EP4627361A1 - Gas hyper-polarizer apparatus - Google Patents

Abstract

A polarizer apparatus comprising: a spin-exchange optical pump for hyperpolarization of a noble gas, the spin- exchange optical pump comprising a field coil arrangement configured to provide, within a static homogeneous-magnetic-field-volume, a static homogeneous magnetic field used by the spin-exchange optical pump; and a storage retainer for storing hyperpolarized noble gas, in gaseous phase, output from the spin-exchange optical pump wherein the storage retainer is located outside an oven of the spin-exchange optical pump and inside the static homogeneous-magnetic-field-volume and wherein the storage retainer provides an associated storage volume for storing and retaining hyperpolarized noble gas produced by the spin-exchange optical pump.

Description

TITLE Gas hyper-polarizer apparatus TECHNOLOGICAL FIELD Examples of the disclosure relate to a polarizer apparatus used to provide hyperpolarized noble gas (a gas hyper-polarizer apparatus). BACKGROUND Nuclear magnetic resonance (NMR) spectroscopy involves applying radio frequency radiation to change and then measure nuclear spin states of atomic nuclei. The strength of signal produced is, for example, dependent upon magnetization density of the atomic nuclei. The magnetization density of atomic nuclei in a physically less dense state (e.g. a gas) can be increased by hyperpolarization. Hyperpolarization increases the net nuclear spin polarization (magnetization) far beyond thermal equilibrium conditions. BRIEF SUMMARY According to various, but not necessarily all, examples there is provided a polarizer apparatus comprising: a spin-exchange optical pump for hyperpolarization of a noble gas, the spin- exchange optical pump comprising a field coil arrangement configured to provide, within a static homogeneous-magnetic-field-volume, a static homogeneous magnetic field used by the spin-exchange optical pump; and a storage retainer for storing hyperpolarized noble gas, in gaseous phase, output from the spin-exchange optical pump wherein the storage retainer is located outside an oven of the spin-exchange optical pump and inside the static homogeneous-magnetic-field-volume and wherein the storage retainer provides an associated storage volume for storing and retaining hyperpolarized noble gas produced by the spin-exchange optical pump. In some but not necessarily all examples, the storage retainer is sized and positioned so that a whole of the storage volume is wholly within the static homogeneous- magnetic-field-volume. In some but not necessarily all examples, a length of the storage volume in a direction parallel to the static homogeneous magnetic field is less than a length of the static homogeneous-magnetic-field-volume in the same direction. In some but not necessarily all examples, the storage volume is neither heated not cooled and, in use, is at ambient room temperature. In some but not necessarily all examples, a whole of the storage volume is outside the oven of the spin exchange optical pump and inside the static homogeneous-magnetic- field-volume. In some but not necessarily all examples, the storage retainer is positioned in a space between an oven of the spin exchange optical pump and field coils of the field coil arrangement. In some but not necessarily all examples, field coils of the field coil arrangement are centered on a longitudinal axis, lie in parallel transverse planes orthogonal to the longitudinal axis, and define a field coil volume that has a constant cross-sectional area defined by a cross-sectional area of the field coils in the parallel transverse planes and a length defined by a maximum distance parallel to the longitudinal axis between field coils of the field coil arrangement, wherein an oven of the spin exchange optical pump extends parallel to the longitudinal axis within the field coil volume and wherein the storage retainer is positioned within the field coil volume, in a transverse relationship to at least a portion of the oven. In some but not necessarily all examples, field coils of the field coil arrangement are cantered on a longitudinal horizontal axis, lie in parallel transverse vertical planes orthogonal to the longitudinal vertical axis, and define a field coil volume that has a constant cross-sectional area defined by a cross-sectional area of the field coils in the parallel transverse vertical planes and a length defined by a maximum distance parallel to the longitudinal horizontal axis between field coils of the field coil arrangement, wherein an oven of the spin exchange optical pump extends parallel to the longitudinal horizontal axis within the field coil volume and wherein the storage retainer is positioned within the field coil volume, vertically above at least a portion of the oven. In some but not necessarily all examples, the storage retainer is thermally isolated from the oven. In some but not necessarily all examples, the polarizer apparatus provides an access route to the storage retainer that enables the placement of hyperpolarized noble gas within the storage retainer in a direction that is parallel to the static homogeneous- magnetic-field. In some but not necessarily all examples, the polarizer apparatus provides an access route into and through the static homogeneous-magnetic-field, in a longitudinal direction that is parallel to the static homogeneous-magnetic-field, to the storage retainer. In some but not necessarily all examples, the polarizer apparatus provides an at least partial non-ferromagnetic enclosure for the storage retainer that prevents access to the storage retainer that is across the static homogeneous-magnetic-field. In some but not necessarily all examples, the polarizer apparatus provides an at least partial non-ferromagnetic enclosure for the storage retainer that prevents access to the storage retainer via a route between adjacent field coils of the field coil arrangement. In some but not necessarily all examples, the polarizer apparatus provides one or more guides that guide user placement of the hyperpolarized noble gas at the storage retainer. In some but not necessarily all examples, the one or more guides are shaped to encourage controlled ingress of hyperpolarized noble gas bag to the storage retainer and controlled egress of hyperpolarized noble gas bag from the storage retainer. In some but not necessarily all examples, the one or more guides are shaped to follow field lines of the field coil arrangement. In some but not necessarily all examples, the polarizer apparatus comprises a stored- gas nuclear magnetic spectrometer for analysis of hyperpolarized noble gas when stored in the storage retainer. In some but not necessarily all examples, the polarizer apparatus comprises a production nuclear magnetic spectrometer for analysis of hyperpolarized noble gas within an optical cell of the spin exchange optical pump, wherein a radio frequency field coil arrangement of the production nuclear magnetic spectrometer is positioned in orthogonal relationship to a radio frequency field coil arrangement of the stored-gas nuclear magnetic spectrometer to enable simultaneous operation of the stored-gas nuclear magnetic spectrometer and the production nuclear magnetic spectrometer. In some but not necessarily all examples, a radio frequency field coil arrangement of the stored-gas nuclear magnetic spectrometer is integrated with the storage retainer. In some but not necessarily all examples, the storage retainer comprises a retainer dish having raised sides. In some but not necessarily all examples, the storage retainer comprises a concave retainer dish. In some but not necessarily all examples, the retainer dish has associated constraints for holding a bag of hyperpolarized noble gas on the retainer dish. In some but not necessarily all examples, the associated constraints for holding a bag of hyperpolarized noble gas on the retainer dish are elastically deformable, wherein the elastic deformation of the constraints provides a holding force for holding the bag of hyperpolarized noble gas on the retainer dish. In some but not necessarily all examples, the associated constraints for holding a bag of hyperpolarized noble gas on the retainer dish comprises a ring for placement on top of the bag of hyperpolarized noble gas and elastic members attached to the ring and sized to be extended when the ring is placed on top of the bag of hyperpolarized noble gas to provide