The abnormal brain scan: specificity of descriptive parameters.

When a brain scan is abnormal, one can often describe the abnormality in terms of its location, shape, sharpness, brightness, and other descriptive parameters. Certain types of abnormality have been linked to certain specific diagnoses: for example, a crescent-shaped lesion would suggest subdural hematoma to many people, a wedge-shaped lesion would suggest cerebral vascular accident, etc. Some features thought to be characteristic of certain diseases are actually quite nonspecific. For example, the "doughnut" sign--at first believed to be characteristic of brain abscess--has also been found in primary and metastatic tumors, CVA, and subdural hematoma. The "crescent" sign that was at first thought to be specific for subdural hematoma occurs also in meningitis, scalp or skull trauma, meningioma en plaque, etc. Some features of abnormalities are highly specific for certain diseases; e.g., wedge- or flame-shaped lesions are rarely seen with disorders other than CVA, and lesions in the midline or in the posterior fossa are almost invariably tumors. This article reviews the features of abnormalities on brain scans that in the literature have been reportedly associated with specific types of disease and explores the strength and validity of the associations.