and communication of the fourth ventricle with the basal cisterns during development and in several disorders. We correlate those features with the expected embryological course of development and illustrate identification of these features and associated abnormalities of the posterior fossa, brain stem, and central nervous system in mid-trimester scans of fetuses with abnormal development. Correlation with contemporaneous ultrasound examinations is demonstrated.

Quality control of prenatal sonography in detecting trisomy 18. The value of perinatal autopsy

AIMS

This study was designed to compare the prenatal ultrasound findings and postmortem pathologic findings of fetuses with trisomy 18.

STUDY DESIGN

Of 22,150 fetal chromosome analyses, 70 fetuses with trisomy 18 were diagnosed between 1990 and 2004. Sonographic and perinatal autopsy findings were compared by organ system and their correlation was assigned to 1 of 3 categories.

RESULTS

There were 164 separate major structural abnormalities found on autopsy. Of them, sonography detected 72 (43.9%). Among major defects the agreement was more than 75% of all abnormalities of these systems: central nervous system (80%), abdominal abnormalities (87.5%) and cystic hygroma (100%). Whereas, the sensitivity of sonography was lower in these organ systems: cardiac system (66.6%), facial abnormalities (26.3%), urinary system (27.3%) and extremities (8.7%). The rate of additional findings at autopsy was 56.1% and involved mainly 2 organ systems: face (including ear) and extremities (including hands and feet). Some ultrasound findings (n=15) were not confirmed at autopsy in our series.

CONCLUSIONS

This study confirms that perinatal autopsy provides additional information in many fetuses with trisomy 18. Besides obstetricians, pediatricians and geneticists, specialized perinatal pathologists have an important role in the multidisciplinary management of prenatally diagnosed fetal malformations. In addition, examining the correlation between sonography and pathologic findings may indicate potential markers for sonographic screening of trisomy 18.

Quantitative MRI measurements of human fetal brain development in utero

Magnetic resonance imaging (MRI) allows for high resolution imaging of the central nervous system. We have tested the feasibility of using MRI in conjunction with quantitative image analysis to perform volumetric measurements of the brain in the developing human fetus in utero. The database comprises MR images of a total of 56 fetuses (gestational age 25-41 weeks) referred because of suspected abnormalities due to ultrasound findings, family history or maternal illness and scanned on a 1.5 T MR system using a single-shot fast spin echo (SSFSE) T2 sequence, slice thickness 3 mm, no gap. Four out of the 56 scans could not be used in the analysis due to poor image quality. Automatic segmentation (using NIH Image routines) was found to be unreliable in these fetal brains, so cerebral, cerebellar and ventricular regions were traced manually. Ventricular volumes did not vary with gestational age in normal fetuses (N=27, R=0.05, p=0.8) while cerebral parenchyma and cerebellum volumes increased significantly during the same period (R=0.67, p=0.0002 and R=0.51, p=0.0066 respectively). Two calculated parameters: percent ventricular asymmetry and volume ratio of ventricles to hemispheric parenchyma were found to be very sensitive to ventricular pathology; such that the mean value of the latter in normal fetuses was 4.4%+/-0.56 (mean+/-SEM, N=27) compared to 34.3%+/-17.6 (N=6, p<0.0001) in fetuses with ventriculomegaly. These results support the use of image analysis and MRI to produce normal growth curves as well as quantitative severity assessments of brain pathologies in the developing human fetus.

Rotation of the vermis as a cause of enlarged cisterna magna on prenatal imaging

OBJECTIVES

Dandy-Walker complex is a continuum of developmental anomalies of the posterior fossa which includes vermian rotation. However, vermian rotation alone may be benign. The aim of this study was to describe our experience with sagittal-plane prenatal ultrasound in the diagnosis of rotation of the vermis in cases of suspected enlarged cisterna magna on routine antenatal imaging, and to describe the follow-up of these patients.

METHODS

Seven women, who were referred to our ultrasound unit for evaluation of an enlarged fetal cisterna magna and suspected agenesis of the vermis on axial-plane imaging, underwent further multiplanar studies of the posterior fossa and measurements of the vermis.

