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.

Non-visualization of choroid plexus of fourth ventricle as first-trimester predictor of posterior fossa anomalies and chromosomal defects.

OBJECTIVE: To assess non-visualization of the choroid plexus of the fourth ventricle (CP-4V) as a simple, qualitative and reproducible first-trimester ultrasound feature of the posterior fossa for the prediction of central nervous system (CNS) anomalies and chromosomal defects. METHODS: First-trimester three-dimensional ultrasound datasets of the fetal brain were obtained prospectively from 65 consecutive normal singletons and retrospectively from 27 fetuses identified as having an abnormal posterior fossa on first-trimester ultrasound examination, and randomly combined to form the final study group. The stored ultrasound volumes were analyzed offline by two accredited sonologists, who were not aware of the final diagnoses. The CP-4V was assessed by multiplanar navigation and classified as visible or non-visible in its normal position depending on whether or not the echogenic structure that separates the fourth ventricle from the cisterna magna was identified in both midsagittal and axial planes. Correlation with subsequent second-trimester ultrasound, fetal magnetic resonance imaging, or postmortem or postnatal findings was performed to determine the predictive value of the first-trimester findings. RESULTS: Among the 92 ultrasound datasets analyzed, 73 (79%) were acquired transabdominally and 19 (21%) transvaginally. The CP-4V was classified as visible in 64 cases and non-visible in 28 cases, with agreement between the two observers in both sagittal and axial planes in all but one case. Twelve of the 28 (43%) fetuses with non-visible CP-4V were subsequently diagnosed as having a CNS malformation (open spina bifida (n = 6), Dandy-Walker malformation (n = 2), Blake's pouch cyst (n = 2), cephalocele (n = 1) and megacisterna magna (n = 1)). In addition, 20 of these 28 (71%) fetuses had aneuploidy (trisomy 18 (n = 10), triploidy (n = 5), trisomy 13 (n = 3), Turner syndrome (n = 1) or trisomy 21 (n = 1)). There was only one false-positive case, in which the CP-4V was classified as absent in a normal fetus. CONCLUSIONS: Non-visualization of the CP-4V in the first trimester appears to be a strong marker of posterior fossa anomalies and chromosomal defects. Qualitative evaluation of this anatomic structure is simple, feasible and reproducible, and its routine assessment during the first-trimester scan may facilitate the early detection of CNS anomalies and associated fetal aneuploidy. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

Normative data for fetal cisterna magna length measurement between 18 and 24 weeks of pregnancy.

PURPOSE: The aim of this study was to determine normative data for fetal cisterna magna length (CML) measurement in a Brazilian population. METHODS: This was a retrospective cross-sectional study on 3,862 normal singleton pregnancies between the 18th and 24th weeks of pregnancy. Fetal CML was measured in the axial plane of the fetal head, at lateral ventricle level, including the cavum septum pellucidum, thalamus, third ventricle, and transverse cerebellar diameter. The anteroposterior measurement was made between the posterior border of the cerebellar vermis and the internal face of the occipital bone. To assess the correlation between CML and gestational age (GA), polynomial equations were calculated, with adjustments using determination coefficient (R2). RESULTS: The mean CML ranged from 4.29±0.93 mm at 18 to 18+6 weeks to 5.58±1.23 mm at 24 to 24+6 weeks of pregnancy. There was a good correlation between CML and GA, best represented by a linear equation: CML=0.535+0.208*GA (R2=0.084). CONCLUSION: We established normative data for fetal CML in the second trimester of pregnancy, in a large Brazilian population.

Reference range of fetal cisterna magna volume by three-dimensional ultrasonography using the VOCAL method.

OBJECTIVE: To determine reference values for the fetal cisterna magna volume by three-dimensional ultrasonography (3DUS) using the virtual organ computer-aided analysis (VOCAL) method. METHODS: A cross-sectional study was conducted with 224 healthy pregnant women between 17 and 29 completed weeks. Measurement of the fetal cisterna magna volume was performed in the standard axial plane of the transverse diameter of the cerebellum through the VOCAL method with a 30° angle. For the determination of reference values, polynomial regressions with adjustments by the coefficient of determination (R(2)) were held. The reliability and agreement were made by the intra-class correlation coefficient (ICC) and limits of agreement of the Bland-Altman graph. RESULTS: The average of the fetal cisterna magna volume ranged from 0.73 ± 0.25 to 3.79 ± 1.10 cm(3) between 17 and 29 weeks, respectively. Correlation was observed between the fetal cisterna magna volume and the gestational age (GA), best represented by a quadratic equation: -1.918+0.0284 × GA + 0.0065 × GA(2) (R(2 )= 0.67). It was observed good reliability and intra-observer agreement, with ICC = 0.92 and 95% limits of agreement (-49.7; 48.4). There was low inter-observer reliability and agreement, with ICC = 0.58 and 95% limits of agreement (-114.0; 80.2). CONCLUSION: Reference values for the fetal cisterna magna volume by 3DUS using the VOCAL method were determined and showed good agreement and intra-observer reliability.

