In vivo MRI of the fetal brain

An ode to fetal, infant, and toddler neuroimaging: Chronicling early clinical to research applications with MRI, and an introduction to an academic society connecting the field

Fetal, infant, and toddler neuroimaging is commonly thought of as a development of modern times (last two decades). Yet, this field mobilized shortly after the discovery and implementation of MRI technology. Here, we provide a review of the parallel advancements in the fields of fetal, infant, and toddler neuroimaging, noting the shifts from clinical to research use, and the ongoing challenges in this fast-growing field. We chronicle the pioneering science of fetal, infant, and toddler neuroimaging, highlighting the early studies that set the stage for modern advances in imaging during this developmental period, and the large-scale multi-site efforts which ultimately led to the explosion of interest in the field today. Lastly, we consider the growing pains of the community and the need for an academic society that bridges expertise in developmental neuroscience, clinical science, as well as computational and biomedical engineering, to ensure special consideration of the vulnerable mother-offspring dyad (especially during pregnancy), data quality, and image processing tools that are created, rather than adapted, for the young brain.

MRI in Pregnancy and Precision Medicine: A Review from Literature

Magnetic resonance imaging (MRI) offers excellent spatial and contrast resolution for evaluating a wide variety of pathologies, without exposing patients to ionizing radiations. Additionally, MRI offers reproducible diagnostic imaging results that are not operator-dependent, a major advantage over ultrasound. MRI is commonly used in pregnant women to evaluate, most frequently, acute abdominal and pelvic pain or placental abnormalities, as well as neurological or fetal abnormalities, infections, or neoplasms. However, to date, our knowledge about MRI safety during pregnancy, especially about the administration of gadolinium-based contrast agents, which are able to cross the placental barrier, is still limited, raising concerns about possible negative effects on both the mother and the health of the fetus. Contrast agents that are unable to cross the placenta in a way that is safe for the fetus are desirable. In recent years, some preclinical studies, carried out in rodent models, have evaluated the role of long circulating liposomal nanoparticle-based blood-pool gadolinium contrast agents that do not penetrate the placental barrier due to their size and therefore do not expose the fetus to the contrast agent during pregnancy, preserving it from any hypothetical risks. Hence, we performed a literature review focusing on contrast and non-contrast MRI use during pregnancy.

Normal development

Numerous events are involved in brain development, some of which are detected by neuroimaging. Major changes in brain morphology are depicted by brain imaging during the fetal period while changes in brain composition can be demonstrated in both pre- and postnatal periods. Although ultrasonography and computed tomography can show changes in brain morphology, these techniques are insensitive to myelination that is one of the most important events occurring during brain maturation. Magnetic resonance imaging (MRI) is therefore the method of choice to evaluate brain maturation. MRI also gives insight into the microstructure of brain tissue through diffusion-weighted imaging and diffusion tensor imaging. Metabolic changes are also part of brain maturation and are assessed by proton magnetic resonance spectroscopy. Understanding and knowledge of the different steps in brain development are required to be able to detect morphologic and structural changes on neuroimaging. Consequently alterations in normal development can be depicted.

Non-central nervous system fetal magnetic resonance imaging

Fetal magnetic resonance imaging (MRI) is currently offered in a limited number of centers but is predominantly used for suspected fetal central nervous system abnormalities. This article concentrates on the role of the different imaging sequences and their value to clinical practice. It also discusses the future of fetal MRI.

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.

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.

Functional imaging in the fetus

This review focuses on the application of magnetic resonance imaging methods in utero studying functional brain development of spontaneous brain activity generated under resting conditions and of task-evoked activity using stimulation. These imaging approaches have been useful to explore the brain's functional organization during development, as already shown in different substantial resting-state studies in preterms. We also discuss emerging future directions regarding analyzing methods and combination of functional and structural connectivity approaches.

