Ethmoid sinus infection. Computerized tomography (CT) scan of an axial section of the brain of a 32-year-old patient with an ethmoid sinus infection. It is a bone window study (grey-level mapping, involving manipulation of the greyscale component of a CT scan to highlight specific structures in the brain), centred on the ethmoid sinus which is infected. The ethmoid sinus is an airspace (lined with a mucosal membrane) in the ethmoid bone of the skull, and is one of the four paranasal (frontal) sinuses. It is located between the nose and eyes. Sinus infection (sinusitis) is where sinuses become filled with fluid, which leads to bacterial growth and viruses causing infection. This patient is prone to a global sinus infection. This is a CT scan without intravenous iodine injection.