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Hyperpolarization Apparatus

The basic concept of a 3He polarizer looks like what is shown in figure below

The system consists of an optical cell containing the rubidium (Rb) metal and gas to be polarized. The cell is heated in an oven to create a very dilute vapor of Rb, which is irradiated by the laser light so that its valence electrons become polarized. Subsequent collisions with 3He or 129Xe atoms transfers the polarization to them. Once the process is complete, the rubidium is removed from the vapor phase by simple cooling, and the polarized gases can be dispensed into the plastic dose bag and transported for MRI.

For the 3He system, polarization builds up over the course of a few hours. The optical cell is filled with 3He, the heat and laser are turned on and polarization proceeds overnight. The following day, the gas is ready for dispensing and use. A typical spin exchange time constant for 3He polarization is 4 hours and peak polarization of 40% is common.

As shown below, the process for polarizing 129Xe is quite similar except that the 129Xe is polarized as it flows continuously through the optical cell. As The 129Xe-Rb spin exchange interaction is much faster than for 3He-Rb. 129Xe is introduced in a dilute 1% mixture along with nitrogen and helium buffer gas into the optical cell. The 129Xe polarizes within a few seconds and then flows out of the optical cell. 129Xe is then cryogenically extracted from the other flowing gases using a cold finger in a liquid nitrogen bath.

The polarization achieved during 129Xe hyperpolarization is affected by both the optical pumping and spin exchange process, as well as the relaxation (loss) of 129Xe polarization in the cryogenic state. This creates optimization curves that depend on the volume of 129Xe being accumulated, the rate of production, and the losses in the solid phase. The theoretically predicted performance for a Polarean 9800 xenon polarizer is shown in the graph below.

A schematic diagram of an actual Polarean 9800 xenon polarizer is shown below showing the major system components. Please refer to technical specifications for more details.

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