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Background and Purpose: In amyotrophic lateral sclerosis (ALS), fiber degeneration within the corticospinal tract (CST) can be quantified by diffusion tensor imaging (DTI) as an indirect marker of upper motor neuron involvement. A new method of measuring quantitative DTI parameters using a probabilistic mixture model for fiber tissue and background in the corticospinal tract of patients with ALS is evaluated., Materials and Methods: Axial echo-planar imaging (EPI) DTI datasets (6 gradient directions, 10 repetitions) were acquired for 10 patients and 20 healthy control subjects. The diffusion tensor was visualized in a multiplanar viewer using a unique color coding method. Pure fiber tissue inside a region is separated from background and mixture voxels using a probabilistic mixture model. This allows for a reduction of errors as a result of partial volume effects and measurement variability., Results: Fractional anisotropy (FA) was measured within the CST at levels ranging from internal capsule to pons. Mean coefficients of variation of intrarater, scan-rescan, and inter-rater reproducibility were 2.4%, 3.0%, and 5.7%, respectively. Optimal measurement positions along the CST with respect to minimum variability and maximum difference between patients and healthy subjects were identified in the caudal half of the internal capsule. Moreover, a negative correlation between the age-corrected FA and the disease duration but not the ALS Severity scale score was found., Conclusion: The new software for fiber integrity quantification is suited to assess FA in the corticospinal tract with high reproducibility. Thus, this tool can be useful in future studies for monitoring disease status and potential treatment efficiency.