RESULTS

The mean maternal age was 27 (range, 20-33) years and the mean gestational age at diagnosis was 19.5 (range, 18-31) weeks. The standard axial plane image showed a 'direct communication' between the cisterna magna and the fourth ventricle. In the mid-sagittal plane, the vermis was clearly delineated, with posterosuperior rotation. Vermis size was within normal limits for gestational age in all cases. Findings were confirmed by prenatal magnetic resonance imaging (MRI) in two cases and postnatal MRI and/or sonography in five. During a mean follow-up of 4.5 (range, 1-7.5) years, all children developed normally, with no neurological complications.

CONCLUSION

The finding of an enlarged cisterna magna on standard- (axial-)plane ultrasound should be evaluated further in the sagittal plane to determine whether the cause is rotation of a normal vermis. This may spare patients unnecessary tests, anxiety and, in some cases, pregnancy termination.

Diagnosis of inferior vermian hypoplasia by fetal magnetic resonance imaging: potential pitfalls and neurodevelopmental outcome

OBJECTIVE

Advances in fetal magnetic resonance imaging allow the detection of subtle anatomic anomalies of unclear long-term clinical significance. The purpose of this study was to examine the accuracy of fetal magnetic resonance imaging in the diagnosis of isolated inferior vermian hypoplasia and to describe the neurodevelopmental outcome.

STUDY DESIGN

We reviewed all cases with fetal and postnatal magnetic resonance imaging studies between 1999 and 2003 and identified 19 cases with a diagnosis of isolated inferior vermian hypoplasia. We compared prenatal and postnatal magnetic resonance imaging studies and evaluated subjects using developmental scales.

RESULTS

Isolated inferior vermian hypoplasia was confirmed by postnatal magnetic resonance imaging in 68% of the patients (13/19); the remaining 6 patients had normal postnatal magnetic resonance imaging results. On developmental testing at mean age 19.8 +/- 4.9 months, 3 infants (23%) with confirmed postnatal diagnosis demonstrated motor and language delays and functional difficulties, and 2 infants (15%) had behavioral problems; none of the infants with normal postnatal magnetic resonance imaging studies were delayed.

CONCLUSION

Isolated inferior vermian hypoplasia in the second trimester may be over-diagnosed by fetal magnetic resonance imaging and therefore warrants postnatal magnetic resonance imaging confirmation.

A pathological approach to anomalies of the posterior fossa

BACKGROUND

Both clinical and postmortem diagnoses of posterior fossa anomalies remain difficult to make and to corroborate. This is particularly true for Dandy-Walker malformation and variant. Difficulties arise for a variety of reasons, including technical and methodological ones, but also because the conditions may overlap anatomically with others, most notably mega cisterna magna, Blake's pouch cyst, and posterior fossa (arachnoid) cysts. Family counseling is difficult and complicated not only by diagnostic uncertainties but by the highly variable prognosis.

METHODS

In this study, a systematic pathologic approach to study of the posterior fossa is put forth. The benefits of postmortem imaging and in situ and ex situ examination of posterior fossa contents are demonstrated.

RESULTS

Normative data for cerebellar width (transverse cerebellar diameter) and height and vermian width and height are derived for the fetal period. These data will help workers recognize changes in these structures, particularly hypoplasia.

CONCLUSIONS

Basic morphologic definitions of posterior fossa anomalies are advanced, in the hopes that better agreement can be reached between clinical and pathologic diagnoses and that better patient and family care will result.

The fetal cerebellar vermis: anatomy and biometric assessment using volume contrast imaging in the C-plane (VCI-C)

OBJECTIVES

To describe the normal appearance and study the biometry of the fetal cerebellar vermis by three-dimensional (3D) volume contrast imaging in the coronal (C-) plane (VCI-C).

METHODS

A total of 203 normally developed fetuses were examined prospectively at 18-33 weeks' gestation. At the level of the view used to measure the transverse cerebellar diameter (TCD), a VCI-C plane was displayed to examine, using a transabdominal probe, the fetal mid-saggital vermis. The volumes acquired were stored for later review and measurement of the anteroposterior (AP) diameter, craniocaudal (CC) diameter and surface area of the cerebellar vermis. Each dataset was evaluated by two independent observers. Measurements as a function of gestational age (GA), biparietal diameter (BPD), head circumference (HC) and TCD were expressed by regression equations. Interobserver variability was evaluated. Nomograms were produced. In order to validate the use of VCI in fetal biometry, datasets from 57 patients were selected arbitrarily for comparison of their VCI-C measurements with those from mid-sagittal sections of a stored 3D multiplanar examination. Intraclass correlation was used to evaluate the agreement between these measurements.

