Midbrain to pons ratio (PSP)

Last revised by Dr Francis Deng on 12 Nov 2021

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.

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Cases and figures

  • Figure 1: annotated cloured image
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  • Case 1: with hummingbird sign
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