Assessment of tissue micro-structure with diffusion weighted magnetic resonance imaging

Based on the pioneer works of Hahn in 1950, Carr and Purcell in 1954, and Stejskal and Tanner in the 1960s, Le Bihan and Breton introduced diffusion weighted imaging (DWI) technique in 1985. Then in 1986, Le Bihan presented the first DWI images of the human brain. From then on, DWI has become a very important area of MRI research.



Measurements of tissue micro-structure can be very important in both clinical and research application. DWI is dependent on specific micro-structural properties of biological tissues, so it is possible to obtain structural information non-invasively from DWI measurements. Diffusion in tissues is slower than in free solutions because tissue compartments hinder or restrict fluid motions, and the reduction in diffusion rate reflects the scale and nature of the tissue environment. Although DWI is becoming a mature method, there are still problems with the validation of this technique needed to be solved.



The first part of this PhD dissertation compares several sequences for the visualization of facial skeletal muscle with diffusion tensor imaging (DTI), which enables the investigation of micro-architecture and connectivity of oriented tissues. Readout segmented echo planar imaging (RS-EPI) has been suggested as an alternative to single-shot EPI (SS-EPI) for DWI with reduced distortion, which shows more reliable facial muscle fiber tracts. While compared to diffusion weighted EPI techniques, diffusion weighted double echo steady state free precession sequence (DW-DESS) is relaxation independent, with higher resolution and considerably higher SNR . By using this approach, the image can achieve a higher resolution, giving more information of the small facial muscle groups.



The second part of this thesis refers to comparison of different diffusion models in renal diffusion imaging. The aim is to obtain additional parameters with stretched-exponential, bi-exponential and kurtosis analysis of multiple b-value DWI and compare these parameters to apparent diffusion coefficient (ADC) obtained with mono-exponential model in their ability to discriminate enhancing from renal imaging.