Superconducting MR yoke magnet and method of energizing same

Abstract: A ferromagnetic flux path (20) extends between pole pieces (30,32) . A superconducting coil (62) in series with a persistence switch (64) encircles the flux path. A pair of resistive coils (50,52) are disposed one at each pole piece. The resistive coils are overdriven near to the point of thermal failure to produce a 0.5 T or other preselected field strength in a gap between the pole pieces. The persistence switch is closed to stabilize and hold the flux through the superconducting coil. The resistive magnets are ramped down or shut off. During imaging, a smaller amount of current is directed to the resistive coils to supplement and focus the magnetic field from the superconducting coil through the gap between the poles. In this manner, high strength magnetic fields are generated in the gap using a relatively inexpensive combination of resistive and superconducting coils.

Description: This invention relates to a magnetic resonance apparatus and a magnetic resonance method. The invention is especially applicable to magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application with magnetic resonance spectroscopy, and other medical and diagnostic techniques, and the like. In general, magnetic resonance entails generating magnetic fields, particularly, strong uniform magnetic fields between opposed pole pieces that are connected by an iron return path. In a magnetic resonance imaging system, a homogeneous magnetic field is typically provided over the imaging volume. A fringe field in excess of five Gauss arising from the system magnet is limited to regions in the immediate neighbourhood of the imager. Heretofore, most magnetic resonance imagers have generated the main or primary magnetic field axially through an air core within a plurality of annular magnetic coils. However, this air core design is relatively inefficient compared to an iron core magnet. An air core design is best suited for superconducting magnets (US-A-5 633 588, which discloses a multi-layer composite member for sustaining the flux-generating currents, and also discloses methods of magnetising the member eg by means of an additional resistive coil) since it requires a large number of ampere-turns compared to an iron core magnet. Iron core magnets can be designed to have well-contained fringe fields, be relatively inexpensive, have improved patient access, and have the ability to easily cycle the energisation. Others have heretofore constructed C, double-C, H and picture frame iron core electromagnets for magnetic resonance applications. However, these are typically very heavy. Even a relatively low field C-magnet, e.g. 0.2 Tesla, has a very large mass in the iron return path or ferromagnetic path. Although the yoke of the iron return path could be positioned close to the poles to minimise weight, the return path effects tended to distort the magnetic field across the gap. Therefore, the return path was positioned well away from the gap thus necessitating an increase in the cross-section, overall length and therefore weight of the magnet. The dual return paths of a double-C, H or picture frame magnet tended to improve the symmetry and hence uniformity in the gap or imaging volume. As the field strength and gap distance increased, the cross-section of the return path was also increased. The weight of the iron core increased approximately as the cube of the pole spacing of the gap and linearly with the field strength. Due to this severe weight penalty, the gap of such magnets was minimised. In magnets built for human imaging, the gaps were arranged vertically to accommodate the smallest front-to-back dimension of a reclining human body. Increasing electrical current through resistive driving coils in a C magnet seems at first a solution to the problem of increasing field strength without decreasing gap size or increasing the iron core dimensions. However, this significantly increases the power consumption of the magnetic resonance system. The magnetic field strength of an iron core electromagnet (units of ampere-turns) can be increased by adding more current or turns. However, the power dissipation in the driving coils increases as the square of the applied current. Additional turns (also with less favourable geometry) increases the resistive loss in the excitation coils. To make matters much worse, the field does not scale with the increased number of amperes or turns because of saturation effects in the iron core. There are practical limits to the field strength of a resistive electromagnet based on acceptable geometry and power dissipation. This also influences system complication as well as cost of manufacture and operation. Further, thermal dissipation from the resistive coils becomes a severe problem. As an example, a prior art C magnet for imaging at 0.2 T having poles of 1.0 m diameter with a 0.5 m gap therebetween requires