Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1989-01-05
1991-10-08
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324309, G01R 3320
Patent
active
050557912
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a nuclear magnetic resonance (NMR) machine phantom and a method of measuring the characteristics of a magnetic field using such a phantom. It finds its use more particularly in the medical field where such machines are used for helping diagnosis in man.
An NMR machine essentially includes means for producing a constant orientator magnetic field, intense and homogeneous in a zone of interest. A body to be examined is placed in this zone and the magnetic moments of the nuclei of the particles of this body tend to be aligned with this orientating field. Under these conditions, the body is subsequently subjected to radiofrequency excitation which causes the orientation of the magnetic moment to swing. When the excitation ceases, the magnetic moments are realigned with the constant field while restituting the energy received during the excitation. The restitution signal is picked up and is processed so as to reveal the structures of the internal parts of the body under examination. The response of the body is a volume response: all the particles subjected to examination emit their restitution signal at the same time. So as to differentiate the signals relative to different parts of the body the excitation is applied in the presence of a space coding. This coding consists in administering additional constant fields, different in different positions in space. In fact, the resonance phenomenon, at each position, depends on the amplitude of the constant field and on the characteristics of the particles examined. In the medical field the particles which it is desired to cause to resonate are the hydrogen atoms contained in water as well as in all organic substances, and whose concentration varies from one tissue to another.
FIELD OF THE INVENTION
The very principle of image forming methods requires the orientating field and coding of the space to be scrupulously accurate. In particular, if the orientating field is not homogeneous, if it is different at different positions of the zone of interest, it introduces parasite coding which falsifies the image. Similarly, the additional coding fields must be known with precision: these coding fields often develop linearly along an axis. Their linear variation is comparable to a gradient: whence their name. The field gradients must be particularly linear. Any distortion of their linearity also introduces parasite coding which falsifies the image. In a first stage, it was considered that the coding parasites affecting the very powerful orientating field were preponderant. Thus, the machines of the prior art include means for correcting the homogeneity of the field, which are adjusted so as to make this field homogeneous. These correction means (called shims in the technique) are adjustable and their value is adjusted on the site as a function of the inhomogeneity discovered.
DESCRIPTION OF THE PRIOR ART
In the prior art, it is known to measure the inhomogeneity of the orientating field in its zone of interest by moving a probe measuring the magnetic field from point to point in this zone. In practice, in a cylindrical zone of interest whose diameter is slightly more than 50 cm and whose height is of the same order, in order to know the homogeneity of the field with sufficient accuracy, it is known to measure its value at a number of points of the order of one or several hundred. Depending on the inhomogeneity discovered, in the known methods, the correction means are adjusted for correcting the homogeneity. For greater accuracy, or quite simply for checking, it may be useful to reiterate, after the first correction, the operation for measuring the homogeneity of the field. More generally, since the intense field is subjected to drifting, it may be useful to re-establish its homogeneity often, for example once per day. So that the measurements are made under good conditions, at each measuring operation the probe must be placed in the same positions of the zone of interest. A mechanical apparatus is then necessa
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Bossaert Jean
LeRoux Patrick
Malgouyres Agnes
General Electric CGR S.A.
Tokar Michael J.
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