Preschooler with developmental delay

MRI of arachnoid cyst of the posterior fossa
Axial (above left), coronal (above right) and sagittal (below) T2 MRI of the brain show a midline to right of midline high signal intensity lesion in the posterior fossa which is inferior to an intact cerebellar vermis and the right cerebellar hemisphere and which is having mass effect on the cerebellum.

The diagnosis was arachnoid cyst of the posterior fossa.