Fetal MRI: the sonographer's view

Fetal magnetic resonance imaging (MRI) is used with increasing frequency as a complementary imaging modality to ultrasound (US) in prenatal diagnosis. Fetal MRI displays the fetal, uterine, and extrauterine anatomy in ways that allow confirmation of normal anatomy and the diagnosis of pathological entities that were formerly very difficult to detect prenatally. Comparison of US views with standard orthogonal plane MR images reinforces the understanding of fetal anatomy as visualized with US. Technological advances in US equipment have allowed the recent description of subtle fetal anatomical structures. Similarly, knowledge of the MRI appearances of pathological conditions has opened opportunities for the sonographic diagnosis of entities such as brainstem malformations and alterations in the normal transient laminar pattern that occur during development of the fetal cerebrum. Fetal MRI can confirm suspicious US findings and thus add confidence in a particular prenatal diagnosis before performing invasive and interventional procedures. Specific MRI sequences can be used to add information about the chemical composition of fetal structures, such as fat, blood, and meconium. Dynamic MRI sequences have increased understanding of gestational age-dependent behavior, and assist the sonographer in assessment of fetal structural anomalies that cause abnormal movement and behavior. The technological ability of US to demonstrate very small structures complements the lower resolution of fetal MR images, whereas the ability of MR to visualize the whole fetus improves the limited views necessitated by US. Therefore, both US and fetal MRI have complementary strengths and weaknesses that can be used to full advantage in prenatal diagnosis.

Fetal neuroimaging

Although ultrasound remains the screening modality of choice in evaluation of the fetal nervous system, magnetic resonance imaging with its multiplanar imaging ability and high signal-to-noise ratio is highly accurate in illustrating the morphologic changes of the developing brain and fetal brain abnormalities. Fetal magnetic resonance imaging is an established powerful tool for obtaining additional information in evaluation of anomalies of the fetal face, neck, and spine. It is helpful to patients and their health care professionals in making vital management decisions and aids in genetic counseling for future pregnancies.

Prenatal ultrasound and fetal MRI: the comparative value of each modality in prenatal diagnosis

Fetal MRI is used with increasing frequency as an adjunct to ultrasound (US) in prenatal diagnosis. In this review, we discuss the relative value of both prenatal US and MRI in evaluating fetal and extra-fetal structures for a variety of clinical indications. Advantages and disadvantages of each imaging modality are addressed. In summary, MRI has advantages in demonstrating pathology of the brain, lungs, complex syndromes, and conditions associated with reduction of amniotic fluid. At present, US is the imaging method of choice during the first trimester, and in the diagnosis of cardiovascular abnormalities, as well as for screening. In some conditions, such as late gestational age, increased maternal body mass index, skeletal dysplasia, and metabolic disease, neither imaging method may provide sufficient diagnostic information.

MR imaging of brain maturation

Magnetic resonance imaging (MRI) is the imaging tool of choice to evaluate brain maturation and especially brain myelination. Magnetic resonance imaging also provides functional insight through diffusion images and proton spectroscopy. In this review the MRI techniques are analyzed for both pre- and postnatal periods. The origin of MR signal changes is also detailed in order to understand normal myelination evolution and the consequences on brain maturation of the different pathologies encountered prior and after birth. Because MRI is "blind" in terms of signal on conventional sequences after 2 years of age, a particular attention is given to diffusion images and proton spectroscopy of the developing brain.

The occurrence of obstructive vs absorptive hydrocephalus in newborns and infants: relevance to treatment choices

BACKGROUND AND OBJECTIVE

The classification of hydrocephalus in newborns and in infants is different from the classification in adulthood. This difference exists due to disparity in the source pathologies that produce the hydrocephalus, and the practical distinctions in prognosis and treatment choices. The objective of this paper is to present the spectrum of obstructive-communicating hydrocephalus, which is more complex in the pediatric group, and to propose the relevance of this particular classification to treatment options.

