PURPOSE: To assess the variability of incidental magnetization transfer effect (MT) by the number of slices and the magnetic field strength. METHODS: Various magnetic resonance images (MRI) were obtained with a phantom containing a series of solutions of gadolinium (Gd) and sucrose in distilled water, agarose gel and two vials with olive oil and distilled water. A diffusion weighted image (DWI) sequence was acquired to determine diffusion coefficient for each component of the phantom. Several inversion recovery (IR) sequences having different TI values were run for single-slice and used to calculate T1 relaxation time with maximum precision and minimizing magnetization transfer effect. The T1 relaxation value resulting from processing IR sequences was used as reference value. The mixed-TSE sequences were used to calculate T1, T2 and PD values and to assess MT effect for single-slice as for multi-slice acquisition. All the DICOM MR images were processed using various algorithms programmed in Mathcad (version 2001i, PTC Needham, MA) by Dr. Hernan Jara. According with the potential of each sequences the programs generated the qMRI maps and values of T1, T2, PD were obtained for all the components of the phantom. Values resulted from Mathcad calculation were used for analysis. All the acquisitions, calculations and measurements were performed for 1.5T and 3T field strength.

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No description available.

Skeletal muscle performs essential functions, including movement and posture. Musculoskeletal disorders can disrupt these functions, leading to temporary or permanent impairment. As most muscle abnormalities will cause morphological and physiological changes in skeletal muscle, identifying diseased or injured skeletal muscle relies on having a frame of reference, i.e. a correct characterization of what is considered healthy or 'normal' skeletal muscle. Non-invasive Magnetic Resonance Imaging (MRI) techniques such as 1H Magnetic Resonance Spectroscopy (1H-MRS) to assess the biochemical environment, Magnetization Transfer (MT) to study water dynamics and Blood Oxygen Level Dependent (BOLD) signal to study blood flow and relative (de)oxy-Hb concentration have yet to be extensively explored in skeletal muscle. Therefore, to improve the knowledge of the biochemical environment of skeletal muscle, a series of experiments were performed using these techniques in calf muscles. 1H-MRS investigations showed high repeatability of metabolite quantification within and across scanning sessions despite its challenges due to the high structural organization of skeletal muscle. Furthermore, differences in the metabolic profile between endurance vs. power-oriented participants at rest were found, suggesting 1H-MRS could be used as a non-invasive technique to assess muscle fiber composition. A multimodal MT, and BOLD study were performed on exercised skeletal muscle to complement the metabolic understanding of skeletal muscle. It was shown that high-quality data could be obtained in simultaneous studies of BOLD/EMG. In addition, during a multimodal MT and BOLD acquisition, MT signal showed a decrease after exercise and was linearly correlated to the BOLD signal activation. The ability of MT to distinguish between highly/lowly activated muscle groups during exercise opens the opportunity to non-invasively investigate muscle group recruitment with a higher spatial resolution compared to EMG, and lower scanning times compared to BOLD. Overall, the main purpose of this thesis was to investigate, characterize and provide unique metrics to study the functional and metabolic profile of healthy skeletal muscle at rest and during exercise. / Thesis / Master of Applied Science (MASc) / Skeletal muscle performs vital functions such as movement, heat generation, and posture. The impact of musculoskeletal disorders, which can disrupt these functions and cause temporary or permanent impairment of physical activity and movement, is expected to grow in the future. Correctly characterizing healthy or 'normal' skeletal muscle is necessary to identify diseased or injured skeletal muscle, as most muscle abnormalities cause changes in morphology and physiology. Non-invasive MRI techniques to assess the biochemical environment, water dynamics, blood flow and relative (de)oxy-Hb concentration have yet to be extensively explored in healthy skeletal muscle. Thus, the primary purpose of this thesis was to investigate, characterize and provide unique metrics to study the functional and metabolic profile of healthy skeletal muscle at rest and during exercise. The metrics investigated can be used to establish a baseline to detect abnormal skeletal muscle.