an elastic force that pulls the ring towards the retainer dish for holding the bag of hyperpolarized noble gas on the retainer dish. In some but not necessarily all examples, a radio frequency field coil arrangement of a stored-gas nuclear magnetic spectrometer is formed as part of the retainer dish. In some but not necessarily all examples, the spin-exchange optical pump comprises: a source of circularly polarized light; an optical cell for housing an alkali metal and positioned for illumination by the source of circularly polarized light; an oven for heating the optical cell to a temperature sufficient to vaporize the alkali metal; an input for ingress of a gas comprising a noble gas; an output for egress of a gas comprising the noble gas after hyperpolarization. In some but not necessarily all examples, the spin-exchange optical pump is configured for the hyperpolarization of ¹²⁹Xe. In some but not necessarily all examples, the spin-exchange optical pump comprises a laser configured for Rb electron spin polarization. In some but not necessarily all examples, the polarizer apparatus is configured for cryogenic collection and bagging of hyperpolarized noble gas prior to storage at the storage retainer. In some but not necessarily all examples, the polarizer apparatus is housed on a transportation trolley. According to various, but not necessarily all, examples there is provided a method comprising: Producing a hyperpolarized noble gas, using a spin-exchange optical pump comprising a field coil arrangement configured to provide, within a homogeneous- magnetic-field-volume, a static homogeneous magnetic field used by the spin- exchange optical pump; Providing a storage container for storing hyperpolarized noble gas, in gaseous phase, output from the spin-exchange optical pump wherein the storage container is located outside the spin-exchange optical pump and inside the homogeneous- magnetic-field-volume. According to various, but not necessarily all, examples there is provided examples as claimed in the appended claims. While the above examples of the disclosure and optional features are described separately, it is to be understood that their provision in all possible combinations and permutations is contained within the disclosure. It is to be understood that various examples of the disclosure can comprise any or all of the features described in respect of other examples of the disclosure, and vice versa. Also, it is to be appreciated that any one or more or all of the features, in any combination, may be implemented by/comprised in/performable by an apparatus, a method, and/or computer program instructions as desired, and as appropriate. BRIEF DESCRIPTION Some examples will now be described with reference to the accompanying drawings in which: FIG.1 shows an example of a polarizer apparatus; FIG.2 shows an example of an optical cell of a spin-exchange optical pump used in the polarizer apparatus; FIG.3 shows an example of a storage retainer of the polarizer apparatus configured to store hyperpolarized noble gas outside the spin-exchange optical pump at room temperature; FIG. 4 shows magnetic fields produced by a field coil arrangement of the spin- exchange optical pump; FIG.5 shows magnetic field strength produced by a field coil arrangement of the spin- exchange optical pump; FIG.6 shows an example of a storage retainer with guide(s); FIG.7 shows an example of a storage retainer with an active retainer; FIG.8 shows another example of a storage container with an integrated field coil(s) linked to a nuclear magnetic spectrometer. FIG.9 shows an example of a method; FIG.10 shows an example of a controller for controlling the polarizer apparatus; FIG.11 shows an example of a computer program for use by the polarizer apparatus. The figures are not necessarily to scale. Certain features and views of the figures can be shown schematically or exaggerated in scale in the interest of clarity and conciseness. For example, the dimensions of some elements in the figures can be exaggerated relative to other elements to aid explanation. Similar reference numerals are used in the figures to designate similar features. For clarity, all reference numerals are not necessarily displayed in all figures. DETAILED DESCRIPTION The nuclear spin is defined by a quantum number (I), which varies depending on the considered isotope. Only atomic nuclei with I ≠ 0 are detectable by NMR spectroscopy. As a result of the nuclear Zeeman effect, nuclear spins of I = ½ adopt two different orientations when they align to an external magnetic field (B0). One orientation corresponds to the lowest energy level of the nucleus (parallel to the external magnetic field), and the other one is associated to the highest energy level of the nucleus (antiparallel to the external magnetic field). The difference between energy levels (ΔE) depends on the magnetic field and the gyromagnetic ratio of the isotope. In thermal equilibrium, there will therefore be a different average population in the different energy levels. Nuclear magnetic resonance, created by applied radiofrequency electromagnetic energy, causes transitions between the nuclear spin-state energy levels that are detected in magnetic resonance imaging (MRI). The strength of signal detected depends upon the difference in populations between the spin states and the density of nuclei. The nuclear polarization caused by the nuclear Zeeman effect can therefore be sufficient to provide a strong signal in, for example, proton (H) nuclear magnetic imaging because of the density of protons in tissue. However, inhaled gases have much lower densities. Therefore to perform nuclear magnetic imaging on inhaled gases it can be desirable to hyperpolarize the gas. Hyperpolarization is an exogenous enhancement of magnetization. This creates a higher magnetization density of the NMR-active isotope. Unlike proton MRI which reports on anatomical features of lung tissues, gas-based MRI reports lung function including gas ventilation, diffusion, and perfusion Hyperpolarization increases the net nuclear spin polarization above thermal equilibrium conditions determined by the Boltzmann equation. The polarization levels can be enhanced by a factor of 10⁴-10⁵ above thermal equilibrium levels. Hyperpolarization is often performed on noble gases using spin-exchange optical pumping. This is because noble gases are chemically inert with respect to alkali metals and have a relaxation time that is long enough to build up polarization but short enough to be used effectively for MRI. In order to interact with the magnetic field in the spectrometer, the nucleus must have an intrinsic nuclear magnetic moment and angular momentum. This occurs when an isotope has a non-zero nuclear spin, meaning an odd number of protons and/or neutrons. Two noble gases, ³He and ¹²⁹Xe, have nuclear spin one-half and gyromagnetic ratios relative to that of the proton of 0.76 and -0.28, respectively. They have long T1 relaxation times after being polarized. Xenon is extracted from the atmosphere by partial distillation with the desired ¹²⁹Xe isotope having significant 26.4% natural abundance. Spin-exchange optical pumping comprises: i) a noble gas with non-zero spin state is placed in an applied external magnetic field. This results in the creation of different energy levels for different nuclear spin states (nuclear Zeeman effect) ii) a laser optical pumping process is used to cause polarization of the electron spin states of a vapor alkali metal, often Rb. Circularly polarized infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in the vaporized alkali metal. The external magnetic field imposes an angular momentum selection rule on the hyperfine electron states of the Rb in order to generate ground state polarization of the Rb electrons. iii) The noble gas and polarized alkali metal vapour are housed in gas phase within the same vessel. Angular momentum is transferred from the alkali metal electrons to the noble gas nuclei through gas-phase collisions. Nitrogen is used as a quenching gas, which improves optical pumping efficiency by preventing the fluorescence of the polarized alkali metal. The alkali metal is continuously optically pumped, and continues to transfer its polarization to the noble gas nucleus to increase the population of the nuclear spin up state. Relaxation does occur but the process accumulates an increase population of spin up nuclear spin states. Relaxation of nuclear polarization and a return to the thermal equilibrium populations starts to occur when illumination is stopped. In order to use hyperpolarized noble gases in applications such as lung imaging, the gas must be transferred from the experimental setup to a patient. As soon as the gas is no longer undergoing spin exchange collisions, the degree of hyperpolarization begins to decrease until thermal equilibrium is reached. However, the hyperpolarization must last long enough to transfer the gas to the patient and obtain an image. FIG 1 illustrates an example of a polarizer apparatus 10. In this example, Rb is used as the alkali metal and ¹²⁹Xe is used as the noble gas isotope. However, in other examples, different alkali metals can be used and/or different noble gases can be used (e.g.