Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/381—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets
  • G01R33/3815—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets with superconducting coils, e.g. power supply therefor
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/3804—Additional hardware for cooling or heating of the magnet assembly, for housing a cooled or heated part of the magnet assembly or for temperature control of the magnet assembly
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F6/00—Superconducting magnets; Superconducting coils
  • H01F6/04—Cooling
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F6/00—Superconducting magnets; Superconducting coils
  • H01F6/06—Coils, e.g. winding, insulating, terminating or casing arrangements therefor

Definitions

• the following relates generally to the superconducting magnet arts, magnetic resonance imaging (MRI) arts, thermal management arts, and related arts.
• the superconducting windings are immersed in liquid helium (LHe) contained in a LHe reservoir surrounded by a vacuum jacket.
• a high conductivity thermal shield of sheet material is disposed in the vacuum jacket to surround the LHe reservoir.
• the vacuum is drawn and the LHe reservoir is filled with LHe.
• a cold head is used to provide refrigeration to the LHe vessel.
• the first stage of the cold head penetrates through into the vacuum volume, and the first stage cold station is connected to the thermal shield by a high thermal conductance link that connects with a thermal bus attached to the thermal shield.
• the second stage of the cold head continues into the LHe volume to be disposed in the gaseous He overpressure above the LHe level in the LHe reservoir.
• the cold head is turned off and the magnet is shipped with the LHe charge loaded.
• the vacuum jacket is relied upon to provide sufficient thermal insulation to maintain the LHe charge in its liquid state during shipping.
• a superconducting magnet in one disclosed aspect, includes a liquid helium reservoir, superconducting magnet windings disposed in the liquid helium reservoir, and a vacuum jacket surrounding the liquid helium reservoir.
• a cold head passes through the vacuum jacket.
• the cold head has a warm end welded to an outer wall of the vacuum jacket and a cold station disposed in the liquid helium reservoir.
• a heat exchanger is disposed inside the vacuum jacket and secured to or integral with the cold head.
• the heat exchanger includes a fluid passage having an inlet in fluid communication with the liquid helium reservoir and having an outlet in fluid communication with ambient air.
• gas helium flows from the liquid helium reservoir to ambient air via the heat exchanger, thereby cooling the non-operating cold head.
• the flowing of gas helium from the liquid helium reservoir to ambient air via the heat exchanger reduces helium boil-off during the transport.
• a cold head comprises: a first stage section having a warm end and an opposite end defining a first stage cold station; a second stage section having a proximate end connected with the first stage cold station and a distal end defining a second stage cold station; and a heat exchanger secured to or integral with at least the first stage section.
• the heat exchanger includes a fluid passage having an inlet and an outlet.
• Another advantage resides in providing a superconducting magnet with reduced likelihood of quench during extended intervals over which the cold head is shut off.
• Another advantage resides in providing a superconducting magnet that can be shipped over longer distances with a LHe charge.
• Another advantage resides in providing a superconducting magnet that can have its cold head shut off for more extended time intervals to facilitate longer-distance shipping, extended maintenance, or so forth.
• Another advantage resides in providing a superconducting magnet with reduced liquid helium evaporation during intervals over which the cold head is turned off or is non- operational.
• a given embodiment may provide none, one, two, more, or all of the foregoing advantages, and/or may provide other advantages as will become apparent to one of ordinary skill in the art upon reading and understanding the present disclosure.
• the invention may take form in various components and arrangements of components, and in various steps and arrangements of steps.
• the drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.
• FIGURE 1 diagrammatically illustrates a side sectional view of a magnetic resonance imaging (MRI) system including a cold head with a heat exchanger secured to or integral with the first stage of the cold head.
• FIGURE 2 diagrammatically illustrates an enlarged view of the portion of the side sectional view of FIGURE 1 depicting the cold head and heat exchanger.
• MRI magnetic resonance imaging
• FIGURE 3 diagrammatically illustrates the enlarged view of FIGURE 2 with a variant embodiment in which the heat exchanger is secured to or integral with both the first stage and the second stage of the cold head.
• FIGURE 4 diagrammatically illustrates a process for charging the superconducting magnet of FIGURE 1 with liquid helium (LHe) and transporting it from the factory to a destination.