MATERIALS AND METHODS

The authors categorized infants with active hydrocephalus at time of presentation into the following four groups along the spectrum of communicating vs obstructive HCP. Group 1: patients with a purely absorptive (communicating) HCP. In these patients, tetraventricular dilatation is usually observed with occasional extraaxial fluid accumulation. An extracranial CSF diversion (shunt) is the treatment of choice. Group 2: patients with an obstructive component together with a persistent absorptive component. In these patients, a technically successful endoscopic procedure will not prevent progression of clinical symptoms of HCP. An extracranial CSF diversion (shunt) should be the treatment of choice even though some of these patients are currently treated by endoscopy. Group 3: patients with an obstructive component together with a temporary absorptive component. In these patients, a technically successful ETV should be followed by temporary CSF drainage [via LP, continuous spinal drainage (CLD), or ventriculostomy] with or without supplemental medical treatment (i.e., Diamox) for several days. Such temporary drainage may decrease failure rate in this subgroup. Group 4: patients with a purely obstructive HCP. In these patients, an endoscopic procedure (ETV) is the treatment of choice. According to this spectrum classification, the authors classify different entities with representative cases and discuss relevancy to treatment options and prognosis.

RESULTS

The data suggest that obstructive hydrocephalus in the very young population may be rather a combination of obstructive and absorptive problem. The outcome of the patient depends mainly not only on the basic pathology causing the hydrocephalus but also on the treatment that is chosen and its complications. While bleeding and infection represent the major causes for communicating hydrocephalus, patients with complex pathologies of congenital type and intra- or interventricular obstructions may reflect obstructive hydrocephalus. Treatment of these patients may be successful by shuntless procedures if the absorptive problem is not the major component. In transient absorptive hydrocephalus, temporary measures were effective in many cases leading to successful procedures of ETV and/or posterior-fossa decompression in selected cases.

CONCLUSIONS

Shuntless procedures are the dream of a pediatric neurosurgeon provided it solves the problem and does not imply unacceptable risk. However, the benefit has to be evaluated years after the procedure is performed, as only prospective multicenter studies will truly show which procedure may have the best overall results in the developing child. Until such studies are available, understanding the basic pathology or the combination of pathologies leading to hydrocephalus in a given child may open the window of opportunities for other than shunt surgery in many hydrocephalic children with major obstructive component.

Dynamic motion analysis of fetuses with central nervous system disorders by cine magnetic resonance imaging using fast imaging employing steady-state acquisition and parallel imaging: a preliminary result

OBJECT

The authors present a novel cine magnetic resonance (MR) imaging, two-dimensional (2D) fast imaging employing steady-state acquisition (FIESTA) technique with parallel imaging. It achieves temporal resolution at less than half a second as well as high spatial resolution cine imaging free of motion artifacts for evaluating the dynamic motion of fetuses in utero. The information obtained is used to predict postnatal outcome.

METHODS

Twenty-five fetuses with anomalies were studied. Ultrasonography demonstrated severe abnormalities in five of the fetuses; the other 20 fetuses constituted a control group. The cine fetal MR imaging demonstrated fetal head, neck, trunk, extremity, and finger as well as swallowing motions. Imaging findings were evaluated and compared in fetuses with major central nervous system (CNS) anomalies in five cases and minor CNS, non-CNS, or no anomalies in 20 cases. Normal motility was observed in the latter group. For fetuses in the former group, those with abnormal motility failed to survive after delivery, whereas those with normal motility survived with functioning preserved. The power deposition of radiofrequency, presented as specific absorption rate (SAR), was calculated. The SAR of FIESTA was approximately 13 times lower than that of conventional MR imaging of fetuses obtained using single-shot fast spin echo sequences.

CONCLUSIONS

The following conclusions are drawn: 1) Fetal motion is no longer a limitation for prenatal imaging after the implementation of parallel imaging with 2D FIESTA, 2) Cine MR imaging illustrates fetal motion in utero with high clinical reliability, 3) For cases involving major CNS anomalies, cine MR imaging provides information on extremity motility in fetuses and serves as a prognostic indicator of postnatal outcome, and 4) The cine MR used to observe fetal activity is technically 2D and conceptually three-dimensional. It provides four-dimensional information for making proper and timely obstetrical and/or postnatal management decisions.

Fetal central nervous system MR imaging

MR imaging of the fetal brain is rapidly being embraced in clinical practice. Fetal MR imaging is proving to be a powerful modality with which to evaluate the fetal brain and is a valuable complement to prenatal ultrasound. Structural abnormalities, such as cerebral malformations and destructive lesions, can be sonographically occult on prenatal ultrasound yet detectable by fetal MR imaging. Moreover, fetal MR imaging offers the promise of contributing to our understanding of normal as well as abnormal brain development with continued advances in MR imaging techniques, such as diffusion-weighted and parallel imaging.

