“Magnetic resonance applications of hyperpolarized xenon at low field strengths”

Low field strength magnetic resonance (MR) systems are appealing because of their lower cost and higher clinical accessibility, but the lower signal-to-noise ratio (SNR) inherent to this regime is a major weakness. In recent years, advances in spin hyperpolarization, pulse sequence design, and instrument sensitivity have at least partially alleviated the latter concern and have reignited interest in low field MR. ¹²⁹Xe is one such hyperpolarized (HP) spin with great utility as a biological MR sensor due to its inert nature, high solubility in tissue, and wide chemical shift range. The feasibility of extending many of the clinical field strength applications of HP ¹²⁹Xe to the low field regime was previously unexplored.

Here, I describe the design and construction of an MR imaging scanner operating at milliTesla (mT) field strengths. With it, I perform the first known measurement of dissolved-phase ¹²⁹Xe longitudinal relaxation times (T₁) in several biologically relevant solvents at low field strengths. Along the way, I develop an improved method of performing these measurements and attempt to reconcile discrepancies between values found in the literature. I then show that HP ¹²⁹Xe is rapidly depolarized in whole blood using a combination of in vivo and in vitro measurements. Next, I investigate the use of superparamagnetic iron oxide nanoparticles (SPIONs) as ¹²⁹Xe T₁ contrast agents in the low field regime. Finally, I present some preliminary results from two promising avenues for future low field HP ¹²⁹Xe studies, including lung imaging and the transfer of polarization to nearby nuclei through the Spin Polarization-Induced Nuclear Overhauser Effect.

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