Abstract： Objective To evaluate the subcortical white matter changes adjacent to focal cortical dysplasia(FCD) by applying diffusion tensor imaging (DTI) and to investigate its potential value in the diagnosis of FCD.Methods The clinical information, MRI images and DTI data of 12 patients with FCD were retrospectively evaluated.The DTI images were postprocessed.The mean diffusivity(MD), fractional anisotrophy (FA) and color-encoded tensor images were acquired.Ellipsoid ROIs were set in the subcortical white matter under FCD lesions and corresponding areas on the contralateral side.The MD value and FA value were measured in each ROI.The paired t test was employed to compare the MD and FA values between the areas ipsilateral and contralateral to the lesion.In diffusion tensor tractography (DTT), the same ROI as DTI measurement was set using T_2 WI images as reference.One ROI served as seed point, all the white matter fiber tracts crossing the ROI were acquired.Results Focal cortical thickening and blurring of the gray-white matter junction were found in all cases of FCD on conventional MRI images.Seven patients demonstrated hyperintensity on T_2 WI images and hypointensity on T_1 WI images in the subcortical white matter adjacent to FCD.Two patients demonstrated focal subcortical white matter atrophy.No abnormal findings were detected in the subcortical white matter in 3 patients.Quantitative analysis of DTI data showed significant fractional anisotrophy (FA) decrease (ipsilateral side to FCD 0.31±0.05, contralateral side 0.42 ±0.08,t=- 3.995, P<0.01), and mean diffusivity (MD) increase in comparison to thecontralateral region [ipsilateral side to FCD (1.02 ±0.16) × 10~(-3) mm~2/s, contralateral side (0.86±0.13) ×10~(-3) mm~2/s,t=2.655,P<0.05].On diffusion tensor tractography (DTT), 10 patients showed definite reduction in the volume of subcortical white matter bundles and the connections between the subcortex and deep white matter were sparse.Two patients showed probable reduction compared with the contralateral normal side.Conclusions FCD not only has cortical abnormalities but also affects local white matter development.DTI is an important supplement to conventional MRI.