An important sign of pneumocephalus is compression and separation of the frontal lobes, resembling the double-peaked silhouete of mount Fuji. The presence of gas within the interhemisferic space indicates that air pressure exceeds the surface tension of CSF, between frontal lobes
Failure to identify the normal "butterfly" appearance of normal hiperecogenic coroid plexus whitin paired lateral ventricles is an early harbinger of holoprosencephaly, in prenatal ultrasound.
The alobar variant is the most severe with holospheric cortical fusion; absent corpus calosum, septum pellucidum and falx cerebri; single monoventricle; and fusion of basal ganglia and thalami.
Other useful findings of extra-axial masses are "CSF cleft" sign and "dural tail" sign. Meningioma is the most common extra-axial mass and comprises up to 15% of all brain tumours. Then, meningioma must be kept in mind when we find out an extra-axial tumour.
Extra-axial masses displace adjacent parenchyma with inward bowing of the giry ("buckling" or "accordion" sign). This usually is associated with some vasogenic edema but less than intra-axial masses.