1. Introduction

Magnetic resonance imaging (MRI) is a powerful tool for diagnostic imaging of soft tissue, combining superior contrast, dynamic capability, and no risk of damage caused by ionizing radiation compared to computerized tomography (CT) [1]. Most MRI techniques conveniently detect the nuclear spin of protons in water, which makes up a significant portion of the human body. This detection modality poses a problem within the lungs, however, as the low proton density translates to greatly reduced signal-to-noise ratios for conventional MRI techniques. Even at high magnetic field strengths, the equilibrium nuclear spin polarization is very low (~10−<sup>4</sup> to 10−<sup>6</sup> ). To facilitate MRI inside the lungs, far greater signal-to-noise ratios are needed, requiring nuclear spin polarizations to be increased far above equilibrium levels—a process known as hyperpolarization. Indeed, inhalation of hyperpolarized gases provides greatly improved sensitivity and bright images, outweighing the low spin densities of the gas phase. Gas hyperpolarization can now be readily achieved using established techniques; however in these approaches, maximizing the available spin polarization is hampered by complex system dynamics and codependence of many variables that are heavily temperature-dependent.

In most studies relevant to lung imaging, this hyperpolarized state is achieved by utilizing circularly polarized photons to shift particles with a nonzero magnetic moment between discrete spin states. During gas-phase collisions, spin angular momentum is transferred from alkali metal electrons to noble gas nuclei, a technique known as spin-exchange optical pumping (SEOP) [2]. Noble gases are used in SEOP because of their lack of reactivity with alkali metal vapors and their ultra-long hyperpolarization lifetimes. Nitrogen is present as a buffer gas to prevent radiation trapping, helping to maximize the alkali metal electron spin polarization—and hence, the noble gas nuclear spin polarization. Thus, inhalation of a hyperpolarized noble gas sample such as 129Xe while inside the MRI scanner enables both structural and functional high-resolution imaging of the lungs in real time. Despite significant advances in polarizer technology in recent years, many underlying aspects of the SEOP process, such as energy- and mass transport mechanisms within the OP cell and the temperature dependences of many parameters, remain poorly understood. Ironically, the desire to maximize the available nuclear spin magnetization leads to a demand for higher pump laser powers and richer noble gas mixtures, which only serve to compound this issue.

In an attempt to overcome these challenges, Raman spectroscopy may be used to complement more commonly used techniques such as optical absorption spectroscopy and low-field NMR in order to optimize the multidimensional SEOP parameter space, thereby improving the 129Xe hyperpolarization process for MR applications. Conventional temperature measurement (e.g., using a thermocouple) would be challenging to implement, given that the SEOP process takes place inside a sealed vessel, within a magnetic field, and under constant uniform illumination by high-power laser light; moreover, temperature measurement of the exterior cell walls may often provide only a poor reflection of the true gas behavior within the cell. However, if optical access is available, Raman spectroscopy can be used to acquire in situ rotational and vibrational temperature measurements of the nitrogen buffer gas—and hence, an accurate picture of the true gas temperature at a given location within the cell. This capability should provide a more complete insight into the energy deposition, transport, and dissipation processes that occur during the SEOP process, which in turn may further enable optimization of the efficiency of noble gas hyperpolarization—thereby improving the diagnostic capability of HP noble gases in clinical settings.
