Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-10-28
2001-12-04
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000, C324S322000
Reexamination Certificate
active
06326788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus which includes a gradient system for generating a magnetic gradient field in an imaging volume of the apparatus, which gradient system includes a primary gradient coil for generating the gradient field and a shielding coil for actively shielding the space outside the imaging volume from the magnetic field generated in said space by the primary gradient coil.
2. Description of the Related Art
An apparatus of this kind is known from U.S. Pat. No. 5,349,297
A magnetic resonance imaging apparatus for medical purposes, also referred to as MRI apparatus, is arranged to form images of cross-sections of a body. To this end, a strong, steady, uniform field is generated in a volume in such an apparatus which is intended for imaging, i.e. the so-called imaging volume. A gradient field is superposed on said uniform field in order to indicate the location of the cross-section to be imaged. The atoms in the tissue present in the imaging volume are then excited by means of an RF field; the spin resonance signal produced upon relaxation of the excited atoms is then used to reconstruct an image of the cross-section indicated by the gradient field. The steady, uniform field, also referred to as the main field, is generated by means of a coil system (superconducting or not). Together with the associated envelope this coil system is shaped as a short tube in which the imaging volume is situated. The diameter of this tube is determined by the dimensions of the patients to be examined and hence has a given minimum value, for example of the order of magnitude of 90 cm.
The gradient system for generating the magnetic gradient field in the imaging volume is arranged within said tube and around the imaging volume. The gradient system includes gradient coils for generating an associated gradient field; for each of the three co-ordinate directions it includes one set, each of which is referred to as a primary gradient coil. Thus three axial fields are produced with gradients in the three co-ordinate directions x, y and the axial direction z. Current pulses are applied to the gradient coils during the imaging process, so that inevitably magnetic stray fields are produced also outside the imaging volume.
These stray fields are capable of inducing eddy currents in the conductive parts of the apparatus which are present in the vicinity of the gradient coil, notably the metal parts provided for generating the main field, for example the tubular part of the envelope for the coil system, any thermal shields situated within the envelope (in the case of a superconducting coil system) or the coils for the main field themselves. The magnetic fields generated by the eddy currents cause distortions of the image to be formed. Moreover, they cause heat dissipation in the parts carrying the eddy current; this is a drawback notably in the case of a superconducting coil system, because the liquid helium acting as the cooling medium then boils off faster. Finally, they also cause an annoying noise in that the parts conducting the eddy current are situated in a magnetic field and hence are subject to Lorenz forces which cause deformation of said parts.
In order to counteract the above-mentioned adverse effects of the eddy currents, attempts are made to shield or compensate the magnetic fields generated outside the imaging volume by the primary coil. From the cited United States patent it is known that in order to compensate said stray fields a shielding coil is arranged around the primary gradient coil, said shielding coil being intended to provide active shielding of the space outside the imaging volume from the magnetic field generated in said space by the primary coil by compensation. Active shielding is to be understood to mean herein the shielding by generating a compensating magnetic field by means of a shielding coil other than the primary coil. Such a shielding coil can be connected in series with the primary coil or be controlled independently of the primary coil; the latter case is referred to as independent active shielding.
Even though a shielding coil for active shielding offers a substantial reduction of the stray field, in practical circumstances a given amount of stray field always remains. This can be explained in that inter alia the shielding coil is made of discrete turns and in that dimensional deviations inevitably occur due to manufacturing tolerances, so that magnetic flux can still escape, between the shielding turns, to the area surrounding the system formed by the gradient coil and the shielding coil.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a magnetic resonance imaging apparatus of the kind set forth in which the actively shielded gradient system offers a substantial reduction of the eddy currents generated in the main field magnet and also a substantial reduction of the noise caused by the gradient system.
To achieve this, the magnetic resonance imaging apparatus according to the invention is characterized in that the gradient system also includes an eddy current shield which is formed by an electrically conductive, substantially closed plate, and that the primary gradient coil and the shielding coil are arranged within the eddy current shield and constitute a mechanically rigid unit in conjunction therewith.
Because the coil system per se already forms a more or less shielded system, the residual stray field is still small in comparison with a non-shielded situation. When the electrically conductive eddy current shield is formed from a substantially closed plate in which the two coils are arranged, the escape of magnetic flux to the environment is prevented so that said problems regarding heat dissipation and production of noise in the main field magnet are avoided.
When the gradient system thus constructed is arranged in the main field of an MRI apparatus, the main field will not exert Lorenz forces on the system as a whole. This can be readily understood, considering the fact that the system does not generate a magnetic field at the area of the main field coils, so that the system cannot exert Lorenz forces on the main field coils. However, this also means that no reactive force (i.e. a force exerted on the gradient system by the main field) can be present either. This itself means that overall the main field does not exert a force (i.e. no moving force) on the gradient system, thus eliminating this cause of annoying noise.
Finally, the two coils and the eddy current shield together form a mechanically rigid unit so that the internal forces in the gradient system constructed according to the invention (which could not cause a displacement of the system as a whole but still a distortion of this system) can cause a slight distortion only of the mechanically rigid assembly. The requirement as regards noise reduction is thus satisfied to a high degree.
Upon installation of known gradient systems it is necessary to adjust the gradient system in dependence on the specific properties of the combination formed by the gradient system and the remainder of the apparatus, notably the main field magnet. This is because of the inevitable manufacturing tolerances, notably in the frequently occurring cases where the manufacturer of the gradient system is not the manufacturer of the system. The manufacturer of the gradient system can still manufacture within the specified tolerances, but when the gradient system is built into the system different adjusting values will be required for each combination. The gradient system constructed according to the invention, however, can be adjusted by the manufacturer as one unit in such a manner that this gradient system does not exhibit a stray field or only an insignificant stray field, so that no further adjustment of the
Ham Cornelis L. G.
Mateboer Aart J.
Mulder Gerardus B. J.
Roozen Nicolaas B.
Verbunt Johannes P. M.
Patidar Jay
Shrivastav Brij B.
U.S. Philips Corporation
Vodopia John F.
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