Magnetic resonance parkinsonism index
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Magnetic resonance parkinsonism index (MRPI) can be used in MRI studies to distinguish patients with classic and brainstem variants of progressive supranuclear palsy (PSP) from those with other movement disorders (e.g. Parkinson disease, clinically unclassifiable parkinsonism, Huntington disease) 3.
MRPI was introduced as an improvement on the simpler midbrain to pons area ratio, however, not all publications have found that it is, in fact, superior 3. It is certainly more complicated whereas midbrain to pons ratio can be easily remembered and used in everyday clinical practice. As PSP results in the progressive atrophy of the midbrain and superior cerebellar peduncles the MRPI of these patients will in turn increase. Conversely, multiple system atrophy (MSA-P) and other forms of parkinsonism capable of producing similar early clinical symptoms will spare these structures. MSA-P in particular rather affects the pons and middle cerebellar peduncle 4-5.
MRPI is calculated by measuring the width of the superior cerebellar peduncles in the coronal plane, the middle cerebellar peduncles in the sagittal plane and the area of the midbrain and pons in the midsagittal plane. It is calculated by multiplying the pons area to midbrain area ratio by the middle cerebellar peduncle width to superior cerebellar peduncle width ratio 1-2:
(P / M) x (MCP / SCP)
MCP = average width of middle cerebellar peduncles
SCP = average width of superior cerebellar peduncles
P = area of pons in midsagittal plane
M = area of midbrain in midsagittal plane
A value of more than 13.55 indicates an abnormal result, and strongly suggests that these patients will go on to develop PSP. One study found that 78.5% of patients with clinically unclassifiable parkinsonism who had abnormal MRPI went on to fulfill criteria for PSP at two years following that measurement 2. It was also shown that MRPI differentiates PSP patients from those with MSA-P or Parkinson disease with a sensitivity, specificity, and positive predictive value of 100%. Furthermore, MRPI has a greater accuracy than clinical features in predicting PSP, and increases with the duration of the disease, thus it can be also used for monitoring disease progression 1.
MRPI 2.0 was introduced in 2018 as an improvement on the initial description by incorporating third ventricular width 4.
It is calculated by multiplying the MRPI by the ratio of third ventricular width to frontal horn width 4.
MRPI x (V3 / FH)
MRPI = (P / M) x (MCP / SCP) - see above
V3 = average width (from three measurements) of the third ventricle on an axial image at the level of anterior and posterior commissures
FH = maximal left to right frontal horn width on axial image in AC-PC plane
Cutoff values have been calculated that depend on the clinical context 4.
PSP-parkinsonian vs Parkinson disease or control: ≥2.18
PSP-Richardson's syndrome vs Parkinson disease or control: ≥2.50
As always, care must be taken in applying these methodologies and values indiscriminately in clinical practice as the high sensitivity, specificity, positive and negative predictive values are rarely reproduced in the wild.
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