RESULTS

The mean maternal age was 32 years. We were able to measure mid-sagittal CC diameter, mid-sagittal AP diameter and cerebellar vermis surface area in all fetuses. Interobserver variability analysis showed no significant differences between the two observers (P > 0.05). Measurements of the cerebellar vermis (AP diameter, CC diameter and surface area) correlated linearly with GA, BPD, HC and TCD (r > or = 0.82, P < 0.0001). CC and AP diameters estimated from the mid-sagittal section of the multiplanar measurements were significantly correlated with VCI-C measurements (r = 0.96, P < 0.00001 and r = 0.95, P < 0.00001, respectively).

CONCLUSIONS

VCI-C is a valuable tool, allowing intrauterine assessment of the normal appearance of the fetal cerebellar vermis. The nomograms developed in this study should enable accurate evaluation of the cerebellar vermis.

Prenatal diagnosis in pregnancies at risk for Joubert syndrome by ultrasound and MRI

OBJECTIVES

To describe the prenatal imaging findings in fetuses at risk for Joubert syndrome (JS), review the literature and propose a protocol for prenatal diagnosis of JS using ultrasound and MRI.

METHODS

We reviewed prenatal ultrasound and fetal MRI studies in two pregnancies at 25% recurrence risk for JS and correlated these findings with gross neuropathology in one affected fetus.

RESULTS

While abnormalities such as occipital encephalocele or enlarged cisterna magna have been identified before mid-trimester, the definitive diagnosis of JS, based on core cerebellar findings, has only been possible after 17 weeks' gestation.

CONCLUSIONS

With longitudinal monitoring, it is possible to diagnose JS in at-risk pregnancies before 24 weeks' gestation. On the basis of our data and review of the literature, we propose a protocol for monitoring pregnancies at risk for JS, utilizing serial ultrasounds combined with fetal MRI at 20-22 weeks' gestation to maximize the accuracy of prenatal diagnosis.

Fetal Brain MRI: Segmentation and Biometric Analysis of the Posterior Fossa

This paper presents a novel approach to fetal magnetic resonance image segmentation and biometric analysis of the posterior fossa's midline structures. We developed a semi-automatic segmentation method (based on a region growing technique) and tested the algorithm on images of 104 normal fetuses. Using the segmented regions of interest (posterior fossa, vermis, and brainstem), we computed four relative area ratios. Statistical and clinical analysis of our results showed that the relative development of these structures appears to be independent of pregnancy term. In an additional study of 23 pathological cases, one of the four measurements was always significantly different from the corresponding value observed in normal cases.

Malformative intracranial cysts: diagnosis and outcome

INTRODUCTION

Prenatal investigations make it possible to follow up malformative intracranial cysts from their detection in utero through the postnatal period. By including those that will remain silent postnatally, precious information can be provided about their real natural history.

DIAGNOSIS

Contrary to common belief, the vast majority of these lesions, if not associated with other fetal anomalies, are benign in nature, remain clinically silent, do not evolve or even frequently regress spontaneously. They are compatible with a strictly normal life, whether requiring postnatal treatment or not.

TREATMENT

Surgery is rarely needed for the treatment of an evolving hydrocephalus or an expanding cyst.

PROGNOSIS

Clinical outcome is not correlated with cyst volume or location. Prognosis, therefore, does not rely upon brain deformation or translation but rather more upon brain integrity. To establish a correct prognosis all efforts must therefore aim to precisely analyze the brain anatomy. In this respect, fetal MRI is mandatory. Although chromosomal anomalies are rarely associated with isolated intracranial cysts, karyotype study is necessary.

