Rapid Production of Hyperpolarized $^3$He Gas for MRI

Hyperpolarized (HP) $^3$He gas created via spin-exchange optical pumping (SEOP) is widely used as a signal source in MRI applications. One drawback to conventional SEOP is the time required for polarization. The process normally requires 10 - 20 hours to achieve 40-50\% polarization in enough gas ($\sim$1 L) for a single imaging experiment. Two recent advances in the physics of SEOP have led to dramatic enhancements in polarization efficiency: the use of spectrally narrowed diode-laser arrays and hybrid SEOP, which employs both potassium and rubidium as alkali-metal intermediaries. We have combined these techniques in constructing two polarizers, a prototype system at Utah and a more fully engineered system at PNNL. We report $>$60\% $^3$He polarization in 0.5 bar{\Large{$\cdot$}}L of gas in valved and refillable glass cells, achieved in under 4 h. With the apparatus described we are able to produce several liters of polarized $^3$He per day.