Ultrafast MR imaging of the normal posterior fossa in fetuses

Evaluation of the height of the corpus callosum body region in fetal meningomyelocele malformation

PURPOSE

To investigate height of the corpus callosum (CC) in order to describe the corpus callosum anomalies in fetuses with meningomyelocele (MMC) and compare these findings with the corpus callosum of healthy fetuses.

METHODS

In this study, fetal MRI examinations were performed on 44 fetal MMC malformation cases. As the control group, 34 fetal MRI examinations, which were anatomically normal, were evaluated retrospectively. In the study group, lateral ventricle diameter, the level and diameter of the MMC defect, and CC height were measured. In the control group, CC height and lateral ventricular diameter were measured.

RESULTS

The mean CC body height was 1.36 mm in the study group, and 2.48 mm in the control group. The height of the CC body region of the study population was inclined to be thinner compared with the control population (p<0.001).

CONCLUSIONS

The fact that the height of the CC body region was found to be thinner in fetal MRI in cases of MMC compared with normal fetuses suggests that various callosal anomalies are uncertain, investigation of additional callosal anomalies may be beneficial in the decision for the continuation of pregnancy, and termination or intrauterine surgery in cases with MMC. Further large case group studies are needed.

Fetal Brain Anatomy

"Fetal brain development has been well studied, allowing for an ample knowledge of the normal changes that occur during gestation. Imaging modalities used to evaluate the fetal central nervous system (CNS) include ultrasound and MRI. MRI is the most accurate imaging modality for parenchymal evaluation and depiction of developmental CNS anomalies. The depiction of CNS abnormalities in a fetus can only be accurately made when there is an understanding of its normal development. This article reviews the expected normal fetal brain anatomy and development during gestation. Additional anatomic structures seen on brain imaging sequences are also reviewed."

Brain fetal magnetic resonance imaging to evaluate maturation of normal white matter during the third trimester of pregnancy

BACKGROUND

Quantitative magnetic resonance imaging (MRI) could improve the estimation of fetal brain maturation and the interpretation of white matter signal intensity in pathological conditions.

OBJECTIVE

To investigate T2-based and diffusion-weighted imaging (DWI) measurements for the evaluation of fetal brain maturation during the last trimester of pregnancy.

MATERIALS AND METHODS

One hundred sixty-eight fetal brain MRIs were retrospectively analyzed (age range: 28-37 weeks of gestation) after ensuring that none of the children developed psychomotor or cognitive impairment (median follow-up: 4.7 years). Bilateral regions of interest were drawn on the frontal, occipital, parietal and temporal lobes from T2-W imaging and DWI, when available, to evaluate signal intensity and apparent diffusion coefficient (ADC) values. Ratios were calculated with two references (pons or thalamus and cerebrospinal fluid) to standardize signal intensities. Reproducibility was evaluated with intraclass correlation coefficients (ICCs) and Bland-Altman plots. Correlations with gestational age were evaluated with univariate and multivariate linear regressions.

RESULTS

T2 measurements were achieved in all cases, and DWI was available in 37 cases. Measurements and ratios were reproducible in eight localizations (i.e. intra- and interobserver ICCs >0.5): frontal T2/thalamus, parietal T2/thalamus, occipital T2/pons, parietal ADC/thalamus, occipital ADC/pons, temporal ADC/pons, occipital ADC and temporal ADC. The frontal T2/thalamus and parietal T2/thalamus correlated with gestational age (P<0.0001 and P=0.014, respectively). In the multivariate modeling, frontal T2/thalamus remained an independent predictor of the gestational age (P<0.0001).

CONCLUSION

The frontal T2/thalamus ratio emerged as a potential additional biomarker of fetal brain maturation during the last trimester of pregnancy.

Ultrasound and MR Imaging of the Normal Fetal Brain

Prenatal imaging of the central nervous system has proven to be essential in the detection of anomalies to guide counseling and intrauterine and postnatal therapies. However, understanding the appearance of normal is important because the fetal brain changes dramatically during the pregnancy. In this review, normal imaging of the brain with ultrasound and MR imaging is discussed. The initial section stresses techniques for both modalities. The second section describes ultrasound and MR landmarks in a normal fetal brain.