• LHe liquid helium
• the cold head is turned off and the MR magnet is shipped, with the LHe charge loaded and the vacuum drawn, to the destination. If shipped by air, the cold head remains off during the entire shipping time interval. If transported by ship, the MR magnet may be refrigerated; however, even in this case there are extended time intervals during loading and offloading and trucking to and from the shipyard during which the cold head is shut off.
• the LHe slowly boils off, e.g. via a provided vent path such as a helium vent bellow.
• the cold head typically comprises a stainless steel cylinder containing a motor- operated displacer executing a refrigeration cycle, e.g. using gas helium as a working cryogenic fluid, and an internal copper screen.
• the cold head installed on the magnet passes through the vacuum jacket, and has a warm end welded to an outer wall of the vacuum jacket and a cold station disposed in the liquid helium reservoir.
• the intermediate cold station is commonly referred to as the first stage cold station
• the cold station disposed in the liquid helium reservoir is referred to as the second stage cold station.
• the first stage cold head is at a higher temperature than the second stage cold station (though still well below ambient temperature).
• the refrigeration cycle operates to chill the stainless steel cylinder to cryogenic temperature, e.g. -4K-10K in some commercially available cold heads, with the distal end in the liquid helium reservoir being chilled to the coldest temperature (thus forming the second stage cold station).
• the stainless steel cylinder, and particularly the first and second stage cold stations warms up. This creates a thermal leakage path that can conduct heat from the warm end welded to the outer wall of the vacuum jacket to the second stage cold station in the liquid helium reservoir, thereby heating the LHe. This results in more rapid boiloff of LHe.
• the thermal leakage path formed by the stopped cold head can therefore limit shipping distance or otherwise constrain shipping options.
• a heat exchanger is secured to the cold head (or, alternatively, may be formed integral with the cold head, e.g. integrated into the stainless steel cylinder).
• the heat exchanger has an inlet connected via a pipe or other fluid conduit to the gas helium overpressure in the LHe reservoir, and an outlet that discharges into the ambient.
• gas He which, within the LHe reservoir, is at a low temperature close to the boiling point of LHe, i.e. ⁇ 4K
• a side sectional view is shown of a magnetic resonance imaging (MRI) device 10, which employs a superconducting magnet.
• the magnet includes superconducting windings 12 disposed in a liquid helium (LHe) reservoir 14 which is mostly filled with LHe; however, there is a gaseous helium (gas He) overpressure present above the LHe level 16.
• LHe liquid helium
• gas He gaseous helium
• the illustrative MRI device 10 employs a horizontal-bore magnet in which the superconducting magnet is generally cylindrical in shape and surrounds (i.e. defines) a horizontal bore 18; however, other magnet geometries are also contemplated.
• a vacuum jacket 20 surrounds the LHe reservoir 14.
• the vacuum jacket 20 includes an outer wall 22 and an inner wall 23.
• the illustrative inner wall 23 is shared between the LHe reservoir 14 and the vacuum jacket 20 (i.e. forms the boundary between the LHe reservoir 14 and vacuum jacket 20).
• the LHe reservoir and vacuum jacket can have separate walls at this boundary that are welded together or otherwise coincident).
• the vacuum jacket 20 further includes side walls 24, 25 or the like sufficient to provide vacuum-tight sealing of its ends.
• the vacuum volume contained by the vacuum jacket 20 is diagrammatically indicated in FIGURE 1 by hatching.
• a thermal shield 26 made of a sturdy thermally conductive material such as aluminum alloy sheet metal (or copper alloy sheet metal or some other high thermal conductivity sheet metal) is preferably disposed in the vacuum volume (that is, inside the vacuum jacket 20) and surrounds the LHe reservoir 14.
• the thermal shield 26 is spaced apart from the inner vacuum jacket wall 23 to avoid thermal conduction from the thermal shield 26 into the LHe reservoir 14.
• the thermal shield 26 may comprise two or more thermal shield layers (variant not shown) spaced apart from each other and with the innermost shield layer spaced apart from the inner vacuum jacket wall 23.
