The measurements should represent the maximum respective widths on the same axial slice where the frontal horns are most prominent.
Anatomic Sentinel Probe
Input the maximum widths of the frontal horns and the internal skull diameter from axial brain imaging to calculate the linear Evans Index.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Evaluating ventriculomegaly on CT or MRI.
Aiding in the diagnosis of Normal Pressure Hydrocephalus (NPH).
Differentiating atrophy from obstructive or communicating hydrocephalus.
Section 2
Literature
Development
Proposed by William Evans in 1942, this ratio remains a cornerstone in neuroradiology. It provides a standardized measure of ventricular size relative to the skull, reducing variability across different patient sizes.
Section 3
Pearls/Pitfalls
NPH Threshold
A ratio > 0.3 is generally accepted as evidence of hydrocephalus. In NPH, this is often seen alongside narrow high-convexity sulci and an acute callosal angle.
Section 4
Evidence Appraisal
Primary Reference
An encephalographic ratio for estimating ventricular enlargement and cerebral atrophy
Evans WA Jr. • Arch Neurol Psychiatry. 1942;47(6):931-937
