Evan's index is a radiological criteria that helps in diagnosis of hydrocephalus. Evan's index is a ratio of width of frontal horns and internal diameter of the skull on the same slice of CT or MRI. Evan's ratio > 0.3 indicates hydrocephalus. Reference: https://radiopaedia.org/articles/evans-index-1#:~:text=day%20reporting%204.-,Measurement,axial%20CT%20and%20MRI%20images. Image via: https://radiopaedia.org/articles/evans-index-1#:~:text=day%20reporting%204.-,Measurement,axial%20CT%20and%20MRI%20images.