Midbrain to pons area ratio (PSP)

Midbrain to pons area ratio as measured on midline sagittal images has been found useful in confirming a small midbrain in the setting of progressive supranuclear palsy (PSP) . This reduction in the size of the midbrain leads to the hummingbird sign.

In the method described by Oba et al. the area of the midbrain and pons are calculated on the midline sagittal image and the ratio midbrain: pons obtained .

The pontomesencephalic junction is defined by a line between the superior pontine notch and the inferior border of the quadrigeminal plate.

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 whereas it is significantly reduced in PSP to 0.12 .