Objective The distinction between idiopathic normal pressure hydrocephalus (iNPH) and hydrocephalus ex vacuo caused by encephalic volume loss remains to be established. This study aims to investigate radiological parameters as clinically useful tools to discriminate iNPH from hydrocephalus ex vacuo caused by Alzheimer’s disease (AD). Methods A total of 54 patients with ventriculomegaly (iNPH, 25; hydrocephalus ex vacuo, 29) were recruited in this study. Consequently, nine radiological parameters were compared between iNPH and hydrocephalus ex vacuo using magnetic resonance imaging (MRI). Results A small callosal angle (CA), the Sylvian fissure dilatation, and absence of narrowing of superior parietal sulci discriminated the iNPH group from the hydrocephalus ex vacuo group (p<0.05). The final binary logistic regression model included narrowing of superior parietal sulci, degrees of the CA, and height of the Sylvian fissure after controlling for age and global Clinical Dementia Rating (CDR). The composite score made from these three indicators (narrowing of superior parietal sulci, degrees of the CA, and height of the Sylvian fissure) was statistically different between iNPH and hydrocephalus ex vacuo. Conclusion The narrowing of the CA, dilatation of the Sylvain fissure, and narrowing of superior parietal sulci may be used as radiological key indices and noninvasive tools for the differential diagnosis of iNPH from hydrocephalus ex vacuo.