The Evans index is the ratio of maximum width of the frontal horns of the lateral ventricles and maximal internal diameter of skull at the same level employed in axial CT and MRI images. This ratio varies with the 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.
Ng SE, Low AM, Tang KK et-al. Value of quantitative MRI biomarkers (Evans' index, aqueductal flow rate, and apparent diffusion coefficient) in idiopathic normal pressure hydrocephalus. J Magn Reson Imaging. 2009;30 (4): 708-15. doi:10.1002/jmri.21865
Toma AK, Holl E, Kitchen ND et-al. Evans' index revisited: the need for an alternative in normal pressure hydrocephalus. Neurosurgery. 2011;68 (4): 939-44. doi:10.1227/NEU.0b013e318208f5e0 - Pubmed citation