Magnetic resonance imaging and the detection of fetal brain anomalies, injury, and physiologic adaptations

PURPOSE OF REVIEW

Magnetic resonance imaging is playing an increasingly prominent role in depicting brain maturation, especially gyral formation that follows a temporospatial pattern, and in detecting developmental abnormalities of the cortex and other brain sectors. Knowledge of the technical advantages and limitations of in-utero magnetic resonance imaging techniques, relative to those of the postnatal period, is essential to optimize magnetic resonance sequences for early diagnosis. This includes an understanding of the changes in both brain anatomy and magnetic resonance signals that occur with an increase in gestational age.

RECENT FINDINGS

Magnetic resonance imaging has evolved has an important adjunct in the diagnosis of brain malformations, particularly in the late-second or third trimester. Noxious conditions elicit more of a chronic rather than acute response in the fetal brain, which differs from that observed postnatally. Clinical applications of proton magnetic resonance spectroscopy may help elucidate fetal brain maturation and its abnormalities from a metabolic point of view.

SUMMARY

Indications for fetal brain magnetic resonance imaging have increased because of improvements in magnetic resonance techniques and the ability to detect subtle changes within the cerebral parenchyma, especially in fetuses at increased risk of brain damage.

Fetal magnetic resonance imaging in the evaluation of fetuses referred for sonographically suspected abnormalities of the corpus callosum

OBJECTIVE

Fetal magnetic resonance imaging (MRI) has been shown to be useful in assessing the developing central nervous system. However, its utility in specific brain disorders has not been well investigated. We hypothesized that fetal MRI can better assess the integrity of the brain in cases with sonographically suspected callosal abnormalities.

METHODS

We retrospectively reviewed fetal MRI and prenatal sonographic studies of 10 fetuses referred for MRI for sonographically suspected callosal abnormalities.

RESULTS

An abnormal corpus callosum was identified on fetal MRI in 80% of cases. The type of callosal abnormality (complete or partial agenesis) was similar on both prenatal sonography and fetal MRI in all cases. All sonographically identified additional brain abnormalities were detected on fetal MRI, with the exception of choroid plexus cysts. Furthermore, in 63% (5 of 8) of cases with a callosal abnormality on both sonography and fetal MRI, additional brain abnormalities were detected on fetal MRI that were not apparent on sonography. These sonographically occult findings were confirmed on postnatal MRI or autopsy in 3 of 5 patients.

CONCLUSIONS

Fetal MRI is an important adjunct to sonography in assessing the corpus callosum and other aspects of brain development when agenesis of the corpus callosum is suspected. It can identify frequent additional findings that are not visible on sonography such as abnormal sulcation. In light of the association between additional brain abnormalities and worse neurodevelopmental outcome, the potential of fetal MRI as an important adjunctive prognostic imaging test in fetuses with callosal agenesis can now be tested.

Assessment of cortical maturation with prenatal MRI: part II: abnormalities of cortical maturation

The fetal cortical maturation is a long process with predefined steps. Abnormalities can occur at different stages of cortical maturation, resulting in various malformations. They can result from disturbance in cell proliferation, cell differentiation, cell migration and in organization of the cortex. Analysis of the different abnormalities of cortical maturation is given with illustrations of the principal malformations encountered in utero and accessible to MRI.

Corroboration of in utero MRI using post-mortem MRI and autopsy in foetuses with CNS abnormalities

AIMS

To corroborate the findings of in utero magnetic resonance imaging (MRI) with autopsy and post-mortem MRI in cases of known or suspected central nervous system (CNS) abnormalities on ultrasound and to compare the diagnostic accuracy of ante-natal ultrasound and in utero MRI.

METHODS

Twelve pregnant women, whose foetuses had suspected central nervous system abnormalities underwent in utero MRI. The foetuses were imaged using MRi before autopsy. The data were used to evaluate the diagnostic accuracy of in utero MRI when compared with a reference standard of autopsy and post-mortem MRI in 10 cases and post-mortem MRI alone in two cases.

