Novel diffusion tensor imaging (DTI) approaches at 3 T

Diffusion tensor imaging (DTI) records random thermal movements of water molecules in the body. In tissues with organized microstructure, such as the white matter of the brain, water tends to move (diffuse) more easily along the main direction of the fibers than in the orthogonal directions, resulting in anisotropic diffusion. The main parameter images of interest are the mean diffusivity (MD) describing the strength of diffusion, fractional anisotropy (FA) describing the asymmetry of diffusion (due to tissue structures), and tractography to visualize the 3D course of fiber tracts. This Thesis arose from the methodological needs related to adoption of the DTI method into our laboratory. In the first study, mechanical vibrations during DTI were demonstrated to be unevenly distributed within the scanner. These vibrations, generally assumed to be negligible, thus have to be taken into consideration, as they may affect the image quality and accuracy of diffusion measurements. In the second study, pre-processing-effects of voxel-based analysis (VBA) of DTI data were evaluated using simulated brain lesions in MD and FA images. Pre-processing and inter-individual variation remarkably affected the outcome of the analysis even to such extent that some lesions were not detected by VBA. The success of lesion detection varied between the brain areas, and it was different for MD and FA. The third study was the first to demonstrate the feasibility of DTI in tracking distal peripheral nerves in both upper and lower limbs. As a continuation, DTI was in the fourth study applied to monitor patients with carpal tunnel syndrome, an entrapment of the median nerve at the wrist, before and after surgical therapy. Comparisons with healthy young control subjects revealed increased MD and decreased FA in patients, whereas results of patients and age-matched control subjects were rather similar. Post-operative healing was reflected in MD but not in FA. Numerous other DTI studies on peripheral nerves, published recently, indicate that this research area initiated by us is becoming increasingly popular.