CT scanning, which delivers high doses of ionising radiation, is increasingly being used in young children after minor head trauma.
The study involved 3,094 men who had received radiation therapy before age 18 months during 1930-59. At age 18 or 19 years, their intellectual capacity was tested and high school attendance was recorded.
The proportion of boys who attended high school decreased with increasing doses of ionising radiation to both the front and back parts of the brain.
A significant dose-related response was also seen for learning ability and logical reasoning, but not for spatial recognition.
Intellectual development could be adversely affected when the infant brain is exposed to ionising radiation at doses equivalent to CT scans of the skull, say the authors.
The risk and benefits of CT scans in minor head trauma need re-evaluating, they conclude.