³He). The apparatus 10 is therefore a Rb ¹²⁹Xe polarizer system. The apparatus 10 comprises a spin-exchange optical pump (SEOP) 20. The SEOP 20 comprises: i) magnetic field coils 30 for producing a static magnetic field 32 for enabling the nuclear Zeeman effect; ii) an optical cell 50 comprising Rb; iii) an oven 46 for heating the optical cell and maintaining the Rb in the vapor state; iv) a laser 42 tuned to excite Rb electrons; v) optics 44 for circularly polarizing the laser light and directing it into the optical cell 50; vi) an input 53 of the optical cell 50 for providing ¹²⁹Xe gas to the optical cell 50; vii) an output 54 of the optical cell 50 for extracting hyperpolarized ¹²⁹Xe gas from the optical cell 50; viii) a controller 60 for controlling the SEOP 20. The SEOP 20 operates to cause polarization of the electron spin states of vapor Rb. Circularly polarized infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in vaporized alkali metal. The external magnetic field ensures that circularly polarized light selectively pumps the Rb electrons into a given angular momentum hyperfine state.¹²⁹Xe and the polarized alkali metal vapour are housed in gas phase within the same vessel, the optical cell 50. Angular momentum is transferred from the alkali metal electrons to the noble gas nuclei through gas-phase collisions. Nitrogen is used as a quenching gas, which prevents the fluorescence of the polarized alkali metal. The alkali metal, Rb, is continuously optically pumped, and continues to transfer its polarization to the noble gas ¹²⁹Xe nucleus to increase the population of the nuclear spin up state. The magnetic field coils 30 produce a single applied external magnetic field Bo used in both the creation of different energy levels for different nuclear spin states (nuclear Zeeman effect) and for imposing an angular momentum selection rule on the hyperfine electron states of the Rb in order to generate ground state polarization of the Rb electrons. FIG 4 illustrates an example of a magnetic field created by the magnetic field coils 30. The magnetic field coils are aligned with a longitudinal axis of the apparatus. Each magnetic field coil is parallel to a transverse plane that is orthogonal to the longitudinal axis. The magnetic field 32 is spatially homogeneous within the homogeneous magnetic field volume 34. The magnetic field 32 does not change, or does not change significantly, with position within the homogeneous magnetic field volume 34. A non- significant change is a change less than 5% or 10%. The magnetic field lines are straight and parallel within the homogeneous magnetic field volume, there is zero curl. FIG 4 illustrates that the magnetic field 32 does not change, or does not change significantly, with distance along a direction orthogonal to the longitudinal axis within the homogeneous magnetic field volume 34. FIG 5 illustrates that the magnitude of the magnetic field 32 does not change, or does not change significantly, with distance along the longitudinal axis within the homogeneous magnetic field volume 34. In the examples illustrated, the magnetic field coils 30 have a four-coil arrangement and are powered as an electromagnet. However, different numbers of coils can be used. In some examples, the coils 30 are square coils. In some examples the square coils have a side length of ~100- 60 cm. The longitudinal spacing between the four coils 30 is optimized for field homogeneity. The longitudinal spacings between the coils 30 are defined such that a/d=0.128 106 and b/d=0.505 492, where a is the distance from the center to the inner coils and b is the distance from the center to the outer coils and d is the side lengths of the coils. The nominal B0 field created by the field coil arrangement 30 can be maintained at 24 mT, corresponding to a 28 kHz resonance frequency for ¹²⁹Xe nuclei. However, other field strengths can be maintained. The optical cell 50 is an enclosed volume loaded with Rb. The optical cell 50 can, for example, be formed from borosilicate glass, In the example illustrated in FIG 2, the optical cell has a cylindrical shape. It has a length of 40cm and a diameter of 7.5cm giving a volume of 1767 cm³. This volume is loaded with ~1 g Rb. The optical cell 50 is placed (not necessarily centered) inside the coils 30 within the homogeneous magnetic field volume 34, as illustrated in FIG 4. The optical cell 50 is placed in an oven 46 which heats the optical cell 50 and its contents. This vaporizes the Rb at temperatures > 40^oC The oven 46 can for example be a ceramic forced-air oven. It can comprise an antireflection glass window at its longitudinal end face nearest the laser 42 for the laser light to pass through. The oven is heated using an air compressor and heating element combination. A compressor forces air through a heating system into the oven 46. A thermocouple in the oven 46 provides a temperature feedback signal that is used to control the compressor system and/or heating system. Using this feedback loop it is possible to maintain a constant temperature of up to 170^oC. The laser 42 is tuned for Rb valence electron D1 line (795 nm). The laser 42 can, for example, be a diode laser. In the example illustrated, the laser 42 is an air-cooled laser diode array providing ~75 W of laser light tuned to the rubidium valence electron D1 line (795 nm). The center wavelength is 794.77nm with a linewidth 0.30nm. The laser 42 is in this example a continuous wave laser. The optics 44 comprise beam shaping optics and polarization optics. The beam shaping optics in this example comprise a beam expander to output a beam diameter that matches a diameter of the optical cell (7.5-cm). For example a focal telescope can be used as the beam expander. In this way, an entirety of the optical cell 50 is illuminated by laser light to provide the greatest polarization possible. The polarisation optics circularly polarise the laser light before it enters the optical cell 50. A circular polarizer can, for example, be formed from a combination of a linear polarizer and a quarter wave plate. The circularly polarised light is a homogeneous circular beam that is incident on the oven window. Stray laser light is mitigated by enclosing the beam, optics and oven with front and back plates, however, laser safety goggles are advised to be worn as an additional precautionary measure. The input 53 comprises a gas manifold that directs a gas flow from a cylinder into the optical cell through a pressure regulator. The gas comprises ¹²⁹Xe. A suitable gas mixture contains 3% isotopically enriched xenon (86% ¹²⁹Xe), 10% N₂ and 87% He. The gas pressure can, for example, be 200 kPa. During continuous-flow operation, the xenon gas mixture is flowed through the cell in a direction counter to the incident laser light at a rate of 1000 sccm (standard cubic cm per minute), corresponding to a xenon flow rate of 1800 mL/h. The polarised gas exits the cell via exit 54. There are two methods of spin-exchange optical pumping: batch mode and continuous-flow mode. Batch mode does not require cryogenic accumulation of xenon, whereas continuous-flow mode does. Batch mode yields higher ¹²⁹Xe polarisation (50%–90%) with slower xenon production rates (100–1000 mL/h) and continuous-flow mode yields lower ¹²⁹Xe polarisation (20%–50%) but with faster production rates (1000–3500 mL/h). In this example, continuous flow mode is used. The faster production rates currently obtainable on continuous-flow hyperpolarisers offer increased potential for routine clinical ¹²⁹Xe MRI when compared to batch mode systems given typical xenon throughput demands. In the illustrated apparatus 10, the hyperpolarized ¹²⁹Xe is cryogenically accumulated. The gas mixture exiting the optical cell 50 is passed through spiral glassware suspended within a liquid-nitrogen cryostat. A permanent magnet is used to provide a magnetic field that helps maintain hyperpolarization. In this example, the spiral glassware is held within a field of from a 250 mT NdBFe horseshoe permanent magnet and submerged in a Dewar containing liquid N2. The ¹²⁹Xe freezing point of ~ 161°K is well above the ~ 77°K temperature of liquid nitrogen. The only gas trapped from the exiting gas mixture at liquid nitrogen temperature is the hyperpolarized ¹²⁹Xe.