Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-12-21
2003-03-11
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S319000
Reexamination Certificate
active
06531870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus.
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. In a magnetic resonance apparatus, 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 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 picks up the generated magnetic resonance signals. A computer produces magnetic resonance images on the basis of these signals.
The gradient coil system thereby usually has three gradient coils. Each of the gradient coil generates a gradient field for a specific spatial direction and, in the desired, ideal case, exclusively generates a main field component that is co-linear with the basic magnetic field, at least within an imaging volume. The main field component has a prescribable principal gradient that, in the desired, ideal case, is of the same magnitude independent of location at any arbitrary point in time, at least within the imaging volume. Since the gradient field is a time-variable magnetic field, these features apply for every point in time; however, the intensity of the principal gradient is variable from one point in time to another point in time. The direction of the principal gradient is usually rigidly prescribed by the design of the gradient coil.
Due to Maxwell's fundamental equations and contrary to the desired, ideal case, however, no gradient coil can be fashioned that exclusively generates the aforementioned main field component across the imaging volume. Among other things, at least one accompanying field component that is directed perpendicularly to the main field component unavoidably accompanies the main 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 100 A. The current rise and decay rates (slew rate) amount to up to several 100 kA/s. The gradient coils are connected to a controlled gradient amplifier for the current supply.
The gradient coil system is usually surrounded by conductive structures, in which eddy currents are induced due to the switched gradient fields. Examples of such conductive structures are a vacuum vessel and/or a cryo-shield of a superconducting basic field magnet system. The fields generated by the eddy currents are unwanted because they weaken the gradient fields and distort its time curve if counter-measures are not undertaken. This leads to a degradations of the quality of magnetic resonance images. Further, the eddy currents induced in components of a superconducting basic field magnet system effect a heating of these components, so that a noticeably increased refrigerating capacity must be employed for maintaining the super-conduction. Given a basic field magnet system with a permanent magnet, the heating as a consequence of eddy currents leads to an unwanted modification of properties of the basic magnetic field, and, further, the eddy currents can even produce a re-magnetization of the permanent magnet.
Such eddy current fields can be compensated to a certain degree by a corresponding pre-distortion of a reference current quantity of the gradient coil. However, only eddy current fields that are similar to image the gradient field, i.e. are like the gradient field in terms of their course, can be compensated by the pre-distortion. (“Similar” is used herein in the geometric sense.) The basic functioning of the known pre-distortion technique is disclosed, for example, in U.S. Pat. Nos. 4,585,995 and in 4,703,275. The calculation of the pre-distortion is thereby essentially based on the perception that induced and decaying eddy currents can be described by a specific number of e-functions of different time constants.
Since, however, there are also eddy currents that are not similar to the gradient field, additional spatial field distortions of a higher order arise. Among other things, actively shielded gradient coils are utilized in order to largely compensate these field distortions. A shielding coil belonging to a gradient coil usually has a lower number of turns and is interconnected with the gradient coil such that the shielding coil has the same current therein as the gradient coil but in the opposite direction. Limits exist on the compensating effect of the shielding coil because, due to a conductor arrangement of the shielding coil, a current flow can be controlled only in the rigidly prescribed paths corresponding to the conductor arrangement. Further, the shielding coil develops its compensating effect and, accompanying this, an attenuation of the gradient field in the imaging volume of the magnetic resonance apparatus, regardless of whether the gradient field is switched rapidly or slowly. Particularly given extremely low frequencies of the gradient field, the compensating effect of the shielding coil is not required since a switched gradient field with very low frequencies causes hardly any eddy currents.
SUMMARY OF THE INVENTION
An object of the invention is to provide an improved magnetic resonance apparatus wherein—among other things—unwanted consequences of a switched gradient field can be controlled in an economic way.
This object is achieved in a magnetic resonance apparatus having a gradient coil system having at least one gradient coil for generating a magnetic gradient field, and an electrically conductive structure that is arranged and fashioned such that the gradient field is damped in a region outside the imaging volume of the magnetic resonance apparatus, and such that a magnetic field caused by the gradient field via induction effects is similar in structure to the gradient field, at least within the imaging volume.
As a result, a gradient coil system wherein shielding coils can be completely foregone can be fashioned for a magnetic resonance apparatus. Compared to a gradient coil system with shielding coils, this means a substantial savings potential in view of volume material, and costs. The inherently unwanted consequences of the switched gradient field are completely governed due to the presence of the structure in combination with the initially described pre-distortion. Compared to the solution with shielding coils, a further advantage of the magnetic resonance apparatus with the electrically conductive structure is that the effect of the structure is merely limited to time-varying gradient fields; a maximally obtainable gradient intensity for a longer time span with a gradient field that does not vary over time is not reduced.
For a gradient coil system that is fashioned approximately hollow-cylindrically, the electrically conductive structure is fashioned approximately like the shell of a barrel and, for example, is arranged between the gradient coil system and a basic field magnet system of the magnetic resonance apparatus. An exact fashioning of the structure is dependent on the conductor arrangement of the gradient coils and can be exactly defined via a numerical optimization method.
The exact fashioning of the structure can be determined, for example, with a procedure wherein conductor arrangements of gradient coils are prescribed as non-dislocatable, and the optimum fashioning of the structure is sought proceeding from a start value of the structure, for example from a fashioning as ideal barrel shell. To that end, an eddy current distribution on the structure that is caused by a flow of current in one of the gradient coils is calculated, with the assistance of a finite element method via the quasi-static Maxwell equations. The eddy current distribution thereby causes a magnetic eddy current field that disturbs magnetic resonance exposures. Further, an evaluation criterion is defined, for example an average
Heid Oliver
Vester Markus
Arana Louis
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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