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
  • G01R33/3806—Open magnet assemblies for improved access to the sample, e.g. C-type or U-type magnets
• • 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

Definitions

• the invention relates to a magnetic resonance imaging system (MRI system) for forming an image of a human or animal patient, or part thereof, including a mainly cylindrical space for accommodating the relevant part of the patient during imaging, said mainly cylindrical space being bounded by an envelope in which there is arranged a magnet for generating a static magnetic field in the cylindrical space.
• MRI system magnetic resonance imaging system
• a magnetic resonance imaging system of this kind is generally known in the field of medical diagnosis and is marketed, for example under the name Gyroscan by Philips Medical Systems.
• a system of this kind includes a large electromagnet which is generally composed of a number of superconducting coils which are accommodated in a cryostat.
• a tube having a diameter large enough to receive a patient is formed in the cryostat. This tube constitutes the cylindrical space in which the relevant part of the patient must be accommodated during imaging.
• the tube typically has a length of 1.8 m or more and an inner diameter of from 60 to 65 cm.
• the magnet coils are arranged around the tube so that, when the coils are energized, a steady magnetic field whose field direction extends in the longitudinal direction of the tube is generated in the tube.
• the aim is to realize a magnetic field of very high homogeneity in the central part of the tube.
• gradient coils are arranged around the tube in known manner in order to apply temporally switched gradients to the static and uniform magnetic field.
• magnetic resonance signals are generated in the body of the patient; these signals are subsequently detected and used to form magnetic resonance images (MR images).
• the long and comparatively narrow tube induces feelings of claustrophobia in a number of patients so that the formation of MR images is impeded or even becomes impossible.
• a first embodiment of the magnetic resonance imaging system according to the invention includes the elements recited in Claim 2.
• This system includes a magnet with a series of coils which are arranged around the cylindrical space for the patient. In comparison with the known MRI systems, this magnet is short and its inner diameter is large so that a general-purpose MRI system can be realized.
• Such an MRI system according to the invention typically has a length of 1.1 m and an inner diameter of 65 cm or more.
• the embodiment of an MRI system as recited in Claim 3 illustrates the feasible dimensions of the coil system ensuring adequate space for the cryostat, the gradient coils and further elements which necessarily have to be arranged between the magnet coils and the cylindrical space for the patient.
• the embodiments according to the Claims 4, 5 and 6 concern an MRI system with a feasible construction of coils whereby a static magnetic field of adequate strength and homogeneity can be achieved.
• MRI systems often involve so-called active shielding; this means that outside the magnet coils for generating the static field at the area where the part of the patient to be imaged is situated there is provided a second system of magnet coils which generates a second magnetic field with an opposed field direction.
• the strength of said second magnetic field is chosen to be such that the two static magnetic fields mainly compensate one another outside the MRI system, so that the area around the MRI system with a relatively large net magnetic field is small.
• extensive and heavy passive shielding for example by applying iron or another ferromagnetic metal, can be dispensed with or such shielding can be constructed so as to be significantly lighter.
• the coils for active shielding in the known MRI systems are arranged over a distance which is substantially equal to the largest distance between the coils forming the magnetic field.
• a further embodiment of the MRI system according to the invention is recited in the Claims 7 and 8.
• Claim 9 recites another embodiment of an MRI system according to the invention.
• This system has a cylindrical patient space whose dimensions correspond substantially to the dimensions of a conventional system, i.e. a typical diameter of from approximately 60 to 70 cm and a length of approximately 1.5 m.
• the accessibility is now achieved by way of the opening formed in the side wall of the cylindrical space, so that the patient is suitably accessible over substantially the entire length and also is in contact with the physician or medical assistant.
• An MRI system of this kind is provided with, for example a coil system as recited in Claim 10.
• This system is capable of forming a magnetic field of adequate strength whose zone of adequate homogeneity extends across a large part of the length of the cylindrical space (approximately 105 cm) and has a diameter of approximately 15 cm.
• a magnet of this kind is particularly suitable for forming images of vascular systems, for example in an arm or a leg of the patient.
