Magnet system for magnetic resonance imaging

Abstract: An extremely efficient magnet system having a comparatively large angle of aperture can be obtained for magnetic resonance imaging by pursuing, instead of elimination of second-order and fourth-order fields in a magnet field in a measuring zone, a reduction of fields of all orders in a common approach affecting active and passive as well as positive and negative oriented coil elements.

Description: The invention relates to a magnet system for generating a steady magnetic field in a measuring space of a magnetic resonance imaging apparatus, comprising a combination of a plurality of coaxial active magnetic coils and a plurality of passive magnetic elements in the form of ring segments of a soft magnetic material. A magnet system of this kind is known from EP-A-0 216 404. A steady magnetic field B generated by a coaxial coil system and extending along an axis Z of the magnetic system generally satisfies: B = A 0 Z 0 + A 1 Z 1 + A 2 Z 2 + A 3 Z 3 + A 4 Z 4 + A 5 Z 5 + A 6 Z 6 + ... Coil systems for magnetic resonance imaging apparatus are customarily designed so that no odd-order terms occur in the magnetic field. In practical magnets this is usually achieved by constructing the coil system using coils which are symmetrically arranged with respect to a central plane and by activating all coils in the same sense. Furthermore, known coil systems are designed so that second-order and fourth-order contributions to the magnetic field are negligibly small in a measuring space to be selected. This condition limits the degree of freedom as regards the positioning of magnet coils of the magnet system with respect to one another in the axial direction, so that a minimum length to diameter ratio is imposed thereon. It is to be noted that in a practical magnet system the length increases and the effective diameter decreases due to necessary minimum dimensions of the cryostat to be used. EP-A-0 216 404 describes how, using ferromagnetic rings, higher-order field contributions, i.e. in this case sixth-order and higher-order fields, are reduced so that a magnetic field having adequate spatial field homogeneity is realised in a measuring space. According to said document a field correction, achieved by means of magnetic material, is used to realise an improved field homogeneity in the measuring space. As mentioned before, a reduction of the second-order and fourth-order field contributions is obtained by the design of the active magnetic coils of the coil system. This condition imposes limitations on the design of the coil system which strongly hamper a reduction of the length of the magnet system. A short magnet system is described in EP-A-0 160 350. Such a system is realised by constructing a magnet system using concentrically situated coils and by activating these coils with respect to one another so that again a measuring space with a spatially homogeneous steady magnetic field is created. However, such a magnet system is very inefficient and hence less suitable for generating comparatively strong steady magnetic fields. It is an object of the invention to provide a magnet system of the kind set forth which can have the advantage of a reduced length without having the disadvantage of being very inefficient. To achieve this, the magnet system in accordance with the invention comprises passive magnetic elements have a smaller radius than the active magnet coils and that are arranged such that the combined effect of the active magnet coils and the passive magnetic elements produces a magnetic field in the measuring space in which second order and higher order local field contributions of the active magnet coils are substantially reduced, the magnet system is characterized in that said plurality of active magnet coils comprises, 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 central angle of aperture of the magnet system amounting to at least 90°. Because second order and fourth order contributions of the active magnet coils are substantially reduced by the combined effect of the active magnet coils and the passive magnetic elements, the condition as regards the second-order and fourth-order field contribution can be ignored in the design of the active magnet coils of the magnet system in accordance with the invention. As a result, a substantially shorter magnet system comprising substantially coaxially arranged coil elements can be realised. This enables the magnet system to have a substantially larger angle of aperture, which angle is defined as the maximum aperture of a cylindrical open space of the magnet, viewed from the magnet centre. By adapting the position of the passive magnetic elements, excessively strong magnetic fields can be locally compensated for. The passive magnetic elements do not require compensation, for example in the form of a Dewar vessel. For that reason, the passive magnetic elements can have a smaller radius than the active magnet coils so that a larger angle of aperture (amounting to at least 90°) can be realised for the magnet system. It is to be noted that an article in Magnetic Resonance in Medicine, 1, 44-46 presents a mathematical framework for the analysis and production of localized magnetic field profiles. However, this article does not disclose how a magnet system can be designed that has the advantage of a reduced length (large angle of aperture) without being very inefficient. A preferred embodiment is characterized in that in order to achieve stray field compensation the diameter of the central magnet coil is substantially larger than that of the outer coils an has a number of ampère-turns adapted for this purpose. In a preferred embodiment, four passive elements in the form of rings of soft magnetic material are included in the described system. The magnetic rings have a diameter of, for example from 0.5 to 0.7 times the inner diameter of the active coils. In a preferred embodiment the length-to-diameter ratio of a central pair of rings is, for example approximately 0.4 and that of an outer pair of rings is approximately 0.8. The length-to-diameter ratio of the coil system as a whole may be, for example approximately 0.6 or less. The pairs of rings need not have the same diameter, it being possible to adapt the cross-section of the rings to desired field contributions or mounting conditions. Moreover, portions of the rings or additional passive elements in the form of rings can be mounted so that they can be trimmed, i.e. axially adjustable to a slight degree in an extremely controlled manner, or they may contain ferromagnetic material that can be replaced. In order to achieve a stray field reduction in a further preferred embodiment, such a diameter is imparted to a centrally disposed active coil element that it can be activated using