Figure 4. CT and MRI images showing blood–fluid levels in cases of primary intracranial malignancies and metastases to the brain. (a) Plain and (b) post-contrast images of the posterior fossa in a 21-year-old man reveal a heterogeneously enhancing mass centred at the right cerebello-pontine angle. Intense and homogeneous enhancement of the solid component of the tumour is seen following contrast administration. The thin-walled peripherally enhancing cystic component has a blood–fluid level and shows no appreciable enhancement of its contents. Sections at a more caudal level reveal gross widening of the ipsilateral internal acoustic canal (not shown) typical of an acoustic schwannoma.