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

AIM

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

MR imaging of the fetal brain

Fetal MRI is clinically performed to evaluate the brain in cases where an abnormality is detected by prenatal sonography. These most commonly include ventriculomegaly, abnormalities of the corpus callosum, and abnormalities of the posterior fossa. Fetal MRI is also increasingly performed to evaluate fetuses who have normal brain findings on prenatal sonogram but who are at increased risk for neurodevelopmental abnormalities, such as complicated monochorionic twin pregnancies. This paper will briefly discuss the common clinical conditions imaged by fetal MRI as well as recent advances in fetal MRI research.

Normal development of the fetal brain by MRI

The fetal brain is a dynamic structure, which can now be imaged using magnetic resonance imaging (MRI). This article will review techniques of fetal MRI as well as several key aspects of brain development and their appearance on MRI. An understanding of normal fetal brain development is essential to correctly identifying developmental abnormalities.

MR volume of the fetal cerebellum in relation to growth

PURPOSE

To quantify fetal cerebellar growth by measuring cerebellar volumes of normal fetuses throughout gestation with MRI.

MATERIALS AND METHODS

A total of 93 fetuses with normal brains ranging in age from 16 to 40 gestational weeks were included in the study. Standard fetal biometric measurements were made on a three-dimensional postprocessing workstation and included the head circumference, transverse cerebellar diameter, biparietal diameter, occipital-frontal diameter, as well as cerebellar volume. The gestational ages were estimated from fetal head circumference measurements. Regression analysis was used to find the best-fit model.

RESULTS

There is a strong correlation describing cerebellar volume and gestational age in fetuses with normal central nervous systems. A second-order polynomial regression model was found to be the most appropriate descriptor of cerebellar volume in relation to normal fetal growth. In addition, the cerebellar volume was also found to correlate strongly with the common fetal biometric measurements of transverse cerebellar diameter, biparietal diameter, and occipital-frontal diameter.

CONCLUSION

Nomograms for fetal cerebellar volume with gestational age derived from head circumference measurements are presented for the first time with MRI. A normal fetal cerebellar volume growth chart is established. These results should prove helpful in defining situations of abnormal growth development and dysmorphology.

MR fetography using heavily T2-weighted sequences: comparison of thin- and thick-slab acquisitions

PURPOSE

To evaluate the use of MR-fetography sequences in identifying the major fetal structures and to compare thick- and thin-slab acquisitions for their diagnostic value.

MATERIALS AND METHODS

Twenty-one consecutive, pregnant women with suspected fetal pathology underwent fetal magnetic resonance imaging (MRI) using a 1.5 T MRI unit. Heavily T2-weighted, single-shot fast spin-echo (SSFSE) sequences with a long echo train (MR-fetography) were acquired in a thick- and thin-slab modus. Thick- and thin-slab acquisitions were reviewed by two experienced radiologists with regard to the overall image quality and landmark anatomical structures (spinal canal, spinal cord, posterior fossa, cerebellum, brainstem, basal cisterns, stomach, urinary bladder and umbilical cord according to a three-scale grading system (good, moderate and poor). Visibility scores were calculated and compared between both sequences.

RESULTS

Overall image quality was graded good in 76.2%, moderate in 19.0% and poor in 4.8% for thick-slab images and good in 81%, moderate in 14.3% and poor in 4.8% for thin-slab images. The visibility scores of the thick/thin-slab images for evaluation of the main fetal structures were as follows: for the spinal canal 2.8+/-0.4/2.9+/-0.54 (p>0.05), spinal cord 2.4+/-0.75/2.7+/-0.66 (p>0.05), posterior fossa components (cerebellum, brainstem and basal cisterns) 2.4+/-0.68/2.8+/-0.54; 2.4+/-0.67/2.7+/-0.66; 2.5+/-0.51/2.7+/-0.56 (p<0.05), stomach 2.8+/-0.44/2.9+/-0.48 (p>0.05), urinary bladder 2.8+/-0.51/2.8+/-0.54 (p>0.05) and umbilical cord 2.9+/-0.30/2.6+/-0.60 (p<0.05).

CONCLUSION

Heavily T2-weighted MR-fetography renders a quick overview of fetal contours, fetal position, amount of amniotic fluid and integrity and presence of several major fluid containing structures. Thick- and thin-slab acquisitions render complementary information. Thick-slab images display the entire fetus in one projection while thin-slab images provide more detailed anatomical information. The short imaging time usually allows measuring both thick- and thin-slab images. MR-fetography is as a helpful addition to conventional fetal MRI. MR-fetography should not be viewed as a single, stand alone sequence but as a supporting fast MR sequence in a well-designed multisequence fetal MRI protocol. Future studies evaluating larger patient groups are mandatory.