approximately 60,000 ampere turns around the ferromagnetic return leg. To drive such a magnet to 0.5 T requires many times the power. At such power levels, thermal dissipation becomes a severe problem. In fact, such an overdriven magnet would be self-destructive. One alternative is to provide a superconducting coil behind each pole piece (US-A-4 985 678). At 0.5 T, the superconducting coil is small, however, the cryostat therefor adds complexity. The cryostat contains liquid helium reserve. The coils are connected in series and suspended to withstand the magnetic forces. With multiple cryostats, persistent mode excitation and coordination leads to complications and expense. Of course, thermal and electrical conduction paths between the superconducting coils are possible but they add further complication and expense to the cryostat. Superconducting magnets behind the pole pieces of a C-magnet have also been disclosed in EP-A-0 424 808. Another alternative is to dispose a superconducting coil along the ferrous return path (EP-A-0 694 789). The invention provides a magnetic resonance apparatus which includes a subject receiving region, first and second pole pieces disposed on opposite sides of the subject receiving region, a ferromagnetic flux path interconnecting the first and second pole pieces, a single superconducting magnet disposed along the ferromagnetic flux path spaced apart from the pole pieces and capable of being magnetically decoupled from and magnetically coupled to the ferromagnetic flux path, first and second resistive magnets disposed adjacent the first and second pole pieces, respectively, and connected to an excitation power source (56), control means (66) adapted for directing the power source (56) to energise the resistive magnets so as to adjust a magnetic field strength through the subject receiving region to a preselected value and for, of the same time, magnetically decoupling the superconducting magnet from the ferromagnetic flux path, means for stabilising the magnetic flux corresponding to said preselected value of the magnetic field strength by magnetically coupling the superconducting magnet to the ferromagnetic flux path, and means for thereafter reducing the magnetic flux produced by the resistive magnets. The invention also provides a method of establishing a static magnetic field in a magnetic resonance system which includes a subject receiving region, first and second pole pieces disposed on opposite sides of the subject receiving region, and a ferromagnetic flux path interconnecting the first and second pole pieces, the method establishing a static magnetic field by the steps of magnetically decoupling a single superconducting coil, which is disposed along the ferromagnetic flux path at a position spaced apart from the pole pieces, from the ferromagnetic flux path, producing a selected magnetic flux in the ferromagnetic flux path with first and second resistive coils disposed adjacent the first and second pole pieces, respectively, magnetically coupling the superconducting coil to the ferromagnetic flux path such that the superconducting coil holds the magnetic flux therethrough stable, and ramping down the magnetic flux produced by the resistive coils. Advantageously, the magnetic resonance apparatus is a magnetic resonance imaging system including a ferromagnetic flux path for producing magnetic flux in an examination region, a radio frequency pulse controller and transmitter for inducing dipoles in the examination region to resonance such that radio frequency resonance signals are generated, gradient magnetic field coils and a gradient magnetic field controller for generating at least phase and read magnetic field gradient pulses in orthogonal directions across the examination region, a receiver for receiving and demodulating the radio frequency magnetic resonance signals, and an image processor for reconstructing image representations. The invention includes two resistive magnets for producing magnetic flux in the ferromagnetic flux path, and a magnetic flux stabilising means in the form of a single superconducting magnet for stabilising the magnetic flux in at least a portion of the ferromagnetic path. In accordance with a more limited aspect of the present invention, the resistive magnets are overdriven near to the point of failure to produce selected magnetic flux in the ferromagnetic flux path. In accordance with another more limited aspect of the present invention, the superconducting coil is connected to a switch which is closed to permit current flow through the coil and open to block current flow. Magnetic resonance apparatus and method in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a diagrammatic illustration of a magnetic resonance imaging apparatus in accordance with the present invention; FIGURE 2 is partial cross-sectional view illustrating the resistive and gradient coil details of the apparatus of FIGURE 1. With