³He spin exchange cells for magnetic resonance imaging

We present a protocol for the consistent fabrication of glass cells to provide hyperpolarized (HP) ³He for pulmonary magnetic resonance imaging. The method for producing HP ³He is spin-exchange optical pumping. The valved cells must hold of order 1 atmṡL of gas at up to 15 atm pressure. Because characteristic spin-exchange times are several hours, the longitudinal nuclear relaxation time T ₁ for ³He must be several tens of hours and robust with respect to repeated refilling and repolarization. Collisions with the cell wall are a significant and often dominant cause of relaxation. Consistent control of wall relaxation through cell fabrication procedures has historically proven difficult. With the help of the discovery of an important mechanism for wall relaxation that involves magnetic surface sites in the glass, and with the further confirmation of the importance of Rb metal to long wall-relaxation times, we have developed a successful protocol for fabrication of ³He spin exchange cells from inexpensive and easily worked borosilicate (Pyrex) glass. The cells are prepared under vacuum using a high-vacuum oil-free turbomolecular pumping station, and they are sealed off under vacuum after ≥100 mg of distilled Rb metal is driven in. Filling of cells with the requisite ³He-N ₂ mixture is done on an entirely separate gas-handling system. Our cells can be refilled and the gas repolarized indefinitely with no significant change in their wall properties. Relaxation data are presented for about 30 cells; the majority of these reach a "40/40" benchmark: T ₁>40 h, and ³He polarizations reach or exceed 40%. Typical polarization times range from 12 to 20 h; 20% polarization can be achieved in 3-5 h.