approximately 30% of the number of ampére-turns of each of the outer coils. Depending on the geometry of the passive elements, the diameter of the central coil then amounts to approximately 1.5 times that of the outer active coil elements. The dimension of a coil system in accordance with the invention can be scaled up or down to a high degree, scaling taking place substantially along radians from the coil centre. For the sake of stability all coils of notably superconducting magnet systems are connected in series. Some preferred embodiments in accordance with the invention will be decribed in detail hereinafter with reference to the drawing. Therein: Fig. 1 diagrammatically shows a magnet system of a magnetic resonance imaging apparatus, and Figs. 2 and 3 show embodiments of magnet systems in accordance with the invention. A magnet system for a magnetic resonance imaging apparatus as shown in Fig. 1 comprises a coil former 2 with electromagnetic coils 4, also referred to as active coil elements hereinafter, and magnetizable coil elements 6, for example consisting of rings or ring segments of a ferromagnetic material such as iron, also referred to as passive coil elements hereinafter. Around the coil former 2 there is provided a shield 8, for example for electrical insulation, for thermal insulation or for both. In customary superconducting magnets, a helium Dewar vessel forms part of the shield and is, therefore, comparatively thick because of the necessity of, for example thermal shields to be evacuated. When use is made of high Tc superconducting current conductors, cooling can be realised, for example by means of liquid nitrogen so that the shielding volume can be reduced, be it only slightly. Such a coil system customarily has a circular-cylindrical construction which is mirror-symmetrical with respect to a symmetry plane 10 extending transversely of an axis 12 through a coil centre 14. A theoretical length-to-diameter ratio of such a coil system is given by a diameter D T of the active coil elements 4, or at least of the extremely outwards situated coil elements, and a length L T measured between the axial ends of the outer active coil elements 4. Said dimensions result in a theoretical angle of aperture α T for the magnet system. In a practical magnet system, a length L, a diameter D and an angle of aperture α are as indicated. A preferred embodiment of a superconducting magnet system in accordance with the invention as shown in Fig. 2 comprises three active coil elements with a central coil 40 and outer coils 44. The coils are accommodated in a helium Dewar vessel 20 which in this case comprises two vacuum walls 22 and 24 and a nitrogen wall 26. The helium space 21 is provided with an opening 28 for replenishing the liquid helium. It is to be noted that a coil system as described thus far will exhibit a second-order and fourth-order with, for example a positive second-order deviation, calculated from the system centre. The central coil 40 may altenatively be subdivided into two coil halves which are preferably symmetrically arranged with respect to the symmetry plane 10; however, in this context it is not the object to eliminate said second-order and fourth-order faults in such a manner. In the described embodiments magnetizable passive elements 60 and 64, having a substantially smaller radius, are included in the magnet system. The coil elements together realise an optimum uniform magnetic field in a measuring space 30. Such a coil design has an angle of aperture α of at least 90°, for example for a measuring space having a diameter of, for example 20 cm. In the present configuration the current direction in the outer coils 44 opposes that in the central coil 40 with, for example approximately 80.000 ampére-turns for the central coil and, for example approximately 250,000 ampére turns for the outer coils. Fig. 3 shows a coil system having a strongly reduced external stray field. Therein, a substantially larger radius is imparted to a central coil 40 and the number of ampére-turns thereof has been increased to, for example approximately 350.000 in order to bring the field contribution thereof to the desired level again. The number of ampére-turns of the outer coils 44, having the opposite direction, then amounts to approximately 30.000. Even though not shown in the diagrammatic representation, the position, the geometry and the iron contents of the passive coil elements have been adapted to the new coil construction in order to optimize field homogeneity. Such a magnet system has approximately the same angle of aperture and permissible measuring volume as the magnet system shown in Fig. 2 and its construction as regards Dewar vessel and shields is also similar. On the basis of the idea of the invention, magnet systems can also be designed for which the symmetry with respect to the symmetry plane 10 is no longer taken into account, for example by imparting a deviating diameter to coil elements, pasive or active, on one side of the symmetry plane. Thus, a magnet system can be designed which need not be larger for an equal or larger measuring volume. Such a measuring volume is in that case accessible, for example from one side only, or may even be completely closed on one side so that it is particularly suitable for examination of peripheral parts of patients, such as arms (hands), legs (feet) and the head.

Claims: A magnet system generating a steady magnetic field in a measuring space of a magnetic resonance imaging apparatus, comprising a combination of a plurality of coaxial active magnet coils and a plurality of passive magnetic elements in the form of ring segments of a soft magnetic material, the passive magnetic elements having a smaller radius than the active magnet coils and are arranged such that the combined effect of the active magnet coils and the passive magnetic elements produces a magnetic field in the measuring space in which second-order and higher-order local field contributions of the active magnet coils are reduced, characterized in that said plurality of active magnet coils comprises, 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, the central angle of aperture of the magnet system amounting to at least 90°. A magnet system as claimed in Claim 1, characterized in that in order to achieve stray field compensation the diameter of the central magnet coil is substantially larger than that of the outer coils and has a number of ampere-turns adapted for this purpose. A magnet system as claimed in Claim 1 or 2, characterized in that passive magnetic elements therein are constructed so that they can be trimmed.

Inventors: OVERWEG JOHANNES ADRIANUS, PEEREN GERARDUS NERIUS, OVERWEG, JOHANNES ADRIANUS, PEEREN, GERARDUS NERIUS
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: G01R33
IPC codes: G01R33, A61B5