Reassessment of the normal fetal cisterna magna during gestation and an alternative approach to the definition of cisterna magna dilatation.

INTRODUCTION: Cisterna magna (CM) measurement constitutes part of the sonographic assessment of the posterior fossa. CM enlargement (ECM) is defined as a measurement exceeding 10 mm, although it has previously been noted that the CM varies in size with gestation. Existing data do not appear to reflect observations regarding CM biometry within our population and this study was therefore undertaken in order to re-evaluate CM biometry. MATERIALS AND METHODS: Data from 4,750 normal pregnancies between 15 and 32 weeks of gestation were collected and used to construct a reference range for the CM. RESULTS: Regression analysis was used to model CM across gestational age and thereby define the upper limits for normal CM measurements across gestation. The CM increases with gestation. These data suggest that a 10-mm cut-off underestimates ECM, notably in the gestational age period below 24 weeks, whilst overestimating isolated ECM beyond this. Differences in CM measurements between genders were confirmed (p < 0.0001). CONCLUSIONS: Defining ECM based upon a cut-off of 10 mm across all gestations may be inappropriate given the variation observed with gestational age. More accurate identification of fetuses with, in particular, isolated ECM may facilitate more precise evaluation of the clinical significance of this finding.

Screening for fetal spina bifida at the 11-13-week scan using three anatomical features of the posterior brain.

OBJECTIVE: To evaluate the contribution of examination of specific anatomical features of the fetal posterior brain on mid-sagittal first-trimester ultrasound examination to the early detection of open spina bifida. METHODS: Four independent observers reviewed a series of 260 mid-sagittal first-trimester ultrasound images from 52 cases of open spina bifida and 208 normal fetuses. The following analysis was performed by each reviewer for each image: Herman score calculation, intracranial translucency score (CFEF-IT) calculation and determination of presence or absence of three anatomical criteria: intracranial translucency (IT), caudal displacement of the brainstem and cisterna magna. The sensitivity and the false-positive rate for spina bifida detection were calculated for each of the latter three criteria. A secondary analysis was performed on the subset of images achieving a Herman score ≥ 7. RESULTS: The highest detection rate for spina bifida was achieved by non-visualization of the cisterna magna, with associated sensitivity of 50-73% and 39-76%, respectively, for all images and for the subset of images achieving a Herman score ≥ 7. Posterior shift of the brainstem achieved the highest detection rate (86%), but for a single reviewer only. The level of variation in performance between observers was also greatest for this sign. Absence of IT was associated with a lower detection rate for all observers. Overall, an abnormal posterior brain presenting at least one of these three criteria was associated with a detection rate ranging from 50 to 90%. CONCLUSION: In the detection of spina bifida, non-visualization of the cisterna magna achieved the best screening performance. Both non-visualization of the IT and posterior shift of the brainstem were associated with acceptable but lower detection rates. A prospective evaluation of changes in the posterior brain is needed to allow assessment of the most pertinent criteria for first-trimester screening for spina bifida.

Cisterna magna width at 11-13 weeks in the detection of posterior fossa anomalies.

OBJECTIVE: To construct reference ranges for cisterna magna (CM) width at 11-13 weeks' gestation in healthy fetuses and determine whether open spina bifida and posterior fossa anomalies could be diagnosed in the first trimester. METHODS: This was a retrospective study. CM width reference ranges were constructed based on the measurements obtained from 80 healthy fetuses with normal postnatal outcome undergoing routine first-trimester ultrasound at 11-13 weeks, using the Lambda-Mu-Sigma method. CM was measured in the fetal mid-sagittal view, as routinely used for nuchal translucency assessment. In addition, first-trimester ultrasound images in 11 fetuses with open spina bifida or posterior fossa anomalies, most of which were diagnosed later in pregnancy, were retrospectively reviewed, and CM measurements were compared against reference ranges. RESULTS: CM width was noted to increase with gestational age in normal fetuses. The anomalies in the 11 fetuses we reviewed were: open spina bifida (n = 5), megacisterna magna (n = 3), Blake's pouch cyst (n = 2) and posterior fossa arachnoid cyst (n = 1). All fetuses with open spina bifida had a CM width below the 5(th) percentile. In the two fetuses with Blake's pouch cyst and in two with megacisterna magna, CM width was above the 95(th) percentile. In one of the fetuses with a megacisterna magna and the one with an arachnoid cyst, CM width was within normal range. CONCLUSION: We have constructed reference ranges for CM width at 11-13 weeks using the mid-sagittal view. It appears that first-trimester CM width can be used as a marker for the early detection of open spina bifida. However, our findings need to be confirmed in prospective large series.

