The MRI and CT demonstrate the classic findings of a neuroglial cyst. The lesion is basically a cyst within the white matter that appears simple, with CSF density on all imaging modalities and is without any associated edema. 

Differentials would include porencephalic cyst (note absence of any volume loss or gliosis) or less likely cystic glioma / metastasis (absence of any edema or enhancing / solid component basically excludes these) or cyst from prior infection.

Presence of weakness is a little concerning however and follow-up would be required.