Production of hyperpolarized gas

Abstract

A method of removing buffer gas from a mixture comprising the buffer gas and hyperpolarized noble gas is described. The method includes reacting the buffer gas to produce a reaction product different to the buffer gas. The buffer gas may be reactively removed by one or more of oxidation, reduction, polymerization and binding reactions with solid surfaces. The buffer gas may be molecular hydrogen and/or molecular nitrogen. Apparatus for carrying out the method are also disclosed.

Claims

1. A method of producing a hyperpolarized noble gas, comprising: mixing a noble gas with an initial buffer gas to form a mixture such that the noble gas is present in the mixture at a first concentration; spin-exchange optical pumping the mixture to hyperpolarize the noble gas; and removing the buffer gas from the mixture so as to isolate the hyperpolarized noble gas whilst maintaining the hyperpolarization by: reacting the buffer gas present in the mixture comprising the buffer gas and hyperpolarized noble gas to produce a reaction product different to the buffer gas, wherein reacting the buffer gas comprises at least one of: (i) oxidising the buffer gas by one or more of combustion and induced plasma; (ii) introducing oxygen (O.sub.2) into the mixture comprising the buffer gas and the hyperpolarized noble gas, and combusting the buffer gas; (iii) introducing oxygen (O.sub.02) into the mixture comprising the buffer gas and the hyperpolarized noble gas in the presence of a catalyst, and catalytically removing the buffer gas; (iv) passing the mixture comprising the buffer gas and the hyperpolarized noble gas over or through an oxidizing agent; and (v) oxidizing an alkaline earth metal using the buffer gas.

2. The method of claim 1, wherein the buffer gas reacts into one or more reaction products which can be removed at temperatures of at least 250 degrees Kelvin.

3. The method of claim 1, wherein the buffer gas is at least one of (i) molecular hydrogen (H.sub.2); (ii) a hydrocarbon; and (iii) molecular nitrogen (N.sub.2).

4. The method of claim 1, wherein the reacting the buffer gas comprises passing the mixture comprising the buffer gas and the hyperpolarized noble gas over or through an oxidizing agent, and wherein: (i) the oxidizing agent is a chemical looping combustion agent, or (ii) the method further comprises discarding the oxidizing agent after use.

5. The method of claim 1, further comprising the step of separating the one or more reaction products from the hyperpolarized noble gas, wherein the buffer gas is molecular hydrogen (H.sub.2) and the reaction product is water vapour (H.sub.2O), and wherein the step of separating comprises condensing the water vapour to separate it from the hyperpolarized noble gas.

6. The method of claim 1, wherein the noble gas is mixed with initial buffer gas before the spin-exchange optical pumping such that the noble gas is present in the mixture at a first concentration, and wherein additional buffer gas is introduced into the mixture during the spin-exchange optical pumping to reduce the concentration of noble gas.

7. The method of claim 6, wherein the additional buffer gas comprises a different gas to the initial buffer gas, such that the composition of the buffer gas is altered during the spin-exchange optical pumping.

8. The method of claim 6, wherein the buffer gas is selected such that it does not react significantly during the spin-exchange optical pumping.

9. The method of claim 6, wherein the spin-exchange optical pumping takes place for ten minutes or less.

10. The method of claim 6, further comprising purging the mixture comprising the buffer gas and the hyperpolarized noble gas from the SEOP cell using further buffer gas.

11. The method of claim 10, wherein the further buffer gas is operable to transport the mixture comprising the buffer gas and the hyperpolarized noble gas from the SEOP cell into a second SEOP cell, and wherein the method comprises further hyperpolarizing the noble gas by second spin-exchange optical pumping to increase the hyperpolarization level.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Examples of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows: (a) percentage spin polarization of .sup.129Xe as a function of cell pressure after 6 minutes SEOP for three mixtures containing 5% Xe and 95% N.sub.2 (squares), CH.sub.4 (triangles), and H.sub.2 (circles), respectively, and for a fourth mixture containing 5% Kr and 95% H.sub.2 (star); and, (b) D.sub.2 fluorescence as a function of cell pressure during optical pumping of the Rb D.sub.1 transition at 3786 K temperature (measured at the front of the SEOP cell) to explore the radiation quenching properties of N.sub.2 (squares), H.sub.2 (circles), and CH.sub.4 (triangles);