Three-dimensional sonography of the posterior fossa in fetuses with open spina bifida at 11-13 weeks' gestation.

OBJECTIVES: To investigate the posterior fossa of normal fetuses and fetuses with open spina bifida in stored three-dimensional (3D) volumes and to describe signs that might allow early detection of this defect. METHODS: A prospective study of 3D volumes of the fetal brain obtained from 10 normal fetuses and three fetuses with open spina bifida was undertaken. Measurements of the anteroposterior diameters of the cisterna magna and fourth ventricle were taken in the tilted axial view. In the mid-sagittal plane the brainstem (BS) diameter and the brainstem-occipital bone (BSOB) distance were measured. The BS/BSOB ratio was calculated. All measurements were expressed as Z-scores. Structural analysis of the differences in the posterior fossa between normal fetuses and fetuses with open spina bifida was undertaken. RESULTS: In normal fetuses all measurements were within ±2.5 Z-scores. In three fetuses with open spina bifida the BS Z-scores were 2.7, 2.8 and 2.8; the BSOB scores were -3.4, -2.8 and -2.9; the cisterna magna scores were -5.6, -3.7 and -4.2; and the BS/BSOB ratio scores were 4.1, 9.7 and 8.9. In normal fetuses the cisterna magna was posterior to the fourth ventricle and extended along its entire length. In fetuses with open spina bifida the cisterna magna was partially or completely obliterated. CONCLUSIONS: Assessment of the cranial posterior fossa is feasible at 11-13 weeks' gestation. There are distinct signs in fetuses with open spina bifida which can be evaluated by ultrasonography.

Appearance of the fetal posterior fossa at 11 + 3 to 13 + 6 gestational weeks on transabdominal ultrasound examination.

OBJECTIVES: To describe the sonographic appearance of the structures of the posterior cranial fossa in fetuses at 11 + 3 to 13 + 6 weeks of pregnancy and to determine whether abnormal findings of the brain and spine can be detected by sonography at this time. METHODS: This was a prospective study including 692 fetuses whose mothers attended Innsbruck Medical University Hospital for first-trimester sonography. In 3% (n = 21) of cases, measurement was prevented by fetal position. Of the remaining 671 cases, in 604 there was either a normal anomaly scan at 20 weeks or delivery of a healthy child and in these cases the transcerebellar diameter (TCD) and the anteroposterior diameter of the cisterna magna (CM), measured at 11 + 3 to 13 + 6 weeks, were analyzed. In 502 fetuses, the anteroposterior diameter of the fourth ventricle (4V) was also measured. In 25 fetuses, intra- and interobserver repeatability was calculated. RESULTS: We observed a linear correlation between crown-rump length (CRL) and CM (CM = 0.0536 × CRL - 1.4701; R2 = 0.688), TCD (TCD = 0.1482 × CRL - 1.2083; R2 = 0.701) and 4V (4V = 0.0181 × CRL + 0.9186; R2 = 0.118). In three patients with posterior fossa cysts, measurements significantly exceeded the reference values. One fetus with spina bifida had an obliterated CM and the posterior border of the 4V could not be visualized. CONCLUSIONS: Transabdominal sonographic assessment of the posterior fossa is feasible in the first trimester. Measurements of the 4V, the CM and the TCD performed at this time are reliable. The established reference values assist in detecting fetal anomalies. However, findings must be interpreted carefully, as some supposed malformations might be merely delayed development of brain structures.

Sex-related differences in the anteroposterior diameter of the foetal cisterna magna.

