EP3114494A1 - Active compensation of magnetic field distortion generated by a recondensing refrigerator - Google Patents

Active compensation of magnetic field distortion generated by a recondensing refrigerator

Info

Publication number
EP3114494A1
EP3114494A1 EP15705950.2A EP15705950A EP3114494A1 EP 3114494 A1 EP3114494 A1 EP 3114494A1 EP 15705950 A EP15705950 A EP 15705950A EP 3114494 A1 EP3114494 A1 EP 3114494A1
Authority
EP
European Patent Office
Prior art keywords
displacers
refrigerator
refrigerators
cryogenic
magnetic field
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP15705950.2A
Other languages
German (de)
French (fr)
Inventor
Marcel Kruip
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Siemens Healthcare Ltd
Original Assignee
Siemens Healthcare Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Siemens Healthcare Ltd filed Critical Siemens Healthcare Ltd
Publication of EP3114494A1 publication Critical patent/EP3114494A1/en
Withdrawn legal-status Critical Current

Links

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
    • G01R33/3804—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
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
    • F25B9/10—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point with several cooling stages
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
    • F25B9/14—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the cycle used, e.g. Stirling cycle
    • 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
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
    • G01R33/56563—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0

Definitions

  • the invention relates to a cryogenic refrigerator having a movable displacer, a cryogenic refrigerator arrangement including a plurality of cryogenic refrigerators and a magnetic resonance imaging apparatus including such a cryogenic refrigerator or such a cryogenic refrigerator arrangement .
  • Cryogenic refrigerators with a second stage temperature of less than 4.2 Kelvin are routinely used to cool superconducting magnets.
  • the most wide-spread application of such magnets are Magnetic Resonance Imaging (MRI) scanners.
  • the commonly used refrigerators are Gifford-MacMahon refrigerators including two cooling stages.
  • the second stage comprises a displacer which moves in a cyclical fashion within a receptacle thereby displacing cold gas between opposite ends of the receptacle.
  • This displacer is commonly made of a material having a high thermal capacity such as Holmium Copper (HoCu) .
  • HoCu Holmium Copper
  • Unfortunately these materials also have magnetic properties which cause them to be magnetised by external magnetic fields such as a stray field in the vicinity of an MRI magnet.
  • JP200906101 OA proposes tilting the refrigerator in such a way that the displacer of the refrigerator moves perpendicular to the line between the refrigerator and the centre of the magnet.
  • Such an arrangement of the refrigerator is disadvantageous because the refrigerator works optimally when oriented vertically.
  • the bottom of the refrigerator should be located above the fill level of the liquid helium used for cooling. Thus, the effectiveness of the refrigerator will be affected when tilted.
  • an object of the invention to provide an enhanced cryogenic refrigerator, an enhanced cryogenic refrigerator arrangement and an enhanced MRI apparatus.
  • the invention provides a cryogenic refrigerator arrangement including a plurality of cryogenic refrigerators.
  • Each of the refrigerators has a movable displacer for displacing a cold gas and comprising a magnetic material.
  • the cryogenic refrigerator arrangement further includes synchronisation means adapted to synchronise respective movements of the displacers of the cryogenic refrigerators.
  • the cryogenic refrigerator arrangement synchronises the various displacers of the refrigerators in such a way that the magnetic fields generated by the displacers mutually cancel out each other.
  • the displacers may be synchronised to move in opposite directions such that the magnetic fields of the two cryogenic refrigerators have opposite polarities and thus cancel out each other.
  • a displacer of one refrigerator may act as a magnetic body for compensating the magnetic field as explained above when referring to a preferred embodiment of the previous inventive aspect.
  • Cryogenic refrigerators including detector means as set forth above are especially suitable for use in such cryogenic refrigerator arrangements because the synchronisation means may rely on the detected positions of the displacers in order to synchronise their movements.
  • the cryogenic refrigerator arrangement may further comprise detector means for detecting a respective position of each of the displacers of the cryogenic refrigerators.
  • the cryogenic refrigerator arrangement includes N cryogenic refrigerators with N being a positive natural number greater than 1.
  • the synchronisation means may be further adapted to synchronise the respective movements of the displacers in such a way that a phase difference of movement between each of the displacers and a corresponding remaining one of the displacers is substantially equal to 360 degrees divided by the number N. In this way three or more refrigerators may be combined.
  • Yet another aspect of the invention provides a magnetic resonance imaging apparatus including a magnet arrangement and at least one cryogenic refrigerator of the invention.
  • a magnetic resonance imaging apparatus may include a magnet arrangement and an inventive cryogenic refrigerator arrangement as set forth above.
  • the refrigerators should be located close to each other.
  • the refrigerators may be located at opposite sides of the MRI apparatus as will be shown for an exemplary embodiment of the invention illustrated below. The invention will be better understood from the following drawings in which a preferred embodiment of the invention will be illustrated by way of example.
  • FIG. 1 A magnetic resonance imaging apparatus may include a magnet arrangement and an inventive cryogenic refrigerator arrangement as set forth above.
  • the refrigerators should be located close to each other.
  • the refrigerators may be located at opposite sides of the MRI apparatus as will be shown for an exemplary embodiment of the invention illustrated below.
  • Figure 1 shows a first embodiment of a magnetic resonance imaging apparatus
  • Figure 2 shows a second embodiment of a magnetic resonance imaging apparatus
  • Figure 3 shows a third embodiment of a magnetic resonance imaging apparatus .
  • Figure 1 shows a first embodiment of a magnetic resonance imaging apparatus.
  • the MRI apparatus includes a magnet arrangement 2 having a substantially tubular shape.
  • An object to be examined may be placed in the volume enclosed by the magnet arrangement 2 and subjected to the strong magnetic fields used for MRI.
  • Such strong magnetic fields can be generated using superconducting magnets. Since superconducting materials only exhibit their superconducting property below a critical temperature which usually is less than 100 Kelvin, the superconducting magnet must be cooled.
  • cryogenic refrigerators 1 are provided to cool the magnet arrangement 2 of the magnetic resonance imaging apparatus.
  • liquid helium cryogenic refrigerators are used even for superconducting materials having a critical temperature far higher than 4.2 Kelvin because the superconductivity of the material benefits from lower temperatures.
  • two refrigerators 1 are arranged side by side on one side of the magnet arrangement 2.
  • the two refrigerators 1 are of identical make such that the respective displacers of the refrigerators 1 have the same magnetic properties.
  • the displacers will be synchronised in such a way that they move in phase opposition. In this way their respective magnetic fields will cancel out each other. The cancellation effect will improve with increasing distance from the refrigerators 1.
  • the refrigerators 1 should be located in proximity to each other.
  • Figure 2 shows a second embodiment of a magnetic resonance imaging apparatus including two refrigerators 1 arranged in such a way.
  • the invention is not limited to using two refrigerators 1.
  • the displacers may be synchronised in such a way that their respective magnetic fields cancel out each other. Shields as proposed in the prior art may be used in combination with the invention.
  • Figure 3 shows a third embodiment of a magnetic resonance imaging apparatus including three refrigerators 1.
  • N being the number of refrigerators or displacers in the arrangement.
  • an arbitrary number of refrigerators 1 may be used.