(3) FIG. 2 shows: (A) a schematic of an experimental setup used for buffer gas combustion; and (B) successive images of the combustion experiment at 100 ms, 110 ms, 120 ms, 130 ms, 140 ms and 150 ms;

(4) FIG. 3 illustrates NMR experiments of hyperpolarized .sup.129Xe, during the catalytic oxidation of H.sub.2, in a 5% Xe/95% H.sub.2 gas mixture: (a) shows separate in situ reactor pressure measurements (diamonds) during the combustion reaction, in which the oxygen reservoir tap was opened at t=40 s; and (b) shows normalized integrated .sup.129Xe NMR signals following a 9 NMR excitation pulse during the combustion reaction in which the oxygen reservoir tap was opened at t=40 s adding 13.40.4 kPa (circles) or 20.50.5 kPa (triangles) partial pressure of oxygen. Control signals for a 5% Xe/95% H2 gas mixture with no combustion (oxygen reservoir tap closed) are shown as black open circles;

(5) FIG. 4 illustrates NMR experiments similar to those shown in FIG. 3, but with hyperpolarized .sup.83Kr, during the catalytic oxidation of H.sub.2, in a 5% Kr/95% H.sub.2 gas mixture: (a) shows in situ reactor pressure measurements (diamonds) during the combustion reaction after opening the oxygen reservoir tap at t=12 s; and (b) shows normalized integrated .sup.83Kr NMR signals following a 12 pulse during the combustion reaction in which the oxygen reservoir tap opened at t=12 s, thereby adding 13.40.4 kPa (circles) and 20.50.5 kPa (triangles) partial pressure of oxygen. Black open circles show control signals for a 5% Kr/95% H.sub.2 gas mixture with no combustion; and

(6) FIG. 5 shows a schematic of an alternative apparatus for producing and purifying hyperpolarized gas.

DETAILED DESCRIPTION

(7) The development of magnetic resonance imaging (MRI) with hyperpolarized (hp) noble gases has resulted in a number of excellent protocols to probe different structural and functional aspects of lungs in health and disease. Technological improvements have enabled pulmonary hp .sup.129Xe MRI at high spatial resolution, thereby reducing the need for usage of the scarcely available .sup.3He isotope. Furthermore, tissue solubility, large chemical shift range, and interaction with specific sensor molecules allow for a variety of biomedical hp .sup.129Xe applications.

(8) Isotopes with nuclear spin I> possess a nuclear electric quadrupole moment. For example, .sup.83Kr (I=9/2) can be hyperpolarized with rubidium (Rb) spin exchange optical pumping (SEOP). .sup.83Kr quadrupolar coupling originating from the surface of the SEOP cell has been observed, and T.sub.2 relaxation can be used as a probe for surfaces. The intriguing properties of .sup.83Kr can be more generally utilized after the removal of the reactive Rb vapor to generate surface sensitive MRI contrast. Most recently, T.sub.1 surface quadrupolar relaxation (SQUARE) MRI contrast with hp .sup.83Kr in lungs was demonstrated to be indicative of surface to volume changes in an animal model of emphysema.

