The last decade has seen a steady increase in interest in using magnetic resonance imaging (MRI) measures to monitor disease activity in multiple sclerosis (MS). While MRI unquestionably provides an important tool for studying the natural history of MS and is useful as a biological marker for disease in studies of disease mechanisms, the use of MRI as a surrogate outcome measure has been problematic. The stringent criteria for a validated surrogate that includes evidence that the surrogate predicts future levels of clinical disease activity and that the effect of various classes of treatments on clinical disease can be accounted for by their effect on the surrogate outcome have not been met at any stage of MS. It is evident that MRI does reflect the underlying pathological conditions of the disease and in this regard may, under some conditions, be acceptable as an invalidated surrogate-outcome measure. In the early stages of relapsing-remitting MS, conventional imaging techniques, such as burden of disease on T2-weighted images and the number of contrast enhancing lesions, appear to have value but more advance imaging techniques will be needed to reflect the pathological changes leading to progression of the disease. The design of clinical trials using MRI as a primary outcome measure present important difficulties and possibilities for error. Future research will need to focus on how MRI can best be used to monitor disease in patients with MS.
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