What are some of the differentials you would entertain, and why wouldn't you list them as most likely?
Medulloblastoma (typically arise from the midline; usually seen in younger patients (2 - 6 years of age)), haemangioblastoma (usually seen in adults; often has large vascular flow voids), ependymoma (tends to fill the fourth ventricle and protrude out of the foramen of Luschka and foramina of Magendie; large cystic component less common).
Why is the midface distorted, and returns poor signal?
The child has braces, which result in field inhomogeneity.
Selected images from an MRI of the posterior fossa demonstrates a cystic mass with peripheral nodular enhancement. It arises form the right cerebellar hemisphere, displacing and compressing the fourth ventricle. It is surrounded by vasogenic oedema. No flow voids are noted.