The false falx sign.

When identified on nonenhanced computed tomography (CT), a long, thin band of increased attenuation in the region of the falx cerebri (the falx sign) has been regarded as evidence of subarachnoid hemorrhage. Shorter, wider, or wedge-shaped interhemispheric fluid collections of blood-equivalent attenuation have been considered representative of a subdural hematoma in an abused child. The superior sagittal sinus, straight sinus, and falx cerebri are identified on unenhanced CT scans of pediatric patients without clinical or radiographic evidence of trauma, subarachnoid hemorrhage, bleeding diathesis, or other abnormalities. These structures are often visualized where atrophy or degenerative diseases of the brain provide an adjacent region of diminished attenuation. With the spatial and density resolution of new CT scanners, visualization of the falx cerebri and its dural sinuses is normal.