Phase-encoded MRI for geometrically undistorted imaging and signal characterization

Abstract

Magnetic resonance imaging (MRI) is a versatile diagnostic modality that has earned its place in clinical practice all over the world. MRI delivers excellent soft-tissue contrast that can be utilized to detect disease and measure physiological properties in a non-invasive manner. As long as the main magnetic field of the MRI scanner is homogeneous this methodology leads to accurate localization of the MR signal, resulting in MR images with high diagnostic quality. In cases of inhomogeneous magnetic fields, the spatial encoding process is affected, resulting in severe image artifacts and altered MR signal behavior. Potential sources of these so-called off-resonance effects are local changes in the main magnetic field, differences in magnetic susceptibility of tissues, chemical shift differences and the presence of paramagnetic materials. In this thesis, the aim was to implement MRI techniques that can be exploited for geometrically undistorted imaging and signal characterization in the presence of off-resonance effects. In this thesis the so-called fully phase-encoded MRI techniques were suggested as a potential solution, because these sequences are not affected by geometrical distortions and have excellent signal characterization properties. In the first part of this thesis the geometrical accuracy of phase-encoded MRI techniques were explored for application in post-operative imaging near orthopedic implants. These implants cause significant degradation of the image quality in conventional MRI, which hampers accurate detection of implant loosening, (pseudo-) tumors, inflammation or metallic wear. In this thesis it was shown that it is possible to acquire signal in proximity to these orthopedic implants within a reasonable timeframe by combining image reconstruction techniques (i.e. compressed sensing, reduced field-of-view imaging) with turbo phase-encoded MRI techniques. In the second part of this thesis, the excellent signal characterization properties of phase-encoded MRI were investigated to perform accurate mapping of MR signal characteristics (e.g. T2*, chemical shift) in proton, sodium and fluorine MRI. For proton MRI it was demonstrated that these properties can be exploited to assess the system performance, quantify high concentrations of paramagnetic particles, and detect current induced disturbances of the MR signal, such as caused by transcranial magnetic stimulation for example. In sodium MRI, the concentrations and signal behavior of sodium are often studied to detect diffuse disease or to enable early structural changes in disease, such as cartilage degradation in osteoarthritis. To accurately map the sodium signal behavior and determine the concentrations in a time-efficient manner, an ultra-short echo-time phase-encoded MRI sequences was used. In the fluorine study, a new setup was presented to detect the presence and metabolic conversion of chemotherapy in the liver and surrounding organs. Such measurements hold potential for improved efficacy assessments, which can prevent toxicity effects in non-responding patients and reduce treatment costs. In conclusion, based on the presented methodology and results, we believe that phase-encoded MRI is a viable strategy for geometrically undistorted imaging near metal implants and signal characterization of nuclei (23Na, 19F) with a low MR sensitivity.