• Fig. 1 is an exterior view of a first embodiment of an MRI system
• Fig. 2 shows diagrammatically a first system of magnet coils for use in the first embodiment
• Fig. 3 shows diagrammatically an alternative system of magnet coils
• Fig. 4 is an exterior view of a second embodiment of an MRI system
• Fig. 5 shows diagrammatically a set of magnet coils for use in the second embodiment.
• Fig. 1 is an exterior view of a first embodiment of an MRI system according to the invention.
• the Figure shows the patient table 10 of the MRI system and also the magnet system which is surrounded by an envelope 20.
• the magnet system is composed of a number of ring-shaped magnet coils which enclose a cylindrical space 22 in order to generate a static and uniform magnetic field in the cylindrical space. Gradient coils for superposing gradients on the static magnetic field are also arranged within the envelope.
• the magnets are generally superconducting electromagnets which are accommodated in a cryostat within the envelope in order to realize and sustain a low temperature for the magnet coils so that superconduction occurs.
• the patient table 10 is supported by a base 12 in which there is provided, for example a drive whereby the patient table and a patient accommodated thereon can be moved into and out of the cylindrical space 22. Further displacements in the vertical and transverse directions are also feasible, depending on the necessity of moving certain parts of the patient into the center of the magnet and/or to facilitate the positioning of the patient on the table.
• the Figure does not show inter alia the equipment necessary for cooling the magnet coils, the power supplies for energizing the magnet coils and gradient coils, the RF coils for realizing and receiving the magnet resonance signals and the data processing equipment for controlling the gradient coils and RF coils and for reconstructing and displaying magnetic resonance images.
• the length of the magnet shown is short and the diameter of the cylindrical space 22 is large.
• the diameter amounts to approximately 70 cm and the length to approximately 110 cm.
• the head of a patient of normal build is situated almost outside the magnet when the heart is positioned at the center.
• the patient can be moved in the vertical and the transverse direction within the cylindrical space in order to position a specific part of the patient, for example, the heart, at the center of the magnet. Because the magnet is short and the diameter of the cylindrical space is large, the patient is rather well accessible for a physician or a medical assistant standing besides the magnet.
• the accessibility can be further enhanced by slightly tilting the patient table or the magnet so that the longitudinal direction of the table no longer extends parallel to the axis of rotation of the cylindrical space.
• Fig. 2 shows diagrammatically an embodiment of a coil system for such a short MRI system.
• the coil system consists of a number of ring-shaped coils and is rotationally symmetrically situated about an axis z-z'; only the cross-section of the coils with a half plane bounded by the axis of rotation is shown.
• a tubular coil is to be understood to mean a ring-shaped coil for which the difference between the radius of the inner surface and that of the outer surface is smaller than the length of the coil in the axial direction
• a disc-shaped coil is to be understood to mean a ring-shaped coil for which the difference between the radius of the inner surface and that of the outer surface is larger than the length of the coil in the axial direction
• a square coil is to be understood to mean a ring-shaped coil for which these two distances are approximately equal.
• the coils need not have a rectangular cross-section (a so-called square coil may also have a round cross-section).
• the coil system which is diagrammatically shown in Fig.
• the 2 includes six coils which form magnetic fields and are denoted by the reference numerals 31, 32, 33, 34, 35 and 36.
• the coils are made, for example of a material which is superconducting at low temperatures, for example NbTi/Cu.
• the two outer coils (31 and 32), viewed in the axial direction, are disc-shaped and constitute the most important coils for the formation of the static magnetic field. In terms of ampere turns these two coils carry the major part of the electric current.
• Two tubular coils 33 and 34 and two square coils 35 and 36 of smaller diameter are arranged within said disc-shaped coils.
• the tubular coils 33 and 34 carry a comparatively small amount of current, be it in the direction opposing the current direction in the coils 31 and 32.
• This current in the opposite direction significantly enhances the homogeneity of the steady magnetic field.
• the current direction in the two square coils 35 and 36 is the same as that in the disc-shaped coils 31 and 32; they carry a current which is slightly larger than that in the tubular coils 33 and 34.
• the tubular coils 37 and 38 are shielding coils and serve to compensate the steady magnetic field outside the MRI system (the stray field); each of these shielding coils carries a current which amounts to approximately half the current in the disc-shaped coils and flows in the opposite direction.
