The air bubble sign is seen on CT of the brain and represents multiple small foci of air within the subarachnoid space, especially the Sylvian fissure.1
Although described as a sign of tension pneumocephalus it is also seen in pneumocephalus without elevated pressures.2
It should not be confused with the similarly named sign of complicated pulmonary hydatid disease: see air bubble sign (pulmonary hydatid).
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