In this Figure, a hyperpolarizer unit is shown which includes several major subsystems, including an MRI gas delivery subsystem through which the polarized gas can be delivered as needed for imaging studies. In FIG. Because of the large spread in wavelength, this type of laser is referred to herein as a "broad-band" laser. In an alternative embodiment, two lasers pumping from opposite sides of the pumping cell 4 can be used, with appropriate redesign of the cell 4 and the optical diagnostics system An aspheric Fresnel lens 2 typically plastic directs most of the light from the diode laser array 1 into the optical pumping cell 4.
An image of the diode face is formed just beyond the end of the optical pumping cell 4. Although the Fresnel lens is inexpensive and well adapted to currently available diode laser arrays, different optics may be more appropriate for future lasers which may have higher intrinsic brightness than those available today. Quarter wave plate 3 converts linearly polarized light from the diode laser array to circularly polarized light. As shown, a plastic quarter wave plate is positioned just past the Fresnel lens 2 where the laser beam has expanded so much that heating of the lens and the wave plate is not a problem.
The light from the laser 1, which is already linearly polarized to a high degree, can be passed through a linear polarizer not shown before it reaches the quarter wave plate 3 if the natural linear polarization is not sufficient. Optical pumping cell 4 is shown, provided with saturated alkali-metal vapor, e.
The cell 4 has the shape of a truncated cone to accommodate the converging light from the lens 2. Refluxed alkali metal from the exit pipe 6 drops back through the cell and collects in the vaporizer 5. The cell and associated piping must withstand the high pressure of premixed gas, typically from above about 1 atmosphere to about 30 atmospheres.
High gas pressure inside the cell is important to permit efficient absorption of the broad-band light from a diode laser.
Hyperpolarized Xenon-129 Magnetic Resonance : Concepts, Production, Techniques and Applications
A vaporizer 5 is provided upstream from the pumping cell 4, for loading the flowing gas mixture with alkali-metal vapor prior to the gas's entry into the cell. The vaporizer 5 can be made of crumpled wires of copper or other non-magnetic metal or sintered metal that is readily wetted by liquid alkali metals e. The vaporizer 5 is soaked with liquid alkali metal, and stuffed into a receptacle of appropriate materials and dimensions to ensure full loading of the gas with vapor. The flow velocity of the gas, the distance through which it flows, and the pore diameter of the "sponge" are adjusted to ensure that the gas is fully saturated with alkali-metal vapor before it enters the optical pumping cell.
The vaporizer 5 is replenished by gravity flow of condensed alkali metal from a refluxing outlet pipe 6, which leaves the cell in a substantially vertical orientation. The refluxing outlet pipe 6 is included to condense alkali metal from the exiting gas. The dimensions and flow velocity are adjusted to ensure that most of the alkali metal condenses and drips back into the optical pumping cell by gravity flow, eventually returning to the vaporizer.
A fluorescence monitoring detector 7, e. The fluorescence monitoring arrangement can be adjusted for use with two lasers pumping from either side of the cell. Insulating window 8 is provided to permit pumping light to enter the oven and the optical pumping cell. This window and other light-transmitting surfaces may be provided with an anti reflection coating. Similar windows are provided for the fluorescence monitor 7 and the optical multichannel analyzer OMA Oven 9 is provided to keep the optical pumping cell at a temperature appropriate for absorbing most of the useful light from the diode laser.
Somewhat lower temperatures are appropriate for cesium which is more easily volatilized. The oven can be heated by flowing hot air or by internal, non-magnetic electrical heaters.
Optical multichannel analyzer OMA 10 for measuring the efficiency of absorption of light from the broad-band diode laser array. A different arrangement of the OMA is required if the cell is pumped from both sides. A high-pressure tank 11 is included to maintain a premixed target gas at a pressure of several hundred atmospheres.
Preferred target gas constituents, by partial pressure, are:. H 2 may be used at somewhat higher partial pressures e. The He gas pressure is adjusted to ensure that it causes negligible spin depolarization compared to the xenon. Other gas mixtures may be employed impart quenching and pressure-broadening qualities to the target gas.
A pressure regulator 12 is employed to reduce the very high pressure of the premixed gas in the storage tank 11 to a pressure appropriate for the optical pumping cell 4. This is typically from about 10 to about 30 atmospheres, depending on how much pressure broadening is needed for optimum use of the broad-band laser light. Gas purifier getter 13 is used to remove trace impurities, mainly water vapor, from the premixed target gas stream.
As shown, the accumulation reservoir 17 includes a counterflow cold trap--cooled by liquid nitrogen or some other cryogen in a Dewar vessel. Closed-cycle refrigerators can also be used for cooling. Detachment point 15, together with the detachment point 20, permits the removal of the accumulation reservoir Valve 14 isolates the optical pumping cell 4 from the detachment point 15, and controls flow therebetween.
Valve 16 is used to isolate the accumulation reservoir 17 from detachment point A permanent magnet 18 is provided to produce a static field of greater than about Gauss 0. A field this large is adequate to obtain the longest possible spin-lattice relaxation times e. For lower condensation temperatures, where much longer spin-lattice relaxation times are attainable, larger magnetic fields are needed.