AIM: To measure the anteroposterior diameter of the foetal cisterna magna and observe whether there are differences according to sex. MATERIALS AND METHODS: Three hundred and thirty-seven Chinese women with low-risk pregnancies and a singleton foetus between 22 and 38 weeks' gestational age were included in this retrospective study. Informed consent of the volunteer subjects and hospital authority approval were first obtained. Double-blinded for gender, the anteroposterior diameter of the cisterna magna of the foetuses was measured by transabdominal sonography. Magnetic resonance imaging (MRI) was used for those foetuses with mega cisterna magna. All of the foetuses were healthy by prenatal and postnatal examination, including physical and imaging examination. RESULTS: The mean anteroposterior diameter of the cisterna magna of all foetuses was 8.01+/-1.79 mm. The anteroposterior diameter of the cisterna magna had no obvious correlation with the gestational age. The mean anteroposterior diameter of the cisterna magna of 179 male foetuses was 8.63+/-2.16 mm, and the mean anteroposterior diameter of the cisterna magna of 158 female foetuses was 7.87+/-1.74 mm. The size difference was statistically significant (p<0.001). In the 33 foetuses with mega cisterna magna, the number of male foetuses was greater than female foetuses, and the proportion of the foetuses with mega cisterna magna in the male group was significantly higher than the foetuses in the female group (p<0.05). CONCLUSION: Male foetuses had slightly larger anteroposterior diameters of the cisterna magna than female foetuses. The study would be useful for creating normal range values for the cisterna magna of male and female foetuses.

The cisterna magna size in normal second-trimester fetuses.

OBJECTIVE: The purpose of this study was to evaluate cisterna magna size of the fetuses between 16 and 24 weeks of gestation, and to establish a normogram for cisterna magna measurements during gestation. METHODS: A prospective study of normal singleton pregnancies was established. Measurement of the fetal cisterna magna was performed by transabdominal ultrasonography between 16 and 24 weeks of gestation in 194 fetuses. RESULTS: A linear correlation was obtained between gestational age (GA) and cisterna magna (R(2)=0.75, P<0.0001). A linear correlation was also found between biparietal diameter and cisterna magna (R(2)=0.74, P<0.0001). The normal mean (+/-SD) for each gestational week was defined. CONCLUSION: The present data offer the normal range of the cisterna magna. Cisterna magna measurement may provide normative data for fetal growth and development. Abnormal measurement of cisterna magna may be a clue for posterior fossa pathologies or a chromosomal disorder.

Infusion of anti-nerve growth factor into the cisternum magnum of chick embryo leads to decrease cell production in the cerebral cortical germinal epithelium.

There has been considerable recent progress in understanding the processes involved in cerebral cortical development. Several mitogenic and trophic factors have been implicated in the processes of cortical cell proliferation and differentiation. Anti-nerve growth factor (NGF) antibody was administered to 15 days chick foetuses through the cisternum magnum. Control group received phosphate buffered saline (PBS). To identify cells born in the cerebral cortex at the time of antibody or PBS injection, 5'-bromo-2'- deoxyuridine was administered to the foetuses by intravenous injection into an outlying vein using micromanipulation. After injection, the foetuses were re-incubated for another 3 days. All the foetuses were collected on day 18, the brains fixed in paraformaldehyde, cut with a microtome and stained with methyl green pyronin and anti-NGF antibody. Quantitative measurements showed that the thickness of the germinal epithelium (GE) and cerebral cortex in the anti-NGF antibody injected foetuses was decreased when compared with normal control embryos. The number of cells produced in the GE of antibody injected foetuses was decreased when compared with normal control embryos. The results from this study using neutralizing antibody suggests that NGF is an important factor in cerebral cortical development, stimulating neuronal precursor proliferation.

The cisterna magna septa: vestigial remnants of Blake's pouch and a potential new marker for normal development of the rhombencephalon.

OBJECTIVE: The purpose of this study was to show the normal sonographic embryologic anatomy of the cisterna magna septa, fourth ventricle, and cerebellar vallecula at various stages of development and our experience with their variable appearance in multiple planes and to discuss the probable relationship between the cisterna magna septa, Dandy-Walker continuum, mega cisterna magna, and persistent Blake's pouch. METHODS: Retrospective and prospective selection of examples of cisterna magna septa was performed over approximately a 12-month period. Standard and nonstandard imaging planes were adopted as necessary. RESULTS: The septa are typically seen inferoposterior to the cerebellar vermis, usually straight and parallel, arising at the cerebellovermian angle and coursing posteriorly to the occipital bone. The cisterna magna septa become contiguous with the roof of the fourth ventricle inferior to the cerebellar vermis. The cerebrospinal fluid space enclosed between the cisterna magna septa is in direct contiguity with the fourth ventricle via the vallecula and is always completely anechoic because it develops intra- and not extra-axially. CONCLUSIONS: We propose that the cisterna magna septa represent the walls of Blake's pouch, a phylogenetic vestigial structure observed during ontogeny. Additionally, our observations support current opinion that a persistent Blake's pouch and mega cisterna magna represent (less severe) abnormalities within the Dandy-Walker continuum. The cisterna magna septa therefore are a marker of normal development of the roof of the rhombencephalon. Deviation from their normal appearances should prompt a closer assessment for associated abnormalities of the cerebellum, vermis, and brain stem by additional imaging in orthogonal planes with either sonography or magnetic resonance imaging.

