Falx and interhemispheric fissure on axial CT: I. Normal anatomy.

To determine the normal appearance of the flax and interhemispheric fissure, 200 consecutive normal CT scans were evaluated prospectively. On unenhanced scans, the normal falx is visualized in 90% of patients and therefore interhemispheric hyperdensity alone should not be considered a sign of subarachnoid hemorrhage. The flax is most often (88%) visualized in the posterior part of the interhemispheric fissure, as a hyperdense, pencil-thin line extending from the calvarium to the splenium of the corpus callosum. In the anterior part of the fissure, the falx is visualized in only 38% of patients, when its appearance differs significantly from that of the fissure. It is seen as a thin, hyperdense line extending posteriorly from the calvarium for a variable distance, but it never reaches the genu of the corpus callosum. The interhemispheric fissure is a hypodense structure broader than the falx with a zigzag configuration due to medial frontal sulci. The difference in configuration between the anterior part of the fissure and the anterior falx is very helpful in differentiating subarachnoid hemorrhage from normal falx visualization.