What are some causes of pneumocephalus?
Mechanical trauma; iatrogenic (e.g. external ventricular drain insertion, sinus surgery, peridural anaesthesia); baro trauma; meningitis from a gas forming organism (rare)
In what anatomic space is the vast majority of gas in this patient?
In the subarachnoid space.
How can you distinguish fat from air on CT?
Although on brain window both fat and gas appear as completely black, they have very different density. Air is very negativ (~ -1000HU) whereas fat is only somewhat negative (~ -90HU). As such measuring the density using a region of interest (ROI) or changing the window level to include fat (e.g. lung or mediastinum window) will easily distinguish between the two.
In what way is the brain like a whale? (I bet you weren't expecting a riddle)
Both are unable to support their own weight, relying rather on the buoyancy provided by the fluid they usually float in. Note how the frontal lobes 'sag' posteriorly.
Extensive pneumocephalus is demonstrated throughout the intracranial compartment with some posterior sagging of the brain.