document: the accuracy to detect fetal pathology depends on the expertise of the sonographer and some limitations inherent to this imaging technique . 
   descent of the fetal head and progressive ossification of the skull bones towards the end of the pregnancy , maternal obesity and oligohydramnios are all well - known limitations affecting the sensitivity to detect central nervous system pathology . 
   the first reported use of fetal magnetic resonance imaging  ( mri ) dates from 1989 . 
   advances in mri technology during the last 2 decades and better knowledge of fetal pathology , have both led to an increasing role for mri in the diagnostic work - up of prenatal anomalies . 
   most fetal mri studies reported so far , have addressed the additional role of mri in the diagnosis and characterization of fetal anomalies detected on ultrasound , with emphasis on central nervous system pathology . in this manuscript 
   we discuss the role of mri as an adjunct to ultrasound for cases diagnosed with ventriculomegaly . 
   the descriptive term cerebral ventriculomegaly is used when the maximal diameter of the atrium of the lateral ventricles  measured in a plane perpendicular to the midsagittal section of the brain  exceeds 10 mm . 
   the prevalence of ventriculomegaly ranges between 0.3 - 1.5 and the severity can be classified in mild ( 10 - 12  mm ) , moderate ( 12 - 15  mm ) and severe ( >  15  mm ) , as illustrated in fig . 1 ( valsky et al . , 2004 ; 
   another classification system is based on measuring the thickness of the overlying brain parenchyma in the category with an atrial diameter of >  15  mm , i.e. >  3  mm thickness indicates a moderate ventriculomegaly and   3  mm a severe form ( levine et al . , 2003 ) . 
   measurement of the lateral ventricle is subject to errors owing to an off - axis image plane of a section , an angled measurement , or improper choice of ventricular boundary giving risk to false - positive test results . 
   therefore , a precise definition of the reference planes and anatomical landmarks is important to avoid inaccurate measurements and facilitate imaging follow - up comparisons ( levine et al . , 2008 ) . 
   ventriculomegaly has a wide range of causes and can roughly be divided in three categories : ( 1 ) an imbalance between the production and absorption of cerebrospinal fluid , of which the obstructive form is most frequently observed , ( 2 )  abnormal cerebral development such as partial or complete agenesis of the corpus callosum and neuronal proliferation / migration disorders and ( 3 )  destructive disease processes leading to loss of neuronal tissue by vascular insults or infectious pathogens . 
   unilateral ventriculomegaly is more often seen in destructive processes , whereas developmental anomalies are more characterized by bilateral broadening of the ventricles ( girard et al . , 2003 ) . 
   the reported incidence of additional anomalies goes up to 70 - 85% , strongly depending on the number of fetuses included in the study and the distribution of the severity of the cerebral ventriculomegaly observed in the study population ( huisman et al . , 2002 ; 
   the term isolated ventriculomegaly is used in case no other structural anomalies are seen at the time of diagnosis . 
   isolated and/or unilateral ventriculomegaly , in particular the mild form , has a lower incidence of peri- and postnatal morbidity and mortality ( senat et al . , 1999 
   fetal mri is mainly performed on 1.5 tesla scanners . to date , no adverse health effects have been demonstrated  neither fetal nor maternal  after short term exposition to electromagnetic fields as used in clinical mri . 
   the haste sequence ( half fourier acquired single shot turbo spin echo ) is nowadays most used and combines short acquisition times ( 1 image in less than 1 second ) with a good signal - to - noise ratio , good t2 contrast and an acceptable spatial resolution ( slice thickness of 3  mm ) . 
   t1 weighted imaging is often used to detect hemorrhages or calcifications . t1-weighted imaging sequences have a lower signal to noise ratio , require longer scanning times ( up to 18 seconds per slice ) and are hence much more susceptible to fetal and maternal motion . 
   novel scanning sequences include the use of diffusion weighted imaging and diffusion tensor imaging for a more functional analysis of the developing brain ( guimiot et al . , 2008 ; 
   mri scans have a big field of view and can be obtained in any given plane 
   . the major limitations of fetal mri are the impact of fetal motion on the image acquisition and its relatively low spatial resolution , in particular compared to ultrasound . 
   contra - indications for mri are the same as for non - pregnant patients ( claustrophobia , metallic brain clips , pacemaker implant ,  ) . 
   opposed to ultrasound , mri has an excellent contrast resolution which enables to differentiate easily between gray and white matter . 
   mri also allows to directly visualize the cortical region and the fossa posterior , without sonographic limiting factors such as maternal obesity , the amount of amniotic fluid , fetal head position or the acoustic shadows of the skull bones . 
   knowledge of the normal appearance and maturation of the developing cerebral gyri and sulci is helpful in the appropriate diagnosis and counseling of anomalous fetuses . at 14 weeks 
   the sylvian fissure and callosal sulcus are respectively visible at 16 and 18 weeks , and the central sulcus is not reliable seen until 24 - 25 weeks of gestation . 
   weeks , numerous new sulci and gyri develop and by the age of 32 - 35 weeks secondary gyri are visible throughout the cerebral cortex ( levine and robson , 2005 ) . 
   an important role in the prenatal diagnostic work - up of ventriculomegaly is the detection and characterization of additional cerebral anomalies ( kubik- huck et al . , 2000 ; 
   , 2002 ; launay et al . , 2002 ; levine et al . , 2003 ; 
   valsky et al . , 2004 ; mehta and levine , 2005 ; zimmerman and bilaniuk , 2005 ; glenn and barkovich , 2006 ; benacerraf et al . , 2007 ; morris et al . 
