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Diffusion weighted imaging (DWI) is a magnetic resonance imaging technique which is sensitive to the molecular motion of water and allows measurements of anisotropy (the directionality) of water diffusion.1 Measurements of anisotropy provides information about tracts, because water molecules travel predominantly parallel but not perpendicular to tracts.2 The anisotropy of diffusion can be calculated on a pixel by pixel basis and calculated images (maps) of anisotropy can be generated. We investigated the pyramidal tract in two patients with hemiparesis using a method to estimate anisotropy proposed by Van Gelderen et al.3 On anisotropy maps the pyramidal tract was visible and symmetric in all six investigated control subjects. In the two patients a reduction of anisotropy suggesting a disruption of the microstructural organisation of the pyramidal tract4 was detected on the clinically abnormal side on anisotropy maps. Our finding suggests that anisotropy measurements can be used as a tool to visualise abnormal tracts in vivo. (A) schematic drawing of a coronal section through the hemisphere. The pyramidal tract consists of projection fibres and forms the posterior limb of the internal capsule. (B) Anisotropy map of a normal control subject. (C) T1 weighted image of a patient with hemiparesis. (D) Anisotropy map of the same patient showing the abnormal pyramidal tract.
Acknowledgments
This research has been kindly funded by an European Community Fellowship (UCW), by the National Society for Epilepsy (MRS and KDB), the Brain Research Trust (CAC and MRS) and the Multiple Sclerosis Society of Great Britain and Northern Ireland (GJB). We thank Dr BE Kendall and Dr JM Stevens for their helpful comments. We thank Dr JS Duncan for permission to study his patients.