reference to FIGURE 1, a support A selectively supports a patient or subject in an air gap defined between pole faces of a C-shaped ferromagnetic flux path B. A magnetomotive force means and magnetic flux stabilising means C creates and stabilises a magnetic field across the air gap between the pole faces and along the ferromagnetic flux path B. A one-time energising and calibration system D is used to set up the magnetic field across the air gap. Magnetic resonance electronics E selectively induce magnetic resonance of dipoles in the image region and process resultant received magnetic resonance signals to create an image or other diagnostic information. The support A includes a supporting surface 10 in a substantially horizontal plane. The supporting surface has a longitudinal axis lengthwise therealong and a perpendicular transverse axis thereacross, both in the horizontal plane. An elevation adjusting means 12 selectively adjusts the relative height of the supporting surface. The supporting surface is slidably mounted on a support frame 14 to provide a means for moving the supporting surface in the horizontal plane. Preferably, a motor drive 16 is mounted to the frame to drive the patient supporting surface therealong. The ferromagnetic flux path B includes a C-shaped ferromagnetic member 20 having a first end 22, a second end 24, and a middle portion 26. A first pole piece 30 and a second pole piece 32 at the first and second ends of the flux path, respectively, define an air gap 34 therebetween. The C-shaped member is configured to minimise the length of the ferromagnetic flux path while spacing the ferromagnetic flux path sufficiently from the gap to minimise distortion to the magnetic field in the air gap. The ferromagnetic flux path can be solid, laminated, include an air core or the like. In the embodiment of FIGURE 2, the ferromagnetic flux path is a C-shaped iron core having flared ends toward the pole pieces to provide a smooth transition between the iron core and the larger cross-section pole pieces which may be circular, oval, rectangular, or the like. The C-shaped member preferably consists of iron or an iron-cobalt alloy but may also consist of another metal, alloy or compound of the transition, rare-earth, and actinide elements. In addition, various other configurations for the ferromagnetic flux path are contemplated including double-C shaped members, which define two return flux paths as well as configurations with a larger number of flux paths. With reference to FIGURE 2, the pole pieces each include a soft ferrite or laminated pole face 36 configured to suppress eddy currents, and passive shims and/or active shim coils 38. The active shim coil is iteratively adjusted until a magnetic field between the pole pieces is optimised. The shimming also compensates for edge effects, pole face irregularities, the presence of the yoke, the influence of the room and other adjoining equipment, and the like. A gradient field coil 40 selectively produces substantially linear gradients aligned with the field or magnetic flux in the air gap. The gradient coil includes appropriate coil segments to create orthogonal gradients in the field within the air gap that are appropriate for imaging. The magnetomotive force means and magnetic flux stabilising means C includes, as the magnetomotive force means, two resistive magnet drivers 50, 52 disposed adjacent pole pieces 30, 32. Each resistive driver includes a resistive coil 54 consisting of a thin, wide, insulated conductor. Each resistive coil wraps annularly around a respective end of the C-shaped member. Optionally, each resistive coil is cooled at its edges using a water-cooling device (not shown) to prevent overheating. The resistive coil is energised and de-energised through a current lead by excitation power source 56 (shown in FIG. 1). The magnetomotive force means and magnetic flux stabilising means C also includes, as the magnetic flux stabilising means, a superconducting cryodriver 60 which encircles a segment of the middle portion 26 of the C-shaped member. As is well known by those skilled in the art, the cryodriver includes a cryostat 62 which houses an annular superconductor magnet that encircles the flux path. The cryostat also holds a coolant around the superconductor to maintain its superconductivity. Preferably, the superconductor is a Niobium-Titanium alloy and the coolant is liquid helium However, it will be appreciated that other superconductors and, if necessary, their coolants may be used without departing from the scope of the invention. The superconductor is in the form of closed circuit including a persistence switch 64 . When the persistence switch is non-resistive (superconducting) or closed, the superconducting magnet maintains constant the magnetic flux passing therethrough. When the persistence switch is resistive or open, the superconducting magnet loses superconductivity and does not