The posterior fossa is the deepest of three of cranial fossae. It’s 1/8 of all the intra-cranial space. It contains the most complex anatomy where we can find pathways regulating consciousness, vital autonomic functions and motor activities.

It is the infra-tentorial compartment of the brain that extends from the tentorium to foramen magnum. Its essential limits are: (figure 1)

- In front: dorsum sellae and the clival part of the occipital bone.

- Behind: the lower portion of the squamosal part of the occipital bone.

- Laterally: by the petrous and mastoid parts of the temporal bone.

- Above and behind: by a small part of a mastoid angle of the parietal bone.

It contains essentially the brainstem, cerebellar hemispheres, the vermis, fourth ventricle and different vascular and nervous structures. (figure 2)

Infra-tentorial tumors account for 50–55% of all brain tumors in children but they are less frequent in adults representing 30 % of all central nervous system (CNS) tumors.

Masses in the posterior fossa may be divided into extra-axial and intra-axial lesions. Extra-axial tumors are more frequent in adults, mainly in cerebellopontine angle (CPA) represented essentially by vestibular schwannoma, meningioma and epidermoid cyst.  Only 15–20% of all intra-axial masses in adults are infra-tentorial, the most common tumor is cerebellar metastasis, and the most common primary tumor is hemangioblastoma. Brainstem gliomas are less frequent. Tumors of fourth ventricle are represented by ependymoma and papilloma.

Conventional MRI is an essential tool for diagnosis, precise location and determinate extension of posterior fossa tumors. Advanced MRI techniques such as diffusion, perfusion and spectroscopy may improve the specific diagnosis of these tumors (grade and type).