LAUSR

Background The falx cerebelli is a retrocerebellar dural reflection. The MR spectrum of the fetal falx cerebelli has not been described. Objective To determine the prevalence of falx cerebelli abnormalities in the context of posterior fossa malformations and compare them to age-matched normal fetal MRI exams. Materials and methods We reviewed all consecutive fetal MRIs performed over 1 year at a children’s hospital. We assessed the falx cerebelli in each examination for location, morphology, size and number. Exams were grouped into (1) normal or non-brain/head abnormalities or (2) abnormal brain or craniofacial structures. We used chi square, linear regression and logistic regression analyses; P <0.05 was considered significant. Results We included 424 examinations (223 controls, 201 malformations) from 378 patients (mean gestational age 27±6 weeks). In the control group, the mean falx size was 2.6±1.2 mm (anteroposterior) × 11.0±3.2 mm (craniocaudal), with 80% retrovermian centered; the falx was linear (23%), Y-shape (15%), V-shape (22%) or U-shape (21%); it was unusually multiplicated (17%) or absent (<2%). Falx cerebellar abnormalities were more common in abnormal exams (59%; 119/201) than in normal exams (19%; 43/223) ( P <0.001). The falx was abnormal with Blake pouch cysts (9/9, 100%) and rhombencephalosynapsis (3/4, 75%), absent in all Chiari II ( n =9) and most Dandy–Walker malformations (5/6, 83%), commonly multiplicated in mega cisterna magna (14/22, 64%), and deviated or absent in cases with arachnoid cysts (3/3, 100%) and adhesions (4/5, 80%). Conclusion Structural alterations of the falx cerebelli are more prevalent in fetuses with brain and craniofacial abnormalities. Specific changes offer clues to posterior fossa diagnoses.