We present a comprehensive theory of nuclear spin polarization of 3 He and 129 Xe gases by spin-exchange collisions with optically pumped alkali-metal vapors. The most important physical processes considered are ͑1͒ spin-conserving spin-exchange collisions between like or unlike alkali-metal atoms; ͑2͒ spin-destroying collisions of the alkali-metal atoms with each other and with buffer-gas atoms; ͑3͒ electron-nuclear spin-exchange collisions between alkali-metal atoms and 3 He or 129 Xe atoms; ͑4͒ spin interactions in van der Waals molecules consisting of a Xe atom bound to an alkali-metal atom; ͑5͒ optical pumping by laser photons; ͑6͒ spatial diffusion. The static magnetic field is assumed to be small enough that the nuclear spin of the alkali-metal atom is well coupled to the electron spin and the total spin is very nearly a good quantum number. Conditions appropriate for the production of large quantities of spin-polarized 3 He or 129 Xe gas are assumed, namely, atmospheres of gas pressure and nearly complete quenching of the optically excited alkali-metal atoms by collisions with N 2 or H 2 gas. Some of the more important results of this work are as follows: ͑1͒ Most of the pumping and relaxation processes are sudden with respect to the nuclear polarization. Consequently, the steady-state population distribution of alkali-metal atoms is well described by a spin temperature, whether the rate of spin-exchange collisions between alkali-metal atoms is large or small compared to the optical pumping rate or the collisional spin-relaxation rates. ͑2͒ The population distributions that characterize the response to sudden changes in the intensity of the pumping light are not described by a spin temperature, except in the limit of very rapid spin exchange. ͑3͒ Expressions given for the radio-frequency ͑rf͒ resonance linewidths and areas can be used to make reliable estimates of the local spin polarization of the alkali-metal atoms. ͑4͒ Diffusion effects for these high-pressure conditions are mainly limited to thin layers at the cell surface and at internal resonant surfaces generated by radio-frequency magnetic fields when the static magnetic field has substantial spatial inhomogeneities. The highly localized effects of diffusion at these surfaces are described with closedform analytic functions instead of the spatial eigenmode expansions that are appropriate for lower-pressure cells. ͓S1050-2947͑98͒07408-3͔
Using a new method of xenon laser-polarization that permits the generation of liter quantities of hyperpolarized 129Xe gas, the first 129Xe imaging results from the human chest and the first 129Xe spectroscopy results from the human chest and head have been obtained. With polarization levels of approximately 2%, cross-sectional images of the lung gas-spaces with a voxel volume of 0.9 cm3 (signal-to-noise ratio (SNR), 28) were acquired and three dissolved-phase resonances in spectra from the chest were detected. In spectra from the head, one prominent dissolved-phase resonance, presumably from brain parenchyma, was detected. With anticipated improvements in the 129Xe polarization system, pulse sequences, RF coils, and breathing maneuvers, these results suggest the possibility for 129Xe gas-phase imaging of the lungs with a resolution approaching that of current conventional thoracic proton imaging. Moreover, the results suggest the feasibility of dissolved-phase imaging of both the chest and brain with a resolution similar to that obtained with the gas-phase images.
Two healthy volunteers who had inhaled approximately 0.75 L of laser-polarized helium-3 gas underwent magnetic resonance imaging at 1.5 T with fast gradient-echo pulse sequences and small flip angles ( < 10 degrees). Thick-section (20 mm) coronal images, time-course data (30 images collected every 1.8 seconds), and thin-section (6 mm) images were acquired. Subjects were able to breathe the gas (12% polarization) without difficulty. Thick-section images were of good quality and had a signal-to-noise ratio (S/N) of 32:1 near the surface coil and 16:1 farther away. The time images showed regional differences, which indicated potential value for quantitation. High-resolution images showed greater detail and a S/N of approximately 6:1.
We report on extensive experimental measurements of the key rates that determine the efficiency for polarizing the nuclei of 3 He by spin exchange with optically pumped Rb vapor. In agreement with recent theoretical predictions, we find a strong temperature dependence of the electron-spin loss rates due to 3 HeRb collisions. We also find that the maximum possible efficiency for spin-exchange polarization of 3 He by K is 10 times greater than for Rb. [S0031-9007(98)05659-2]
Resonances in the magnetic decoupling curves for the spin relaxation of dense alkali-metal vapors prove that much of the relaxation is due to the spin-axis interaction in triplet dimers. Initial estimates of the spin-axis coupling coefficients for the dimers (likely accurate to a factor of 2) are |lambda|/h = 290 MHz for Rb; 2500 MHz for Cs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.