| Abstract |
The use of MRI as a quantitative tool has attracted great interest and has a significant impact on diagnosis of tissue abnormalities. In conventional MRI the image contrast is primarily based on tissue T1 (longitudinal relaxation time), T2 (transverse relaxation time), T2*(overall transverse relaxation time) and water/blood molecular diffusion and flow. The final 2D/3D MR image voxel signal is obtained from the indirect contributions of all the above parameters and is presented as a 2D/3D MR image with contrast reflecting the relative parameter weight (T1-W, T2-W, T2*-W, Diffusion-W etc). A map can be generated by fitting the signal curve of each contrast related parameter. Therefore, the accuracy and precision of the fitting method is of particular interest, so that the created map is accurate. Also, many advanced methods exist in literature for these measurements, providing additional information, but not in the current exams in the hospitals. Therefore, the challenges in quantitative MRI (qMRI) can be split into analysis challenges, including the accuracy and precision of the method used for map calculation, but also system challenges, considering that currently an external software of high cost is required for each individual measurement in the hospitals. A tool to be a part of an already installed workstation does not exist, to include all the optimized advanced techniques under its umbrella. Following an analysis of the theoretical background, in order to address these challenges the subject of this dissertation and the methodology can be separated into four parts. 1. Phantom Studies. Special tissue mimicking phantoms were fabricated. These phantoms were simulating the relaxometric and diffusion characteristics of selected pathologies and diseases. The phantoms were measured repeatedly for a period of six months. All relaxometric and diffusion algorithms were used for the assessment of T1, T2, T2* and ADC of the phantoms via the 2D/3D parametric mapping techniques. Short and long term precision figures for the measurement methodologies were estimated. Different algorithms were tested in terms of measurement accuracy, reproducibility, reliability and speed. 2. Optimization of the algorithms, for fast and reliable ADC, T2 and T2* measurements via ADC, T2 and T2* parametric maps. In the case of T2 measurements, except from the ordinary approximations such as single exponential behavior, a more complete and complicated analysis considering multi exponential T2 analysis and T2 distribution maps was performed. This appropriate and state of the art algorithm which was designed and developed within the frame of this thesis, gave valuable extra information for the T2 distribution values, describing different molecular environments from a single set of TE values. 3. Application to human subjects. Multi-exponential T2 analysis and T2 distribution parametric maps were used for the quantification of myelin bound and free water fractions in Multiple Sclerosis (MS) patients. Additionally, applications of fast 2D/3D T2 and T2* parametric mapping included also liver hemosiderosis for the estimation of the iron burden in multi-transfused thalassemic patients. Furthermore, innovative methods for the calculation of Apparent Diffusion Coefficient (ADC) were implemented using multiple b-value EPI and HASTE MRI sequences. Micro-perfusion (D*) and true diffusion (D) parametric maps were generated utilizing double exponential decay analysis algorithms applied on multi-b (10 b) diffusion MRI signals. Applications of 2D ADC (D*) and ADC (D) parametric mapping were focused on Multiple Sclerosis. ADC (D*) maps were correlated with DCE-MRI parametric maps and DSC-MRI parametric maps. In the final part, these algorithms were implemented in a 4. PACS – Workstation for the daily clinical practice.
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