Figure 13d. Medulloblastoma in a 3-year-old boy with a 1-week history of headache, vomiting, and difficulty walking at night. Papilledema was noted on physical examination. (a) Axial CT image shows a homogeneous hyperattenuated mass arising in the cerebellar vermis. A thin crescent of the fourth ventricle is visible along the ventral margin of the mass. (b) Axial T1-weighted MR image reveals the nearly homogeneous hypointense mass. Note a small focal area of marked hypointensity (arrowhead). (c) Axial T2-weighted MR image demonstrates diffuse hyperintensity of the mass and focal marked hyperintensity (arrowhead) of the area previously noted in b. This area represents either cystic change or necrosis. There is minimal surrounding vasogenic edema. (d) Contrast-enhanced axial T1-weighted MR image shows linear enhancement within the mass, but a large portion of it does not enhance.