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.

Evolution of subarachnoid space in normal fetuses using magnetic resonance imaging.

OBJECTIVE: The purpose of this study was to measure the fetal subarachnoid spaces at different sites of the brain using magnetic resonance (MR) images and analyze them in relation to gestational age. METHODS: Fetal MR images were obtained from 158 fetuses between 18 and 39 weeks of gestation who later showed no neurological problems. We bilaterally measured the distance between the superoanterior gyrus and the cranium as the frontal subarachnoid space (FSS) and the distance between the cortex in the parieto-occipital sulcus and the cranium as the parietal subarachnoid space (PSS). We also measured the cisterna magna between the cerebellar vermis and the cranium. Each of these was analyzed in relation to gestational age. RESULTS: The width of the FSS began to decrease significantly at 32 weeks of gestation (P < 0.05). The width of the PSS started to decrease significantly at 34 weeks of gestation (P < 0.05). There was no difference between the right and left sides (P < 0.05). The size of the cisterna magna showed a positive correlation with gestational age (P < 0.05). CONCLUSION: Measurement of the subarachnoid space is potentially useful for evaluating fetal development.

Sonographic measurement of the fetal cerebellum, cisterna magna, and cavum septum pellucidum in normal fetuses in the second and third trimesters of pregnancy.

PURPOSE: Absence of the cavum septum pellucidum (CSP), the cisterna magna (CM), or both, and enlargement of either or both structures are associated with various central nervous system malformations. In an effort to determine normal sizes and relationships between these cranial structures, we measured the CSP and CM in normal fetuses in the second and third trimesters of pregnancy using transabdominal sonography. METHODS: Women with uncomplicated pregnancies and normal singleton fetuses between 16 and 38 weeks' menstrual age were included in this prospective study. The width and anteroposterior (AP) diameters of the CSP were measured on the transverse transventricular plane, and the AP diameter of the CM was measured on the transcerebellar plane from the posterior aspect of the cerebellar vermis to the inner edge of the cranium. The transverse and AP diameters of the cerebellum were also measured. In addition, we measured the biparietal diameters (BPDs). RESULTS: In total, 130 women participated; 64 were examined in their second trimester and 66 in their third trimester. The mean age of the women was 27.4 +/- 4.8 years (range, 18-38 years), and the mean menstrual age of the fetuses was 26.9 +/- 6.7 weeks (range, 16-38 weeks). The mean BPD was 66.8 +/- 18.7 mm (range, 30-96 mm). The mean width and AP diameter of the CSP and the mean AP diameter of the CM differed significantly between the second and third trimesters (p < 0.001). All measured parameters correlated significantly with menstrual age and BPD. CONCLUSIONS: In normal fetuses, the CSP and CM should be visible on transabdominal sonography between 16 and 38 weeks' menstrual age. Because abnormalities in these cranial structures may be indicative of central nervous system malformations, the availability of mean sonographic measurements from normal fetuses should be helpful in determining the need for additional testing in fetuses with abnormal measurements.

Fetal central nervous system ventricle and cisterna magna measurements by magnetic resonance imaging.

OBJECTIVE: Our purpose was to evaluate the ventricular atria and cisterna magna in fetuses with and without suspected central nervous system (CNS) anomalies by magnetic resonance (MR). STUDY DESIGN: Measurements of the right and left ventricular atria and cisterna magna were obtained by MR in two groups: those with and without CNS anomalies. Published mean ultrasound measurements of the far field atrium were compared with MR. RESULTS: MR measurements were obtained in 23 fetuses without and 37 fetuses with CNS anomalies. Atrial measurements were independent of gestational age in healthy subjects. MR atrial widths were larger in abnormal compared with normal subjects (P <.05). The atrial cutoff value derived by 2 SDs above the mean with MR is 10 mm. MR cisterna magna measurements increased with gestational age (P =.005). CONCLUSION: The cutoff value for ventriculomegaly on MR is >10 mm. MR cisterna magna measurements are dependent on gestational age. Both ventricular atria and cisterna magna are readily measured with MR.