Disproportionate atrophy of midbrain is seen with concavity of its superior and dorsolateral margins.
Midline sagittal midbrain to pons area ratio is ~ 0.12 (reduced; normal value is ~ 0.24). Mid brain to pons width ratio is approx. 0.4 (reduced).
 

These findings along with clinical history of the patient suggest possibility of progressive supranuclear palsy. The above measurements are important to objectively assess for the disease in a patient with clinical suspicion as the so called 'humming bird' sign of progressive supranuclear palsy can be seen in a lot of patients of older age group.