Polarization Process

³He and ¹²⁹Xe polarization processes are both based on the spin exchange optical pumping technique (figure on the right). Since the body collisions between rubidium atoms and noble gases during the spin exchange are different (see figure below), the time necessary to hyperpolarize the noble gas (³He or ¹²⁹Xe), as well as the amount of gas produced and the process used to collect it, is different.

For the ¹²⁹Xe, large liter quantities of hyperpolarized gas can be formed. In a flowing gas stream, ¹²⁹Xe is polarized via spin exchange with optically pumped rubidium (Rb) vapor. Approximately 100 watts of circularly polarized light centered at 795 nm from a diode laser are used to excite the rubidium absorption resonance.

The flow rate is set such that a given ¹²⁹Xe atom spend 1-2 spin exchange time constants in contact with the the polarized rubidium vapor. Upon exiting the pumping chamber, the hyperpolarized xenon gas is trapped as a solid in a cold finger cooled to liquid nitrogen temperature. Under suitable circumstances, the polarization is maintained during freezing and the T1 relaxation time of solid ¹²⁹Xe at 77 K is approximately three hours in a small magnetic field.

For the ³He, the quantity of hyperpolarized gas is dependent on the pump chamber volume (first figure) and the initial fill pressure (usually 8.0 - 8.5 atm). Spin exchange takes place when the polarized rubidium atoms undergo binary collisions with the ³He atoms (first figure). During the collisions, the rubidium valence electron penetrates to the core of the ³He atom, and through a hyperfine interaction, transfers angular momentum to the spin -½ of the ³He nucleus. In the ³He, hyperpolarization saturation occurs after six to twelve hours (pump chamber dependent).

After the gas is sufficiently polarized, it is placed into a special plastic bag (Tedlar bags - Jensen Inert Products, FL). Because the strength of the signal remains sufficient for only a short period, the gas must be administered to the patient immediately after it is dispensed. As shown to the right, the patient is placed on the MRI table (in either a supine or prone position), between the two loops of the ³He coil (one of the loops is under the patient and is not shown).