maintain the magnetic field. Returning to FIGURE 1, to set up the main magnetic field in the C-shaped member, the energising and calibration system D is used. The system includes a set-up control 66 which directs the power source 56 to energise or apply current to the resistive drivers 50 , 52 while the persistence switch 64 is resistive. For the full benefit of the invention, the resistive drivers are overdriven near to the point of overheating. Failure control 68 ensures that the resistive drivers do not overheat to the point of failure. The failure control may incorporate one of a number of devices such as a timer, a thermocouple or field strength indicator located adjacent the resistive coils, or a computer simulator which monitors the power or field strength at the coils and computes the point of temperature failure. The set-up control is also attached to the persistence switch 64 . When a desired magnetic field, such as 0.5 T, is achieved in the gap or in the superconducting driver, the persistence switch is closed. This in effect freezes the strength of the magnetic field in the portion of the flux path surrounded by the superconducting driver. Any changes in the magnetic field strength induces opposing currents in the superconductor, thus stabilising the field strength. To focus as much of the magnetic field in the middle portion of the C-shaped ferromagnetic member, the superconducting coil should be as close as possible to the member. One way to achieve this is to incorporate a section of the middle portion of the C-shaped ferromagnetic member into the cryodriver, such as in the cold shields. Optionally, a magnetic field strength detector can be used to monitor the field in the air gap and indicate when the persistence switch should be closed. After the persistence switch is closed, the resistive drivers are ramped down to a lower level or to zero which lowers, but does not extinguish, the field strength in the air gap. Although the resistive drivers were overdriven to initialise the superconducting magnet, only moderate power in the resistive coils reestablishes the desired field strength in the air gap. During imaging, a smaller amount of current may be directed to the resistive coils to supplement and focus the magnetic field from the superconducting coil through the gap between the poles. The resistive coils, their configuration, and any cooling system are selected to permit the resistive coils to generate, at least for a moment, the desired magnetic field in the gap without assistance from the superconducting magnet Once the desired magnetic field strength is established in the superconductor, the energising and calibration system D is disconnected from the magnetomotive force and magnetic flux stabilising means C. Should the need arise to remagnetise and stabilise the magnetic flux stabilising means, the system D is reconnected. The electronics section E includes an radio frequency (RF) transmitter means 70 which selectively applies radio frequency pulses to a radio frequency coil 72 to excite magnetic resonance of dipoles in the gap magnetic field. A receiver 74 receives magnetic resonance signals from the region of interest using the radio frequency coil 72 as an antenna. A gradient coil control 76 applies electrical pulses to the gradient field coil 40 to cause gradients across the gap magnetic field to encode the magnetic resonance signals. An image reconstruction processor 78 performs an inverse two-dimensional Fourier transform or other known algorithm to reconstruct an image representation from the received magnetic resonance signals. The image representations are stored in a memory 80, displayed on a video monitor 82, further processed, or the like. A central magnetic resonance controller 84 controls the excitation power control 56 to the resistive drivers, the RF transmitter 70, the gradient field control 76 to implement a preselected magnetic resonance imaging sequence as is conventional in the art. Rather than using the same coil for transmission and reception, surface coils such as a coil loop 84 or solenoid coil 86 may be disposed in the image region and used to receive the magnetic resonance signals. A wide variety of radio frequency coils are contemplated including crossed ellipse quadrature coils, saddle quadrature coils, solenoid coils, saddle coils, butterfly coils, phased array coils, or the like. A primary advantage of the above-described embodiment with multiple driven C magnet is that it generates a strong, uniform, and stable magnetic field using a relatively inexpensive combination of resistive and superconducting coils. Another advantage resides in the open space around the patient which facilitates range of motion studies and other procedures, invasive procedures, connection of life support equipment, and reduces patient claustrophobia. A further advantage resides in its compact size. Another advantage resides in its moderate cost.

