= embryonic midline outpouching of superior medullary velum extending inferior + posterior to vermis into cisterna magna

Blake pouch = rudimentary 4^th ventricular tela choroidea membrane as a normal transient structure that regresses by 12 weeks GA to form the foramen of Magendie (communication between 4^th ventricle + subarachnoid space)

Cause: failure of fenestration of foramen of Magendie

Age: neonate

• retrocerebellar / infraretrocerebellar cyst = diverticulum of enlarged 4^th ventricle
• enhancing structure along anterosuperior aspect of cyst inferior to vermis = displaced choroid plexus (on SAG contrast-enhanced T1WI)
• mild indentation of inferior vermis / caudomedial aspects of cerebellar hemispheres ← mass effect of cyst
• tetraventricular hydrocephalus

Pertinent negatives:

• NO supratentorial morphologic abnormalities aside from hydrocephalus
• posterior fossa + cerebellum of normal size + shape

Prognosis: favorable (shunt-related complications possible)

DDx: megacisterna magna