(9) Although hp .sup.129Xe can be obtained through dynamic nuclear polarization (DNP) with high spin polarization levels of up to P=30%, at present only SEOP can produce hp .sup.129Xe with P90%. Furthermore SEOP is the currently the only method to provide hp .sup.83Kr for viable MRI applications. To obtain high spin polarization the noble gas needs to be diluted with a buffer gas, usually .sup.4He and/or N.sub.2, during SEOP. Following SEOP, hp .sup.129Xe is cryogenically (typically at 77K) separated from the gas mixture under carefully chosen conditions to prevent polarization loss. Cryogenic separation is cumbersome for biomedical hp .sup.129Xe applications (or costly if automated) and is not practical for hp .sup.83Kr due to the fast quadrupolar relaxation of hp .sup.83Kr. To avoid cryogenic separation, .sup.129Xe SEOP at high noble gas mole fraction has been explored in the past. Nevertheless, gas dilution is still necessary to obtain high spin polarization and thus reduces MRI signal intensity per unit volume of inhaled gas. A good measure for the resulting signal intensity is the apparent polarization, P.sub.app, i.e. the spin polarization P scaled by the gas dilution factor. Without cryogenic separation, the best apparent polarization for hp .sup.129Xe currently reported is P.sub.app=37% obtained after t>1 h of SEOP. For hp .sup.83Kr, the highest polarization achieved was P=26% after 8 min of SEOP but, with no method available for gas separation, the highest apparent polarization, to date is P.sub.app=4% resulting to P.sub.app=3% after recompression of the gas to ambient pressure.

(10) In an effort to improve the apparent polarization of a hp noble gas, we have attempted SEOP using noble gas mixtures containing alternative buffer gases. We have realized that if a reactive buffer gas is used, then this can subsequently be removed from the hp gas mixture reactively (for example, through catalytic combustion). Our initial experiments, as described herein, concentrated on reactive separation of a buffer gas comprising molecular hydrogen and/or a hydrocarbon from .sup.129Xe and .sup.83Kr, which have a naturally abundant isotope distribution (i.e. 26.4% .sup.129Xe and 11.5% .sup.83Kr). However, it will be appreciated that the process of reactively removing a buffer gas from a hp noble gas mixture, as described in more detail below, can be used with other hp noble gases if required, and with other buffer gases.

(11) To produce the initial hpNG/buffer gas mixture, SEOP was conducted at 0.05 T field strength in a 120 mm long cylindrical Pyrex cell with 28 mm inner diameter. FIG. 1a shows the .sup.129Xe nuclear spin polarization, P, after 6 min of SEOP with a 0.2 nm linewidth laser of 23 W incident power at an external cell temperature of 383K as function of pressure of the mixture within the cell. It will be appreciated that the above parameters are particular to the experimental set up which was chosen, and need not be utilized in a clinical setting if not required. The invention described herein focusses on the separation of the hpNG from a buffer gas, rather than on the specifics of how the hpNG is produced initially.

(12) FIG. 1a shows that the spin polarization achieved for a 5% Xe-95% H.sub.2 mixture (circles) is strikingly similar to the one produced with a 5% Xe-95% N.sub.2 mixture (squares) at 373 K under otherwise identical conditions. This shows that molecular hydrogen works well as a buffer gas. For comparison, the results obtained using a 5% Xe-95% CH.sub.4 mixture (triangles) are also shown, and it can be seen that a much lower spin polarization was achieved when using CH.sub.4 as a buffer gas. Even so, some increase in polarization was obtained, suggesting that CH.sub.4 could also serve as a useful, if less effective, buffer gas.

(13) FIG. 1a also shows the spin polarization achieved in a 5% Kr-95% H.sub.2 mixture. It can be seen that a .sup.83Kr polarization of P=17.50.2% was obtained after 10 min of SEOP with 95% H.sub.2. This was carried out at 433 K temperature and 2.1 kPa SEOP pressure. Using N.sub.2 as buffer gas results similar spin polarization (P=15%) at this pressure after 8 min of SEOP, suggesting that H.sub.2 is also effective as a buffer gas for Kr.