RESULTS

The diagnostic accuracy of antenatal ultrasound and in utero MRI in correctly characterizing brain and spine abnormalities were 42 and 100%, respectively.

CONCLUSION

In utero MRI provides a useful adjuvant to antenatal ultrasound when assessing CNS abnormalities by providing more accurate anatomical information. Post-mortem MRI assists the diagnosis of macroscopic structural abnormalities.

Fetal brain injury

Improvements in MRI techniques widen the indications for fetal brain imaging and fetal brain injury represents the third indication of fetal brain magnetic resonance imaging (MRI) after the evaluation of suspected central nervous system (CNS) malformations and ventricular dilatation. Optimal MR imaging technique is necessary in order to collect as much data as possible about the fetal brain. Diffusion images can be used routinely in addition to the standard protocol of fetal brain MRI that consists of T1 and T2 weighted images of the fetal brain. Monovoxel proton magnetic resonance spectroscopy can also be performed in utero, but this technique is still more part of research protocol than of routine clinical protocol. Fetal brain injury includes hypoxia-ischemia, congenital infections (especially toxoplasmosis and cytomegalovirus infections), brain damage due to malformation such as vascular brain malformation and heart malformation, pregnancies at risk of fetal brain damage, and even inherited metabolic diseases, especially mitochondrial diseases. MRI findings in fetal brain injury consist of acute or chronic lesions that can be seen alone or in combination. Acute response of the fetal brain is less commonly seen than the chronic response compared to the brain response encountered in the postnatal period.

Ultrafast magnetic resonance imaging of central nervous system abnormalities in utero in the second and third trimester of pregnancy: comparison with ultrasound

OBJECTIVE

To assess the ability of ultrafast magnetic resonance imaging to visualise abnormalities in the central nervous system of third trimester fetuses in utero and to compare the results with the current 'reference standard' of ultrasound and postnatal imaging or post-mortem data.

DESIGN

A prospective, observational study comparing the diagnostic accuracy of two imaging methods: antenatal ultrasound and antenatal magnetic resonance with each other and postnatal or post mortem data.

POPULATION

Twenty-one pregnant women of 19-36 weeks of gestation whose fetus were thought to have a central nervous system abnormality on the basis of antenatal ultrasound. The women had either not been offered or had refused a termination and were willing to have a magnetic resonance scan.

METHODS

A 1.5T magnetic resonance scanner used a single shot fast spin echo sequence, in three image planes. The results were compared with the ultrasound results obtained by an experienced investigator independently. A series of 21 patients, with a range of pathologies of central nervous system, were imaged. Postnatal ultrasound and/or magnetic resonance imaging, or post-mortem data were used for additional confirmation of the pathology in all cases.

RESULTS

The magnetic resonance report was different to the ultrasound in 10/21 (47.6%), magnetic resonance provided information additional to the ultrasound in 5/21 (23.8%), ultrasound and magnetic resonance results agreed in 6/21 cases (28.6%).

CONCLUSION

Magnetic resonance in the third trimester provides a useful adjuvant to ultrasound imaging of the fetus when assessing abnormalities of the central nervous system after 19 weeks of gestation particularly if the abnormality involves the posterior fossa.

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.

Normal fetal brain development: MR imaging with a half-Fourier rapid acquisition with relaxation enhancement sequence

PURPOSE

To analyze normal maturation of the fetal brain with half-Fourier rapid acquisition with relaxation enhancement (RARE) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

The normal brains of 25 fetuses of 12-38 weeks gestational age were examined in utero with half-Fourier RARE imaging. Gyrus maturation, gray and white matter differentiation, ventricle-to-brain diameter ratio, and subarachnoid space size were evaluated with respect to gestational age.

RESULTS

At 12-23 weeks, the brain had a smooth surface, and two or three layers were differentiated in the cerebral cortex. At 24-26 weeks, only a few shallow grooves were seen in the central sulcus, and three layers, including the immature cortex, intermediate zone, and germinal matrix, were differentiated in all fetuses. At 27-29 weeks, sulcus formation was observed in various regions of the brain parenchyma, and the germinal matrix became invisible. Sulcation was seen in the whole cerebral cortex from 30 weeks on. However, the cortex did not undergo infolding, and opercular formation was not seen before 33 weeks. At 23 weeks and earlier, the cerebral ventricles were large; thereafter, they gradually became smaller. The subarachnoid space overlying the cortical convexities was slightly dilated at all gestational ages, most markedly at 21-26 weeks.

