What sequences are most useful in making the diagnosis?
DWI / ADC distinguishes an epidermoid from CSF filled lesions and post contrast T1 ensures that this does not represent a partially cystic tumour (e.g. pilocytic astrocytoma or haemangioblastoma).
Is FLAIR also helpful?
Yes, as the epidermoid cyst does not completely suppress / attenuate on FLAIR, rather remaining somewhat dirty in appearance.
Within the inferior aspect of the fourth ventricle is a mass with very high T2 signal (similar to CSF) and low T1 signal (again, similar to CSF). On FLAIR, the mass does not fully attenuate, and no contrast enhancement is present. Importantly on DWI/ADC, the mass demonstrates restricted diffusion.
Despite its size, the mass does not result in hydrocephalus. It is an isolated abnormality.