The magnet may also be contained inside the cryogenic assembly and kept cool along with xenon accumulation reservoir. Valve 19 is employed to isolate the xenon condenser 17 from the detachment point 20, which together with detachment point 15 permits removal of the xenon condenser Valve 21 is used to release sublimed hyperpolarized Xe gas to transfer bag 22 or to any other container for transport of hyperpolarized Xe gas at atmospheric pressure for various uses, e.
Hard-walled containers can be used to transport the hyperpolarized Xe gas at other pressures.
Hyperpolarized Xenon-129 Magnetic Resonance
Valve 23 isolates the xenon accumulator 17 during sublimation of the condensed xenon and gas transfer to the bag or other receptacle Glass-to-metal seal 24 is provided, with the piping on the pump side of the seal preferably being stainless steel or other metal. On the xenon-condenser side of the seal, the piping is glass. Similar glass-to-metal seals on the input side of the gas flow and appropriate stress-relieving bellows are not shown, but are normally to be preferred.
Pump 27, isolated by valve 26, is used for evacuating any remaining He and N 2 from the xenon condenser 17 at the end of the accumulation period. A needle valve 28 or other flow control device is included to permit waste He and N 2 gas to vent to the room or to a recovery container for reuse.
This valve 28 controls the flow rate through the optical pumping cell 4. The venting rate is adjusted to optimize the preparation of hyperpolarized Xe according to principles we have developed.
Publications | Hyperpolarized Gas MRI Lab
Flow of the gas is monitored by flow meter A vent 30 is provided, leading to the atmosphere or to a collection receptacle for spent He and quenching gas e. Port 31 is included for purging the gas lines with clean gas e. Vent 33 permits release of the purging gas introduced at the port Attachment point 32 is supplied for connecting the premixed gas supply to the optical pumping cell. Valve 34 isolates the optical pumping cell during purging of the gas-supply piping. A nuclear magnetic resonance pickup coil 35 is also included to monitor Xe polarization in the pumping chamber, which is useful for optimizing the gas flow rate.
Temperature sensor 36, e. A static magnetic field 37 is also illustrated. The source is not shown, but we have successfully used either Helmholtz coils or the fringing fields of a magnetic resonance imaging magnet or a combination of the two. A control subsystem not shown is generally desirable as a unified computer-software and hardwired subsystem which is used to control and monitor the different processes occurring in the various subsystems.
While described with particularity hereinbelow, the manufacture and operation of high capacity hyperpolarization systems, such as are suitable for use in conjunction with the accumulator apparatus of the invention, are described in more detail in co-pending U. One implementation of a polarization chamber is shown in FIG. As the chamber in which the optical pumping and spin exchange takes place it must satisfy a number of requirements. For example, the pumping chamber must hold an appropriate amount of polarizable gas in a substantially leak-tight environment.
The gas pressure in the chamber is maintained according to the requirements of the apparatus, preferably being maintained at a pressure above atmospheric pressure also designated herein "hyperbaric" up to about 30 atmospheres atm , and more preferably from about 8 atm to about 12 atm for a glass cell. The gas pressure may be outside above or below this range, as required.
A presently preferred pressure is about 10 atm, which reflects the structural limitations of glass, the material most typically used in the manufacture of polarization chambers. Higher pressure or gas density could be used in other polarization chamber structures.
The pumping chamber should admit hyperpolarizing radiation from the laser source s. Preferred structures of the chamber include conical or truncated conical frustoconical structures, although in certain configurations a cylindrical cell is suitable. Preferably, the chamber is designed in conjunction with the laser system to maximize light delivery into the chamber and throughout its interior, to maximize the efficiency of the hyperpolarizing procedure.
A preferred type of pumping chamber not shown in FIG. For example, the wavelength of the D 1 transition in rubidium is Optimization of pumping efficiency would require that the light ports be as transparent as possible to light of the requisite wavelength, i. They may be antireflection-coated to maximize light transmission. When two ports are employed in conjunction with opposed lasers, the interior surfaces of the ports may be made reflective to retain the light in the interior of the chamber.
The volume-averaged relaxation time of the nuclear polarization of a gas in the pumping chamber must be sufficiently slow compared to the spin-exchange rate between the alkali metal atom and the noble gas nucleus to allow the desired level of polarization in the cell to be attained.
The materials and design of the polarization chamber must, therefore, be selected with care. For example, the pumping chamber should be chemically compatible with alkali-metals, preferably being compatible with alkali metals at the elevated temperatures appropriate for optical pumping e. In addition, if an NMR polarimetry system is used to monitor the hyperpolarization procedure, it is preferred that the pumping chamber walls not interfere substantially with the rf field required for polarimetry. The particular implementation of the pumping chamber will depend on the type of gas being polarized.
As noted above, polarization chambers suitable for use in accordance with the invention are typically made of glass. The glass should be resistant to the alkali metal s employed in the spin exchange process. Such glasses are exemplified by aluminosilicate glasses such as Corning , or metal-sealing borosilicate glasses such as Corning or Schott For lower temperature applications, standard borosilicate laboratory glassware, e. Other pumping chamber designs, capable of higher pressure operation, include metal structures fitted with glass light ports.
Moreover, in another approach, a cell can be surrounded with hot high pressure gas or a transparent liquid e.
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As shown in FIG. Any suitable gas ports permitting flow control can be employed. Alternatively, the chamber can have a single gas port through which gas is flowed into and out of the cell periodically.