Direct detection of polarization transfer from hyperpolarized 129Xe to thermally polarized 1H at 2 mT

We present a simple protocol for polarizing 1H at ultra-low field. By simply bubbling hyperpolarized Xe gas in a solution containing thermally polarized 1H spins, the proton thermal polarization can be enhanced by more than 100-fold. This enhancement is clearly observed, even in absence of saturation pulses, at ultra-low field where thermal polarization is close to background noise levels. Simultaneous detection of 1H and 129Xe magnetizations shows that the transfer of polarization from 129Xe to 1H spins is a continuous process that lasts until the dissolved-phase 129Xe polarization is relaxed back to thermal equilibrium.