¹²⁹Xe in the flowing gas mixture is cryogenically separated from He and N₂, which are removed as exhaust gases through a vacuum line. The relaxation time of polarized xenon in the frozen state at 77°K at 250 mT is T₁ ~2.5 hours The gas flow rate from the optical cell 50 is controlled using a mass flow meter in line with a diaphragm vacuum pump which generates a downstream pressure of ~200 Pa. Once sufficient hyperpolarized ¹²⁹Xe snow is deposited in the spiral glassware, it is defrosted by immersion in room-temperature water. The frozen hyperpolarized ¹²⁹Xe sublimates to the gas phase and flows into a bag 80, for example a polyvinyl fluoride film bag. A B1 field surface coil(s) of an NMR spectrometer can be secured to the outside surface of the optical cell 50, this B1 field is orthogonal to the B0 field direction. The spectrometer B1 field coil(s) allow RF excitation pulses to be produced to change the population of the split nuclear spin states of ¹²⁹Xe and then to detect the free induction decay of the transverse signal produced by the polarized nuclear spins. The optical cell spectrometer field coil can therefore be used to test the size of the hyperpolarization of the ¹²⁹Xe as it is directly proportional to the longitudinal and transverse components of the induced magnetisation in the B1 coil detected during free induction decay. When the ¹²⁹Xe is sufficiently polarized the hyperpolarized can be removed from the optical cell 50 via the output 54 as described above. In order to further increase polarization, a reflective mirror can be placed behind the optical cell 50 in order to reflect the laser light back through the optical cell 50. A light detector can be placed behind the optical cell 50 to measure laser light absorption by Rb. The difference in the spectrum between a room temperature spectrum and a spectrum taken while the cell is heated can be used to calculate and estimated Rb polarization value. In at least some examples, the apparatus 10 is a self-contained, stand-alone and transportable apparatus that can quickly be installed on-site or be taken temporarily to another facility where there is interest in using hyperpolarised ¹²⁹Xe. The apparatus 10 is compact with no need for additional site infrastructure (i.e. it should run on mains electricity without the need for compressed air supplies. It can provide doses of ¹²⁹Xe for high-quality clinical lung MRI in <20 min. The apparatus 10 can be manually transported in a lightweight van and rapidly installed on a small footprint in a hospital setting. The apparatus 10 can for example occupy a volume of 1.34m length x 0.72m width x 1.2m height, and is powered by 3 × 240 V / 50 Hz AC mains sockets, and weighs less than 150 kg. Relaxation of the hyperpolarization of the ¹²⁹Xe starts to occur after removal of the ¹²⁹Xe from the SEOP cell 20. In order to concentrate the xenon mixture form e.g.3% to 100% the gas mixture needs can be cryogenically distilled to separate out the helium and nitrogen buffer gases. The rate of relaxation increases when the frozen ¹²⁹Xe is thawed as the xenon undergoes a phase transition. The rate of relaxation could be controlled by storing the ¹²⁹Xe in a frozen or cooled state. However, this requires specialist skill and equipment. It does not lend itself well to easy storage and immediate use within a hospital environment. It would be desirable to store the hyperpolarized ¹²⁹Xe at ambient (room temperature) without the cryogenic distillation and also to store cryogenically distilled gas doses. The rate of relaxation decreases when the energy gap between nuclear spin states is increased (other parameters remaining the same). The apparatus 10 uses the nuclear Zeeman effect to maintain a constant energy gap between the nuclear spin states of the hyperpolarized ¹²⁹Xe during storage. The apparatus 10 comprises a storage facility 70 that stores the hyperpolarized ¹²⁹Xe in a constant magnetic field. The storage facility 70 is an in-situ storage facility that is an integral part of the polarizer apparatus 10. The storage facility 70 has an integral B1 RF coil for measuring the polarisation (dose equivalence) of the dispersed gas dose. The inventors have also realized that magnetic flux change has a deleterious effect on the hyperpolarization of ¹²⁹Xe. The inventors have therefore taken steps to store the hyperpolarized ¹²⁹Xe in a homogenous magnetic field. The inventors have also taken steps to enable (and encourage) the movement of the hyperpolarized ¹²⁹Xe into storage within the homogenous magnetic field in a manner that avoids, prevents or discourages crossing of magnetic flux and, in some examples, in a manner that directs, forces or encourages movement parallel to the magnetic flux. The inventors have created a polarizer apparatus 10 that comprises an integrated storage facility for ¹²⁹Xe that reduces the rate of relaxation. As illustrated in FIG 1, the polarizer apparatus 10 comprises: a spin-exchange optical pumping system 20 for hyperpolarization of a noble gas 52 (e.g.¹²⁹Xe), the spin-exchange optical pumping system 20 comprising a field coil arrangement 30 configured to provide, within a static homogeneous-magnetic- field-volume 34, a static homogeneous magnetic field 32 used by the spin-exchange optical pumping system 20; and a storage retainer 70 for storing hyperpolarized noble gas 52, in gaseous phase, output from the spin-exchange optical pump 20 wherein the storage retainer 70 is located outside an oven 46 of the spin-exchange optical pump 20 and inside the static homogeneous-magnetic-field-volume 34 and wherein the storage retainer 70 provides an associated storage volume 71 for storing and retaining hyperpolarized noble gas 52 produced by the spin-exchange optical pump 20. The storage system has an integrated B1 RF coil for measuring the signal of the dispensed gas dose. In the example illustrated, the storage volume 71 is neither heated nor cooled and, in use, is at ambient room temperature. The storage retainer 70 is thermally isolated from the oven 46. In the example illustrated, a whole of the storage volume 71 is outside the oven 46 of the spin exchange optical pump 20 and inside the static homogeneous-magnetic-field- volume 34. The storage retainer 70 is positioned in a space between an oven 46 of the spin exchange optical pump 20 and within the field coil arrangement 30. In the example illustrated, the field coils of the field coil arrangement 30 are centered on a longitudinal (horizontal) axis, lie in parallel transverse (vertical) planes orthogonal to the longitudinal (horizontal) axis, and define a field coil volume that has a constant cross-sectional area defined by a cross-sectional area of the field coils in the parallel transverse (vertical) planes and a length defined by a maximum distance parallel to the longitudinal (horizontal) axis between field coils of the field coil arrangement 30, wherein an oven 46 of the spin exchange optical pump 20 extends parallel to the longitudinal (horizontal) axis within the field coil volume and wherein the storage retainer 70 is positioned within the field coil volume, in a transverse relationship to (vertically above) at least a portion of the oven 46. In the example illustrated, the polarizer apparatus 10 is housed on a transportation trolley 100 that comprises a chassis with wheels. The orientation of the polarizer apparatus 10 in the trolley 100 is such that the longitudinal axis is horizontal and the transverse plane, orthogonal to the longitudinal axis is vertical. However, other orientations are possible. In the example illustrated, the hyperpolarized noble gas 52 (e.g.