• a cold head 30 executes a refrigeration cycle using a working fluid such as helium to provide active cooling of the LHe reservoir 14 and, in the illustrative embodiment, also provides active cooling of the thermal shield 26.
• the cold head 30 passes through the vacuum jacket 20.
• a warm end 32 of the cold head 30 is welded to the outer wall 22 of the vacuum jacket by one or more welds 33. (Note, some features of the cold head 30 are labeled with reference numbers only in the enlargement shown in FIGURE 2).
• a motorized drive assembly 34 is connected to the warm end 32 of the cold head 30 (and may be viewed as part of the warm end), and includes a motor that drives a displacer (internal components not shown) to cause cyclic compression and expansion of the working fluid in accord with a refrigeration cycle. At least a distal end of the motorized drive assembly 34 is outside of the vacuum jacket 20 and hence exposed to ambient air, and this exposed end includes connectors 36 for attachment of one or more electrical power cables and one or more hoses for injecting the working fluid (cables and hoses not shown).
• the illustrative cold head 30 is preferably a cylindrical cold head, although other geometries are contemplated.
• the illustrative cold head 30 is a two-stage design that includes: a first stage section 40 having one end being the warm end 32 and an opposite end defining an intermediate (or first stage) cold station 42; and a second stage section 44 connected with the intermediate (or first stage) cold station 42 and penetrating into the liquid helium reservoir 14 to define a second stage cold station 46 disposed in the liquid helium reservoir 14.
• the first stage section 40 and the second stage section 44 each comprise a stainless steel cylinder housing through which the displacer passes, with the second stage section 44 typically having a smaller diameter than the first stage section 40. (That is, the first stage section 40 is cylindrical with a first diameter and the second stage section 44 is cylindrical with a second diameter smaller than the first diameter).
• the penetration of the second stage section 44 through the inner wall 23 is suitably sealed using an annular weld or other vacuum-tight seal.
• the first stage cold station 42 is connected with the thermal shield 26 by a high conductance thermal link 50 that connects with a thermal bus 52 that is welded, brazed, or otherwise secured to the thermal shield 26.
• the second stage cold station 46 is disposed in gaseous He overpressure above the LHe level 16 in the LHe reservoir 14.
• the cold head 30 is designed and operated to cool the second stage cold station 46 to below the liquefaction temperature of helium, and the first stage cold station 42 to a higher temperature (albeit cool enough for the thermal shield 26 to provide effective thermal shielding of the LHe reservoir 14).
• the cold head 30 is typically welded to the outer vacuum wall 22 and to the inner vacuum wall 23.
• a LHe charge is loaded into the LHe reservoir 14 via a suitable fill line (not shown).
• the fill line or another ingress path also provides for inserting electrical conductive leads or the like (not shown) for connecting with and electrically energizing the magnet windings 12.
• a static electric current flowing through these windings 12 generates a static Bo magnetic field, which is horizontal as indicated in FIGURE 1 in the illustrative case of a horizontal bore magnet.
• the LHe charge in the LHe reservoir 14 should be maintained; otherwise, the superconducting windings 12 may warm to a temperature above the superconducting critical temperature for the magnet windings 12, resulting in a quench of the magnet. (To provide controlled shut-down in the event the LHe charge must be removed, the leads are preferably re-inserted and the magnet current ramped down to zero prior to removal of the LHe charge).
• the MRI device optionally includes various other components known in the art, such as a set of magnetic field gradient coils 54 for superimposing selected magnetic field gradients onto the Bo magnetic field in the x-, y-, and/or z-directions, a whole-body radio frequency (RF) coil (not shown) for exciting and/or detecting magnetic resonance signals, a patient couch (not shown) for loading a medical patient or other imaging subject into the bore 18 of the MRI device 10 for imaging, and/or so forth.
• RF radio frequency
• the cold head 30 beneficially chills the LHe reservoir 14 when the cold head is operational. However, the cold head is occasionally turned off. This may be done intentionally to prepare for maintenance, shipping of the magnet, or so forth, or may occur unintentionally due to some malfunction. Any time the cold head is turned off for any extended period of time, it will begin to warm up and create a thermal leakage path by which heat from ambient air contacting the warm end 32 and the motorized drive unit 34 can conduct into the LHe reservoir 14. Thus, the cold head 30 when turned off becomes a thermal liability.
