The infratentorial region is a cavity delimited superiorly by the tentorium cerebelli,
inferiorly by the foramen magnum,
anteriorly by the apex of petrous temporal,
posteriorly by occipital bone and laterally by the squamous and mastoid portions of temporal bones.
It contents the cerebellum,
midbrain,
medulla and pons.
The best imaging modality to study the infratentorial region is the MR.
The basic radiological structure of cerebellum includes hemispheres,
vermis and pedicles.
Cerebellar hemispheres and vermis is are divided into lobules,
which are fully demarked on MR.
In the brainstem,
MR clearly depicts midbrain,
superior and inferior colliculus,
red nucleus,
subthalamic nucleus and substantia nigra.
MR also permits the study of the ventricular system,
exposing in detail the Sylvian aqueduct,
4º ventricle,
foramina of Luschka as well as the peritroncular cisterns.
The standard protocol of study in our institution,
using a 1,5T MR Scanner,
includes the acquisition of 4mm axial and sagittal T1-WI; intravenous administration of Gadolinium (in selected cases); axial T2 and T2*-WI; DWI and ADC map; and post-contrast axial and coronal T1-WI.
Coronal FLAIR,
perfusion and spectroscopy are used occasionally.
The latter two are considered advanced techniques.
We divide the broad spectrum of pathologies that can affect this region in adults in relation to their etiology: inflammatory-demyelinating (multiple sclerosis/ADEM,
Behçet disease,
sarcoidosis,
vasculitis,
Wallerian degeneration),
infectious (listeria,
tuberculosis,
pneumococcus,
chronic lymphocytic meningitis),
tumoral (primary such as brainstem and cerebellar gliomas/secondary such as methastases),
vascular (infarction,
hemorrhage,
vascular malformation),
metabolic (Central pontine myelinolysis) and traumatic (Diffuse axonal injury).
We describe the MRI features (conventional and advanced) of some of the most frequent lesions.
Finally,
we propose a scheme based on such MRI features,
to classify lesions into one of the above-mentioned groups.
This scheme is intended to be used as a tool to narrow the differentials and to help making a specific diagnosis.