• Fig. 3 shows diagrammatically a second embodiment of a coil system for an
• This system again includes six current-carrying coils which form magnetic fields and are denoted by the reference numerals 41, 42, 43, 44, 45 and 46.
• the two outer coils 41 and 42 viewed in the axial direction, are disc-shaped and again constitute the most important coils for building up the static magnetic field and carry the major part of the electric current in terms of ampere turns.
• Within these disc-shaped coils there are arranged four tubular coils 43, 44, 45 and 46 of smaller diameter.
• the coils 43 and 44 notably serve to enhance the homogeneity and in terms of ampere turns they carry a comparatively small amount of current in the direction opposing the current direction in the coils 41 and 42.
• the current direction in the two tubular coils 45 and 46 is the same as that in the disc-shaped coils 41 and 42; they carry a current which is slightly smaller than that in the tubular coils 43 and 44.
• the properties of the static magnetic field can be further improved by the inclusion of annular conductors which do not carry a current, for example iron tubular conductors 47 and
• Fig. 4 shows a second embodiment of an MRI system according to the invention.
• the static magnetic field is generated by means of a magnet which is accommodated in a C-shaped or U-shaped envelope 52 having a length which is comparable to the length of an adult.
• the C-shaped or U-shaped envelope is open at its ends and on one side also over a substantial part of its length, being the entire length in the Figure.
• a patient on a patient table 51 can be moved into the magnetic field via one of the ends. Because the magnet is open on one side, the patient is accessible for a physician or medical assistant and the feelings of claustrophobia are also reduced.
• the zone of homogeneity of such a magnet is very long and its diameter can be sufficient for the simultaneous imaging of a large part of an arm, leg or part of the torso of the patient.
• the MRI system may be provided with a display screen 55.
• the display screen should evidently be of a type which operates in the presence of a magnetic field, for example an LCD, and it should also be adequately shielded so as to avoid disturbances in the RF receiving coils.
• this Figure does not show the equipment for cooling the magnet coils, the power supplies for energizing the magnet coils and gradient coils, the RF coils for realizing and receiving the magnetic resonance signals, and the data processing equipment for controlling the gradient coils and RF coils and for reconstructing magnetic resonance images.
• FIG. 5 shows a feasible configuration of a magnet coil system for use in the above MRI system.
• the system is composed of a large number of parallel conductors, eight of which are shown and denoted by the reference numerals 61 to 68 in the Figure.
• the parallel conductors are interconnected in a two by two fashion by arc-shaped (C-shaped or U-shaped) conductors which are denoted by the reference numerals 71 to 78.
• the parallel, substantially straight conductors 61 and 62 which are arranged to both sides of a symmetry plane of the magnet system are interconnected by the C-shaped conductors 71 and 72.
• the straight conductors 63 and 64 are interconnected by the arc-shaped conductors 73 and 74 which cover a smaller arc than the conductors 71 and 72.
• the arc-shaped conductors 75 and 76 constituting a coil in conjunction with the straight conductors 65 and 66, have a shorter arc again, like the conductors 77 and 78 which constitute a coil in conjunction with the straight conductors 67 and 68.
• the direction of the steady magnetic field in such a configuration will be parallel to the symmetry plane of the coil system, perpendicular to the straight conductors 61 to 68. Adequate homogeneity is achieved when the current in the coils gradually increases as the distance between the symmetry plane (small current) and the straight conductors increases.
• the maximum current strength occurs in the coils whose straight conductors enclose an angle of approximately 65° relative to the symmetry plane, viewed from the axis of the cylindrical space.
• the coils which enclose a larger angle with respect to and are situated at a larger distance from the symmetry plane carry somewhat less current again. In the vicinity of the side opening the current must be slightly stronger again in order to compensate for the absence of conductors in the opening; consequently, it may be that the maximum current intensity occurs in the conductor situated nearest to the opening.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

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• 1999
  • 1999-11-15 EP EP99958093A patent/EP1051636A1/de not_active Withdrawn
  • 1999-11-15 JP JP2000585685A patent/JP2002531196A/ja active Pending
  • 1999-11-15 WO PCT/EP1999/008950 patent/WO2000033100A1/en not_active Application Discontinuation

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