¹²⁹Xe) output from the spin-exchange optical pump 20 is collected in a bag 80 as described above. The bag 80 containing the dose of hyperpolarized noble gas 52 (e.g. ¹²⁹Xe) is stored in the storage retainer 70. In the example illustrated, the polarizer apparatus 10 provides an access route 74 to the storage retainer 70 that enables the user-placement of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) within the storage retainer 70 in a direction that is parallel to the static homogeneous-magnetic-field. In the example illustrated, the access route 74 is in a direction substantially parallel to the longitudinal axis and has enough clearance (space) to allow passage of the bag 80 of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) to the storage retainer 70. In the example illustrated, the storage volume 71 of the storage retainer 70 is accessed via an aperture 72. In some examples, the aperture has a door. FIG 3 illustrates an example of a storage retainer 70 for storing hyperpolarized noble gas 52, in gaseous phase, output from the spin-exchange optical pump 20. A container 80, for example a bag, holds hyperpolarized noble gas (e.g.¹²⁹Xe). The container 80 is moved into the storage retainer 70, through aperture 72, along access route 74. The polarizer apparatus 10 provides a non-ferromagnetic enclosure 76. The enclosure 76 is at least a partial enclosure in the sense that it can have permanently open apertures 72 to allow ingress and egress of the container 80 or for other purposes such as mass reduction. For example, the enclosure can, in some examples, be a metallic (Faraday) cage. The enclosure 76 for the storage retainer 70 is designed to prevent access to the storage retainer 70 that is across the magnetic flux lines of the static homogeneous magnetic field 32. The polarizer apparatus 10 comprises a stored-gas nuclear magnetic spectrometer for analysis of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) when stored in the storage retainer 70. The stored-gas nuclear magnetic spectrometer is connected to the radio frequency B1 field coil arrangement 96. The radio frequency B1 field coil arrangement 96 of the stored-gas nuclear magnetic spectrometer is integrated with the storage retainer 70. In this example it is a part of the enclosure 76. As illustrated in FIG 1, the enclosure 76 for the storage retainer 70 is positioned to prevent access to the storage retainer 70 via a route between adjacent field coils of the field coil arrangement 30. In FIG 1, the apparatus 10 can comprise a production nuclear magnetic spectrometer for analysis of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) within an optical cell 50 of the spin exchange optical pump 20. The production nuclear magnetic spectrometer comprises a radio frequency field coil arrangement. The radio frequency field coil arrangement of the production nuclear magnetic spectrometer is positioned in orthogonal relationship to the radio frequency field coil arrangement 96 of the stored-gas nuclear magnetic spectrometer. This enables simultaneous operation of the stored-gas nuclear magnetic spectrometer and the production nuclear magnetic spectrometer. For example a field coil(s) of the production nuclear magnetic spectrometer can lie in a substantially horizontal plane and the field coil(s) of the stored-gas nuclear magnetic spectrometer can lie in a substantially vertical plane, or vice versa. The field coil(s) of the production nuclear magnetic spectrometer can lie on top of the optical cell 50 inside the oven 46. The B1 field coil(s) of the stored-gas nuclear magnetic spectrometer can lie on an interior wall of the enclosure 76. FIG 4 illustrates the static magnetic field 32 and its associated static homogeneous magnetic field volume 34. The storage retainer 70 is sized and positioned so that a whole of the storage volume 71 is wholly within the static homogeneous-magnetic-field-volume 34. Thus the whole of the container 80, for example a bag, holding hyperpolarized noble gas (e.g.¹²⁹Xe) is wholly within the static homogeneous-magnetic-field-volume 34. A length of the storage volume 71 in the longitudinal direction parallel to the static homogeneous magnetic field 34 is less than a length of the static homogeneous- magnetic-field-volume 34 in the same direction. The oven 46 of the spin-exchange optical pumping system 20 and the optical cell 50 within are located inside the static homogeneous-magnetic-field-volume 34. The storage retainer 70 is located outside the oven 46 of the spin-exchange optical pump 20 and inside the static homogeneous-magnetic-field-volume 34. The access route 74 to the storage retainer 70 is along the static homogeneous- magnetic-field, and is in a longitudinal direction that is parallel to the static homogeneous-magnetic-field 32. In some examples, as illustrated in FIG 6, the polarizer apparatus 10 provides one or more guides 78 that guide user placement of the hyperpolarized noble gas 52 (e.g. ¹²⁹Xe) at the storage retainer 70. The one or more guides 78 are shaped to encourage controlled ingress of a container 80 comprising hyperpolarized noble gas 52 (e.g.¹²⁹Xe) to the storage retainer 70 and controlled egress of the container 80 comprising hyperpolarized noble gas 52 (e.g. ¹²⁹Xe) from the storage retainer 70. The one or more guides 78 are shaped to follow field lines of the magnetic field 32 produced by the field coil arrangement 30. As illustrated in FIG 7 and 8, in at least some examples, the storage retainer 70 comprises a retainer dish 90 having raised sides. The storage retainer 70, in these examples, comprises a concave retainer dish 90. The raised sides create a local potential energy minimum that retains the container 80, for example a bag, of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) within the storage retainer 70. It also prevents the container 80 rolling out of the storage retainer 70. The retainer dish 90 can be a passive retainer that relies upon the local potential energy minimum to retain the container 80. However, in other examples, the retainer dish 90 is additionally associated with an active retainer that applies a force to hold the container 80, for example tension in extended elastic constraints to hold a bag 80 in position. The retainer dish 90 has associated constraints 94 for holding a container 80, for example a bag, of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) on the retainer dish 90. The associated constraints 94 for holding the container 80 of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) on the retainer dish 90 are elastically deformable. The elastic deformation of the constraints 94 provides a holding force for holding the container (e.g. bag) 80 of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) on the retainer dish 90. The associated constraints 94 for holding a bag 80 of hyperpolarized noble gas 52 (e.g.¹²⁹Xe) on the retainer dish 90 comprises a ring 92 for placement on top of the bag 80 of hyperpolarized noble gas 52 and elastic members 94 attached to the ring 92 and sized to be extended when the ring 92 is placed on top of the bag of hyperpolarized noble gas 52 while it rests in the retainer dish 90 to provide an elastic force that pulls the ring 92 towards the retainer dish 90 for holding the bag 80 of hyperpolarized noble gas 52 on the retainer dish 90. As illustrated in FIG 8, a radio frequency field coil arrangement 96 of a stored-gas nuclear magnetic spectrometer can be formed as part of the retainer dish 90. FIG 9 illustrates an example of a method 500 for storing hyperpolarized noble gas 52 (e.g.