Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-03-10
2001-09-18
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S319000, C324S313000
Reexamination Certificate
active
06291998
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnet assembly for generating the basic magnetic field in a magnetic resonance imaging (MRI) device.
2. Description of the Prior Art
A magnet is a component of a magnetic field generation unit of an MRI device. The magnet in such an assembly has a patient tube in which a patient lies during his/her examination. The magnetic field generation unit generates a substantially homogenous and temporally constant (main) magnetic field (also called a basic magnetic field or B
0
field) in an examination volume (imaging volume) situated inside the patient tube. Because of the necessary homogeneity of the examination volume, this is also called a homogeneity volume.
The minimum required extent of the generally spherical homogeneity volume is determined by the fact that signal pickups must be possible at spatially varied positions of the patient. Thus, for example, head surveys are performed centered with respect to the magnetic longitudinal axis, while heart imaging occurs outside the magnetic longitudinal axis. The required minimum extent of the homogeneity volume thus must be relatively large, although the examined organs or body regions themselves would require only a small homogeneity volume.
The extent of the homogeneity volume is essentially determined by the length of the magnet. The size (length and diameter) of the magnet determines the cost of the magnet, with a larger size naturally resulting in a correspondingly higher cost due to the basis of the required basic homogeneity. Furthermore, in long patient tubes, which are at least as long as the patient himself or herself, the patient may experience anxiety during the examination.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnet assembly which enables the construction of a cost-effective and patient-friendly MRI device.
This object is inventively achieved in a magnet assembly having a magnet according with a patient receptacle inside of which the homogeneity volume is situated. This homogeneity volume is inventively displaceable in space by an auxiliary means. In the inventive magnet according to the invention, a &Dgr;B
0
field is easily impressed on the B
0
field (main magnetic field), thus making it possible to achieve a spatial displacement, and thus more than one single positioning, of the homogeneity volume.
The inventive solution is suitable for a number of MRI devices. For example, the magnet can be constructed as a cylindrical magnet (solenoid) or as a horseshoe magnet (C-arm MRI device). Given cylindrical magnets, the patient receptacle is constructed as a patient tube.
In the inventive magnet, the homogeneity volume is adapted exclusively to the size of the examined organs or body regions, such as the heart or head. Thus, the inventive magnet has a considerably smaller homogeneity volume than conventional MRI magnets, since the varying position of the examined organs or body regions no longer needs to be considered in the dimensioning of the homogeneity volume. In the inventive magnet, the varying position of the examined organs or body parts is taken into account by the displaceability of the homogeneity volume. Accordingly, the smaller homogeneity volume compared to known magnets is inexpensive to produce, since the inventive magnet is constructed smaller, particularly shorter. The shorter length of the inventive magnet compared to conventional magnets also leads to a correspondingly shorter patient tube in cylindrical magnets, resulting in a greater comfort for the patient and easier access.
Any field non-homogeneities which may arise in a displacement of the homogeneity field can be arithmetically determined and compensated by the usual measures.
In the context of the invention, the homogeneity volume can be displaced in all directions. In general, however, a displaceability along the x-axis should be sufficient. A displaceability along the y-axis (perpendicular to the z-axis and to the x-axis) is only necessary given a very small homogeneity volume. A displaceability of the homogeneity volume in the direction of the z-axis (i.e. in the direction of the longitudinal axis of the magnet) is usually not necessary, since, in this case, a displacement of the patient bed in the patient tube is simpler, circumstances permitting.
The auxiliary means for the spatial displacement of the homogeneity volume can be formed by displacement coils or ferromagnetic displacement elements. A combination of displacement coils and displacement elements also can serve for the spatial displacement of the homogeneity volume.
REFERENCES:
patent: 4015196 (1977-03-01), Moore et al.
patent: 4240439 (1980-12-01), Abe et al.
patent: 4737754 (1988-04-01), Goldie
patent: 4985679 (1991-01-01), McDougall
patent: 5121060 (1992-06-01), Doddrell et al.
patent: 5365172 (1994-11-01), Hrovat et al.
patent: 5646532 (1997-07-01), Knuttel et al.
patent: 5969525 (1999-10-01), Van Driel et al.
patent: 0 710 851 (1996-05-01), None
“Dynamic Shimming for Multi-Slice Magnetic Resonance Imaging,” Morrell et al., MRM vol. 38 (1977), pp. 477, 483.
Fetzner Tiffany A.
Oda Christine
Schiff Hardin & Wait
Siemens Aktiengesellschaft
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