Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-02-28
2004-02-24
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S318000
Reexamination Certificate
active
06696836
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance apparatus with a gradient system containing at least one gradient coil for generating a gradient field at least within an imaging volume and at least one shield coil operable independently of the gradient coil for generating a shielding field with which the gradient field can be counteracted in a prescribable region, and is also directed to a magnetic resonance apparatus for the implementation of the method.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for acquiring images of the inside of the body of an examination subject. Rapidly switched gradient fields that are generated by a gradient system are superimposed in a magnetic resonant apparatus on a static basic magnetic field that is generated by a basic field magnet system. The magnetic resonance apparatus further has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and that registers the generated magnetic resonance signals, from which magnetic resonance images are produced.
The gradient system contains a gradient coil system and a gradient amplifier and control unit. The gradient coil system usually has three gradient coils. Each of the gradient coils generates a gradient field for a specific spatial direction that, in the desired ideal case, is exclusively formed by a primary field component that is collinear with the basic magnetic field at least within an imaging volume. The main field component has a prescribable gradient that, in the desired ideal case, is of the same magnitude independently of location at every arbitrary point in time, at least within the imaging volume. Since the gradient field is a time-variable magnetic field, this in fact applies for every point in time; however, the intensity of the gradient is variable from one point in time to another. The direction of the gradient is usually permanently prescribed by the design of the gradient coil. As a result of Maxwell's fundamental equations, however, and contrary to the desired ideal case, no gradient coils can be formed that produces only the aforementioned primary field component over the imaging volume. Among other things, at least one accompanying field component that is directed perpendicularly to the primary field component unavoidably accompanies the primary field component.
Appropriate currents are set in the gradient coil for generating the gradient field. The amplitudes of the required currents amount to up to several hundred amperes. The rise and decay rates of the current amount to up to several hundred kA/s. For power supply, the gradient coils are connected to the gradient amplifier and control unit.
The gradient coil system usually is surrounded by conductive structures wherein eddy currents are induced by the activated gradient fields. Examples of such conductor conductive structures are the vacuum vessel and/or cryoshield of the superconducting basic field magnetic system, copper foil of the radio-frequency shielding and the gradient coil system itself. The fields generated by the eddy currents are unwanted because, without counter-measures, they attenuate the gradient field and distort it in terms of its time curve. This leads to degradation of the quality of the magnetic resonance images. Further, the eddy currents induced in the superconducting basic field magnet system cause a heating of the basic magnetic system, so that a considerably increased cooling power must be exerted for maintaining the super-conduction. In the case of a basic field magnetic system with a permanent magnet, the heating as a consequence of the eddy currents leads to an unwanted modification of the properties of the basic magnetic field and, further, the eddy currents can even produce a demagnetization of the permanent magnet.
Such eddy current fields can be compensated to a certain degree by a suitable pre-distortion of a reference current quantity of the gradient coil. With the pre-distortion, however, only eddy current fields can be compensated that image the gradient field similarly in the mathematical sense, i.e. are the same as the gradient field in terms of their spatial course. The principle functioning of such pre-distortion is disclosed in U.S. Pat. No. 4,585,995. The calculation of the pre-distortion is thereby essentially based on the perception that excited and decaying eddy currents can be described by a specific number of exponential functions having different time constants. Transferred to an electrical network for the compensation of eddy current fields, this means that the pre-distortion can be implemented with filters having different limit frequencies. The setting of the time constants or limit frequencies ensues, for example, by an operator who determines the optimum values at the installed magnetic resonance apparatus by step-by-step variation of settings of the pre-distortion and repeated checking. In another embodiment, the setting of the time constants or limit frequencies ensues automatically. The latter is disclosed, for example, in U.S. Pat. No. 4,928,063.
When implementing a sequence, the pre-distortion of the reference current quantity should be continuously implemented during the entire time execution of the sequence. Due to the pre-distortion of the gradient field amplifier and control unit, power reserves must be kept available that generate a higher power and thus resulting in a more costly dimensioning of the gradient amplifier and control unit.
Since, however, the gradient field also produces eddy current fields whose spatial curves are not the same as the gradient field, additional spatial field distortions of a higher order arise. In order to largely compensate these field distortions, actively shielded gradient coils are among the measures utilized. A shield coil belonging to the gradient coil is designed for this purpose such that the gradient field can be neutralized (counteracted) in a prescribable region, usually in a vacuum container surrounding the gradient coil system or a cryoshield of a superconducting basic field magnet system. To this end, the shield coil usually has a lower number of turns than the gradient coil and is interconnected with the gradient coil so that the shield coil has the same current therein as the gradient coil, but flowing in the opposite direction. Further, the shield coil has an attenuating effect on the gradient field in the imaging volume; an attenuation of the actually useful gradient field of up to half in the imaging volume must be accepted. A gradient coil with an appertaining shield coil for neutralizing a gradient field on a defined area is disclosed, for example, in British Specification 2 180 943.
Further, German OS 34 11 222 discloses a magnetic resonance apparatus that has three gradient coils for generating gradient fields and at least one further coil arrangement operable independently of the gradient coils for generating a magnetic field that proceeds in the direction of a basic magnetic field. The further coil arrangement is designed such that the magnetic field changes in a spatially non-linear fashion and such that a superimposition of the magnetic field with the gradient fields yields a defined, time-spatial modification of the magnetic flux density. The further coil arrangement is fashioned such in one embodiment so that the magnetic field has a spatial course that corresponds to a spherical function of the second or third order. In particular, the unwanted eddy current effects caused by the gradient fields can be eliminated with the further coil arrangement.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved method for operating a magnetic resonance apparatus as well as a magnetic resonance apparatus for the implementation of the method with which, among other things, high gradient intensities can be achieved.
In the inventive method for operating a magnetic resonance app
Fetzner Tiffany A.
Gutierrez Diego
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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