Bi-exponential diffusion signal decay in normal appearing white matter of multiple sclerosis.

Abstract Purpose Our aim was to characterize bi-exponential diffusion signal changes in normal appearing white matter of multiple sclerosis (MS) patients. Methods Diffusion parameters were measured using mono-exponential (0–1000 s/mm 2 ) and bi-exponential (0–5000 s/mm 2 ) approaches from 14 relapsing-remitting subtype of MS patients and 14 age- and sex-matched controls after acquiring diffusion-weighted images on a 3T MRI system. The results were analyzed using parametric or nonparametric tests and multiple linear regression models. Results Mono-exponential apparent diffusion coefficient (ADC) slightly increased in controls ( P =.09), but decreased significantly in MS as a function of age, nonetheless an elevated ADC was observed with increasing lesion number in patients. Bi-exponential analyses showed that the increased ADC is the result of decreased relative volume fraction of slow diffusing component (f s ). However, the fast and slow diffusion components (ADC f , ADC s ) did not change as a function of either age in controls or lesion number and age in MS patients. Conclusions These data demonstrated that the myelin content of the white matter affects diffusion in relapsing-remitting subtype of multiple sclerosis that is possibly a consequence of the shift between different water fractions.