FIGURE 96.20 Congenital aqueductal stenosis and hydrocephalus Congenital aqueductal stenosis Midsagittal T1-weighted MR image shows marked dilatation of the lateral and third ventricles with a normal-sized fourth ventricle (Reprinted with permission from Eisenberg RL An Atlas of Differential Diagnosis 4th ed Philadelphia, PA: Lippincott Williams & Wilkins; 2003 Original: Reprinted with permission from Castillo M Neuroradiology Companion Philadelphia, PA: Lippincott-Raven; 1999.)