Methodological developments of low field MRI: Elasto-graphy, MRI-ultrasound interaction and dynamic nuclear polarization

This thesis deals with two aspects of low field (0.2 T) Magnetic Resonance Imaging (MRI): the research of new contrasts due to the interaction between Nuclear Magnetic Resonance (NMR) and acoustics (elasto-graphy, spin-phonon interaction) and enhancement of the signal-to-noise ratio by Dynamic Nuclear Polarization (DNP). Magnetic Resonance Elasto-graphy (MRE) allows to assess some viscoelastic properties of tissues by visualization of the propagation of low frequency acoustic strain waves. A review on MRE is given, as well as a study on local measurement of the acoustic absorption coefficient. The next part is dedicated to MRI-ultrasound interaction. First, the ultrasonic transducer was calibrated for power and acoustic field using the comparison of two methods: the radiation force method (balance method) and laser interferometry. Then, we tried to modify the T1 contrast of tissues by spin-phonon interaction due to the application of ultrasound at the resonance frequency at 0.2 T, which is about 8.25 MHz. No modification of T1 contrast has been obtained, but the acoustic streaming phenomenon has been observed in liquids. MRI visualization of this streaming could make possible to calibrate transducers as well as to assess some mechanical properties of viscous fluids. The goal of the last part was to set up DNP experiments at 0.2 T in order to enhance the NMR signal. This double resonance method is based on the polarization transfer of unpaired electrons of free radicals to the surrounding protons of water. This transfer occurs by cross relaxation during the saturation of an electronic transition using Electronic Paramagnetic Resonance (EPR). Two EPR cavities operating at 5.43 GHz have been tested on oxo-TEMPO free radicals (nitroxide). An enhancement of the NMR signal by a factor 30 was obtained during these preliminary experiments. (author)