The Evans index is defined as the ratio of which measurements?

• A. The ratio of the width of the lateral ventricles to the skull diameter
• B. The ratio of brain parenchymal volume to skull volume
• C. The ratio of the width of the frontal horns to the maximum internal diameter of the skull
• D. The ratio of hippocampal height to skull width

Answer: (C)

The key idea is a simple, head-size–normalized measure of how dilated the ventricles are. Evans index uses the width of the frontal horns of the lateral ventricles on an axial image and divides it by the maximum internal diameter of the skull on the same plane. This ratio flags ventriculomegaly when the frontal-horn width is large relative to skull size, which helps distinguish hydrocephalus-related dilation from normal-sized ventricles in a small head or from brain atrophy. In practice, a value around 0.3 or higher suggests ventriculomegaly.

This is why the defined measurement is the width of the frontal horns to the maximum internal skull diameter. Other measurements involve brain parenchymal volume, hippocampal height, or a different ventricular measurement, which do not define Evans index.