Objective To measure the extracerebral space width of normal infants with magnetic resonance imaging(MRI)and observe the variation tendency with age for quantitative diagnosis of hydrocephalus.Methods We measured the distance of interhemispheric fissure and bilateral frontal subarachnoid space on MRI images of 213 normal infants.The width of extracerebral space was expressed as mean ± standard deviation.The gender and bilateral differences were compared by rank sum test,and the variation trend of extracerebral space with age was observed by the scatterplots fitting regression curve.Results The width of infant extracerebral space had two milestones,4-month-old extracerebral space was the widest,and the amplitude of variation tended to be mild after 1 year of age.There was no significant difference between the genders in gap of extracerebral space(P＞ 0.05). However,bilateral frontal subarachnoid space differed significantly between genders(P＜0.05).Conclusion The width of extracerebral space,a dynamic process with age in infants,is most pronounced under 10 months old.The accurate diagnosis of pathological external hydrocephalus should be based on the reference value of the extracerebral space at different months,and it is very important to combine it closely with clinical history.