CONCLUSION

The accuracy of diagnosis and prognosis depends mainly upon our ability to correctly interpret images, which in turn depends on the resolution of prenatal imaging and on experience. Median retrocerebellar fluid collections remain the most difficult to prognosticate, retrocerebellar cysts often being difficult to differentiate from mega cisterna magna and Dandy-Walker complex. In our hands, prenatal prognostication was correct in approximately 90% of the cases.

Ultrasonographic nomograms of the fetal fourth ventricle: additional tool for detecting abnormalities of the posterior fossa

OBJECTIVE

To characterize normal growth of the fetal fourth ventricle on ultrasonography throughout pregnancy.

METHODS

Consecutive biometric measurements and fetal organ scans were obtained from 299 patients undergoing fetal anatomic surveys between 13 and 40 weeks' gestation. Using 7- and 3.5-MHz transducers for early (13- to 17-week) and late (>17-week) examinations, respectively, we scanned the fetal head in the axial plane with special focus on the posterior fossa of the brain. The fourth ventricle was identified, and its anteroposterior diameter and width were measured. A "triangle" formula was used for calculating its circumference and area.

RESULTS

The fourth ventricle was shown as a hypoechoic triangle below the level of the cerebellum. A linear regression line of the fourth ventricle was observed across gestational age, and a first-degree correlation was found between gestational age and anteroposterior diameter of the fourth ventricle (r = 0.894; P < .0001; y = -0.84 + 0.23 x gestational age), its width (r = 0.657; P < .0001; y = 3.82 + 0.14 x gestational age), its circumference (r = 0.843; P < .0001; y = 5.11 + 0.58 x gestational age), and its area (r = 0.844; P < .0001; y = -10.11 + 1.17 x gestational age). Twelve enlarged fourth ventricles were found between 14 and 16 weeks, but results of follow-up scans at 20 weeks were normal.

CONCLUSIONS

An isolated enlarged fourth ventricle in the early second trimester might represent a normal variant; it should be followed, but decisions about the fate of the pregnancy should not be based solely on this finding.

Intracranial neonatal neurosonography: an update

This article reviews salient features of a normal neonatal cranial ultrasound examination with suggestions concerning techniques that take advantage of new developments in ultrasound technology. It also illustrates pathologic findings in such areas as congenital abnormalities, intracranial hemorrhage, and infection. Recent publications on the subject of neurodevelopmental outcome are explored, pointing out how varying descriptions of intraventricular hemorrhages affect their results.

Sonography of fetal cerebrospinal anomalies

Congenital anomalies of the central nervous system (CNS) are among the most frequent malformations. Current ultrasound equipment can give a precise diagnosis of many of these lesions from early gestation. High-resolution transvaginal probes play a major role both in allowing an early diagnosis and for better defining subtle details of both normal and abnormal cerebral anatomy. The diagnostic accuracy of prenatal ultrasound is, however, heavily dependent upon the expertise of the sonologist, the type of equipment employed, and the time dedicated to the scan. Fetal sonography is effective in identifying neural tube defects, although alpha-fetoprotein screening seems to give a greater sensitivity. The accuracy of ultrasound in the identification of CNS malformations other than neural tube defects remains unclear because of the ascertainment biases of the few large prospective studies that have been conducted to date.

The fetal cerebellar vermis: normal development as shown by transvaginal ultrasound

OBJECTIVE

To determine the normal appearance and development of the fetal cerebellar vermis using mid-sagittal planes obtained by transvaginal sonography.

SUBJECTS AND METHODS

A total of 101 fetuses were evaluated by transvaginal sonography between 21 and 39 weeks of gestation. The mid-sagittal antero-posterior and cranio-caudal vermian diameters, circumference and surface area were measured and calculated. Nomograms were produced.

RESULTS

The vermis was observed in 96% of the cases and all the measurements were performed in 92%. The vermis grows in a linear fashion throughout pregnancy. The growth pattern correlates well with gestational age, biparietal diameter, head circumference and transverse cerebellar diameter.

CONCLUSION

Transvaginal sonography is a valuable tool in the study of the fetal vermis during the second half of pregnancy. Knowledge of its normal appearance may help identify developmental anomalies and enable accurate prenatal counseling.

