ObjectiveTo measure the ventricular size and extracerebral space of normal neonates and infants, and discuss the diagnostic criteria of MRI in external hydrocephalus. MethodsNeonates and infants suspected to have brain diseases treated in our hospital from January 2009 to January 2013 were selected, and through strict inclusion and exclusion, 671 neonates and infants within 3 years of age and without neural system disease were included in our study. They were divided into 9 age groups. The ventricular size was measured on T1WI, that was bifrontal index, bibody index and the ratio of ventricle and cranial cavity diameter (VT/ST). And the width of extracerebral space, intercerebral fissure and sylvian fissure were measured on T2WI. A total of 149 neonates and infants with external hydrocephalus underwent the same measurement, and the changes in external hydrocephalus were dynamically observed. ResultsVentricular volume increased gradually with age, but the bifrontal index[(0.32±0.05) mm] and the ratio of VT/ST (14.09%±1.56%) were relatively constant. The extracerebral space widened with age during 0-6 months, after which it began to narrow, and the age of 3-6 months was the turning point for this trend. The frontal subarachnoid, intercerebral fissure, and sylvian fissure of infants with external hydrocephalus were wider than standard values of the corresponding age group, and the ratio of VT/ST was less than 15%. ConclusionThe standard values of ventricular size and extracerebral space of each age group provide the basis for the evaluation of brain development. Among them, the bifrontal index and the ratio of VT/ST are reliable indicators to determine the ventricular size. Subarachnoid depth, intercerebral fissure, and the ratio of VT/ST play an important role in the diagnosis of EH. But for secondary external hydrocephalus, early treatment should be carried out.