¹²⁹Xe), in gaseous phase. At block 502, the method 500 comprises producing a hyperpolarized noble gas 52 (e.g. ¹²⁹Xe), using a spin-exchange optical pump 20 comprising a field coil arrangement 30 configured to provide, within a homogeneous-magnetic-field-volume 34, a static homogeneous magnetic field used by the spin-exchange optical pump 20. At block 502, the method 500 comprises providing a storage container for storing hyperpolarized noble gas 52 (e.g. ¹²⁹Xe), in gaseous phase, output from the spin- exchange optical pump 20 wherein the storage container is located outside the spin- exchange optical pump 20 and inside the homogeneous-magnetic-field-volume 34. Fig 10 illustrates an example of a controller 60 suitable for use in an apparatus 10. Implementation of a controller 60 may be as controller circuitry. The controller 60 may be implemented in hardware alone, have certain aspects in software including firmware alone or can be a combination of hardware and software (including firmware). As illustrated in Fig 10 the controller 60 may be implemented using instructions that enable hardware functionality, for example, by using executable instructions of a computer program 406 in a general-purpose or special-purpose processor 402 that may be stored on a computer readable storage medium (disk, memory etc) to be executed by such a processor 402. The processor 402 is configured to read from and write to the memory 404. The processor 402 may also comprise an output interface via which data and/or commands are output by the processor 402 and an input interface via which data and/or commands are input to the processor 402. The memory 404 stores a computer program 406 comprising computer program instructions (computer program code) that controls the operation of the apparatus 10 when loaded into the processor 402. The computer program instructions, of the computer program 406, provide the logic and routines that enables the apparatus to perform the methods illustrated in the accompanying Figs. The processor 402 by reading the memory 404 is able to load and execute the computer program 406. As illustrated in Fig 11, the computer program 406 may arrive at the apparatus 10 via any suitable delivery mechanism 408. The delivery mechanism 408 may be, for example, a machine-readable medium, a computer-readable medium, a non- transitory computer-readable storage medium, a computer program product, a memory device, a record medium such as a Compact Disc Read-Only Memory (CD- ROM) or a Digital Versatile Disc (DVD) or a solid-state memory, an article of manufacture that comprises or tangibly embodies the computer program 406. The delivery mechanism may be a signal configured to reliably transfer the computer program 406. The apparatus 10 may propagate or transmit the computer program 406 as a computer data signal. The computer program instructions may be comprised in a computer program, a non- transitory computer readable medium, a computer program product, a machine- readable medium. In some but not necessarily all examples, the computer program instructions may be distributed over more than one computer program. Although the memory 404 is illustrated as a single component/circuitry it may be implemented as one or more separate components/circuitry some or all of which may be integrated/removable and/or may provide permanent/semi-permanent/ dynamic/cached storage. Although the processor 402 is illustrated as a single component/circuitry it may be implemented as one or more separate components/circuitry some or all of which may be integrated/removable. The processor 402 may be a single core or multi-core processor. References to ‘computer-readable storage medium’, ‘computer program product’, ‘tangibly embodied computer program’ etc. or a ‘controller’, ‘computer’, ‘processor’ etc. should be understood to encompass not only computers having different architectures such as single /multi- processor architectures and sequential (Von Neumann)/parallel architectures but also specialized circuits such as field- programmable gate arrays (FPGA), application specific circuits (ASIC), signal processing devices and other processing circuitry. References to computer program, instructions, code etc. should be understood to encompass software for a programmable processor or firmware such as, for example, the programmable content of a hardware device whether instructions for a processor, or configuration settings for a fixed-function device, gate array or programmable logic device etc. The blocks illustrated in the accompanying Figs may represent steps in a method and/or sections of code in the computer program 406. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some blocks to be omitted. Where a structural feature has been described, it may be replaced by means for performing one or more of the functions of the structural feature whether that function or those functions are explicitly or implicitly described. As used here ‘module’ refers to a unit or apparatus that excludes certain parts/components that would be added by an end manufacturer or a user. The storage retainer 70 can be a module. The retainer dish 90 can be a module. The whole of the apparatus 10 can be modular. The apparatus can be provided in an electronic device, for example, a mobile terminal, according to an example of the present disclosure. It should be understood, however, that a mobile terminal is merely illustrative of an electronic device that would benefit from examples of implementations of the present disclosure and, therefore, should not be taken to limit the scope of the present disclosure to the same. While in certain implementation examples, the apparatus can be provided in a mobile terminal, other types of electronic devices, such as, but not limited to: mobile communication devices, hand portable electronic devices, wearable computing devices, portable digital assistants (PDAs), pagers, mobile computers, desktop computers, televisions, gaming devices, laptop computers, cameras, video recorders, GPS devices and other types of electronic systems, can readily employ examples of the present disclosure. Furthermore, devices can readily employ examples of the present disclosure regardless of their intent to provide mobility. The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one...” or by using “consisting”. In this description, the wording ‘connects’, ‘couple’ and ‘communication’ and their derivatives mean operationally connected/coupled/in communication. It should be appreciated that any number or combination of intervening components can exist (including no intervening components), i.e., so as to provide direct or indirect connection/coupling/communication. Any such intervening components can include hardware and/or software components. As used herein, the term "determine/determining" (and grammatical variants thereof) can include, not least: calculating, computing, processing, deriving, measuring, investigating, identifying, looking up (for example, looking up in a table, a database or another data structure), ascertaining and the like. Also, "determining" can include receiving (for example, receiving information), accessing (for example, accessing data in a memory), obtaining and the like. Also, " determine/determining" can include resolving, selecting, choosing, establishing, and the like. In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example. Although examples have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims. Features described in the preceding description may be used in combinations other than the combinations explicitly described above. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not. The term ‘a’, ‘an’ or ‘the’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a/an/the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’, ‘an’ or ‘the’ with an exclusive meaning then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning. The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and also to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result. In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as described. The above description describes some examples of the present disclosure however those of ordinary skill in the art will be aware of possible alternative structures and method features which offer equivalent functionality to the specific examples of such structures and features described herein above and which for the sake of brevity and clarity have been omitted from the above description. Nonetheless, the above description should be read as implicitly including reference to such alternative structures and method features which provide equivalent functionality unless such alternative structures or method features are explicitly excluded in the above description of the examples of the present disclosure. Whilst endeavoring in the foregoing specification to draw attention to those features believed to be of importance it should be understood that the Applicant may seek protection via the claims in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not emphasis has been placed thereon. In this description words derived from ‘polarise’ (UK spelling) can be interchanged with words derived from ‘polarize’ (US spelling). In this description words derived from ‘centre’ (UK spelling) can be interchanged with words derived from ‘center’ (US spelling). In this description words derived from ‘vapour’ (UK spelling) can be interchanged with words derived from ‘vapor’ (US spelling). In this description words with an ending ‘isation’ (UK spelling) can be interchanged with words with an ending ‘ization’ (US spelling). I/we claim:

