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Purpose: To prospectively assess if middle cerebellar peduncle (MCP) atrophy, evaluated at magnetic resonance (MR) imaging, can help differentiate multiple system atrophy (MSA) from Parkinson disease (PD)., Materials and Methods: All participants provided informed consent for participation in the study, which was approved by the institutional review board. Sixteen consecutive patients with MSA, 26 consecutive patients with PD, and 14 healthy control subjects were examined with MR imaging. Images were interpreted independently by two experienced neuroradiologists blinded to clinical information, who visually inspected the images for the presence or absence of putaminal atrophy, putaminal hypointensity, slitlike hyperintensity in the posterolateral margin of the putamen, brainstem atrophy, hyperintensity of the MCP, and cruciform hyperintensity of the pons. Measurements of MCP width on T1-weighted volumetric spoiled gradient-echo images were performed in all subjects. Differences in MCP width among the groups were evaluated by using the Kruskall-Wallis test, followed by the Mann-Whitney U test for multiple comparisons and Bonferroni correction., Results: All patients (mean age, 63.88 years; range, 55-72 years) with MSA had at least one of the features commonly observed in this disease on MR images, whereas control subjects (mean age, 66.93 years; range, 61-77 years) and all but one patient with PD (mean age, 65.31 years; range, 51-79 years) had normal MR images. The average MCP width was significantly smaller in patients with MSA (6.10 mm+/-1.18 [standard deviation]) than in those with PD (9.32 mm+/-0.77, P<.001) or control subjects (9.80 mm+/-0.66, P<.001)., Conclusion: Measurement of MCP width on MR images may be useful for distinguishing patients with MSA from those with PD., (Copyright (c) RSNA, 2006.)