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As currently implemented, magnetic resonance imaging (MRI) relies on the protons of water molecules in tissue to provide the NMR signal. Protons are, however, notoriously difficult to image in some biological environments of interest, notably the lungs and lipid bilayer membranes such as those in the brain. Here we show that 129Xe gas can be used for high-resolution MRI when the nuclear-spin polarization of the atoms is increased by laser optical pumping and spin exchange. This process produces hyperpolarized 129Xe, in which the magnetization is enhanced by a factor of about 10(5). By introducing hyperpolarized 129Xe into mouse lungs we have obtained images of the lung gas space with a speed and a resolution better than those available from proton MRI or emission tomography. As xenon (a safe general anaesthetic) is rapidly and safely transferred from the lungs to blood and thence to other tissues, where it is concentrated in lipid and protein components, images of the circulatory system, the brain and other vital organs can also be obtained. Because the magnetic behaviour of 129Xe is very sensitive to its environment, and is different from that of 1H2O, MRI using hyperpolarized 129Xe should involve distinct and sensitive mechanisms for tissue contrast.

A method is described for producing several liters of nuclear spin polarized 129Xe gas via spin exchange with an optically pumped Rb vapor. We use a 140 W AlGaAs laser diode array whose broad spectral output is efficiently absorbed by employing ∼10 atm of 4He to pressure broaden the Rb D1 absorption profile. 129Xe is polarized in a continuous gas flow and is then cryogenically accumulated and stored. Extensions of this technique should enable the production of tens of liters of 129Xe with a nuclear spin polarization of order 50%. Production of laser-polarized 129Xe in liter quantities is important for the continued development of magnetic resonance imaging using spin-polarized 129Xe.

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