Hyperpolarized gas polarimetry and imaging at low magnetic field

MR imaging with polarized noble gases has shown promise in both, biomedical and material's imaging applications. Its advantage over the conventional proton MRI lies in its ability to produce high signal-to-noise ratio (SNR), high-resolution images at low magnetic field strengths. In this work: (1) We implemented and studied in detail two methods for detecting hyperpolarization levels of 129Xe and 3He: NMR (Nuclear Magnetic Resonance) and EPR (Electron Paramagnetic Resonance). The 3He NMR and EPR data allowed for a comparison of these two polarimetry methods, while 129Xe NMR and EPR data showed promise for the calibration of 129Xe EPR shifts. (2) We investigated the possibility of using a pulsed resistive low-field MR scanner for spin echo imaging of hyperpolarized gases. By collecting CPMG spin echo trains containing 4096 echoes and lasting over 30 seconds, we demonstrated a high degree of stability for the pulsed resistive low-field scanner. (3) We developed a single-shot PGSE sequence for measuring diffusion coefficients of hyperpolarized gases which removed the effects of background gradients, thus allowing a separation of the $TCPMG2$ relaxation from diffusion-induced signal loss. The theoretical estimations of 3He and 129Xe diffusion coefficients which were based on the Lennard-Jones potential agreed well with our measured 3He and 129Xe diffusion coefficients within the experimental errors. (4) We determined the inherent T₂ relaxation times of 3He and 129Xe by varying the interecho time in the conventional CPMG spin echo sequence and by modelling the functional dependence of the $TCPMG2$ relaxation time on the interecho spacing. (5) We collected first ever 3He gradient echo images on a pulsed resistive low-field scanner. In addition, we modelled numerically the effects of flip angle, diffusion and relaxation rates on signal decay during gradient echo imaging with hyperpolarized gases. (6) We show, with simulations and experiments, that central ordering of RARE k-space acquisition significantly reduces diffusion-induced signal loss. The 1-D RARE images of 3He show a factor of a 100 improvement in the SNR (for 1.6 mm resolution) when using centrally ordered phase-encode gradients.