T1 relaxation time mapping of white matter tracts in multiple sclerosis defined by diffusion tensor imaging

T(1) relaxation time (T(1)) is a quantitative magnetic resonance measure that enables a global evaluation of white matter disease in multiple sclerosis (MS). We aimed to investigate whether mapping of T(1) values in critical white matter tracts, defined by diffusion tensor (DT) imaging, could provide a stronger surrogate marker of disability. 25 patients with relapsing-remitting MS and 14 healthy controls were imaged with a dual-echo T(2)-weighted sequence. Whole brain T(1) maps were acquired using a multi-slice inversion recovery sequence and DT images generated from a spin-echo, echo-planar diffusion weighted sequence. Trajectories were defined to follow the course of white matter fibre tracts in the pyramidal pathways and corpus callosum. T(1) values were sampled along these trajectories. Total white matter T(1) was sampled by defining white matter masks on axial slices of the T(1) maps. Median T(1) in the pyramidal tracts, corpus callosum and total white matter of MS patients was significantly longer than in controls (p < 0.0001). Median pyramidal tract T(1) correlated significantly with the pyramidal Kurtzke Functional Systems Score (r = 0.64, p = 0.0007) and the Expanded Disability Status Scale (r = 0.55, p = 0.005). By contrast, no correlation with disability was observed for corpus callosum T(1) or total white matter T(1). Our findings show that quantifying pathology within the pyramidal tracts, by utilizing T(1), provides a strong correlate of disability compared with the overall white matter burden of disease. Pyramidal tract T(1) may also provide an objective, sensitive measure for monitoring the progression of motor deficits and disability.