(14) The high polarization with the .sup.129Xe/H.sub.2 mixture in FIG. 1a was achieved repeatedly over the course of at least 6 hours despite the formation of rubidium hydrides (RbH) during on-resonance D.sub.1 laser irradiation. However, the SEOP cell needed to be kept under operational conditions as cycling to room temperature (and back to SEOP temperature) reduced the spin polarization by a factor of ten, presumably because of a visible thick RbH surface coating within the cell that required thorough cleaning and refilling of the SEOP cell with Rb for further usage. Note that RbH will disassociate to Rb and H.sub.2 at higher temperatures (>443 K) and this process may be used for SEOP cell recycling, although this was not further investigated. The .sup.131Xe T.sub.1 relaxation time increases for this spin I=3/2 isotope because of RbH buildup, and RbH surface coating may also reduce the .sup.83Kr T.sub.1 relaxation rates. The effect of RbH surface deposition on the T.sub.1 relaxation of .sup.129Xe as a function of field strength (0.08 T) and temperature (340K) in a spherical 25 mm diameter cell has been explored, and the results indicate for the current work that .sup.129Xe would likely exhibit T.sub.1 times in excess of 400 s at the high temperature condition during fast SEOP of the Xe/H.sub.2 mixtures. Therefore, relaxation by itself should not limit the reachable spin polarization below that obtained in SEOP with Xe/N.sub.2 mixtures.

(15) Usually, SEOP mixtures contain at least 5-10% molecular nitrogen for radiation quenching, i.e. to dissipate the energy from electronically excited Rb into the vibrational modes of N.sub.2 and therefore to prevent radiation trapping of arbitrarily polarized fluorescence photons that reduce Rb polarization. This is a particular concern at high SEOP temperatures with associated high Rb densities. The efficacy of H.sub.2 as a radiation quencher has been studied, and SEOP of dissociated atomic hydrogen has been explored, usually at low SEOP temperature and very low H.sub.2 partial pressure. The results presented here in FIG. 1b, monitoring the Rb D.sub.2 fluorescence, demonstrate that H.sub.2 (circles) serves as an efficient radiation quenching agent. It can be seen that H.sub.2 has a sufficient quenching cross section to prevent radiation trapping and, remarkably, H.sub.2 is able to effectively prevent radiation trapping even at high temperatures of 383 K and 433 K (for .sup.129Xe and .sup.83Kr SEOP, respectively) with associated high rubidium density and 23 W of laser power. At pressures above 40 kPa, it can be seen that there is little difference between N.sub.2 (squares) and H.sub.2 (circles) as an Rb radiation quenching agent, in agreement with the polarization curves shown in FIG. 1a. The D.sub.2 fluorescence recorded when using CH.sub.4 as a quenching agent is shown for comparison (triangles), and it can be seen that CH.sub.4 is less effective as a quenching agent.

(16) Successful SEOP using buffer gases other than N.sub.2, and particularly H.sub.2, opens the path for oxidative removal of the buffer gas as an alternative for cryogenic separation.

(17) Furthermore, as discussed in more detail below, we have found that N.sub.2 itself can be reactively removed if an appropriate reactant is provided.

(18) To test the effect of such oxidative removal on the noble gas spin polarization, a catalytic combustion setup was devised as sketched in FIG. 2a.

(19) The experimental apparatus shown in FIG. 2a includes a reaction chamber 10 having an inlet 12. Fluidly connected to the inlet 12 via a conduit 14 are an oxygen supply 16 and a hpNG mixture supply 18. The hpNG mixture is produced by SEOP in a polarization cell (not shown).

(20) The pressure and delivery of the hpNG gas is controlled by a pair of valves A, B, and monitored by a pressure gauge 20. Similarly, pressure and delivery of the O.sub.2 gas is controlled by a pair of valves C, D, and monitored by a pressure gauge 22. A catalyst 24 is provided inside the reaction chamber, for example a Pt/Al.sub.2O.sub.3 catalyst powder (25 mg, 5 wt. % dry loading Pt on alumina). The reaction chamber itself is, in this example, a 1.5 mm thick glass vessel. The reaction chamber 10 is located within an MRI detection system 26, so that the effect of the combustion on the polarization of the mixture within the reaction chamber can be measured.