Landscapes

  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • General Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Combustion & Propulsion (AREA)
  • Chemical & Material Sciences (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)
  • Epidemiology (AREA)
  • Health & Medical Sciences (AREA)
  • Electromagnetism (AREA)
  • Power Engineering (AREA)

Abstract

A cryogenic refrigerator arrangement including a plurality of N cryogenic refrigerators each of which having a movable displacer for displacing a cold gas and comprising a magnetic material. A phase difference of movement between each of the displacers and a corresponding remaining one of the displacers is substantially equal to 360 degrees divided by the number N. Therefore, a magnetic field distortion generated by the motion of one of the displacers is compensated by the magnetic field distortions generated by the motion of the N-1 remaining displacers. As a consequence, when using the refrigerator in an MRI environment, image artifacts caused by the operation of the refrigerator are reduced.

Description

ACTIVE COMPENSATION OF MAGNETIC FIELD DISTORTION GENERATED BY A RECONDENSING REFRIGERATOR
The invention relates to a cryogenic refrigerator having a movable displacer, a cryogenic refrigerator arrangement including a plurality of cryogenic refrigerators and a magnetic resonance imaging apparatus including such a cryogenic refrigerator or such a cryogenic refrigerator arrangement .
Cryogenic refrigerators with a second stage temperature of less than 4.2 Kelvin are routinely used to cool superconducting magnets. The most wide-spread application of such magnets are Magnetic Resonance Imaging (MRI) scanners. The commonly used refrigerators are Gifford-MacMahon refrigerators including two cooling stages. The second stage comprises a displacer which moves in a cyclical fashion within a receptacle thereby displacing cold gas between opposite ends of the receptacle. This displacer is commonly made of a material having a high thermal capacity such as Holmium Copper (HoCu) . Unfortunately these materials also have magnetic properties which cause them to be magnetised by external magnetic fields such as a stray field in the vicinity of an MRI magnet. Since the refrigerator needs to be mounted in proximity to the magnet, the stray field will magnetise the displacer and the displacer will thus, in good approximation, behave like a dipole. As the displacer moves during operation of the refrigerator with a typical amplitude of about 35 millimetres, a field variation caused by the magnetised displacer can affect the image quality of the MRI scanner. This is because the angle between the line connecting the centre of the displacer to the centre of the magnet and the line along which the displacer moves does not equal 90 degrees. This problem has been mitigated as proposed in US 7,196,600 B2 by using a shield arranged around the second stage of the refrigerator and made of a meta material. However, the effectiveness of this approach is limited by the fact that the shield cannot enclose the refrigerator completely to the effect that oscillating magnetic flux may still leak from the refrigerator .
JP200906101 OA proposes tilting the refrigerator in such a way that the displacer of the refrigerator moves perpendicular to the line between the refrigerator and the centre of the magnet. However, such an arrangement of the refrigerator is disadvantageous because the refrigerator works optimally when oriented vertically. Furthermore the bottom of the refrigerator should be located above the fill level of the liquid helium used for cooling. Thus, the effectiveness of the refrigerator will be affected when tilted.
Accordingly it is an object of the invention to provide an enhanced cryogenic refrigerator, an enhanced cryogenic refrigerator arrangement and an enhanced MRI apparatus.
To achieve the foregoing objective, the invention provides a cryogenic refrigerator arrangement including a plurality of cryogenic refrigerators. Each of the refrigerators has a movable displacer for displacing a cold gas and comprising a magnetic material. According to the invention the cryogenic refrigerator arrangement further includes synchronisation means adapted to synchronise respective movements of the displacers of the cryogenic refrigerators.
The cryogenic refrigerator arrangement synchronises the various displacers of the refrigerators in such a way that the magnetic fields generated by the displacers mutually cancel out each other. For example, in a cryogenic refrigerator arrangement including two cryogenic refrigerators the displacers may be synchronised to move in opposite directions such that the magnetic fields of the two cryogenic refrigerators have opposite polarities and thus cancel out each other. In this way a displacer of one refrigerator may act as a magnetic body for compensating the magnetic field as explained above when referring to a preferred embodiment of the previous inventive aspect.