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.

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.

Investigation of normal organ development with fetal MRI

The understanding of the presentation of normal organ development on fetal MRI forms the basis for recognition of pathological states. During the second and third trimesters, maturational processes include changes in size, shape and signal intensities of organs. Visualization of these developmental processes requires tailored MR protocols. Further prerequisites for recognition of normal maturational states are unequivocal intrauterine orientation with respect to left and right body halves, fetal proportions, and knowledge about the MR presentation of extrafetal/intrauterine organs. Emphasis is laid on the demonstration of normal MR appearance of organs that are frequently involved in malformation syndromes. In addition, examples of time-dependent contrast enhancement of intrauterine structures are given.

Outcome of fetal cerebral posterior fossa anomalies

INTRODUCTION

Limited data exist on the outcome of Dandy-Walker malformation (DWM), Dandy-Walker variant (DWV) and mega-cisterna magna (MCM). We report the first population-based study of posterior fossa anomalies from the northern region of England.

METHODS

Cases were identified from the Northern Congenital Abnormality Survey (NorCAS) and regional Fetal Medicine Unit databases for the period 1986-2004 for DWM/V and 1995-2005 for MCM (defined as a cisterna magna > or =10 mm). Outcome data was obtained from pediatric records and/or general practitioner/health visitor questionnaires for all survivors.

RESULTS

A prenatal diagnosis of a posterior fossa abnormality was made in 91 cases, with a further 12 cases of DWM/V diagnosed postnatally, giving incidences of DWM/V and MCM of 1/11574 and 1/8268 births respectively. In five cases where DWM/V was suspected prenatally, the diagnosis was not confirmed. Of the 47 with DWM/V, 41 (87%) had additional anomalies. There were three survivors, all with neurodevelopmental disability. Of the 39 cases of MCM, 24 (62%) had additional anomalies. There were 30 survivors; one child died at 3 months and the outcome was normal in 25 children including 12/13 (92%) with isolated MCM.

SUMMARY

Posterior fossa anomalies are relatively common. The outcome is very poor in DWM/V owing to the high rate of associated anomalies. The outcome appears better with MCM, especially if this is an isolated finding.

Liver, meconium, haemorrhage: the value of T1-weighted images in fetal MRI

BACKGROUND

Ultrafast T2-weighted (T2-W) MRI sequences are currently considered a routine technique for fetal MR imaging. Limited experience exists with fetal T1-weighted (T1-W) imaging techniques.

OBJECTIVE

To determine MRI patterns of some fetal abdominal or haemorrhagic disorders with particular respect to the diagnostic value of T1-W images.

MATERIALS AND METHODS

In addition to standard T2-W single-shot sequences, T1-W single-shot and/or multislice sequences were employed in 25 MR examinations performed in 23 fetuses between 20 and 36 weeks of gestation for more detailed assessment of liver, meconium-filled digestive tract, haemorrhage, or further characterization of a fetal abdominal mass. Diagnostic value and presence of motion artefacts on T1-W images was recorded in each case.

RESULTS

T1-W images enabled superior delineation of fetal liver and large intestine. They provided additional diagnostic information in 9 (39%) of 23 fetuses. One false-positive and one false-negative MRI diagnosis of malrotation anomaly were encountered. Use of single-shot T1-W sequences reduced the occurrence of motion artefacts in 64%.

CONCLUSION

Our results suggest that the specific signal properties of methaemoglobin, meconium and liver are sufficiently important for T1-W sequences to become a routine part of fetal MRI protocols when dealing with digestive tract anomalies, diaphragmatic and abdominal wall defects, intraabdominal masses, and fetal haemorrhage.

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 Posterior Fossa Volume: Assessment with MR Imaging

PURPOSE

To retrospectively determine the relationship between posterior fossa volume (PFV) and estimated gestational age (EGA) and/or femur length (FL) during pregnancy for the purpose of developing a normal growth curve.

