Blake's Pouch Cyst MRI

•  The Blake's pouch cyst is characterized by the presence of a CSF-filled cyst in a purely infravermian or infra-retrovermian location. This cyst is essentially a diverticulum of the inferior fourth ventricle, which is variably enlarged, ranging from mild to marked enlargement. 
• The posterior fossa is typically normal in size.
• The cerebellar vermis, however, is normal in size and not rotated, allowing the differentiation between BPC and Dandy walker malformation 
• The consistent presence of a tetraventricular  hydrocephalus allows the differentiation between BPC and posterior fossa arachnoid cysts as well as Mega cisterna magna.
• In Blakes pouch cyst dynamic CSF flow studies may show no flow between the cyst and the posterior cervical subarachnoid space and hyperdynamic pulsatile flow within the third ventricle, Sylvian aqueduct, and fourth ventricle due to tetraventricular hydrocephalus