1. Field of the Invention
The invention is directed to an electrical conductor arrangement, particularly for a magnetic resonance apparatus.
2. Description of the Prior Art
Electrical conductor arrangements are utilized in many areas of technology, for example for generating predetermined magnetic fields. In a magnetic resonance apparatus, thus, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic magnetic field magnet system. A conductor arrangement of a gradient coil of the gradient coil system can be defined, for example, according to a method as described in German OS 42 03 582 and/or German OS 197 26 332. Further, the magnetic resonance apparatus has a radiofrequency system that emits radiofrequency signals into an examination subject for triggering magnetic resonance signals and that picks up the generated magnetic resonance signals on the basis of which magnetic resonance images are produced.
A gradient coil of the gradient coil system generates a gradient field for a specific spatial direction. This gradient field has only one principal field component that is co-linear with the basic magnetic field, at least within a linearity volume in the desirable, ideal case. The principal field component has a prescribable principal gradient that, independent of location, is of approximately the same size at every arbitrary point in time, at least within the linearity volume. Since the gradient field is a temporally variable magnetic field, the above in fact applies for every point in time; an intensity of the principal gradient, however, is variable from one point in time to another point in time.
Due to Maxwell""s fundamental equations and contrary to the desired ideal case, however, no gradient coils can be fashioned that exclusively produce the principal field component over the linearity volume. At least one accompanying field component that is directed perpendicularly to the principal field component accompanies the principal field component. In some applications, the accompanying field components of gradient fields lead to a falsification of examination results.
The gradient coil system is usually surrounded by conductive structures wherein eddy currents are induced by the switched gradient fields. Examples of such conductive structures are the inner cryoshield of a superconductive basic field magnet system, the copper foil of the radiofrequency shielding and the gradient coil system itself. The fields generated by the eddy currents are undesired because, among other things, they weaken the gradient field unless counter-measures are taken and distort it in terms of its time curve. For example, this leads to degradations in the quality of magnetic resonance images.
The eddy current fields can be compensated to a certain extent by a corresponding pre-distortion of a reference current quantity of the gradient coil. However, only eddy current fields that similarly image the gradient field in a mathematical sense, i.e. are similar to the gradient field in terms of their field course, can be compensated by the pre-distortion. Since the eddy currents also do not similarly image the gradient field, additional, spatial field distortions of a higher order arise. Given actively shielded gradient coil systems, these latter field distortions can be compensated by a corresponding arrangement of conductors of shielding coils. This requires a correspondingly high mechanical precision in the structure of the gradient coil system, which involves high manufacturing costs.
In magnetic resonance technology, a homogeneity of the basic magnetic field is a decisive factor for the quality of the magnetic resonance images. Field inhomogeneities of the basic magnetic field within an imaging volume of the magnetic resonance apparatus thereby cause geometrical distortions of the magnetic resonance image that are proportional to the field inhomogeneities. The field inhomogeneity is especially important in fast pulse sequences, for example in the echo planar method.
Measures for improving the basic magnetic field homogeneity are referred to as shimming measures. A distinction is made between passive and active shimming measures. In the active shimming measure, correction coils, referred to as shim coils, are utilized in a shim coil arrangement, these homogenizing the basic magnetic field. A power pack device that delivers highly constant and reproducibly adjustable dc currents is required for the operation of the shim coil arrangement.
As disclosed, for example, in German AS 195 11 791. the basic magnetic field within he imaging volume can be described with coefficients of spherical harmonic functions. This German Patent also discloses that linear basic magnetic field deviations, i.e. field disturbances of the first order, can be compensated by charging gradient coils with an offset current. The offset current is a constant current that is superimposed on a current implementing a gradient sequence. For compensating field inhomogeneities of a higher order, a shim coil that essentially compensates the corresponding field disturbance and that is charged with a suitable current for this purpose is respectively provided according to the order to be compensated. Since a shim coil is to be provided for each order to be compensated but the space that is available for shim coils, for example within the gradient coil system, is tightly limited, limits are placed on the aforementioned compensation.
An object of the invention is to provide an improved electrical conductor arrangement, particularly for a use in a magnetic resonance apparatus, with which, among other things, the aforementioned disadvantages of the known arrangements can be alleviated.
This object is achieved in accordance with the principles of the present invention in an electrical conductor arrangement, particularly for a magnetic resonance apparatus, having a number of conductor meshes which are arranged in areas limiting lines defined by a network structure, and a number of control devices electrically connected in the respective conductor meshes for generating respective currents in the conductor arrangement.
A selected planar current distribution can be set and regulated as desired in a broad range as a result of an inventive electrical conductor arrangement that has conductor meshes that are arranged in surfaces whose limiting lines are determined by a network structure and into which control devices that are fashioned for the control of currents within the conductor arrangement are electrically linked. For example, the inventive electrical conductor arrangement represents a flexibly employable magnetic field generating unit wherein the field properties can be freely parameterized, at least within a broad range.
In an especially advantageous way, the inventive electrical conductor arrangement can be utilized in a magnetic resonance apparatus, for example within a gradient coil system, for compensating field errors as a consequence of eddy currents and/or of accompanying field components of gradient fields and/or of inhomogeneities of a basic magnetic field. For setting the corresponding planar current distribution, the control devices are parameterized or controlled using known pulse sequences for location-dependent field dynamics and known design methods for determining an optimum planar current distribution given gradient coils. The method for setting currents in a shim coil arrangement according to the initially cited German AS 195 11 791 is referenced by an example of known pulse sequences for location-dependent field dynamics. German OS 42 03 582, which has likewise already been cited, is an example of known design methods for determining an optimum planar current distribution. With the inventive electrical conductor arrangement, which, for example, always has an identical structure in hardware terms, magnetic field distortions as a consequence of individual manufacturing tolerances of magnetic field-generating components of a magnetic resonance apparatus can be individually compensated in a simple way by a corresponding parameterization of the control devices of the conductor arrangement.
In one embodiment, the inventive electrical conductor arrangement is fashioned as a flexible printed circuit board constructed single-layer or multi-layer. Given use in a gradient coil system of a magnetic resonance apparatus, for example, this can be cast together with further components of the gradient coil system.
In addition to being employed as a field-correcting device, one or more of the inventive electrical conductor arrangements can be utilized in a magnetic resonance apparatus as magnetic field-generating components in the sense of a gradient coil and/or of a basic field magnet and/or as an apparatus in the sense of a radiofrequency antenna for transmitting radiofrequency signals and/or for receiving magnetic resonance signals.