Imaging techniques provide a method for non-invasive longitudinal monitoring of cancer therapies, but common metrics such as tumour size are late markers and do not indicate heterogeneity of response. Apoptotic cell death is an earlier marker of tumour response and produces molecular and cellular-level changes (macromolecular breakdown, membrane changes and cell shrinkage) that may be detectable by magnetic resonance imaging (MRI). Previous studies using conventional MRI methods have shown little sensitivity to apoptosis. In this thesis it is hypothesized that, using an extracellular contrast agent to affect the MRI property of relaxation for extracellular water preferentially, parameters related to water in the intracellular and extracellular environments and the exchange between them can be obtained and will be sensitive to apoptosis. It is also hypothesized that membrane changes and macromolecular breakdown are detectable by the technique of magnetization transfer. Measurements of relaxation in the presence of contrast agent in vitro demonstrated a decrease in extracellular water fraction and an increase in the rate of water exchange across the plasma membrane during apoptosis. In vivo, this method was complicated by the difficulty of delivering contrast agent to the tumour, but regions with good delivery showed correlation between high water exchange rates from MRI and apoptosis in histology. Magnetization transfer studies indicated only small changes in vitro during apoptosis and these were largely related to changes in the free water, so this method was not investigated further. Further work is required to determine the tumour lines where the water exchange methods may be applied reliably. Nevertheless, the method of measuring water exchange presented in this thesis can be performed in a clinically-feasible amount of time (~20 minutes). It therefore has potential in detecting apoptosis and predicting therapy response. It also emphasizes the role of water exchange in conventional MRI relaxation experiments.

Imaging techniques provide a method for non-invasive longitudinal monitoring of cancer therapies, but common metrics such as tumour size are late markers and do not indicate heterogeneity of response. Apoptotic cell death is an earlier marker of tumour response and produces molecular and cellular-level changes (macromolecular breakdown, membrane changes and cell shrinkage) that may be detectable by magnetic resonance imaging (MRI). Previous studies using conventional MRI methods have shown little sensitivity to apoptosis. In this thesis it is hypothesized that, using an extracellular contrast agent to affect the MRI property of relaxation for extracellular water preferentially, parameters related to water in the intracellular and extracellular environments and the exchange between them can be obtained and will be sensitive to apoptosis. It is also hypothesized that membrane changes and macromolecular breakdown are detectable by the technique of magnetization transfer. Measurements of relaxation in the presence of contrast agent in vitro demonstrated a decrease in extracellular water fraction and an increase in the rate of water exchange across the plasma membrane during apoptosis. In vivo, this method was complicated by the difficulty of delivering contrast agent to the tumour, but regions with good delivery showed correlation between high water exchange rates from MRI and apoptosis in histology. Magnetization transfer studies indicated only small changes in vitro during apoptosis and these were largely related to changes in the free water, so this method was not investigated further. Further work is required to determine the tumour lines where the water exchange methods may be applied reliably. Nevertheless, the method of measuring water exchange presented in this thesis can be performed in a clinically-feasible amount of time (~20 minutes). It therefore has potential in detecting apoptosis and predicting therapy response. It also emphasizes the role of water exchange in conventional MRI relaxation experiments.

