A 7-month-old infant developed Haemophilus influenzae meningitis that failed to respond to appropriate therapy. Anterior fontanel subdural taps were negative. A subsequent sodium pertechnetate Tc 99m brain scan demonstrated a subdural fluid collection on the left, manifested as a "doughnut" sign. Following surgical evacuation of the effusion, the child recovered uneventfully.
Trackler RT, Miller KE, Cohen ML. The "Doughnut Sign" in Subdural Effusion. Am J Dis Child. 1975;129(3):373-374. doi:10.1001/archpedi.1975.02120400073019