(21) In the first instance, the hpNG mixture made up of 95:5 H.sub.2:hpNG was delivered into the reaction chamber 10 by opening valve B. The signal decay over time was monitored through conventional NMR spectroscopy at 9.4 T in order to provide baseline data 30, shown as open circles in FIG. 3b for hp .sup.129Xe and in FIG. 4b for hp .sup.83Kr. This allows the decay of the polarization over time to be compared with the reduction in polarization (if any) resulting from the reaction.

(22) For oxidative H.sub.2 removal, molecular oxygen, O.sub.2, was added to the hpNG mixture by opening valve C. This led to complete hydrogen combustion within <140 ms, as depicted in FIG. 2b, in which it can be seen that the H.sub.2/O.sub.2 mixture ignites and burns out completely between t=120 ms and t=140 ms. Note that the pressure of the H.sub.2/hp noble gas mixture delivered to the reactor was kept below 30 kPa to avoid excessively high temperatures and associated pressure bursts that might have compromised reactor integrity.

(23) The reactor pressure during this process was monitored (see FIGS. 3a and 4a) but potential short pressure increases during the reaction were not detected at the time resolution of the pressure gauge. Upon adding O.sub.2, slightly above the stoichiometric ratio, the pressure decreases within 15 s as the sole reaction product, H.sub.2O, condenses rapidly upon cooling. The reactor (outside) temperature increase was limited to 5K and the final reactor pressure observed (4.70.5 kPa) was close to that of water vapor at ambient temperature.

(24) Monitoring the hp .sup.129Xe signal intensity 32 (FIG. 3b, filled circles), an initial signal increase is observed upon O.sub.2 delivery, caused by additional hp .sup.129Xe in the connecting tubing that is pushed into the reaction (and NMR detection) chamber by the O.sub.2 gas. Within 20 s, the signal returns approximately to the baseline due to gas convection and diffusion, thereby suggesting that the nuclear spin state experiences no significant depolarization during the catalytic reaction. However, after the reaction, the .sup.129Xe relaxation is accelerated due to a small excess of paramagnetic O.sub.2 (ca. 0.7 kPa partial pressure). Increasing the O.sub.2 excess to 7.50.5 kPa (filled triangles, 34) leads to further accelerated .sup.129Xe signal decay. Note that without O.sub.2 excess, hp .sup.129Xe may be stored and accumulated at low pressure.

(25) Turning to FIG. 4, the hp .sup.83Kr data (filled circles, 36) in FIG. 4b shows a different behavior. Firstly, .sup.83Kr remaining in the connecting tubing will have completely depolarized due to fast quadrupolar T.sub.1 relaxation in the presence of the Teflon surface. Therefore, the signal intensity will not display a short term rise as in the case of hp .sup.129Xe upon O.sub.2 gas delivery. This simplifies the data interpretation and the .sup.83Kr data demonstrates clearly that no signal loss is caused by the combustion. In contrast to .sup.129Xe, the .sup.83Kr gas phase relaxation even slows down after the reaction due to the reduced overall pressure and the strong pressure dependence of .sup.83Kr gas phase T.sub.1 relaxation. Due to krypton's very low gyromagnetic ratio , its T.sub.1 relaxation is only marginally affected by paramagnetic O.sub.2, even at higher (7.50.5 kPa) oxygen partial pressure (FIG. 4b, filled triangles, 38).