   the incidence of additional malformations detected on mri following normal ultrasound findings varies greatly between several reported studies and depends on the severity of ventriculomegaly , the expertise of the sonographer and the selected patient population . 
   percentages of detection range between 5% for mild ventriculomegaly and up to 50% for moderate and severe cases ( valsky et al . , 2004 ; salomon et al . , 
   numbers should be interpreted with caution , because of the systematic lack of postnatal / postmortem ( imaging ) correlates , the uncertainty between the time span between prenatal ultrasound and mri in several studies , the a priori knowledge of ultrasound findings by the fetal mri specialist and the lack of data investigating the sonographic detection / confirmation of the fetal anomalies following the mri scan . 
   address several items why the added value of mri is difficult to statistically demonstrate : an overenthusiastic attitude towards new technologies , comparing a first line routine ultrasound exam with a mri exam in a tertiary centre , the lack of description of the ultrasound technique and checklist , suboptimal or insufficient ultrasound images in the reported studies and no information with respect to the time span between the ultrasound and mri exam ( malinger et al . , 2004 ) . 
   at last , it is very important to stress that the interobserver agreement , i.e. the interpretation agreement between 2 observers using the same modality , for both imaging modalities can vary greatly . in a study of levine et al . , 
   in which the agreement between experts using the same imaging modality was investigated , overall consensus was reached with respect to normal and abnormal findings in only 60% of the cases for ultrasound and in 53% for mri ( levine et al . , 2008 ) . the numbers even drop below 50% for the detection of cerebellar and gyral anomalies . 
   important to note is that the readers in this study could not indicate their uncertainty in each finding , but were only allowed to indicate the presence or absence of an anomaly . given the often subtle findings of fetal central nervous pathology , such as cerebellar of gyral anomalies , this probably has led to an overestimate of the discrepancies . 
   another important finding is that the level of subspecialty training of the individuals interpreting the mri examination , in particular the involvement of pediatric neuroradiologists , significantly improved the interobserver agreement . 
   below , we will discuss some disease entities often associated with ventriculomegaly , with emphasis on the added value of mri ( launay et al . 
   , 2002 ; glenn and barkovich , 2006 part 2 ; benacerraf et al . , 2007 ) . 
   mri is a very sensitive technique to detect small deposits of intraventricular blood , which may suggest an intraventricular hemorrhage as cause of the hydrocephaly ( fig . 
   mri has also a high accuracy to detect corpus callosum dysgenesis and allows to screen for additional malformations such as cortical anomalies , periventricular of subependymal nodular heteropia and interhemispheric cysts ( fig . 
   accurate characterization of those additional anomalies is important in terms of prognosis and parental counseling . 
   mri is equally good for a morpho- and volumetric assessment of the fossa posterior in case of suspicious ultrasound findings . 
   it allows a good evaluation of the vermis , the cerebellar hemispheres , the pedunculi , the brainstem , the 4th ventricle , the tentorium cerebelli and the retrocerebellar space , which enables to detect developmental anomalies such as vermis hypoplasia , dandy walker malformations , chiari malformations and other fossa posterior malformations  ( fig . 
   a recurring indication for ventriculomegaly reported in the literature , is the detection of cortical malformations , such as polymicrogyria ( fig .  5 ) , 
   the excellent tissue contrast of mri and the absence of bony interference , makes this technique ideal for cortical evaluation . 
   congenital infections such as cytomegalovirus and toxoplasmosis , are also good candidates to refer for mri , in particular to detect pathological white matter changes and cortical malformations ( fig . 
   focal loss of brain tissue , hemorrhages and sequelae of ventriculitis can also easily be detected on mri . 
   on the basis of reported studies , fetal mri has a limited role over ultrasound in assessing the size of the cerebral ventricles , except for cases where fetal position and calvarial ossification cause problems . in the second trimester , in particular the 20 - 24 weeks group , 
   mri is useful to detect small foci of brain hemorrhage or callosal anomalies in fetuses diagnosed with cerebral ventriculomegaly ( griffiths et al . , 2010 ) . later in pregnancy ( over 25  weeks ) 
   , mri adds information about normal and pathological cortical development and maturation , depicts fossa posterior anomalies and is a more sensitive imaging modality to detect white matter pathology ( griffiths et al . , 2010 ; yi et al . , 
   referring prenatal cases of ventriculomegaly for fetal mri is a topic of debate , not only due to questions related to the diagnostic accuracy of mri and its added value , but also because of practical issues such as the availability of mri , the cost and available expertise of fetal radiologists . 
   most studies reported in the literature advocate the added value of mri in cases of moderate and severe ventriculomegaly , not only to confirm the severity , but mainly to further detect and characterize additional anomalies . in cases of mild unilateral ventriculomegaly , mri might demonstrate additional findings which are difficult to detect with sonography such as leucomalacy and neuronal migration disorders . in both categories ,
summary: the indication for fetal magnetic resonance imaging ( mri ) remains a subject of debate , partly because of questions concerning its diagnostic accuracy compared to ultrasound , partly because of practical factors such as accessibility , high costs and available expertise . 
   most studies advocate an added value for mri in cases diagnosed with central nervous system pathology . 
   mri is a good modality to detect small foci of brain hemorrhage , to depict callosal anomalies , to add information about normal and pathological cortical development , and is a more sensitive imaging method to detect white matter pathology . 
   this manuscript discusses the role of mri as an adjunct to ultrasound for cases diagnosed with cerebral ventriculomegaly .