These findings occur as the ipsilateral, lateral ventricle is compressed with subsequent dilatation of the contralateral ventricle to maintain the same volume. The ipsilateral cistern is widened because of the fact that the brain stem is inferiorly contiguous with the spinal cord leading to a long rigid structure as shown in the coronal CT image . Note the mass on the right with widening of the ipsilateral, right ambient cistern. As the supratentorial brain shifts say to the right, the superior aspect of this long column of mid brain and cord also shifts to the right. This will narrow the contralateral cistern and widen the ipsilateral cistern at the anterolateral aspects of the brain stem .
Uncal herniation is a subset of descending transtentorial herniations.
The uncus is displaced into the suprasellar cistern. The usual six pointed star appearance of the suprasellar cistern then becomes truncated on the ipsilateral side of the herniation . Coronal imaging can also demonstrate uncal herniation either by CT or MRI . A gross image of descending transtentorial herniation can demonstrate the cleft or crease on the parahippocampal gyrus as the brain herniates through the incisura. The uncus is also filling the left aspect of the suprasellar cistern.