Thesis (M.S.)--Rochester Institute of Technology, 1991. / Spine title: MRI studies of porous materials. Includes bibliographical references (leaves 83-84).

Thesis (Ph. D. in Physics)--Vanderbilt University, Dec. 2007. / Title from title screen. Includes bibliographical references.

No description available.

Thesis (Ph.D.)--Rochester Institute of Technology, 2009. / Typescript. Includes bibliographical references (leaves 169-176).

Thesis (M.S.)--Michigan State University. Dept. of Physics, 2008. / Title from PDF t.p. (viewed on Aug. 7, 2009) Includes bibliographical references (p. 55-57). Also issued in print.

By sensitising the magnetic resonance signal to the diffusion of water molecules in tissue, diffusion-weighted magnetic resonance imaging provides a means of assessing tumour microstructure non-invasively. Such measurements have the potential to provide important information about tumour development and the response of tumours to treatment, but the way in which different tissue properties affect the diffusion-weighted signal remains unclear. Through simulations, in vivo studies and phantom experiments, this thesis investigates the relationship between the diffusion-weighted signal, the pulse sequence parameters used for acquisition, and microstructural properties of tumours. The use of oscillating gradient pulse sequences on a clinical scanner was investigated initially, with theoretical and practical considerations leading subsequent work to focus on pulsed gradient sequences. The forward problem of predicting the diffusion-weighted signal for given combinations of tissue properties and sequence parameters was addressed numerically through Monte Carlo simulations, focussing on how tumour cell size, intracellular volume fraction and membrane permeability affect the signal. These simulations allowed the sensitivity of the signal to changes in these tissue properties to be investigated, revealing how sensitivity depends on sequence parameters as well as the specific microstructural configuration. By repeating the simulations using the specific sequence parameters used in a clinical and preclinical study, the sensitivity of the implemented protocols was assessed, and linked to the experimental findings. The preclinical study illustrated the importance of the diffusion time in determining the sensitivity to treatment-induced changes in tumours, with larger post-treatment signal changes observed at longer diffusion times. These trends were qualitatively reflected in the sensitivity analysis derived from the simulations. Finally, the inverse problem of estimating microstructural properties from the diffusion-weighted signal was addressed using a physical phantom designed as a simple mimic of tumour tissue. By fitting a biophysical model to the diffusion data, the size and volume fraction of the approximately spherical 'cells' were estimated. The radius was slightly underestimated compared with that determined from independent measurements, the fitted volume fraction was plausible, and parameters were found to be estimated with reasonably good precision.

Thesis (Ph.D)--Biomedical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Hu, Xiaoping; Committee Member: Brummer, Marijn; Committee Member: Duong, Tim; Committee Member: Keilholz, Shella; Committee Member: Schumacher, Eric. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Multiple myeloma and other plasma cell disorders infiltrate the bone marrow in different patterns. While some patients show a homogeneous distribution of the clonal plasma cells others present with focal accumulations, commonly called focal lesions. Novel imaging techniques can provide information on these infiltration patterns and, due to their low invasiveness, can be performed repeatedly and therefore be used for monitoring. Conventional magnetic resonance imaging (MRI) has a high sensitivity for bone marrow assessment but cannot safely differentiate between active and inactive lesions. Therefore, positron emission tomography, especially combined with computed tomography (PET/CT), has been more widely used, at least for the monitoring of treatment response. Comparative, but mostly retrospective studies, have shown that functional MRI techniques, namely diffusion-weighted imaging (DWI), which assesses the movement of water molecules, can evaluate tissue cellularity with high sensitivity, which challenges the dominance of PET/CT in treatment response assessment. This review will discuss the benefits and challenges of DWI and compare it to other available imaging techniques used in patients with monoclonal plasma cell disorders

