Notes
The formula
The magnetic resonance parkinsonism index (MRPI) is calculated using the following equation:
MRPI = (P / M) x (MCP / SCP)
- Where
- P = area of pons in midsagittal plane
- M = area of midbrain in midsagittal plane
- MCP = average width of middle cerebellar peduncles
- SCP = average width of superior cerebellar peduncles
How to measure the required parameters
The above illustration shows the proper measurement technique:
- the area of pons and midbrain are both measured in the midsagittal plane
- the middle cerebellar peduncle in the sagittal plane (both sides, averaged)
- the superior cerebellar peduncle is assessed in the coronal plane (both sides, averaged)
In the original study volumetric T1-weighted images acquired at 1.5T were used for analysis.
Since progressive supranuclear palsy (PSP) results in midbrain and superior cerebellar peduncle atrophy, this results in an increase of the MRPI score. In contrast, multiple system atrophy (MSA-P) typically spares these structures but results in atrophy of the pons and middle cerebellar peduncle.
Prognostic value
- In the original study MRPI differentiated patients with PSP from those with MSA-P or Parkinson’s disease with a sensitivity and specificity of 100%, and a positive predictive value of 100%
- It has been shown that 78.5% of patients with clinically unclassifiable parkinsonism and an MRPI >13.55 fulfilled the criteria for PSP at the two years later follow-up
- MRPI also showed a higher accuracy (92.9%) than clinical features in predicting PSP
- MRPI tends to increase with the duration of PSP, thus it can be used to quantify disease progression
MRPI 2.0
MRPI 2.0 was introduced in 2018, and further improves MRPI by incorporating third ventricular width relative to the frontal horn width.
It is calculated using the following formula:
MRPI 2.0 = MRPI x (V3 / FH)
- Where
- MRPI = the original MRPI score calculated using the above discussed formula
- V3 = average width (from three measurements) of the third ventricle on an axial image at the level of the anterior and posterior commissures
- FH = maximal left to right frontal horn width on an axial image in the anterior to posterior commissure plane
Illustrations of the measurement technique can be found on this site maintained by the authors of the study.
Prognostic value
- MRPI 2.0. was found to be more sensitive than MRPI, while maintining the speicifity of the original score
- The cutoff values of MRPI 2.0 depend on the clinical context:
- PSP-Parkinsonism vs Parkinson’s disease or normal control: ≥2.18
- PSP-Richardson’s syndrome vs Parkinson’s disease or normal control ≥2.50
Further reading
- Radiopaedia article
- MRPI online automatic tool – provides automated MRPI calculations from T1-weighted images. Site maintained by the authors of the original study. Registration needed, each user can submit up to 4 cases monthly
Last updated: 2022-11-24