The interpretation of functional imaging data from patients with brain lesions will be influenced by the site and extent of lesions. Two separate imaging techniques can provide accurate objective anatomical data that can be used to address hypotheses about the structure-function relationships of surviving brain regions. Voxel-based morphometry (VBM) and diffusion tensor imaging (DTI) are both MRI-dependent techniques.
VBM is used to characterise grey and white matter differences in structural MRI scans (Ashburner and Friston, 2000) thus providing objective anatomical data about which regions of the brain have been damaged. A number of studies have validated VBM techniques against more traditional region of interest volumetric approaches (Good et al., 2002; Maguire et al., 2003). In stroke patients VBM has been used to identify structural changes in peri-lesional tissue and at distant sites compared to controls (Schormann and Kraemer, 2003). It is this ability to search for structural differences across the whole brain in an objective, automated and therefore in a reproducible manner that makes VBM such a valuable technique.
DTI is a technique that provides quantitative information about the integrity and orientation of white matter fibre tracts in the brain. It is based on the principle that water diffuses preferentially along nerve axons rather than across nerve bundles and that the direction of water diffusion within a brain voxel (typically 2-3 mm3) can be assessed and quantified. This parameter is used to test the direction of most coherent diffusion in all voxels and therefore the likelihood of connection to a specified starting point (a technique termed tractography) (Parker et al., 2002). Construction of statistical maps of the likelihood of connections between cortical areas, deep nuclei, and motor output pathways is then possible (Behrens et al., 2003; Ciccarelli et al., 2003; Guye et al., 2003). The technique is ideal for examination of the structural integrity of the corticospinal tract in health and disease (Kunimatsu et al., 2003; Toosy et al., 2003; Sach et al., 2004).
These precise and objective measurements of anatomical damage will become invaluable for the interpretation of functional imaging data.
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