On utilise en IRM clinique T2, T1 et la densité de protons comme biomarqueurs de diagnostic et de suivi. Cependant, seuls les tissus à T2 long sont visibles par IRM classique. La séquence UTE (Ultra-short TE) a été récemment développée pour des études quantitatives de l’os cortical. Nous avons dans une première étape confronté des mesures de porosité de l’os cortical par IRM-UTE et par microtomographie par rayonnement synchrotron, car la porosité est un paramètre déterminant de la qualité osseuse. L’étude a été menée sur 38 échantillons de diaphyses fémorales humaines en collaboration avec une équipe du B2OA (UMR7052). La porosité par IRM-UTE à 4.7 T (TE = 51 µs) est entre 18 et 43% (moyenne 30%). La porosité par microtomographie (résolution spatiale : 6.5 µm) est entre 3 et 27% (moyenne 14%). Aucune corrélation n’a pu être observée entre les deux mesures. Une importante dispersion a été observée sur les valeurs de T1 entre les échantillons, que nous proposons d’attribuer à des effets de transfert d’aimantation (MT) entre les protons de l’eau liée au collagène et les protons des terminaisons méthylène du collagène. Pour confirmer cette interprétation, nous avons dans une seconde étape confronté plusieurs méthodes d’évaluation de la relaxation longitudinale dans des échantillons d’os bovin. Les mesures réalisées par différentes séquences (inversion-récupération, saturation hors-résonance, saturation par répétition de binomiales et angle de bascule variable) confirment des effets de MT importants. Les méthodes les plus robustes pour évaluer les paramètres sont la saturation hors-résonance et par répétition de binomiales, ce qui suggère leur utilisation pour de futures applications in vivo. / Longitudinal and transverse relaxations are quantitative tools used in MRI for diagnosis and follow up. However only tissues with long T2 can be detected with MRI. Quantitative evaluation of cortical bone porosity is now feasible with UTE.In this work, porosity measurements from UTE in human cortical bone samples were compared with those from micro-computed tomography (µCT). 38 human cortical bone samples (upper diaphysis) were examined in collaboration with a team from B2OA (UMR7052). Porosity from UTE (TE = 51 µs) was between 18% and 43% (mean 30%) and from µCT (spatial resolution = 6.5 µm) between 3% and 27% (mean 14%). No correlation could be established between the two measurements. T1 values from few samples were dispersed; a possible explanation could be the magnetization transfer (MT) between collagen-bound water protons and collagen methylene protons.For a quantitative interpretation of this phenomenon, 11 bovine cortical bone samples were examined. Several sequences (inversion-recovery, off-resonance saturation, repeated binomial excitations, variable flip angle) were implemented at 4.7 T to assess MT parameters. The aim was to compare which method may provide accurate parameter estimation. Off-resonance saturation and repeated binomial excitation seem to be more suitable for in vivo MT quantification.

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L’imagerie par résonance magnétique (IRM) est une technique d’imagerie médicale largement utilisée pour son caractère non-invasif et pour sa capacité à explorer les tissus mous. Des techniques IRM avancées et innovantes ont été développées de manière à améliorer la spécificité du signal des techniques conventionnelles et ainsi accéder à de nouvelles informations. Un axe de recherche particulièrement important en IRM concerne la possibilité d’accéder in vivo à des informations sur la myéline. Cette dernière est un constituant majeur du système nerveux central qui assure la bonne conduction nerveuse. Sa dégradation est l’une des caractéristiques de la sclérose en plaques, qui est la première cause de handicap sévère non traumatique chez le jeune adulte. Imager la myéline par IRM demeure néanmoins un challenge du fait du temps de relaxation T2 très court des protons la constituant. Le transfert d’aimantation inhomogène (ihMT) est une technique récemment découverte qui permet d’isoler le signal de composantes macromoléculaires grâce à leurs propriétés de relaxation dipolaire, caractérisées par la constante T1D. L’objectif de ce travail de thèse a concerné la validation de la technique ihMT comme biomarqueur de la myéline et l’évaluation de la spécificité du signal pour la myéline, sur des modèles murins (souris). / Magnetic resonance imaging (MRI) is a non-invasive medical imaging technique, widely used to explore soft tissues. Advanced and innovated MRI techniques have been developed to improve the specificity of conventional MR sequences thus allowing accessing new information. A particularly important research topic concerns the ability to in vivo access myelin information. Myelin is a major component of the central nervous system responsible for a good nerve conduction. Myelin alteration occurs in multiple sclerosis, one of the main cause for young adult permanent disability. However, myelin MRI is challenged by the very short relaxation time, T2, of myelin protons. Inhomogeneous magnetization transfer (ihMT) is a recent technique, which allows assessing macromolecular tissue component by exploiting their dipolar order relaxation properties, characterized by the time constant T1D. The objective of this thesis concerned the validation of ihMT as a myelin biomarker and the evaluation of the specificity of ihMT for myelin on mouse models.