A new view of the neonatal brain: clinical utility of supplemental neurologic US imaging windows

Anterior fontanelle imaging has traditionally been the mainstay of neonatal cranial ultrasonography (US). However, this traditional approach has limited diagnostic accuracy, which has led to increasing use of two alternate neurologic US imaging techniques: posterior fontanelle imaging and mastoid fontanelle imaging. These alternate techniques can help detect pathologic conditions and structural malformations in the neonatal brain. Posterior fontanelle imaging allows improved detection of intraventricular hemorrhage. This technique better demonstrates subtle differences in echogenicity between clot and a choroid glomus and depicts clot extending into the occipital and temporal horns. Mastoid fontanelle imaging is particularly useful in detecting hemorrhage involving the brainstem, cerebellum, and subarachnoid cisterns. It greatly facilitates clot detection in the fourth ventricle and cisterna magna because the tissues surrounding these structures are normally echogenic. Mastoid fontanelle imaging can also help distinguish holoprosencephaly from aqueductal stenosis and identify small malformations of the posterior fossa. Color Doppler US may help identify normal variants such as calcar avis and lobular choroid plexus, and adjunct magnetic resonance imaging can help distinguish normal structures from a true Dandy-Walker variant. Use of posterior fontanelle imaging and mastoid fontanelle imaging can significantly augment the diagnostic power of neurologic US.

Magnetic resonance techniques in the evaluation of the fetal and neonatal brain

Magnetic resonance imaging (MRI) has contributed dramatically to our understanding of the newborn with neurologic problems. Recently developed magnetic resonance techniques, such as fetal MRI and MR spectroscopy, offer additional insight into normal and pathologic processes affecting the fetal and neonatal CNS. This article examines developmental abnormalities as reflected in neuroimaging studies and discusses some of the newer MR modalities and their capabilities.

Ultrafast MR imaging of the normal posterior fossa in fetuses

OBJECTIVE

The purpose of our study was to determine if a standard imaging protocol using ultrafast MR sequences could adequately reveal normal posterior fossa anatomy in fetuses and, if so, to document a template on MR imaging for normal posterior fossa development.

MATERIALS AND METHODS

A retrospective review found 66 MR imaging studies of 63 fetuses, 16-39 weeks' gestation age (mean, 25 weeks' gestation), who were referred between June 1996 and May 1999 for evaluation of non-central nervous system anomalies revealed on prenatal sonography. All fetuses had normal brains and spines on prenatal sonography. The standard MR imaging protocol included axial, sagittal, and coronal half-Fourier acquisition single-shot turbo spin echo (HASTE); sagittal and coronal two-dimensional fast low-angle shot (FLASH); and axial turbo T1-weighted FLASH images through the fetal brain. Structures that we analyzed were the fourth ventricle, the cisterna magna, the vermis, the cerebellar hemispheres, and the brainstem. Using the HASTE sequences, we documented gestational age-specific signal intensity changes in the cerebellar hemispheres and the brainstem.

RESULTS

The posterior fossa anatomy was sufficiently well defined to exclude abnormalities of the fourth ventricle and cerebellar vermis in all cases. Because of high T2-weighting, good contrast enhancement, and good signal-to-noise ratios, HASTE images provided the best anatomic definition of the posterior fossa.

CONCLUSION

Normal posterior fossa anatomy can be adequately shown on ultrafast MR images, which can be helpful when prenatal sonography is equivocal.

Dandy-walker malformation: prenatal diagnosis and outcome

Prenatal ultrasound identified Dandy-Walker malformation (DWM) in ten singleton pregnancies with concurrent central nervous system (CNS) anomalies and extra-CNS anomalies in eight cases. DWM was confirmed by postnatal magnetic resonance imaging (MRI) or pathological examination in nine cases. Karyotypes were normal in the seven infants tested. Postnatal neurological and developmental testing in the five survivors showed a spectrum of clinical outcome from minor defects to severe handicap. Postnatal investigation also disclosed additional CNS and extra-CNS findings not detected on ultrasound, as did autopsy in the other five infants. However, ultrasound diagnosis of DWM is accurate and is an indication for exhaustive screening for concurrent anomalies both within and outside the CNS and in chromosome structure and number, as the prognosis is heavily dependent on associated malformations and karyotype.