• the thermal leakage path presented by the non-operational cold head 30 is alleviated by providing a heat exchanger 60, which is disposed inside the vacuum jacket 20 and is secured to or integral with the cold head 30.
• the heat exchanger 60 includes a fluid passage 62 having an inlet 64 in fluid communication with the LHe reservoir 14, and having an outlet 66 in fluid communication with ambient air.
• an inlet fluid conduit 70 passes through the common wall 23 shared between the vacuum jacket 20 and the LHe reservoir 14.
• the inlet fluid conduit 70 provides fluid communication between the inlet 64 of the fluid passage 62 of the heat exchanger 60 and the LHe reservoir 14.
• an outlet fluid conduit 72 passes through the outer wall 22 of the vacuum jacket 20 and provides fluid communication between the outlet 66 of the fluid passage 62 of the heat exchanger 60 and ambient air.
• gas helium from the LHe reservoir 14 is injected by the gas helium overpressure into the inlet fluid conduit 70 and flows through the fluid passage 62 and thence into the outlet fluid conduit 72 to be discharged into ambient air. As the gas helium flows through the fluid passage 62, it absorbs heat from the cold head 30.
• the fluid passage 62 of the heat exchanger 60 is preferably serpentine or spirals around the cylindrical cold head 30 to provide a large contact area. Additionally or alternatively, the fluid passage 62 may be a multi-channel fluid passage, i.e. the fluid passage 62 may provide multiple paths for gas helium to flow from the inlet 64 to the outlet 66.
• the heat exchanger 60 can employ any conventional heat exchanger design for enhancing this heat transfer.
• the heat exchanger comprises a metal shell wrapped around the cold head 30, and the fluid passage 62 is drilled, milled, or otherwise formed into this metal shell. In this approach, the metal shell provides a thermally conductive path from the fluid passage 62 to the cylindrical cold head 30.
• the heat exchanger 60 preferably wraps around the entire circumference of the (illustrative cylindrical) cold head 30.
• the metal shell may be divided into discrete segments, e.g. six arcuate segments each extending over a 60° arc, with tube connections between inlets and outlets of neighboring segments.
• the heat exchanger 60 may employ a shell or segments of another thermally conductive material that is more flexible, such as silicon type or acrylic type thermal conductive sheeting, with the fluid passage 62 being a tube embedded into the sheeting.
• the heat exchanger 60 is integral with the cold head 30.
• the cold head 30 may employ a housing made of stainless steel cylinders, e.g. a larger diameter cylinder forming the housing of the first stage section 40, and a smaller diameter cylinder forming the housing of the second stage section 44.
• the cylindrical stainless steel housing 40, 44 of the cold head 30 suitably has embedded tubing forming the fluid passage 62 of the heat exchanger, and the cylindrical stainless steel housing 40, 44 of the cold head 30 also forms the body of the heat exchanger 60.
• the heat exchanger 60 may comprise stainless steel tubing that is wrapped around the cold head 30 and is welded, brazed, or otherwise secured to outer surfaces of the cylindrical stainless steel housing 40, 44 of the cold head 30. This approach is straightforward to manufacture or even retrofit to an existing cold head, but has less thermal transfer surface area compared with other illustrative designs.
• the heat exchanger 60 is secured to the first stage section 40 of the cold head 30, but is not secured to the second stage section 44 of the cold head 30. Since heat flows from the ambient air into the warm end 32 of the cold head 30, providing cooling via the heat exchanger 60 of the first stage section 40 only (without also cooling the second stage section 44) provides substantial benefit.
• FIGURE 3 a variant embodiment also provides cooling via the heat exchanger of the second stage section 44.
• the embodiment of FIGURE 3 includes the same superconducting magnet as in FIGURE 1 and the same cold head 30 as in FIGURES 1 and 2.
• the embodiment of FIGURE 3 differs from that of FIGURE 2 in that, in the embodiment of FIGURE 3, the heat exchanger 60i, 60 2 includes a first heat exchanger section 60i (with a first fluid passage 62i) secured to or integral with the first stage section 40 of the cold head 30, and also an added second heat exchanger section 60 2 (with a second fluid passage 62 2 ) which is secured to or integral with the second stage section 44 of the cold head 30.
