The Evans' index is the ratio of maximum width of the frontal horns of the lateral ventricles and the maximal internal diameter of the skull at the same level employed in axial CT and MRI images. This ratio varies with age and sex.
It is useful as a marker of ventricular volume and thus has been proposed as a helpful biomarker in the diagnosis of normal pressure hydrocephalus (NPH) . Unfortunately it is a very rough marker of ventriculomegaly, and varies greatly depending on the location and angle of the slice . As such Evans' index has little role to play in day-to-day reporting.
A normal Evans' index is less than 0.3, but it can be higher in a normal healthy elderly individual .