(26) Because of the low total gas pressure after catalytic buffer gas removal, the hp gases will require recompression to (slightly above) ambient pressure for biomedical application. Recompression was recently demonstrated with little polarization loss for hp .sup.129Xe and acceptable polarization loss for hp .sup.83Kr. In conclusion, H.sub.2 is a very efficient Rb D.sub.1 radiation quenching agent even for very high Rb density at 433 K and can therefore be used as the sole buffer gas in noble gas SEOP. As a consequence, catalytic H.sub.2 combustion becomes an alternative to cryogenic hp noble gas separation after SEOP. This suggests that P.sub.app50% might be possible in cryogenics-free hp .sup.129Xe production using high temperature SEOP at the associated very short pumping times below 10 minutes. The capability of rapid H.sub.2 removal also opens up the possibility of hydrogen gas assisted recovery of hp noble gases from equipment, for example through purging of connecting pipelines. In addition, dilution with H.sub.2 may reduce relaxation during hp .sup.129Xe storage at ambient pressure, similar to storage at low pressure. Perhaps the most important result is that hp .sup.83Kr has been purified without depolarization for the first time, suggesting that P.sub.app>15% has now become feasible (after recompression to ambient pressure) with 23 W laser power. This constitutes a five-fold improvement in MRI signal intensity over previous results that enabled non-slice selective images of ex vivo rodent lungs with 0.7950.635 mm.sup.2 resolution in pre-clinical work.

(27) Turning now to FIG. 5, an example flow-through system 40 is depicted. Similar to FIG. 2, the system 40 includes a reaction chamber 42 having an inlet 44 which is fluidly connected to a source of hp gas mixture 45 and to a source of molecular oxygen 46. The delivery of the hp gas mixture and the oxygen is controlled by respective valves 48, and flow controllers 50.

(28) The hp gas mixture is produced via SEOP in a cell 52. A supply of noble gas for polarization 54 and a buffer gas 56 (in this case H.sub.2) are provided. A mixing chamber 58 is fluidly connected to the noble gas supply 54 and the buffer gas supply 56 by respective valves. In the mixing chamber, noble gas from the supply 54 is mixed with buffer gas from the supply 56, at high pressure, in this case approx. 230 kPa, as indicated by pressure gauge 72a. If required, the mixing chamber can also contain alkali metal vapor (in this case rubidium) to purify the gases and, in some cases, pre-saturate with alkali metal in preparation for pumping.

(29) After mixing, the gas mixture is expanded into the SEOP cell, leading to a lower pressure in the SEOP cell (in this case approx. 0.9 kPa, as indicated by gauge 72b). A further supply 60 of buffer gas (also H.sub.2, although a different buffer gas could be used if required) is provided between the mixing chamber 58 and the SEOP cell 52. This allows the ratio of the buffer gas:noble gas to be adjusted if desired during SEOP and, if a different buffer gas is provided in buffer gas source 60, may also allow the composition of the buffer gas to be adjusted during SEOP. Alternatively, or additionally, the mixture ratio can be adjusted using buffer gas source 56. In the example shown, the initial SEOP ratio is 95:5 buffer gas:noble gas.

(30) In the SEOP cell the noble gas/buffer gas/alkali metal vapor mixture is irradiated with circularly polarized laser light 62, resulting in hyperpolarization of the noble gas. Irradiation is conducted with inlet valve 64 and outlet valve 66 closed. When sufficient polarization has been achieved (e.g. after about 10 minutes), outlet valve 66 is opened, and the hp gas mixture is drawn out of the SEOP cell into the reaction chamber 42, via an alkali metal removal trap 68.

(31) If it is required to adjust the temperature in the SEOP cell during SEOP, the temperature can be temporarily changed through the addition of the buffer gas and/or through temperature regulation of the SEOP cell using a heater (not depicted).

(32) After SEOP the gas mixture is released through opening of valve 66 and the rubidium vapor is removed through a filter 68. The gas streams through a flow regulator 50 into the reactor chamber 42. Gas transport is accomplished through pressure equalization (discussed more fully below). In addition H.sub.2, or a hydrocarbon gas, (e.g. from buffer gas source 60 or 56) can be used to purge remaining hpNG and to transport hpNG through the connecting tubing.

(33) As discussed previously in relation to FIG. 2, in the reaction chamber the hp noble gas/buffer gas mixture is mixed with molecular oxygen to reactively remove the buffer gas. In this case, the buffer gas (hydrogen) is removed by catalytic oxidation: a catalyst, typically platinum or palladium, or an oxide thereof present in the reaction chamber facilitates the reactive removal of the buffer gas without combustion.

