D.D.Posterior fossa cyst

1-Look to the vermis of the cerebellum and according to its condition you can reach a proper diagnosis.

2-If the vermis is anaplastic with evidence of a canal connecting between the cyst and fourth ventricle, this condition is diagnosed as Dandy-Walker malformation.

3-If the vermis is hypo plastic(small in size but still separating the cyst from the fourth ventricle),

Now you are facing with Dandy-Walker variant.

4-If the vermis is of normal morphology and size, now you have two possibilities, the first one is arachnoid cyst and the second one is mega cisterna magna. You could find with former supra tentorial hydrocephalus while the latter do not shows that.