What is the key sequence for this tumour?
DWI/ADC. The presence of diffusion restriction of this "dirty" cystic lesion makes an epidermoid cyst the likely diagnosis.
There is a midline posterior fossa tumour favoured to be extra-axial and located posterior to the vermis and in between the cerebellar hemispheres, pushing against them. It appears to be predominantly cystic, showing heterogenous signal intensity, with the more solid portions having an intense intrinsic high T1 signal and patchy low T2 signal, with no convincing internal enhancement. There are marginal linear blooming artefacts on SWI, and flecks of calcifications have been shown on the previous CT. Heterogeneous restriction diffusion is noted throughout the lesion. The lesion does not cause a significant mass effect in the posterior fossa; there are no cerebellar signal abnormalities and the fourth ventricle is preserved. The remainder of the brain appears normal.