Production of highly polarized $^{3}$He for magnetic resonance imaging

Spin-exchange optical pumping (SEOP) is the main method used to laser-polarize liter quantities of $^{3}$He gas for MRI applications. A major drawback is that 12-24 hours are normally required to achieve about 40{\%} polarization. We describe a high-throughput SEOP apparatus with rapid $^{3}$He polarization times (2-3 hours) and high polarizations ($>$50{\%}) by using a 60 W spectrally narrowed diode-array laser and hybrid optical spin-exchange (Rb-K). Individually, a narrowed diode laser has been shown to improve polarization levels by about 20{\%} [1], and hybrid SEOP increases spin-exchange efficiency by about 10 times [2]. Here, simultaneous use of both advances is described. We also examine the use of $^{3}$He dilution with a heavy buffer gas to slow $^{3}$He diffusion and thereby improve spatial resolution in $^{3}$He MRI experiments. [1] B. Chann, E. Babcock, L.W. Anderson, T.G. Walker, W.C. Chen, T.B. Smith, A.K. Thompson, T.R. Gentile, J. Appl. Phys. \textbf{94}, 6908 (2003). [2] E. Babcock, I. Nelson, S. Kadlecek, B. Driehuys, L.W. Anderson, F.W. Hersman, T.G. Walker, Phys. Rev. Lett. \textbf{91}, 123003 (2003).