MATERIALS AND METHODS

Advance institutional review board approval was obtained for this HIPAA-compliant study, and the need for parent informed consent was waived. A cross-sectional retrospective study was performed to measure PFV on in vivo magnetic resonance (MR) images obtained in 76 fetuses of 18-36 weeks gestation who had a morphologically normal CNS. Because this was a retrospective series, MR imaging techniques varied slightly, but all fetuses underwent imaging at contiguous 3-5-mm intervals in at least two orthogonal planes, with repetition time msec/echo time msec, 5-12/62-95; number of signals acquired, one; flip angle, 150 degrees -180 degrees; and matrix, 128-192 x 256. Posterior fossa areas were manually traced on half-Fourier rapid acquisition with relaxation enhancement in utero fetal MR images by one observer. PFVs were then calculated by manually summing areas from the contiguous sections and multiplying the total area by the section thickness. An average PFV (APFV) across orthogonal planes was calculated for each fetus, and the relationship between APFV and EGA was mathematically modeled. Coronal, transverse, and sagittal views were compared with correlations and Bland-Altman plots. Two additional observers repeated the measurements for a small subset of fetuses (n = 5). Paired t test analyses were also performed to determine significant differences between sagittal, transverse, and coronal measurements, as well as to determine preliminary intraobserver and interobserver variability of measurements in a subset of cases.

RESULTS

The relationship between APFV (in cubic centimeters) and EGA (in weeks) was well described by a single exponential function [APFV = 0.689 exp(EGA/9.10)]. APFV doubling time was 6.31 weeks. Root-mean-square variation of values around the model line was 1.63 cm(3). There was no statistically significant intra- or interobserver variation (P > .16 for all fetuses) at preliminary analysis. No correlation between APFV and FL could be found.

CONCLUSION

The normal fetal PFV growth curve generated in this study may have potential as a model for clinical application.

Fetal Magnetic Resonance Imaging of an Intracranial Venous Thrombosis

OBJECTIVE

To describe the magnetic resonance imaging findings of an antenatal intracranial venous thrombosis.

METHODS

Prenatal ultrasound (US) at 22 weeks' gestational age (GA) in a 27-year-old patient (G2 P1) revealed a nonspecific avascular echogenic mass above the cerebellum evaluated with MRI at 23 weeks' GA.

RESULTS

With MRI an enlarged occipital pericerebral space with a layered ovoid mass at the torcular herophili was demonstrated and MRI characteristics suggested a subacute cloth. In follow-up, an important enlargement of the mass was noticed on US examinations. After multidisciplinary consult and parental consent, a prostaglandin induction was performed at 25 weeks' GA. The baby died immediately after birth. Pathology confirmed the diagnosis of a venous thrombosis at the torcular herophili with extension into the superior sagittal sinus.

CONCLUSION

In our case, MRI findings were helpful in diagnosing an intracranial fetal venous thrombosis.

MRI supported diagnosis and counselling in a family with a probably autosomal recessive form of pachygyria

OBJECTIVES

To describe the use of fetal MRI as an adjunct to high-resolution ultrasound in parental counselling in a family with a central nervous system (CNS) abnormality, resembling the agyria-pachyria complex, of probably autosomal recessive inheritance.

METHODS

Description of the use of fetal MRI as an adjunct to high-resolution ultrasound in three consecutive pregnancies in a family with a CNS abnormality.

RESULTS

Fetal MRI, by showing more detailed abnormalities of the CNS, proved to be very useful in parental counselling because of the definitive diagnosis available early in pregnancy. In the first two pregnancies, ultrasound examination revealed mild pyelectasis, megacystis and an enlarged posterior fossa with small cerebellum. MRI at 22-33 weeks confirmed these abnormalities, but also showed pachygyria. In the third pregnancy, MRI at 19 weeks showed no abnormalities, a finding of decisive importance for counselling.

CONCLUSION

Fetal MRI was useful both for early diagnosis and prenatal management in this family with an agyria-pachyria complex CNS abnormality.

Magnetic resonance evaluation of fetal ventriculomegaly-associated congenital malformations and lesions

Fetal MRI provides diagnostic quality images of the brain which permit differentiation between the various etiologies of ventriculomegaly: hydrocephalus, congenital malformation, and destructive processes.

Fast Fetal Magnetic Resonance Imaging

Fetal magnetic resonance imaging (MRI) can be used as a problem-solving tool when ultrasonic findings are equivocal. The role of fetal MRI has increased as obstetricians become aware of its potential and in utero therapy for anomalies becomes increasingly sophisticated. In this pictorial essay, we present a wide range of anomalies diagnosed or confirmed using MRI and discuss findings that help in the differential diagnosis.