Zielsetzung: Die diffusionsgewichtete Bildgebung ist wesentlicher Bestandteil der Magnetresonanz-tomographie des Prostatakarzinoms (PCa). Aus entsprechenden Rohdaten, aufgenommen bei verschie-denen b-Werten (Diffusionswichtungsfaktoren), kann der Diffusionskoeffizient (apparent diffusion coefficient - ADC) abgeschätzt werden, der ein sensitiver Indikator für maligne Veränderungen der Gewebearchitektur ist. Die absoluten ADC-Werte sind allerdings stark von der Wahl der zugrundeliegenden b-Werte abhängig und darüber hinaus gibt es Hinweise, dass die Wahl der b-Werte einen signifikanten Einfluss auf die visuelle Analyse, insbesondere auf die Abgrenzbarkeit der Läsion von der Umgebung und auf den Kontrast hat. Es wurde daher untersucht, inwieweit die Wahl der b-Werte den diagnostischen Wert des ADC im Hinblick auf die Detektion und Beurteilung des PCa hat. Methodik: 41 konsekutive Patienten mit gesichertem PCa erhielten eine multiparametrische, endorektale MR-Bildgebung bei 3 Tesla. Die ADC-Karten wurden retrospektiv auf Basis vier verschiedener Kombinationen von b-Werten (0-800 s/mm2) berechnet. Drei Untersucher bestimmten die „führende“ Läsion und beurteilten dann den diagnostischen Wert der jeweiligen ADC-Karten (Visual Score - VS) mit sehr gut (2), befriedigend (1) oder schlecht (0). Für die quantitative Auswertung wurden der mittlere ADC für gesundes und für Tumorgewebe bestimmt. Unterschiede in Abhängigkeit von den gewählten b-Werten wurden mittels statistischer Tests (einseitige ANOVA, Faktor Methode, Signifikanzniveau 5 %) ausgewertet. Ergebnisse: 85 % der Tumoren wurden von den Auswertern richtig erkannt. Die Wahl der b-Werte hatte hochgradig signifikanten (P<0,001) Einfluss auf die absoluten ADC-Werte in gesundem und verändertem Gewebe. ADC-Karten auf Basis von b=[50, 800] und [0, 800] wurden am besten (VS=1,6±0,3) und zweitbesten (VS=1,1±0,3; P<0,001) bewertet. Insbesondere für niedrig-gradige Karzinome (Gleason Score ≤ 6, 13/41 Patienten), wurde nur die Kombination [50, 800] besser als befriedigend (VS=1,4±0,3) bewertet. Der mittlere Tumor-ADC zeigte eine moderate aber signifikant negative Korrelation (Spearman ρ: -0,38 bis -0,46; P<0,05) mit dem Gleason Score. Schlussfolgerung: Absolute ADC-Werte sind stark von der Wahl der zugrundeliegenden b-Werte abhängig und eignen sich daher nicht zur allgemeingültigen Charakterisierung von Prostatakarzinomen. Ein minimaler b-Wert > 0 s/mm2 wird für die Berechnung der ADC-Karten im Hinblick auf eine nachweislich verbesserte visuelle Auswertbarkeit empfohlen.

Diffusion-weighted magnetic resonance imaging can be used to estimate microstructural parameters of white matter in the brain. Two complementary techniques are investigated: the use of the temporal diffusion spectrum to explore small length scales and the STEAM technique to probe larger features. The diffusion spectrum has the potential to be more sensitive to small pores compared to conventional time-dependent diffusion. However, analytical expressions for the diffusion spectrum of particles only exist for simple geometries such as cylinders, which are often used as a model for intra-axonal diffusion. We propose a mathematical model for the extra-axonal space with parameters that are related to the microstructural properties of pore size, tortuosity, and surface-to-volume ratio. Measurements were made with an extra-axonal space phantom to validate the model. Fitted values for the phantom pore size match those from simulation. We extend the model to include the intra-axonal signal contribution. However, the parameters used to describe the intra- and extra-axonal spaces are related and it is important to remove redundant parameters to avoid overparameterization, which would make the model less robust. We propose analytical expressions to simplify the model. The model was then applied to measurements on fixed corpus callosum, which is a model system consisting of parallel axons. The estimated values of the axon volume fraction and mean and standard deviation of the axon radius distribution are comparable to those found in literature. Temporal diffusion spectra are useful for measuring the geometric properties of small spaces such as axon radii. However, longer diffusion times accessible using the STEAM sequence are necessary to probe structures with longer diffusion distances such as those parallel to the direction of axons. We used a model from the literature originally developed for use with animal magnetic resonance scanners and simplified it to quantify axial hindrance from data acquired on healthy volunteers in a clinical scanner. The interpretation of axial hindrance, which is a largely unexplored area of research, is discussed.