Magnet system for magnetic resonance imaging
AssigneeU.S. Philips Corporation
InventorsJohannes A. Overweg, Gerardus N. Peeren
Life of this patent
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Abstract
An extremely efficient magnet system having a comparatively large angle of aperture can be obtained for magnetic resonance imaging by a reduction of fields of all orders in a common approach affecting active and passive as well as positive and negative oriented coil elements. Passive soft-magnetic ring segments are arranged in two pairs and located within a plurality of larger diameter active magnetic coils. The active coils include a central coil and outer coils wherein the outer coils are smaller than the central coil. The coils are in a helium Dewar vessel and arranged such that the system has an aperture of about 90.degree.. The central active coil may have a larger radius than the smaller outer coils.
Claims13
A magnet system for generating a steady magnetic field in a measuring space of a magnetic resonance imaging apparatus, comprising:
- a coil system including a plurality of coaxial active magnet coils, a plurality of passive magnetic coil elements comprising closed ring elements forming uninterrupted ring-shaped paths of soft ferromagnetic material which have a diameter less than 0.8 than the inner diameter of said coil system, the passive magnetic coil elements being arranged so as to reduce second-order and higher-order local field contributions of the active magnet coils, said system including means for activating said active magnet coils in a mutually opposed sense for reducing external stray field that otherwise might occur, at least one of the active magnet coils activated in one sense being central to at least two outer active magnet coils activated in the opposing sense, wherein when viewed radially, the passive magnetic coil elements are mounted substantially within the active coil elements, and the magnet system has a central angle of aperture (.alpha.) which is at least approximately 45.degree. and a shield positioned between the active magnet coils and the passive magnetic coil elements such that the angle of aperture of an axially outer active magnet coil is greater than the angle of aperture of the shield.
A magnet system for generating a steady magnetic field in a measuring space of a magnetic resonance imaging apparatus comprising:
- a plurality of coaxial active magnet coils arranged to provide a wide aperture angle but which produce in the measuring space a magnetic field with significant second order and fourth order local field contributions,
- a plurality of passive magnetic coil elements arranged so that the combined effect of the active magnet coils and the passive magnetic coil elements together produce a magnetic field in the measuring space in which said second order, fourth order and higher order local field contributions of the active magnet coils are substantially reduced, and wherein said plurality of active magnet coils comprise, in the axial direction of the magnet system, two outer active magnet coils energized in the same sense and a central active magnet coil energized in the opposite sense relative to the two outer active magnet coils.
A magnet system for generating a steady magnetic field in a measuring space of a magnetic resonance imaging apparatus, comprising a plurality of coaxial active magnet coils and a plurality of passive magnetic coil elements comprising closed ring elements forming uninterrupted ring-shaped paths of ferromagnetic material, wherein said passive elements include two pairs of passive coil elements with an axial-length-to-ring-radius ratio of approximately 0.4 for, as viewed along an axial direction of the magnet system, an inner pair of passive coil elements and approximately 0.8 for an outer pair of passive coil elements, the passive magnetic coil elements being arranged for reducing second-order and higher-order local field contributions of the active magnet coils, said system including means for activating said active magnet coils in a mutually opposed sense for reducing external stray field that otherwise might occur, at least one of the active magnet coils activated in one sense being central to at least two outer active magnet coils activated in the opposite sense.
A magnet system for generating a steady magnetic field in a measuring space of a magnetic resonance imaging apparatus, comprising:
- a coil system including a plurality of coaxial active magnet coils, a plurality of passive magnetic coil elements comprising closed ring elements forming uninterrupted ring-shaped paths of ferromagnetic material, the passive magnetic coil elements being arranged so as to reduce second-order and higher-order local field contributions of the active magnet coils, said system including means for activating said active magnet coils in a mutually opposed sense for reducing external stray field that otherwise might occur, at least one of the active magnet soils activated in one sense being central to at least two outer active magnet coils activated in the opposing sense, and wherein the magnet system has a central angle of aperture (.alpha.) which is at least approximately 45.degree., and said passive elements include two pairs of passive magnetic coil elements wherein one pair is inner and the other pair outer as viewed along an axial direction relative to a reference position, said elements having an axial-length-to-ring-radius ratio of approximately 0.4 for the inner pair and approximately 0.8 for the outer pair.
Dependencies are inferred from the claim text.