A hypothetical 3-year-old child presents with new-onset ataxia. A magnetic resonance image of the brain is pursued, consisting of a single axial T1 slice that shows the ventricles to be normal in size. The child returns several weeks later, and owing to concern about the cerebellum, another single coronal T1 slice is obtained, which is also normal. A third scan later that month, this time a coronal T2 image including the basal ganglia, reveals bilateral injury. Further diagnostic tests proceed from there.