Claims

CLAIMS 1. A polarizer apparatus comprising: a spin-exchange optical pump for hyperpolarization of a noble gas, the spin- exchange optical pump comprising a field coil arrangement configured to provide, within a static homogeneous-magnetic-field-volume, a static homogeneous magnetic field used by the spin-exchange optical pump; and a storage retainer for storing hyperpolarized noble gas, in gaseous phase, output from the spin-exchange optical pump wherein the storage retainer is located outside an oven of the spin-exchange optical pump and inside the static homogeneous-magnetic-field-volume and wherein the storage retainer provides an associated storage volume for storing and retaining hyperpolarized noble gas produced by the spin-exchange optical pump.

2. A polarizer apparatus as claimed in claim 1, wherein the storage retainer is sized and positioned so that a whole of the storage volume is wholly within the static homogeneous-magnetic-field-volume.

3. A polarizer apparatus as claimed in claim 1 or 2, wherein a length of the storage volume in a direction parallel to the static homogeneous magnetic field is less than a length of the static homogeneous-magnetic-field-volume in the same direction.

4. A polarizer apparatus as claimed in any preceding claim, wherein the storage volume is neither heated not cooled and, in use, is at ambient room temperature.

5. A polarizer apparatus as claimed in any preceding claim, wherein a whole of the storage volume is outside the oven of the spin exchange optical pump and inside the static homogeneous-magnetic-field-volume.

6. A polarizer apparatus as claimed in any preceding claim, wherein the storage retainer is positioned in a space between an oven of the spin exchange optical pump and field coils of the field coil arrangement.

