In the method described by Oba et al the area of the midbrain and pons are calculated and the ratio midbrain : pons obtained 1.
The pontomesencephalic junction is defined by a line between the superior pontine notch (A) and the inferior border of the quadrigeminal plate (B).
The pontomedullary junction is defined by a line parallel to the first line, at the level of the inferior pontine notch.
A normal value is approximately 0.24 where as it is significantly reduced in PSP to 0.12 1.
Diagram by Frank Gaillard.