(34) Alternatively, a chemical looping agent is used that serves as oxidation agent and that can be recycled later with molecular oxygen gas. In this way the buffer gas is never mixed with molecular oxygen providing a very safe reaction system. If a metal oxide is used as an oxidation agent, O.sub.2 gas may be used to regenerate the metal oxide at a later time after buffer gas removal is completed.

(35) The resulting mixture, which is substantially free from buffer gas, is drawn through a condensation chamber 70, in which water vapor resulting from the reaction, and any remaining alkali metal, is removed via condensation. If a hydrocarbon was used as buffer gas (or one of the buffer gases), a CO.sub.2 getter (not shown) will also need to be included.

(36) Because the buffer gas has been removed the pressure drops significantly (see pressure gauge 72c, approx. 0.1 kPa). This pressure drop enables gas transport though pressure equalization until the gas mixture is almost entirely removed from the SEOP cell. As noted above, further buffer gas from source 56 or 60 can be used to purge remaining hpNG from the cell and connecting tubing if required. The purified hpNG that is substantially free from buffer gas continues to flow through a hydrogen detector 73 (and if needed through a CO detector) into a pre-evacuated storage volume 74 that also serves as a pneumatically operated, single piston recompression unit. Alternatively, other pumps, such as a peristaltic pump, could be used for recompression of the hpNG to the desired high pressure (for example, slightly above ambient pressure as indicated by the pressure gauge 72d).

(37) If it is intended to use the purified hp noble gas in a clinical setting (e.g. for lung MRI), the hp noble gas can be mixed with oxygen from a compression oxygen supply 76. The hp gas/oxygen mixture can then be delivered directly to the application (e.g. to a patient for inhalation).

(38) Before recompression, the hpNG may be stored for some time at low pressure. Furthermore, before recompression, O.sub.2 may be added to produce a breathable mixture after recompression. If a mixture containing hpNG and O.sub.2 is produced for biomedical applications, another reactor 78 may be used to further ensure a very high level of H.sub.2 removal (and if applicable, CO removal to physiologically safe levels). A final H.sub.2 detector 80 (and, if applicable, CO detector) ensures production of a physiologically safe, non-reactive gas that is released for MRI usage and other applications. Other applications may include NMR spectroscopy, NMR relaxometry, and usage of hpNG as nuclear spin polarized targets.

(39) Although the present invention has been described above primarily with respect to oxidative removal of a hydrogen buffer gas, it will be appreciated that other reactive buffer gases could be used, so long as those gases provide an effective buffer during SEOP, and effective quenching. If required, a mixture of reactive buffer gases could be used to achieve the desired properties. Depending on the buffer gas selected, a reaction other than oxidation might be appropriate, such as polymerisation or reaction with a solid surface. Hydrogen and oxidation work well together, however, as the reaction product (water) is easily removed and is not biologically harmful.

(40) One such alternative buffer gas is molecular nitrogen (N.sub.2). As discussed above, N.sub.2 is known to be an effective quenching agent and dilutant. However, it has previously been thought necessary to remove N.sub.2 cryogenically. In contrast, we have realised that N.sub.2 can be removed reactively, resulting in a cheaper and quicker method for separating hpNG from the N.sub.2 buffer gas.

(41) Unlike H.sub.2, N.sub.2 is not reacted with an oxidation agent, rather N.sub.2 itself serves as the actual oxidation agent of a suitable substance, such as a metal that serves as a reducing agent. Reactive N.sub.2 removal thus replaces cryogenic N.sub.2 removal, currently being used.

(42) As an example, N.sub.2 may be reacted with magnesium metal at high temperatures to produce magnesium nitride. Other alkaline earth metals may also be used for this purposefor example Ca, Sr, Ba. This reaction can be accomplished in a flow through reactor similar to the one presented in FIG. 5 for the hydrogen gas removal.

