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

Abstract

T₁ relaxation time (T₁) 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 T₁ 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 T₂-weighted sequence. Whole brain T₁ 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. T₁ values were sampled along these trajectories. Total white matter T₁ was sampled by defining white matter masks on axial slices of the T₁ maps. Median T₁ 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 T₁ 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 T₁ or total white matter T₁. Our findings show that quantifying pathology within the pyramidal tracts, by utilizing T₁, provides a strong correlate of disability compared with the overall white matter burden of disease. Pyramidal tract T₁ may also provide an objective, sensitive measure for monitoring the progression of motor deficits and disability.