Magnetic resonance imaging of lungs at ultra-low magnetic field strength using hyperpolarized xenon-129 gas

Conventional proton Magnetic Resonance (MR) imaging has been limited to water rich organs and tissues, with limited success in lungs and airways. With the introduction of Hyperpolarized Noble Gases (HNG's), the low spin concentration in gas-spaces is compensated by the extremely high non-equilibrium magnetizations achievable. This magnetization increase allows the use of field strengths much lower than those currently used in clinical MR systems with advantages such as: increased patient accessibility, reduced cost and image artifacts and less siting restrictions.;Until now, the choice of field strength for HNG imaging has been mostly determined by the existing conventional MR systems or on hardware limitations. This work provides a theoretical framework for selection of the optimum field strength for clinical HNG MR imaging systems based on models of the field strength dependence of the achievable signal-to-noise ratio (SNR) and spatial resolution.;A methodology for MR imaging of the lungs at very low field strengths is developed by building a system for hyperpolarized 129Xe (HXe) rat lung imaging that uses the fringe field of a superconducting magnet as the source of the static magnetic field. A passive shimming procedure that improves the fringe field homogeneity to the levels required for imaging is theoretically described and used at two field strengths (8.5 mT and 17 mT).;Spectra and images of HXe gas in phantoms and excised rat lungs are presented and used to validate the obtained theoretical SNR field dependence for small samples. The issues related to clinical HNG imaging at low fields for both gas-space and dissolved-phase imaging are investigated and possible ways to address them in future are discussed.