7. A polarizer apparatus as claimed in any preceding claim, wherein field coils of the field coil arrangement are centered on a longitudinal axis, lie in parallel transverse planes orthogonal to the longitudinal axis, and define a field coil volume that has a constant cross-sectional area defined by a cross-sectional area of the field coils in the parallel transverse planes and a length defined by a maximum distance parallel to the longitudinal axis between field coils of the field coil arrangement, wherein an oven of the spin exchange optical pump extends parallel to the longitudinal axis within the field coil volume and wherein the storage retainer is positioned within the field coil volume, in a transverse relationship to at least a portion of the oven.

8. A polarizer apparatus as claimed in any preceding claim, wherein field coils of the field coil arrangement are centered on a longitudinal horizontal axis, lie in parallel transverse vertical planes orthogonal to the longitudinal vertical axis, and define a field coil volume that has a constant cross-sectional area defined by a cross-sectional area of the field coils in the parallel transverse vertical planes and a length defined by a maximum distance parallel to the longitudinal horizontal axis between field coils of the field coil arrangement, wherein an oven of the spin exchange optical pump extends parallel to the longitudinal horizontal axis within the field coil volume and wherein the storage retainer is positioned within the field coil volume, vertically above at least a portion of the oven.

9. A polarizer apparatus as claimed in any of claim 5 to 8, wherein the storage retainer is thermally isolated from the oven.

10. A polarizer apparatus as claimed in any preceding claim, wherein the polarizer apparatus provides an access route to the storage retainer that enables the placement of hyperpolarized noble gas within the storage retainer in a direction that is parallel to the static homogeneous-magnetic-field.

11. A polarizer apparatus as claimed in any preceding claim, wherein the polarizer apparatus provides an access route into and through the static homogeneous- magnetic-field, in a longitudinal direction that is parallel to the static homogeneous- magnetic-field, to the storage retainer.

12. A polarizer apparatus as claimed in any preceding claim, wherein the polarizer apparatus provides an at least partial non-ferromagnetic enclosure for the storage retainer that prevents access to the storage retainer that is across the static homogeneous-magnetic-field.

13. A polarizer apparatus as claimed in any preceding claim, wherein the polarizer apparatus provides an at least partial non-ferromagnetic enclosure for the storage retainer that prevents access to the storage retainer via a route between adjacent field coils of the field coil arrangement.

14. A polarizer apparatus as claimed in any preceding claim, wherein the polarizer apparatus provides one or more guides that guide user placement of the hyperpolarized noble gas at the storage retainer.

15. A polarizer apparatus as claimed in claim 14, wherein the one or more guides are shaped to encourage controlled ingress of hyperpolarized noble gas bag to the storage retainer and controlled egress of hyperpolarized noble gas bag from the storage retainer.

16. A polarizer apparatus as claimed in claim 14 or 15, wherein the one or more guides are shaped to follow field lines of the field coil arrangement.

17. A polarizer apparatus as claimed in any preceding claim, comprising a stored-gas nuclear magnetic spectrometer for analysis of hyperpolarized noble gas when stored in the storage retainer.

18. A polarizer apparatus as claimed in claim 17, comprising a production nuclear magnetic spectrometer for analysis of hyperpolarized noble gas within an optical cell of the spin exchange optical pump, wherein a radio frequency field coil arrangement of the production nuclear magnetic spectrometer is positioned in orthogonal relationship to a radio frequency field coil arrangement of the stored-gas nuclear magnetic spectrometer to enable simultaneous operation of the stored-gas nuclear magnetic spectrometer and the production nuclear magnetic spectrometer.

19. A polarizer apparatus as claimed in claim 17 or 18, wherein a radio frequency field coil arrangement of the stored-gas nuclear magnetic spectrometer is integrated with the storage retainer.

20. A polarizer apparatus as claimed in any preceding claim, wherein the storage retainer comprises a retainer dish wherein the retainer dish has raised sides; and/or the retainer dish is concave; and/or the retainer dish has associated constraints for holding a bag of hyperpolarized noble gas on the retainer dish; and/or the retainer dish has associated constraints for holding a bag of hyperpolarized noble gas on the retainer dish wherein the associated constraints for holding a bag of hyperpolarized noble gas on the retainer dish are elastically deformable, wherein the elastic deformation of the constraints provides a holding force for holding the bag of hyperpolarized noble gas on the retainer dish; and/or the retainer dish has associated constraints for holding a bag of hyperpolarized noble gas on the retainer dish, wherein the associated constraints for holding a bag of hyperpolarized noble gas on the retainer dish comprises a ring for placement on top of the bag of hyperpolarized noble gas and elastic members attached to the ring and sized to be extended when the ring is placed on top of the bag of hyperpolarized noble gas to provide an elastic force that pulls the ring towards the retainer dish for holding the bag of hyperpolarized noble gas on the retainer dish.

21. A polarizer apparatus as claimed in claim 20, wherein a radio frequency field coil arrangement of a stored-gas nuclear magnetic spectrometer is formed as part of the retainer dish.

22. A polarizer apparatus as claimed in any preceding claim, wherein the spin- exchange optical pump comprises: a source of circularly polarized light; an optical cell for housing an alkali metal and positioned for illumination by the source of circularly polarized light; an oven for heating the optical cell to a temperature sufficient to vaporize the alkali metal; an input for ingress of a gas comprising a noble gas; an output for egress of a gas comprising the noble gas after hyperpolarization.

23. A polarizer apparatus as claimed in any preceding claim, wherein the spin- exchange optical pump is configured for the hyperpolarization of ¹²⁹Xe and/or wherein the spin-exchange optical pump comprises a laser configured for Rb electron spin polarization.

24. A polarizer apparatus as claimed in any preceding claim, configured for cryogenic collection and bagging of hyperpolarized noble gas prior to storage at the storage retainer and/or housed on a transportation trolley.

25. A method comprising: Producing a hyperpolarized noble gas, using a spin-exchange optical pump comprising a field coil arrangement configured to provide, within a homogeneous- magnetic-field-volume, a static homogeneous magnetic field used by the spin- exchange optical pump; Providing a storage container for storing hyperpolarized noble gas, in gaseous phase, output from the spin-exchange optical pump wherein the storage container is located outside the spin-exchange optical pump and inside the homogeneous- magnetic-field-volume.