Cryogenic refrigerators including detector means as set forth above are especially suitable for use in such cryogenic refrigerator arrangements because the synchronisation means may rely on the detected positions of the displacers in order to synchronise their movements. In the same way the cryogenic refrigerator arrangement may further comprise detector means for detecting a respective position of each of the displacers of the cryogenic refrigerators.
Preferably the cryogenic refrigerator arrangement includes N cryogenic refrigerators with N being a positive natural number greater than 1. The synchronisation means may be further adapted to synchronise the respective movements of the displacers in such a way that a phase difference of movement between each of the displacers and a corresponding remaining one of the displacers is substantially equal to 360 degrees divided by the number N. In this way three or more refrigerators may be combined.
Yet another aspect of the invention provides a magnetic resonance imaging apparatus including a magnet arrangement and at least one cryogenic refrigerator of the invention.
A magnetic resonance imaging apparatus may include a magnet arrangement and an inventive cryogenic refrigerator arrangement as set forth above. As above the refrigerators should be located close to each other. However, in some arrangements the refrigerators may be located at opposite sides of the MRI apparatus as will be shown for an exemplary embodiment of the invention illustrated below. The invention will be better understood from the following drawings in which a preferred embodiment of the invention will be illustrated by way of example. In the drawings:
Figure 1 shows a first embodiment of a magnetic resonance imaging apparatus;
Figure 2 shows a second embodiment of a magnetic resonance imaging apparatus; and
Figure 3 shows a third embodiment of a magnetic resonance imaging apparatus . Figure 1 shows a first embodiment of a magnetic resonance imaging apparatus. The MRI apparatus includes a magnet arrangement 2 having a substantially tubular shape. An object to be examined may be placed in the volume enclosed by the magnet arrangement 2 and subjected to the strong magnetic fields used for MRI. Such strong magnetic fields can be generated using superconducting magnets. Since superconducting materials only exhibit their superconducting property below a critical temperature which usually is less than 100 Kelvin, the superconducting magnet must be cooled. For this purpose cryogenic refrigerators 1 are provided to cool the magnet arrangement 2 of the magnetic resonance imaging apparatus. Usually liquid helium cryogenic refrigerators are used even for superconducting materials having a critical temperature far higher than 4.2 Kelvin because the superconductivity of the material benefits from lower temperatures.
In the first embodiment of the magnetic resonance imaging apparatus two refrigerators 1 are arranged side by side on one side of the magnet arrangement 2. Preferably the two refrigerators 1 are of identical make such that the respective displacers of the refrigerators 1 have the same magnetic properties. According to one aspect of the invention the displacers will be synchronised in such a way that they move in phase opposition. In this way their respective magnetic fields will cancel out each other. The cancellation effect will improve with increasing distance from the refrigerators 1. Thus, the refrigerators 1 should be located in proximity to each other. However, as illustrated by Figure 2, it is also possible to arrange the refrigerators 1 in a symmetric manner, e.g. on opposite sides of the magnet arrangement 2. Figure 2 shows a second embodiment of a magnetic resonance imaging apparatus including two refrigerators 1 arranged in such a way.
Generally the invention is not limited to using two refrigerators 1. When a plurality of refrigerators 1 are present, the displacers may be synchronised in such a way that their respective magnetic fields cancel out each other. Shields as proposed in the prior art may be used in combination with the invention.
Figure 3 shows a third embodiment of a magnetic resonance imaging apparatus including three refrigerators 1. In such a case the displacers of the refrigerators 1 are synchronised to have a phase difference of 360 / N = 120 degrees with N being the number of refrigerators or displacers in the arrangement. Using the same phase relation, an arbitrary number of refrigerators 1 may be used.
While the invention has been described in connection with preferred embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the scope of the invention as defined by the appended claims.