• the second heat exchanger section 60 2 includes the inlet 64 of the heat exchanger 60i, 60 2 in fluid communication with the LHe reservoir via the inlet fluid conduit 70.
• the first heat exchanger section 60i includes the outlet 66 of the heat exchanger in fluid communication with ambient air via the outlet fluid conduit 72.
• the heat exchanger 60i, 60 2 further includes a fluid conduit 74 connecting the first heat exchanger section 60i and the second heat exchanger section 60 2 in series. That is, the gas helium flows into the inlet 64, through the second heat exchanger section 60 2 , then through the fluid conduit 74 and into the first heat exchanger section 60i, and finally exits from the outlet 66 of the first heat exchanger section 60i and discharged into ambient air.
• the disclosed heat exchanger 60 has the dual benefits of providing a gas helium overpressure vent path and leveraging the sensible cooling capacity of the cold gas He in the LHe tank 14 to provide cooling of the cold head 30 over time intervals when the cold head 30 is turned off (or, more generally, not operating to provide cryogenic cooling).
• the heat exchanger 60 should be helium leak-tight because any gas helium leaking out of the heat exchanger 60 will enter the vacuum contained by the vacuum jacket 20. Excessive gas leakage into this vacuum space can compromise the thermal insulation of the LHe reservoir 14, which in an extreme case can lead to rapid boiling of the liquid helium and potential magnet quench or damage.
• FIGURE 4 a process for loading a LHe charge and transporting the superconducting magnet of the MRI device 10 of FIGURE 1 is described.
• the vacuum jacket 20 is evacuated using suitable vacuum couplings (not shown in FIGURE 1) on the outer vacuum wall 22.
• the liquid helium reservoir 14 is evacuated.
• the cold head 30 is turned on and in an operation 84 the liquid helium (LHe) charge is loaded via a fill line (not shown in FIGURE 1) passing through the outer vacuum wall 22.
• the operations 82, 84 may be performed in a different order, and/or additional operations known in the art may be performed.
• the operation 84 entails evacuating air from the LHe reservoir 14 prior to flowing the LHe into the LHe reservoir 14.
• the cold head 30 is turned off preparatory to transport operation(s) 90 in which the superconducting magnet (filled with the LHe charge) is transported.
• the heat exchanger 60 operates to provide cooling of the cold head 30, as well as to provide a vent path for overpressure of gas helium in the LHe reservoir 14. Because the gas helium in the LHe reservoir 14 is an overpressure above the LHe level 16, the gas helium is at a temperature above, but relatively close to, the boiling temperature of the LHe, i.e.
• the heat exchanger 60 operates to provide a passive mechanism for cooling the non-operating cold head 30, which in turn reduces the rate of evaporation of the LHe in the LHe reservoir 14. This reduction in LHe evaporation rate allows for longer transport times and consequently longer achievable transport distances.
• the cold head 30 is turned back on, thereafter providing active cooling of the LHe reservoir 14. While advantages of the disclosed heat exchanger 60 thermally coupled with the cold head 30 accrue during magnet transport as described with reference to FIGURE 4, it will be appreciated that analogous benefit is obtained for any procedure or situation in which the cold head 30 is turned off or otherwise non- operational for an extended time period, e.g.
• the reduced LHe evaporation reduces the likelihood that the LHe charge will be unduly depleted, and reduces the likelihood that LHe depletion may lead to magnet quenching.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)
• Containers, Films, And Cooling For Superconductive Devices (AREA)

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• 2018
  • 2018-07-17 EP EP18742474.2A patent/EP3655978B1/en active Active
  • 2018-07-17 US US16/630,500 patent/US11199600B2/en active Active
  • 2018-07-17 CN CN201880053275.8A patent/CN110998759B/zh not_active Expired - Fee Related
  • 2018-07-17 WO PCT/EP2018/069343 patent/WO2019016180A1/en not_active Ceased
  • 2018-07-17 JP JP2020502182A patent/JP6901622B2/ja not_active Expired - Fee Related

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