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MRPI is calculated by measuring the width of the superior cerebellar peduncle in the coronal plane, the middle cerebellar peduncle 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: (P / M) x (MCP / SCP)
- MCP = width of middle cerebellar peduncle
- SCP = width of superior cerebellar peduncle
- P = area of pons in midsagittal plane
- M = area of midbrain in midsagittal plane