Diffusion tensor imaging and the measurement of diffusion tensors in biological systems.

摘要

This dissertation revisits the theory of diffusion-weighting factor of gradients for any sequence and presents a new comprehensive formula which not only unifies all former results, but also predicts new results for unsolved pulse sequences. Then, on the basis of this new formula, two diffusion tensor imaging techniques were built, tested, and applied to the rat spinal cord and brain, thus showing the biological and clinical significance of diffusion tensor imaging.;There have been many previous attempts to derive an expression of diffusion-weighting factor of gradients in different pulse sequences, but until now, nobody has been able to derive a formula for any double-quantum and multiple-quantum coherence sequences, only under in case had this problem been solved experimentally. This dissertation introduces a new method which not only unifies all different expressions for single quantum coherence, but also predicts the expressions for double-quantum and multiple-quantum coherence sequences. Next, using the new formula for diffusion-weighting factor, two diffusion tensor imaging techniques were built: spin echo diffusion tensor imaging and multi-stimulated echo diffusion tensor imaging. The Echo-Plan Imaging (EPI) spin echo diffusion tensor imaging had been built in 1994, but the multi-echo stimulated echo diffusion tensor imaging technique, along with Basser's EPI stimulated echo diffusion tensor imaging technique, were the first two techniques in the world that used stimulated echo sequence in diffusion tensor imaging.;In this dissertation, diffusion tensor imaging techniques were first applied to normal and injured rat spinal cords and brain, color trace images were presented which clearly show the fiber orientations of the spinal cord that were not accomplished by any other MRI. We found that the injury of the spinal cord causes the loss of anisotropy of white matter, which is consistent with other results using apparent diffusion coefficient (ADC). It is interesting that there were substantial changes in diffusion characteristics in the injured area which appeared normal by conventional imaging. These results imply that there are consequences of spinal cord injury which dramatically alter axon structure but do not change water content, so are not detected by conventional imaging. We found that the hydrocephalus of the rat brain causes the loss of anisotropy of white matter, and hydrocephalus results in increased water content in the brain surrounding the ventricles. This has been confirmed by independent measurements.;This is a new direction in the noninvasive investigation of the normal and abnormal brain and spinal cord using a new imaging technique to study tissue organization, pathological changes, and recovery with treatment. The technique could ultimately be applied to clinical situations (in the same way as ADC is now being applied to measure the progress of clinical stroke). The dissertation concludes with a theoretical interpretation of the measured diffusion constants.