Figure 11a. Medulloblastoma in a 4-month-old boy with irritability and vomiting. Physical examination revealed increased head circumference, bulging fontanelle, and left gaze preference. (a) Axial CT image shows a hyperattenuated heterogeneous mass in the midline of the posterior fossa with nearly complete effacement of adjacent cisternal spaces. (b) On an axial T1-weighted MR image, the mass is heterogeneous with focal areas of hyperintensity mixed with isointense signal. (c) On an axial T2-weighted MR image, the mass has mild hypointensity compared with gray matter and contains scattered areas of moderate hypointensity that correspond to regions of hemorrhage. (d) Contrast-enhanced sagittal MR image shows intense plaquelike enhancement of the mass with superior displacement of the straight sinus (arrowheads).