CONCLUSION

Changes in brain maturation proceed through stages in an orderly and predictable fashion and can be evaluated reliably with half-Fourier RARE MR imaging.

MRI does not change fetal cardiotocographic parameters

The aim of this study was to determine whether magnetic resonance imaging (MRI) has any effect on fetal cardiotocographic (CTG) parameters or movement incidence. Sixteen mothers were examined during the last trimester at 28-39 weeks (mean 33 weeks; SD 4) of gestation due to a suspected fetal anomaly found in antenatal ultrasonography (US). MR imaging was performed using Siemens Magnetom Vision 1.5 T equipment with a 25 mT/m peak gradient amplitude. T2-weighted images were produced with HASTE and TRUE-FISP sequences and T1-weighted images with a 2D FLASH sequence. A four-element phase-array coil was used as the receiver. Before and after MRI-examination, a computerized analysis of the fetal heart rate (FHR) was produced. Basal FHR, short-term variation (STV) and fetal movements were calculated. The mean basal FHR was 136 beats/min (SD 11.6) before, and 133 beats/min (SD 8.9) after (P = 0.158). Short-term variation was in the normal range for both CTG-tracings: mean 9.7 ms (SD 2.7) and 8.8 ms (SD 2.8) (P = 0.196). The median for fetal movements before MRI was 48/h, and after MRI 24.5/h (P = 0.98). MRI at high field strength with powerful gradients did not affect fetal heart activity or movement incidence.

A magnetic resonance template for normal cerebellar development in the human fetus

OBJECTIVE

Although ultrasound is the primary imaging modality for prenatal anatomic evaluation, some central nervous system malformations may be better defined with high-resolution magnetic resonance imaging (MRI). MRI allows us to visualize the features of brain development that were previously only seen histologically by embryologists and anatomists. Although there are several reports of the postnatal development of the cerebellum as revealed on magnetic resonance (MR) images, systematic MR studies of cerebellar development during the fetal period are lacking. Our objective was to use high-resolution MRI to provide a template of cerebellar development during the late first and early second trimesters, a period when the diagnosis of congenital malformations is most medicoethically relevant. The MR findings were then correlated with histological data.

METHODS

Twenty-six normal formalin-fixed fetal specimens with a gestational age of 9 to 24 weeks were examined with high-resolution MRI using a conventional clinical magnet and pulse sequences. The MR findings were correlated with the whole-mount histological specimens catalogued in a well-known fetal atlas.

RESULTS

Resolution of the morphological features of cerebellar development in fetuses greater than 10 weeks gestational age was possible. Development of the rhombic lips, vermis, fourth ventricular roof, foramen of Magendie, and the cerebellar fissures was documented. Development of the cerebellum as revealed on MR images lagged behind the known stages of development by as much as 5 weeks. Features of cerebellar histogenesis were beyond the resolution of MRI. However, differences in signal intensity between gray and white matter of the developing cerebellum were detected and are postulated to represent differences in cellularity and water content of the constituent tissues.

CONCLUSION

Direct correlation of MR images of fetuses during the late first and early second trimesters with anatomic atlases could result in a mistaken diagnosis of delayed or abnormal development of the posterior fossa contents because of a time lag in the detection of structures on MR images. An MR template of normal cerebellar development would be useful to avoid confusion of normal development with abnormal development and to identify the expected developmental features when provided the estimated gestational age of a fetus.

Neonatal seizures associated with cerebral lesions shown by magnetic resonance imaging

AIM

To determine the diagnostic potential of magnetic resonance imaging (MRI) in neonatal seizures; to elucidate the aetiology, timing, and prognosis of the cerebral lesions detected.

