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Objective: To determine the usefulness of MRI measurements in patients with pure akinesia with gait freezing (PAGF), Richardson's syndrome, and Parkinson's disease for diagnosis., Methods: We obtained MRI measurements for patients with PAGF, Richardson's syndrome, or Parkinson's disease: 9 patients with PAGF, 26 with Richardson's syndrome, and 93 with Parkinson's disease. We measured the area of the pons and midbrain on midsagittal MRIs and the midbrain width on axial MRIs. We also calculated the mean values of the superior cerebellar peduncle, middle cerebellar peduncle, and cerebral crus width; the pons area-to-midbrain area ratio; the middle cerebellar peduncle width-to-superior cerebellar peduncle width ratio; and the magnetic resonance (MR) Parkinsonism index., Results: The Richardson's syndrome group had the highest pons area-to-midbrain area ratio and MR Parkinsonism index; the Parkinson's disease group had the lowest values. The Parkinson's disease group also had the highest midbrain width and cerebral crus width, with the lowest values being seen in the Richardson's syndrome group. The PAGF group had the intermediate values of the pons area-to-midbrain area ratio and MR Parkinsonism index between the Richardson's syndrome group and the Parkinson's disease group, whereas significant differences were found only in the pons area-to-midbrain area ratio. Results from receiver operating characteristic curve analyses showed that the pons area-to-midbrain area ratio has a higher sensitivity, specificity, and accuracy than the MR Parkinsonism index., Conclusions: The pons area-to-midbrain area ratio is more useful to distinguish PAGF from Richardson's syndrome and Parkinson's disease than the MR Parkinsonism index.