Assessment of cortical maturation with prenatal MRI. Part I: Normal cortical maturation

Cortical maturation, especially gyral formation, follows a temporospatial schedule and is a good marker of fetal maturation. Although ultrasonography is still the imaging method of choice to evaluate fetal anatomy, MRI has an increasingly important role in the detection of brain abnormalities, especially of cortical development. Knowledge of MRI techniques in utero with the advantages and disadvantages of some sequences is necessary, in order to try to optimize the different magnetic resonance sequences to be able to make an early diagnosis. The different steps of cortical maturation known from histology represent the background necessary for the understanding of maturation in order to be then able to evaluate brain maturation through neuroimaging. Illustrations of the normal cortical maturation are given for each step accessible to MRI for both the cerebral hemispheres and the posterior fossa.

MRI of the fetal posterior fossa

MRI is a useful tool to complement US for imaging of the fetal posterior fossa (PF). In France, the discovery of a PF malformation in the fetus frequently leads to termination of pregnancy (80% in a personal series). However, despite improved accuracy in the diagnosis of PF abnormalities, prognosis remains uncertain. The first objective of this review is to document the normal MRI landmarks of the developing fetal PF. Because of their thinness, the visibility of the cerebellar fissures is dramatically delayed on MRI compared to macroscopic data. An important landmark is identification of the primary fissure of the vermis, normally seen at around 25-26 weeks' gestation (WG) on the sagittal slice, separating the larger posterior lobe from the anterior lobe (volume ratio around 2:1). The prepyramidal and secondary fissures are usually only identifiable after 32 WG and the hemispheric fissures are difficult to see until the end of pregnancy. Considering the signal changes, high signal on T2-weighted (T2-W) sequences is seen from 25 WG in the posterior part of the brain stem (tegmentum and ascending sensory tracts) related to myelination. The low signal intensities seen within the cerebellum on T2-W images correspond to high cellularity of grey matter (deep nuclei), as there is no myelination within the white matter before 38 WG. The second objective is to highlight the signs highly predictive of a poor neurological prognosis. Lack of pontine curvature or vermian agenesis without a PF cyst (small volume of PF) is greatly associated with poor neurological status. The third objective is to propose a diagnostic strategy in difficult cases where prognosis is important, e.g. the Dandy Walker continuum. Analysis of the cerebellum is often impossible if a PF cyst is present (whatever its nature) as the mass effect usually blurs the foliation and even impairs evaluation of the normal ratio between the posterior and anterior lobes of the vermis. Isolated cerebellar hypoplasias raise the question of prognosis and genetic counselling. Such uncertainties require an amniocentesis and a careful search for other anomalies (cerebral and extracerebral). Unilateral abnormalities of a cerebellar hemisphere can be associated with good neurological status if they are isolated. The final objective is to discuss other rare PF fetal abnormalities, such as vascular malformations and tumours.

Fetal imaging of central nervous system abnormalities

Fetal MR imaging is complementary to obstetric ultrasonography. The additional information provided by in utero MR imaging may alter prenatal, perinatal, or immediate postnatal management. For example, the MR imaging findings may affect the decision to continue the pregnancy, change the mode, timing, or location of delivery, or modify decisions regarding the necessity of immediate postnatal surgery. Finally, the information contributed by MR may permit a better assessment of the risk of recurrent defects in subsequent pregnancies.

MR imaging of fetal abnormalities

Our purpose was to evaluate the capability of ultrafast single-shot fast spin-echo MR imaging to assess normal fetal anatomy and abnormalities of different fetal organ systems. Fetal MR imaging was performed prospectively in consecutive 40 pregnant women because of abnormal findings or suspected fetal abnormalities on prenatal US. No statistically significant difference between US and MR imaging was found for the detection of abnormality in any organ system. MR imaging was slightly superior to US with regard to cerebral abnormalities only. In four (10%) of 40 fetuses, additional information provided by MR imaging altered counseling. However, MR imaging of the extremities-face and soft tissues was limited because of the lack of real-time information.

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 fetal MRI and prenatal diagnosis

Improvements in magnetic resonance imaging (MRI) technology continue to provide faster scan times and higher resolution increasing the applications for fetal imaging. MRI is an adjunct to good prenatal ultrasound scan (US). It provides significant additional information that improves diagnostic accuracy in evaluation of the fetal brain, spine, neck, chest, abdomen, and urinary tract. MRI provides important anatomic information that helps in planning delivery and surgical procedures.

Fetal central nervous system biometry on MR imaging

OBJECTIVE

We sought to compare the biometry of the fetal head on MR imaging with sonographic measurements in fetuses with and without suspected central nervous system abnormalities.

