| Water diffusion in human tissue was investigated in this thesis. Tissue structures and fluids in the body have a pronounced effect on the mobility of water at the microscopic level. They restrict the motion of water molecules which would otherwise move according to free diffusion. The result is that the diffusion coefficient appears to be smaller. A comparison of the measured apparent diffusion coefficient, D;From current analytical equations, the diffusion profiles in the white matter reveal that the membrane permeability of the axons falls within lower and upper limits of P = 0.002 cm/s and P = 0.019 cm/s. However, this thesis also presents a Monte Carlo analysis and its results suggest a prominent mechanism affecting water mobility is missing from these theoretical equations. It is found that fluid motion at the cellular level (probably originating from cerebrospinal fluid pressure) is most likely a prominent factor affecting the motion of water molecules. The experimental findings in this thesis also reveal that the diffusion anisotropy of white matter is removed at low diffusion times. In the skeletal muscle, this work has found a very small restricted diffusion effect in the longitudinal direction of muscle fibre, probably due to the Z-line of the sarcomere.;Diseases such as stroke and multiple sclerosis may benefit from clinical applications of diffusion imaging. More information about the basic processes of water diffusion in tissue is needed before the full potential of diffusion MRI can be realized. |