(43) In such a system the reactive removal of N.sub.2 would still take place in the reaction chamber 42. However, rather than providing an external source of oxygen gas 46 or an oxidizing agent within the reaction chamber, a reducing agent (i.e. substance to be oxidized), such as an alkaline earth metal, would be provided in the reaction chamber 42 instead. The N.sub.2 buffer gas can thus be used to oxidize the metal within the reaction chamber, thus reactively removing the N.sub.2 from the hpNG.

(44) All other components of the flow through reactor 40 would remain unchanged, and therefore will not be described again here.

(45) An example of an oxidation reaction for the reactive removal of nitrogen would be:
HPNG+N.sub.2+3Mg.fwdarw.HPNG+Mg.sub.3N.sub.2(s)

(46) This is a combustion reaction in which N.sub.2 reacts with metallic magnesium to produce magnesium nitride (s=solid). As discussed above, heating can be provided for the reaction chamber, for example in the form of inductive heating, to maintain the chamber at an appropriate temperature for the reaction.

(47) Magnesium metal may be provided in a temperature resistant ceramic tube as a wire, surface coating, surface wash of magnesium powder or magnesium nano particles, or just as a tube filled with magnesium powder or nano particles. Generally, the higher the surface area of the magnesium the better, as this improves the contact area with the N.sub.2 gas and thus increases the efficiency of the reaction.

(48) Magnesium nitride is typically a solid at room temperature, and thus can easily be separated from the hpNG after the reaction is complete.

(49) If required, the magnesium metal can be recycled after use via reduction of the magnesium nitride. An example recycling process might involve the following reactions:
Mg.sub.3N.sub.2(s)+6H.sub.2O(g).fwdarw.3Mg(OH).sub.2(s)+2NH.sub.3(g)1.)
heating: Mg(OH).sub.2(s).fwdarw.MgO(s)+H.sub.2O(g)2.)
reduction: MgO+H.sub.2(g).fwdarw.Mg+H.sub.2O3.)

(50) Note that magnesium hydride decomposes above 300 C. and should therefore not be formed if the temperature is kept high enough. (s=solid, g=gas).

(51) It can thus be seen that reactive separation of nitrogen buffer gas from hpNG is a viable alternative to the cryogenic separation process which is traditionally used.

(52) The invention has been described primarily in relation to a buffer gas/noble gas mixture having a ratio of 5% noble gas to 95% buffer gas. Other ratios could be used if required. For example, the very high laser power that has become available recently enables a high level of polarization in mixtures containing 50% xenon. Higher noble gas concentration reduces the required volume of the SEOP cell and can be advantageous.

(53) With the ratios and gases discussed above, we have found that SEOP times of less than 15 minutes produce sufficient polarization for use clinically. Indeed SEOP of less than 10 minutes, and in some cases less than 8, 6 or 5 minutes is sufficient. However, for other applications, where more polarization is required, longer SEOP times may be used.

(54) One path to expedite the SEOP time is to change the gas mixture during SEOP. This can be further assisted by selecting a temperature that optimises SEOP for the particular mixture. For example, lower levels of polarization can be reached with less than ideal mixtures and temperatures but at a high production rate. Once a certain polarization level is reached the mixture is optimized for slower SEOP that further increases the polarization. For example a 50% NG, 50% H.sub.2 mixture at low pressure of 50 kPa could be used to reach an initial spin polarization of P=5-10% for .sup.83Kr (or of P=30-50% for .sup.129Xe) after which the total pressure can be increased by further H.sub.2 addition leading to a 25% NG; 75% H.sub.2 mixture. SEOP can then continue until the desired polarization level is reached. Alternatively, the obtained 25% NG; 75% H.sub.2 mixture may be transferred into a second SEOP cell of larger volume to continue the SEOP process at lower pressure.

(55) When recompressed, we have been able to produce hyperpolarized .sup.129Xe having an apparent polarization of greater than 40%, and in some cases greater than 50%, by this process using relatively low laser power (i.e. 23 W). Similarly, when recompressed, we have been able to produce hyperpolarized .sup.83Kr having an apparent polarization of greater than 10%, and in some cases greater than 15%, by this process.