"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)"
"SCP = average width of superior cerebellar peduncles"
"MRPI x (V3 / FH)"
"MRPI = (P / M) x (MCP / SCP) - see above"
"FH = maximal left to right frontal horn width on axial image in AC-PC plane"
"FH = maximal left to right frontal horn width on axial image in AC-PC plane"
"P = area of pons in midsagittal plane"
"PSP-Richardson's syndrome vs Parkinson disease or control: ≥2.50"
Expected headings
"Measurement"
"MRPI"
"MRPI 2.0"
"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."