Figure 9e. Medulloblastoma in a 4-year-old boy with a 2-month history of ataxic gait. On day of admission, he hit his head on the floor and presented comatose to the emergency department. (a) Axial CT image shows a heterogeneous mass in the cerebellar vermis. Areas of hyperattenuation (arrowheads) are secondary to hemorrhage. The fourth ventricle is not seen. (b) Axial T1-weighted MR image shows mild hyperintensity in the hemorrhagic regions; otherwise the mass is predominantly hypointense. (c) Axial T2-weighted MR image reveals marked hypointensity in the hemorrhagic zones. These features are consistent with intracellular methemoglobin. (d) Contrast-enhanced axial T1-weighted MR image demonstrates heterogeneous but intense enhancement of the nonhemorrhagic portions. (e) Sagittal T1-weighted MR image shows complete filling of the fourth ventricle and upward extension of the posterior fossa mass through the cerebral aqueduct (arrow) into the third ventricle.