Image Processing

We processed all images using the FSL brain image analysis package (version 6.0.1, Oxford Centre for Functional MRI of the Brain, Oxford, UK).²⁵⁾ The methods used have been described elsewhere.²¹⁾ In summary, the DTI data were initially corrected for motion and eddy current distortions via alignment of later images to the first image (b=0 s/mm²). Extracerebral regions were then removed from the images. Next, regional FA values for the brain were calculated to generate an FA brain map. This map was subsequently converted to a standard stereotaxic space. Regions of interest (ROIs) were set within the corticospinal tracts in the right and left cerebral peduncles (Fig. 1). These regions were selected because of the potential for magnetic resonance susceptibility effects of supratentorial stroke lesions to interfere with the validity of DTI data.²⁶⁾ The ROIs for the cerebral peduncles were set with reference to the International Consortium for Brain Mapping DTI-81²⁷⁾ (Fig. 1). FA values were calculated for the left and right ROIs with subsequent estimation of mean values for single voxels. The ratio of FA values between the ipsilesional and contralesional hemispheres (rFA) was calculated as the index of neural degeneration in the corticospinal tracts for each patient (Fig. 1).^5,28)

 Regions of interest in cerebral peduncles and examples of DTI-FA images from a patient with hemorrhagic stroke and a patient with ischemic stroke transformed into the standard space. CT, computed tomography; DWI, diffusion-weighted imaging.