Claims: A magnetic resonance apparatus which includes a subject receiving region (34), first and second pole pieces (30, 32) disposed on opposite sides of the subject receiving region, a ferromagnetic flux path (B) interconnecting the first and second pole pieces, a single superconducting magnet (60) disposed along the ferromagnetic flux path (B) spaced apart from the pole pieces and capable of being magnetically decoupled from and magnetically coupled to the ferromagnetic flux path, first and second resistive magnets (50, 52) disposed adjacent the first and second pole pieces (30, 32), respectively, and connected to an excitation power source (56) control means (66) adapted for directing the power source (56) to energise the resistive magnets so as to adjust a magnetic field strength through the subject receiving region (34) to a preselected value and for, at the same time, magnetically decoupling the superconducting magnet from the ferromagnetic flux path, means for stabilising the magnetic flux corresponding to said preselected value of the magnetic field strength by magnetically coupling the superconducting magnet to the ferromagnetic flux path, and means for thereafter reducing the magnetic flux produced by the resistive magnets. A magnetic resonance apparatus as claimed in claim 1, in which the superconducting magnet includes a superconducting coil (60) which surrounds the ferromagnetic flux path (B). A magnetic resonance apparatus as claimed in claim 2, in which the superconducting coil (60) further includes a switch (64) which in a closed state permits superconducting current to flow through the superconducting coil and in an open state prevents current from flowing through the coil. A magnetic resonance apparatus as claimed in any one of claims 1 to 3, in which there is provided a magnet control system (D) for overdriving the resistive magnets (50, 52) with the superconducting magnet (60) magnetically decoupled from the ferromagnetic flux path until a preselected magnetic field strength is achieved in the subject receiving region (34), whereby the superconducting magnet is magnetically coupled to the ferromagnetic flux path when the preselected field strength is achieved, and the magnetic flux produced by the resistive magnets reduced. A magnetic resonance apparatus as claimed in claim 4, including a failure control (68) to ensure that the resistive magnets do not overheat to the point of failure. A method of establishing a static magnetic field in a magnetic resonance system which includes a subject receiving region (34), first and second pole pieces (30, 32) disposed on opposite sides of the subject receiving region, and a ferromagnetic flux path (B) interconnecting the first and second pole pieces, the method establishing a static magnetic field by the steps of magnetically decoupling a single superconducting coil (60) , which is disposed along the ferromagnetic flux path at a position spaced apart from the pole pieces, from the ferromagnetic flux path, producing a selected magnetic flux in the ferromagnetic flux path with first and second resistive coils (50, 52) disposed adjacent the first and second pole pieces, respectively, magnetically coupling the superconducting coil to the ferromagnetic flux path such that the superconducting coil holds the magnetic flux therethrough stable, and ramping down the magnetic flux produced by the resistive coils. A method as claimed in claim 6, including the step of overdriving the resistive coils (50, 52) near to the point of failure to produce the selected magnetic flux in the ferromagnetic flux path (B). A method as claimed in claim 7, in which the superconducting coil (60) is connected to a switch (64) which is closed to permit current flow through the coil and open to block current flow, the method including the steps of opening the switch (64) to decouple the superconducting coil (60) from the ferromagnetic flux path (B), and closing the switch to magnetically couple the superconducting coil to the ferromagnetic flux path. A method as claimed in any one of claims 6 to 8, wherein the step of producing the selected magnetic flux includes overdriving the resistive coils near to the point of failure due to overheating. A magnetic resonance method using a method of establishing a static magnetic field as claimed in any one of claims 6 to 9, including the steps of, exciting resonance in selected dipoles in the subject receiving region and spatially encoding resonance signals produced by the resonating dipoles, and reconstructing the spatially encoded resonance signals into an image representation (82).

Inventors: DEMEESTER GORDON D, MORICH MICHAEL A, DEMEESTER, GORDON D., MORICH, MICHAEL A.
Applicants: MARCONI MEDICAL SYS INC, MARCONI MEDICAL SYSTEMS, INC.
Application year: 1997
Main classification: G01R33
IPC codes: G01R33, A61B5