He can also be hyperpolarized using metastability exchange optical pumping (MEOP). This process is able to polarize He nuclei in the ground state with optically pumped He nuclei in the metastable state. MEOP only involves He nuclei at room temperature and at low pressure (≈a few mbars). The process of MEOP is very efficient (high polarization rate), however, compression of the gas up to atmospheric pressure is needed.