MRI of brain with T1 weighted echo train spin echo, T2 weighted
 echo train spin echo, T2 weighted fluid-attenuated inversion
 recovery echo train spin echo, MR diffusion imaging, T2 weighted
 short inversion time inversion recovery echo train spin echo, and
 without intravenous contrast:

1. Focal large amount cerebrospinal fluid accumulation in right
   retrocerebellar space with mild across midline, may due to
   large arachnoid cyst (figure 1).
2. Presence of mass effect of the above lesion with compression of
   adjacent cerebellum and medulla oblongata, and suspicious
   foramen of Magendie.
   Downward displacement of cerebellar tonsil (1.3cm below foramen
   magnum), suggesting tonsillar herniation (arrows in figure 2).
3. Marked dilatation of ventricles with stretched corpus callosum,
   possibly associated with severe obstructive hydrocephalus from
   compressed foramen of Magendie.

Impression:
1. Right retrocerebellar large arachnoid cyst, suspicious tonsillar
   herniation and severe obstructive hydrocephalus.
2. Recommendation: neurosurgery consultation. Thank you!