METHODS

Thirty one term neonates with clinical seizures underwent ultrasonography between days 1-7 (mean 2.5 days) and a high field spin-echo MRI scan on days 1-30 (mean 8.1 days), both of which were repeated at 3 months of age. Routine investigation excluded, as far as possible, infection, haematological, and metabolic-toxic causes as causes of the neonatal seizures.

RESULTS

Brain abnormality was demonstrated by MRI in 68% of infants and ultrasonographically in 10%. Diffuse brain lesions (present in 29%) were associated with high mortality (58%) and morbidity (42%), whatever the aetiology. In contrast to a better short term prognosis for neonates with focal lesions where no infants died, 33% had a handicap, and the rest were normal at a mean follow up age of 2 1/2 years. Cerebral lesions were presumed to have antepartum origin in 43% of cases. Seizure aetiology was considered to be hypoxic-ischaemic in 35%, haemorrhagic in 26%, metabolic disturbances and cerebral dysgenesis in 16% and unknown in 23%.

CONCLUSIONS

MRI detected a remarkably high incidence of brain lesions in neonatal seizures. Almost half of these were of prenatal origin and pathogenesis may essentially be attributed to hypoxic and/or haemodynamic causes.

A magnetic resonance template for normal neuronal migration in the fetus

OBJECTIVE

Although the features of neuronal migration have been known since the turn of the century, the serial features of neuronal migration as seen with magnetic resonance imaging (MRI) have not been described. Our objective was to provide a template of the normal appearance and the temporal pattern of neuronal migration in the human fetal brain early in the second trimester as seen with MR imaging and to correlate our findings with histological sections and atlases.

METHODS

Twenty-eight normal fetal specimens, which ranged from 9 to 24 weeks of gestational age, were imaged with a 1.5 T clinical MRI unit by use of conventional spin echo, fast spin echo, and three-dimensional Fourier transformation spoiled gradient refocussed pulse sequences.

RESULTS

The three-dimensional Fourier transformation spoiled gradient refocussed pulse sequence provided the highest resolution images of neuronal migration. At 13 weeks of gestational age, the germinal matrix was identified. A five-layer pattern of the fetal forebrain, which included layers of neuroblast formation and migration, could be identified at 16 to 18 weeks by MRI. The germinal matrix and layers of migrating neurons diminished considerably in size by 21 weeks. Histological studies and correlation with anatomic atlases confirmed the MRI findings.

CONCLUSION

Images obtained by use of MRI with standard clinical pulse sequences can document the appearance and the temporal patterns of neuronal migration in postmortem fetal specimens. With the evolution of high-resolution MRI and faster scanning techniques, these findings may serve as a template for the in utero MRI appearance of neuronal migration and thereby compliment the antenatal ultrasonic investigation of congenital anomalies.

Short communication: MR imaging of fetal brain abnormalities using a HASTE sequence

HASTE (half-Fourier acquisition single-shot turbo spin echo) is a sequence that enables T2 weighted magnetic resonance (MR) images to be obtained in a few seconds. The purpose of this study was to evaluate the usefulness of this sequence in the diagnosis of fetal cerebral abnormalities. Five fetuses suspected of having cerebral abnormalities on ultrasound examination were studied by MR imaging using the HASTE sequence in utero. We compared the images with post-natal MR images or computed tomography (CT) scans. In four fetuses, the abnormality was diagnosed correctly, and the diagnosis was almost correct in the remaining fetus. This sequence is useful because it provides images of diagnostic quality in a very short scanning time.

MRI in obstetrics: a supplementary method for ultrasonography

The use of magnetic resonance imaging (MRI) as a complementary diagnostic method was evaluated in a group of 36 complicated pregnancies during the last trimester. Most of the examinations were carried out by two specific MRI sequences, the T1-weighted gradient echo FLASH sequence (TR 120, TE 12) and the T2-weighted fast-spin echo sequence (TR 2000, TE 90). Information additional to that obtained by ultrasonic diagnosis was obtained by MRI in 16 of the 25 cases of foetal malformation and in 5 of the 11 cases with other obstetric problems. Foetal curarization was not necessary in most of the cases. A good MRI technique, for selected obstetric problems and when used in combination with detailed antepartal ultrasonic examinations, can give useful diagnostic information to an obstetrician.