Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-11-14
2001-10-23
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S003100, C062S919000
Reexamination Certificate
active
06305190
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the collection and accumulation of polarized noble gases, and relates more particularly to the production of hyperpolarized gases for use in medical diagnostic procedures such as magnetic resonance imaging (“MRI”) and spectroscopy applications.
BACKGROUND OF THE INVENTION
Conventionally, MRI has been used to produce images by exciting the nuclei of hydrogen molecules (present in water protons) in the human body. However, it has recently been discovered that polarized noble gases can produce improved images of certain areas and regions of the body which have heretofore produced less than satisfactory images in this modality. Polarized Helium 3 (“
3
He”) and Xenon-129 (“
129
Xe”) have been found to be particularly suited for this purpose. Unfortunately, as will be discussed further below, the polarized state of the gases is sensitive to handling and environmental conditions and can, undesirably, decay from the polarized state relatively quickly.
Hyperpolarizers are used to produce and accumulate polarized noble gases. Hyperpolarizers artificially enhance the polarization of certain noble gas nuclei (such as
129
Xe or
3
He) over the natural or equilibrium levels, i.e., the Boltzmann polarization. Such an increase is desirable because it enhances and increases the Magnetic Resonance Imaging (“MRI”) signal intensity, allowing physicians to obtain better images of the substance in the body. See U.S. Pat. No. 5,545,396 to Albert et al., the disclosure of which is hereby incorporated herein by reference as if recited in full herein.
In order to produce the hyperpolarized gas, the noble gas is typically blended with optically pumped alkali metal vapors such as rubidium (“Rb”). These optically pumped metal vapors collide with the nuclei of the noble gas and hyperpolarize the noble gas through a phenomenon known as “spin-exchange”. The “optical pumping” of the alkali metal vapor is produced by irradiating the alkali-metal vapor with circularly polarized light at the wavelength of the first principal resonance for the alkali metal (e.g., 795 nm for Rb). Generally stated, the ground state atoms become excited, then subsequently decay back to the ground state. Under a modest magnetic field (10 Gauss), the cycling of atoms between the ground and excited states can yield nearly 100% polarization of the atoms in a few microseconds. This polarization is generally carried by the lone valence electron characteristics of the alkali metal. In the presence of non-zero nuclear spin noble gases, the alkali-metal vapor atoms can collide with the noble gas atoms in a manner in which the polarization of the valence electrons is transferred to the noble-gas nuclei through a mutual spin flip “spin-exchange”.
Conventionally, lasers have been used to optically pump the alkali metals. Various lasers emit light signals over various wavelength bands. In order to improve the optical pumping process for certain types of lasers (particularly those with broader bandwidth emissions), the absorption or resonance line width of the alkali metal can be made broader to more closely correspond with the particular laser emission bandwidth of the selected laser. This broadening can be achieved by pressure broadening, i.e., by using a buffer gas in the optical pumping chamber. Collisions of the alkali metal vapor with a buffer gas will lead to a broadening of the alkali's absorption bandwidth.
For example, it is known that the amount of polarized
129
Xe which can be produced per unit time is directly proportional to the light power absorbed by the Rb vapor. Thus, polarizing
129
Xe in large quantities generally takes a large amount of laser power. When using a diode laser array, the natural Rb absorption line bandwidth is typically many times narrower than the laser emission bandwidth. The Rb absorption range can be increased by using a buffer gas. Of course, the selection of a buffer gas can also undesirably impact the Rb-noble gas spin-exchange by potentially introducing an angular momentum loss of the alkali metal to the buffer gas rather than to the noble gas as desired.
In any event, after the spin-exchange has been completed, the hyperpolarized gas is separated from the alkali metal prior to introduction into a patient. Unfortunately, after and during collection, the hyperpolarized gas can deteriorate or decay relatively quickly (lose its hyperpolarized state) and therefore must be handled, collected, transported, and stored carefully. Thus, handling of the hyperpolarized gases is critical, because of the sensitivity of the hyperpolarized state to environmental and handling factors and the potential for undesirable decay of the gas from its hyperpolarized state.
Some accumulation systems employ cryogenic accumulators to separate the buffer gas from the polarized gas and to freeze the collected polarized gas. Unfortunately, reductions in polarization of the gas can be problematic as, after final thawing of the frozen gas, the polarization level of the gas can potentially be undesirably reduced by as much as an order of magnitude. Further and disadvantageously, the extremely low operating temperatures of the accumulator near the cryogen source can sometimes clog the collection area of the accumulator, thereby decreasing the rate of, or even preventing, further collection.
OBJECTS AND SUMMARY OF THE INVENTION
In view of the foregoing, it is therefore an object of the present invention to extend the polarization life of collected polarized noble gases and to reduce the amount of de-polarization in the collected polarized gas prior to the end use point.
It is another object of the present invention to provide an improved cryogenic accumulator which can be used in a substantially continuous production environment.
It is a further object of the present invention to provide an improved collection device and method which reduces the amount of polarization lost during processing.
It is yet another object of the invention to provide a method which will minimize the de-polarization effects attributed to thawing a frozen polarized gas product prior to delivery to an end user.
These and other objects are satisfied by the present invention by a cryogenic accumulator with an internal heating jacket. In particular, a first aspect of the invention is directed to a cryogenic accumulator for collecting polarized noble gases which includes a primary flow channel having opposing first and second ends configured to direct polarized gas therethrough, and an outer sleeve positioned around the primary flow channel. The outer sleeve has a closed end defining a collection chamber positioned below the flow channel second end. The accumulator also includes a secondary flow channel positioned intermediate of the primary flow channel and the outer sleeve. The secondary flow channel has a closed end positioned in close proximity to the primary flow channel second end.
In a preferred embodiment, the outer sleeve and the outer wall of the secondary flow channel define a buffer gas exit channel therebetween and the (circumferentially extending) inner wall of the secondary flow channel defines the primary flow channel. It is also preferred that the primary flow channel second end be configured as a nozzle and that the secondary flow channel be configured as a warming or heating jacket to direct circulating room temperature dry gases such as nitrogen therethrough. The circulating nitrogen is separate from the flow channel and acts to compensate or protect the nozzle area against the cold buffer gas exiting along the outside of the primary flow channel and the cryogenic temperatures associated with the cryogen bath. Advantageously, such a secondary flow channel can reduce the likelihood that the primary flow nozzle will freeze and clog from sublimation of the noble gas.
Further and preferably, the accumulator includes first and second isolation valves in communication with the primary flow channel and the buffer gas exit channel. The first isolation valve is positioned at the first end of the
Deaton Daniel
Driehuys Bastiaan
Hasson K. C.
Langhorn Alan
Zollinger David
Capossela Ronald
Medi--Physics, Inc.
Myers Bigel Sibley & Sajovec P.A.
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