MATERIALS AND METHODS

Blinded retrospective measurements of biparietal diameter, head circumference, and cerebellar width obtained on MR imaging were assigned a gestational age on the basis of median sonographic measurements and compared with sonographic and clinical assignment of gestational age in fetuses with no central nervous system abnormalities. In fetuses with central nervous system abnormalities, the same MR measurements were compared with sonographic measurements obtained within 1 week. Single-shot fast spin-echo sequences were obtained. Pearson's product moment correlation coefficients and paired sample t tests were performed.

RESULTS

In 22 fetuses with no suspected central nervous system abnormalities, significant correlation was seen in the assignment of gestational age by MR measurements and sonographic gestational age. In 25 fetuses with central nervous system abnormalities, significant correlation was also seen between biparietal diameter and head circumference measurements. The mean biparietal diameter on MR imaging was greater than on sonography in those fetuses with central nervous system abnormalities (p = 0.038).

CONCLUSION

MR imaging measurements of biparietal diameter, head circumference, and cerebellar width are strongly correlated to gestational age in fetuses without central nervous system abnormalities. Significant correlation is found between MR imaging and sonographic measurements of biparietal diameter and head circumference in fetuses with central nervous system abnormalities. Larger biparietal diameter measurements were seen with MR imaging than with sonography in the abnormal group. Fetal central nervous system biometry can be performed as part of the MR imaging evaluation of the fetal central nervous system.

Fetal imaging

Presently, MRI is an adjunct to prenatal sonography. It provides information that can aid in the diagnosis of fetal anomalies, affect prenatal counseling and management of the pregnancy, and guide prenatal intervention and delivery planning. With further advances in technology, particularly shorter scan times and better image resolution, the applications for fetal imaging are likely to increase.

Ultrafast MRI of the fetus

During the last decade there has been increasing interest in MRI for the evaluation of ultrasound-detected fetal abnormalities. Motion artefacts previously precluded detailed imaging, but this is now possible using single-shot rapid acquisition sequences. These ultrafast techniques with subsecond images capture 'snap-shot' views of the fetus. By virtue of the infinitely long relaxation time (TR) these images are heavily T2-weighted so that fluid around and within the fetus delineates the anatomy. Currently, fetal MRI has shown to be most beneficial in the investigation of cerebral abnormalities suspected from sonography, and in the detection of subtle associated anomalies. It clearly has a role in the evaluation of complex somatic abnormalities, and is helpful in the evaluation of fetuses prior to surgery. We present a pictorial review of our experience with single-shot fast spin-echo (SSFSE) imaging, demonstrating normal anatomy of the fetal brain and body. In addition we present cerebral and somatic abnormalities to illustrate cases where we have found fetal MRI to be a useful adjunct to sonography.

Fetal magnetic resonance imaging

Ultrafast magnetic resonance imaging (MRI) sequences have changed the use of MRI to evaluate fetal abnormalities. Currently, the best application is the evaluation of suspected brain abnormalities found on ultrasound. MRI differentiates the various types of fetal ventriculomegaly. Superior posterior fossa visualization allows differentiation of Dandy-Walker malformation from a large cisterna magna. Anomalies of the corpus callosum can be seen. MRI also is valuable in the evaluation of fetal giant neck masses for planning delivery of the baby and surgery for life-threatening airway obstruction. In the chest, MRI differentiates masses such as diaphragmatic hernia, cystic adenomatoid malformation, and sequestration, and it aids in planning fetal surgery because MRI directly visualizes the position of the lung, liver, and bowel.

Normal fetal anatomy as visualized with fast magnetic resonance imaging

The development of ultrafast sequences has revolutionized fetal magnetic imaging (MRI). Fetal anatomy can be characterize precisely by MRI. This is particularly true for evaluation of the central nervous system, for which magnetic resonance (MR) evaluation is superior to ultrasound evaluation. MR allows analysis of development of the cerebrum as well as posterior fossa structures. Awareness of the normal sequence of maturation as well as the approximate timing of development of key structures is useful for evaluating the developing fetus. MRI also permits evaluation of the oropharynx, lungs, diaphragm, and intra-abdominal and retroperitoneal structures. Occasionally, beautiful depiction of smaller structures, including extremities and details of the face and neck may be obtained, but this is dependent on the orientation of the fetus and the plane of imaging as well as the section thickness. MRI is useful in assignment of fetal gender as well as in evaluation of the chorionicity of twin gestations. This report illustrates the normal appearance of the developing fetus as seen on fast sequences during the second and third trimesters.