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Computed tomography of meningiomas often demonstrates a zone of diminished density surrounding the actual tumor. This zone may be related to widening of the subarachnoid space, cerebral edema, tissue necrosis, or demyelination. A case is presented in which the anatomic basis of the zone of diminished density was shown on pneumoencephalography and at surgery to be an enlarged subarachnoid space.

The normal computed tomography (CT) appearances of the posterior third ventricle and related structures are presented. Seventy-six patients with lesions directly involving this ventricle were studied by CT, and results of other neuroradiological procedures evaluated. Primary tumors, metastases, hematomas, infarcts, and cysts may be reliably distinguished by CT, particularly if contrast enhancement is utilized. Size and density resolution limit the direct visualization of the aqueduct of Sylvius; stenosis however may be inferred. Small infarcts may be difficult to demonstrate by the use of CT scans without contrast enhancement, yet be shown by isotope scans.