There is enlargement of brainstem mainly pons and lesser degree midbrain, which appears heterogeneously hypointense on T1W and hyperintense on both T2W and FLAIR sequences. It shows areas of mild enhancement on T1W post contrast.
It causes mass effect in the form of effacement of the prepontine cistern, displacing and partially encasing the basilar artery anteriorly, flattening the fourth ventricle posteriorly and obliteration of perimesencephalic cisterns laterally.