**Midbrain to pons ratios** as measured on midline sagittal images have been found useful in confirming midbrain atrophy in the setting of progressive supranuclear palsy (PSP).

##### Area ratio

A measurement method was described by Oba et al. on the midsagittal MR image using free-hand regions of interest to define the ratio of areas (Figure 1) ^{1}:

- the pontomesencephalic junction is defined by a straight line between the superior pontine notch and the inferior border of the quadrigeminal plate
- the pontomedullary junction is defined by a straight line parallel to the first line, at the level of the inferior pontine notch
- the area of the midbrain is traced above the first line, excluding the tectum (Figure 1, red)
- the area of the pons is traced between the two lines (Figure 1, blue)
- the midbrain to pons area ratio is the area of the midbrain divided by the area of the pons

In the original study ^{1}, patients with progressive supranuclear palsy had a ratio of <0.16, while patients who were normal or had multiple system atrophy–parkinsonian type or Parkinson disease had a midbrain/pons ratio greater than this. Other studies have suggested an optimal cutoff of <0.20-0.22 depending on the comparison group ^{2}.

Note that the inverse of the midbrain/pons area ratio is a component of the magnetic resonance parkinsonism index.

As manual measurement can be laborious, segmentation and calculation of these areas have been automated, taking a full volumetric T1-weighted MR dataset as the input, with preserved diagnostic accuracy ^{3,4}.

##### Short-axis diameter ratio

An alternative measurement method was described by Massey et al. on the midsagittal T1-weighted MR image using elliptical regions of interest to define the ratio of short-axis diameters ^{5}:

- the major axis of the midbrain is defined by the longest diameter in the oblique superior-inferior direction, including the midbrain tegmentum but not the tectal plate
- the major axis of the pons is defined by the longest diameter in the oblique superior-inferior direction, including the basis pontis but not the pontine tegmentum
- the short axes are the maximal diameter in the above-defined areas that are perpendicular to the major axes
- the midbrain to pons ratio is the midbrain short-axis divided by the pontine short-axis

Note that this method measures only the basis pontis whereas the method of Oba et al. also includes the area of the pontine tegmentum.

In the original study ^{5}, an optimal cutoff of the midbrain to pons ratio was <0.52 for progressive supranuclear palsy when compared with multiple systems atrophy.