Sonographic visualization of neonatal posterior fossa abnormalities through the posterolateral fontanelle

OBJECTIVE

This study was performed to determine whether imaging through the posterolateral fontanelle in addition to the anterior fontanelle during neonatal cranial sonography improves diagnostic accuracy or examiner confidence in the diagnosis of neonatal posterior fossa abnormalities.

MATERIALS AND METHODS

In 1995 we changed our protocol of neonatal cranial sonography to include imaging through the posterolateral fontanelle in all patients. The reports of all sonography performed in the first 15 months of this protocol were reviewed, and two radiologists reviewed the images of all patients in whom a posterior fossa abnormality was diagnosed with posterolateral fontanelle images masked and then with posterolateral fontanelle images available.

RESULTS

In total, 1292 sonograms were obtained in 462 patients. In 200 patients, the sonographic findings were abnormal; of these 200 patients, 24 (12%) had posterior fossa abnormalities (nine posterior fossa hemorrhages, four Arnold-Chiari malformations (type II), two posterior fossa arteriovenous malformations, and nine partial vermian defects). The posterolateral fontanelle images showed the posterior fossa abnormality better than the anterior fontanelle images did in 23 (96%) of the 24 patients, increased confidence in the diagnosis of 18 (75%) of the 24 patients, and was the only technique to reveal the posterior fossa abnormality in 11 (46%) of the 24 patients. Nearly all pathologic correlations with imaging confirmed the posterolateral fontanelle findings except for the diagnosis of inferior vermian agenesis, which was presumed to be a false-positive diagnosis in four patients in whom MR imaging showed no abnormalities.

CONCLUSION

Additional imaging through the posterolateral fontanelle during routine neonatal cranial sonography added considerable benefit. False-positive diagnosis of vermian defects is a troubling problem but may be avoided with careful attention to the midline sagittal sonographic images of the vermis and fourth ventricle.

Ultrasound evaluation of the fetal central nervous system

Evaluation of the fetal central nervous system is an integral part of any obstetric examination. Critical to the diagnosis of central nervous system abnormalities is a basic understanding of the normal fetal anatomy. Anatomic features of a normal ultrasound examination of the fetal central nervous system are discussed. In addition, the sonographic findings associated with a variety of major central nervous system anomalies are reviewed.

The fetal head: borderlines

After more than 20 years of using ultrasound to examine the fetal head, there are still areas of controversy. The size of the ventricular atrium or the anteroposterior measurement of the posterior fossa have been shown to be relatively stable throughout pregnancy. However, there are instances in which intracranial malformations occur with normal ventricular atrial measurements. Although there are more obvious posterior fossa defects such as Dandy-Walker malformation, there are other abnormalities such as Dandy-Walker variant that are difficult to detect. Another area of controversy surrounds dealing with the fetus with a choroid plexus cyst. Review and understanding of the current literature are needed to best manage these fetuses with choroid plexus cysts. Knowledge of central nervous system embryology and of technical pitfalls of sonography, and understanding the overlap between normal and abnormal anatomy are needed to obtain a more precise central nervous system diagnosis. This article will review some of the borderlines in examination of the fetal brain.

Prenatal sonographic diagnosis of vermal agenesis

Agenesis of the vermis as detected during gestation by ultrasonography may indicate the existence of various malformation arrays or syndromes. We report on our observations of five cases of complete vermal agenesis that were detected at 22-31 weeks of gestation. All had a vertex presentation and transvaginal sonography established the diagnosis of vermal agenesis. Two of the vermal agenesis cases had no associated anomalies outside the central nervous system (CNS). In one, the cerebellar cleft was the only abnormality present and the other also had lobar holoprosencephaly. The three remaining fetuses had trisomy 13 and featured various additional extra-CNS anomalies. The association of complete vermal agenesis and trisomy 13 has not been previously reported. Our experience with this series suggests that supplementation with vaginal fetal sonography is a valuable tool for obtaining a more accurate view of the posterior fossa whenever a cyst or a cyst-like abnormality is detected by transabdominal sonography. A finding of isolated vermal agenesis appears to mandate a careful search for additional anomalies and the performance of karyotype analysis.