Midline sagittal T2-weighted image in case 4 showing midbrain atrophy with preservation of the normal pons. The maximal measurement [perpendicular to the major axis of the midbrain (white line A, 8.2 mm)] and pons (white line B, 17.2 mm) were taken, and the ratio of A to B of 0.48 was calculated. The combination of a midbrain measure of < 9.35 mm combined with a midbrain-to-pons ratio has been shown to be specific for the diagnosis of PSP [8].