Claims

Claims
1. A cryogenic refrigerator arrangement including a plurality of N cryogenic refrigerators each of which having a movable displacer for displacing a cold gas and comprising a magnetic material, characterised by synchronisation means adapted to synchronise the respective movements of the displacers of the cryogenic refrigerators in such a way that a phase difference of movement between each of the displacers and a corresponding remaining one of the displacers is substantially equal to 360 degrees divided by the number N.
2. The cryogenic refrigerator arrangement of claim 1, further comprising detector means for detecting a respective position of each of the displacers of the cryogenic refrigerators.
3. A magnetic resonance imaging apparatus including a magnet arrangement and a cryogenic refrigerator arrangement as claimed in one of the claims 1-2.
EP15705950.2A 2014-03-04 2015-02-11 Active compensation of magnetic field distortion generated by a recondensing refrigerator Withdrawn EP3114494A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB1403757.6A GB2523762A (en) 2014-03-04 2014-03-04 Active compensation of magnetic field generated by a recondensing refrigerator
PCT/EP2015/052868 WO2015132055A1 (en) 2014-03-04 2015-02-11 Active compensation of magnetic field distortion generated by a recondensing refrigerator

Publications (1)

Publication Number Publication Date
EP3114494A1 true EP3114494A1 (en) 2017-01-11

Family

ID=50490737

Family Applications (1)

Application Number Title Priority Date Filing Date
EP15705950.2A Withdrawn EP3114494A1 (en) 2014-03-04 2015-02-11 Active compensation of magnetic field distortion generated by a recondensing refrigerator

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EP (1) EP3114494A1 (en)
GB (1) GB2523762A (en)
WO (1) WO2015132055A1 (en)

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116772446B (en) * 2023-06-28 2025-07-15 包头稀土研究院 Synchronous control method and system of magnetic refrigerator and magnetic refrigerator

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Also Published As

Publication number Publication date
GB2523762A (en) 2015-09-09
GB201403757D0 (en) 2014-04-16
WO2015132055A1 (en) 2015-09-11

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