Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-11-29
2001-10-02
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06297633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance tomography system of the type having gradient coils arranged in a basic field magnet, an RF transmission and reception coil, and a drive and evaluation circuit.
2. Description of the Prior Art
In all conventional magnetic resonance tomography systems, particular value is placed thereon making the magnetic field of the basic field magnet as uniform as possible in the scan volume, i.e. the volume roughly corresponding to a sphere having a diameter of 50 cm within which one strives to obtain an exact image. This volume is also referred to as the DSV (design shim volume). For example, a uniformity of less than 3 ppm required in the DSV. Achieving this uniformity, however, requires an extremely high outlay in the design of the basic field magnet, which is composed of a plurality of superconducting coils. Typically, six coils are required in order to achieve the uniformity cited above as an example.
However, European Application 0 399 789 discloses a magnetic resonance tomography apparatus that makes use of inhomogeneities of the basic magnetic field for imaging purposes. Gradients of the basic magnetic field on the order of 0.1 T/cm are thereby generated, these being superimposed with switched gradient fields having values of, typically, 0.2 T/cm.
German OS 33 04 461 discloses a magnetic resonance tomography apparatus wherein the homogeneity range of the basic magnetic field is less than the measuring field. The gradient field, however, proceeds monotonously over the entire measuring field, preferably increasing/decreasing linearly. A nuclear magnetic resonance tomogram thus exhibits less resolution in the edge region than in the central, diagnostically interpretable region.
SUMMARY OF THE INVENTION
An object of the present invention is to provide magnetic resonance tomography system which can be operated with a more simply constructed, and thus significantly less expensive, basic field magnet, without any noteworthy loss in the quality of the images obtained.
This object is inventively in a magnetic resonance tomography system having a basic field magnetic that is designed such without taking the absolute field uniformity into consideration, and which produces a basic field having a field gradient dB
0z
/du in the direction u in the scan volume that is smaller in the slice-selection direction and in the readout direction than a field gradient G
u
activated in these respective directions with the gradient coils.
The optimization of the magnet so that, instead of the absolute field deviations being taken into consideration, only the adherence to a maximum amount of the field gradient is sought, leads to a magnet structure that is significantly simplified; for example, a basic field magnet having two or three coils can be employed instead of a basic field magnet having six coils. The invention is based on the perception that the image creation, and thus the quality of the image, does not depend on the absolute field uniformity in the scan volume, but only how large the field gradient is. As long as the field gradient of the basic field magnet is smaller in the slice-selection direction and in the readout direction than the field gradient that can be maximally obtained with the gradient coils, unambiguity of the spatial resolution is assured, even though certain distortions may be present.
Optimum unambiguities, and thus optimum imaging conditions, are achieved in an embodiment of the invention wherein the field gradient dB
0z
/du of the basic field magnet is made smaller than the location error of the field gradient of the gradient coils. The location error generated by the field gradient of the basic magnetic field is thus small than the location error generated by unavoidable errors of the field gradient of the gradient coils.
Given employment of line-by-line K-sampling under a readout gradient, an image distortion occurs in the readout direction to the field inhomogeneity (the phasecoding directions, by contrast, are free of distortions). In order also to be able to correct greater distortions, i.e. frequency offsets, with post-processing, the receiver and sampling bandwidth are selected, according to a further embodiment of the invention, so that the regions of greatest B
0
inhomogeneity can still be received as well.
This can be achieved with respect to the sampling bandwidth in a further embodiment of the invention by over-sampling of the time signal. The entire readout time derives from the desired resolution and the minimally achieved readout gradient, i.e. the gradient in the region where readout and field inhomogeneity gradient partially cancel.
It is also within the scope of the invention to calculate the Fourier transformation, which is employed for the date processing to produce an image in the evaluation circuit over the entire sampled time signal, and the image is distortion-corrected by interpolation with the pixel offset known from the field inhomogeneity curve.
REFERENCES:
patent: 5309101 (1994-05-01), Kim et al.
patent: 5739688 (1998-04-01), Krieg
patent: OS 33 04 461 (1984-08-01), None
patent: 0 399 789 (1990-11-01), None
Arana Louis
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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