Hyperpolarization of a gas

Details Hyperpolarization of a gas Hyperpolarization of a gas

2002-11-25

2003-11-25

Bennett, Henry (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Solidification

Reexamination Certificate

active

06651459

ABSTRACT:

The present invention relates to a method and apparatus for hyperpolarizing a gas sample. In particular, the present invention relates to hyperpolarizing a noble gas for use in MRI and NMR experiments.
Conventional MRI techniques exploit the interaction of the intrinsic magnetic moment or spin of nuclei with an applied magnetic field. Nuclei whose spin is aligned with the applied magnetic field have a different energy state to nuclei whose spin is aligned opposed to the applied magnetic field. Accordingly, by applying a radio frequency radiation to the nuclei in a magnetic field, nuclei can be made to jump from a lower energy state to a higher energy state. The signals produced when the nuclei return to the lower energy state can then be measured, thereby providing information concerning the nature of the physical properties of the object being measured.
In most cases, MRI and NMR imaging is carried out using hydrogen nuclei which are present in water and fat. However, suitable nuclei are not always naturally present to enable measurements to be made. Thus, for example, in the lungs there are too few protons to generate a clear image. In addition to this microscopic air/tissue interfaces of the lung produce magnetic field variations that cause the already weak signal to decay even more rapidly. The problem is further exasperated by normal breathing and cardial motion.
A proposed solution to this is for the patient to inhale a mixture of a buffer gas, such as Nitrogen or Helium, and a strongly polarised sample gas, such as a noble gas. The hyperpolarized noble gas, as it is known, is a gas which includes an induced polarization, and hence an induced magnetic moment in the atomic nuclei. This allows MRI and NMR experiments to be performed in the normal way, even if the normally used hydrogen nuclei are not present.
Currently there are two main techniques for generating hyperpolarized gases. The first technique is described for example in U.S. Pat. No. 5,809,801, U.S. Pat. No. 5,617,860 and U.S. Pat. No. 5,642,625.
The technique described in these documents is the indirect hyperpolarization of Xenon or Helium3 (
3
He). This is achieved by mixing the gas with a small amount of an alkaline-metal vapour, such as rubidium. A weak magnetic field is applied to the vapour mixture to cause splitting of the alkaline-metal electron energy levels.
The vapour mixture is then optically pumped using a laser to cause a build-up of electron polarization in the higher energy sub-level of the metal vapour. Nuclei of the noble gas atoms then become polarized by collisions with the alkaline-metal which causes transfer of angular momentum from the polarized alkali electrons to the nuclei spin of the noble gas.
An alternative method of hyperpolarizing
3
He is achieved by direct optical pumping of a metastable state of the helium. In this method an electrical discharge and a low pressure cell are used to create atoms of
3
He in a metastable state. These metastable atoms are then exposed to circularly polarized laser light, from a high powered LNA-laser, which causes the transfer of polarization from the electrons to the helium nuclei via coupling with the unpaired neutron.
Both of the above mentioned methods rely on optical pumping techniques and are therefore extremely inefficient.
Other methods have been considered which involve the use of solidified Xenon. However, the Xenon has a long spin-lattice relaxation time and therefore must be kept in a strong magnetic field, at a low temperature, for long periods of time to result in any useful level of polarization. This direct approach is therefore impractical.
In order to overcome this, the document “High Equilibrium Spin-Polarizations in Solid
129
Xenon” by Honig et al, proposes mixing the Xenon with bulk amounts of oxygen to help improve the polarization. Meanwhile “The Brute Force
129
Xe and D
2
Polarization at low temperature” by Usenko et al describes achieving polarization by inducing an electron current in the solidified Xenon. Again however, these techniques have proved to be extremely inefficient.
FROSSATI G: “Polarisation of He, D2 (and possibly Xe) using cryogenic techniques” NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH, SECTION—A: ACCELERATORS, SPECTROMETERS, DETECTORS AND ASSOCIATED EQUIPMENT, NORTH-HOLLAND PUBLISHING COMPANY AMSTERDAM, NL, vol. 402, no. 2-3, Jan. 11, 1998, pages 479-483 discloses a method of hyperpolarizing a gas sample, the method comprising the steps of cryogenically forming a solidified gas structure from the sample gas, the solidified gas structure being surrounded by 3He, applying a magnetic field to the solidified gas structure and the 3He to thereby polarize the solidified gas structure, and removing the 3He to thereby leave a solidified gas structure of hyperpolarized sample gas.
In accordance with a first aspect of the present invention, we provide a method of hyperpolarizing a gas sample, the method comprising the steps of:
a. cryogenically forming a solidified gas structure from the sample gas, the solidified gas structure being surrounded by
3
He;
b. applying a magnetic field to the solidified gas structure and the
3
He to thereby polarize the solidified gas structure; and
c. removing the
3
He to thereby leave a solidified gas structure of hyperpolarized sample gas, characterized in that step (c) comprises the steps of:
i. increasing the temperature of the solidified gas structure;
ii. introducing
4
He into the region surrounding the solidified gas structure to thereby displace the
3
He; and
iii. pumping the
3
He and the
4
He away from the solidified gas structure.
In accordance with a second aspect of the present invention, we provide apparatus for hyperpolarizing a gas sample, the apparatus comprising:
a. cryogenic apparatus for forming a solidified gas structure from the sample gas, the solidified gas structure being surrounded by
3
He;
b. magnetic field generating assembly for applying a magnetic field to the solidified gas structure and the
3
He to thereby polarize the
3
He and the solidified gas structure; and
c. a removal system for removing the
3
He to thereby leave a solidified gas structure of hyperpolarized noble gas, characterized in that the removal system is adapted to carry out the steps of:
i. increasing the temperature of the solidified gas structure;
ii. introducing
4
He into the region surrounding the solidified gas structure to thereby displace the
3
He; and
iii. pumping the
3
He and the
4
He away from the solidified gas structure.
Accordingly, the present invention provides a method and an apparatus for producing a hyperpolarized sample gas. In this technique, a solidified gas structure is formed which is surrounded by
3
He. A magnetic field is then applied to the gas structure and the
3
He which causes polarization of the solidified gas structure. Under conditions of low temperature, solidified gases normally have an extremely low relaxation rate. However, in the present invention magnetic dipole—dipole coupling at the gas structure/
3
He interface leads to an increase in the relaxation rate of the solidified gas, thereby increasing the rate at which the solidified gas is polarized. The
3
He is then removed to leave behind a solidified gas structure of hyperpolarized noble gas. This can then be used in NMR and MRI experiments as required.
The step of cryogenically forming a solidified gas structure surrounded by
3
He usually comprises the steps of forming a solidified gas structure and, introducing the
3
He into the regions surrounding the solidified gas structure. Alternatively however the solidified gas structure is formed in an environment including
3
He.
Typically the step of forming a solidified gas structure comprises the step of cooling a substrate within a chamber and, introducing the sample gas into the chamber thereby causing the sample gas to condense onto the substrate. However, alternatively the gas structure may be formed by introducing the sample gas into an environment including a substrate and then cooling the entire environment to cause the

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