The fetal falx cerebelli.

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) x 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.