Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1998-11-25
2001-02-20
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S320000
Reexamination Certificate
active
06191584
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a permanent magnet for nuclear magnetic resonance image detection, either of the total-body type, for receiving a considerable part of the body, or the whole body, or of the dedicated type, i.e., for detecting nuclear magnetic resonance images only relating to certain parts of the human body.
2. Description of Related Art
In order to obtain spin-echo signals for reconstructing sufficiently faithful images of the part under examination, i.e., valid images, therefrom, the body, or the part thereof under examination must be exposed, in nuclear magnetic resonance machines, to a static magnetic field, having, in the image detection region, certain intensity and homogeneity characteristics. These characteristics are determined by predetermined tolerances.
As extensively discussed in U.S. Pat. No. 5,495,222, an ideal static magnetic field is obtained through a magnetic structure which closes the cavity or the volume for receiving the body or the part thereof on all sides.
Theoretically, the provision of magnets of this type is possible, but they must be considerably large, so as to allow the whole body, for example of a patient, to be received therein. Such a machine is definitely expensive and cumbersome, and involves safety and comfort drawbacks for the patient.
Therefore, prior art machines have cavity-delimiting magnetic structures with at least one open side, with two open sides, generally opposite to each other, or with three or more open sides, like in C-shaped magnetic structures, and in structures having only two opposite and spaced poles and being open all along the peripheral shell of the two poles, except for supporting members, like columns.
Owing to the provision of one or more open sides of the magnetic structure, in that region the magnetic field is diffused out of the structure. Here, the field lines are outwardly swelled, and the field intensity and homogeneity progressively decrease, as the field approaches the open side or sides of the structure. The magnetic field volume having sufficient intensity and homogeneity characteristics for image detection is thus reduced, with respect to the total volume of the cavity defined by the magnet, and with respect to the total volume of the structure thereof.
The immediate solution thereto would be an enlargement of the magnet structure, so as to keep the useful image detection volume as distant as possible from the open side or sides. However, also owing to the necessary relationship between the useful volume and the volume or extension of the body, or the part thereof, to be examined, this solution involves very unfavorable ratios between the total magnet size and the useful image detection volume, i.e., the volume within which the static field has sufficient characteristics for detecting valid nuclear magnetic resonance images. In practice, the magnet structure must have a huge size, and be expensive, both as regards construction and as regards purchasing costs and location of the equipment at the final user's site.
Although this drawback is less serious in the total-body machines, i.e., those having a magnet structure which receives the whole patient body or at least a considerable part thereof, the situation described above is much more burdensome in dedicated machines, i.e., small machines for detecting images of certain localized, specific body parts.
Dedicated machines use small magnetic structures, which must be relatively handy and light. The size compaction of the magnetic structures requires a high ratio between the space requirement of the structure and the useful image detection volume, and between the total cavity volume defined by the magnet and the useful image detection volume. Said useful volume should fill up a considerable part of the total cavity volume.
The problems related to dedicated machines are still more serious because, in order to provide highly flexible machines for several body parts having different anatomic and morphologic characteristics, the magnet structure must have an increasingly great number of open sides.
In certain types of permanent magnets, such as in the magnet described in U.S. Pat. No. 5,495,222, that is a C-shaped magnet with two open sides transverse to the axis of its C shape and one open side parallel thereto, in order to compensate field aberrations there are provided correction means at one open side, consisting in this case of extensions being directed to partially close the opening on the side parallel to the axis. The aberrations at the sides transverse to the axis of the C shape are not compensated, since the magnet is long enough to keep the open sides sufficiently distant from the relevant volume.
The magnet according to this document is of the total-body type, and the patient body is introduced therein in the direction of the axis of the C shape, whereas the partially open side, parallel to the axis, has the function to allow physicians or paramedics to perform operations on the patient.
OBJECTS AND SUMMARY
Therefore, the present invention aims at providing a magnet of the type described hereinbefore, which allows for a size reduction of the magnetic structure, while providing at least the same useful image detection volume, or even increasing the ratio between the useful image detection volume and the total space requirement of the magnet, allowing it to be made of such a shape that all its open sides may be indiscriminately used for introducing the patient therein, and having a small number of parts, with a simple geometry, and a relatively easy fabrication with sufficient tolerances.
The invention achieves the above purposes, with a magnet of the type described hereinbefore, in which in the vicinity of the open side or sides, the magnet has means for correcting the static magnetic field generated between the main poles, which correction means are intended to increase the magnetic potential near the opening, and over a predetermined depth therefrom, transverse to the open side, without reducing the span of said opening.
Hence, at the open side or sides, there is provided a pair of opposite auxiliary poles, inwardly penetrating over a predetermined distance toward their respective main poles, which auxiliary poles are provided with magnetized material, according to two alternative or combined methods. The magnetized material associated with the auxiliary poles is in larger quantity and of different quality with respect to the magnetized material associated with the main poles, in such a way as to obtain an increase of the magnetic potential in the area between said auxiliary poles with respect to that between the main poles. Alternatively, the magnetized material associated with the auxiliary poles is equal as regards quantity and type to the magnetized material associated with the main poles, whereas, between the main pole and the adjacent auxiliary pole, a magnetized insert is interposed, so as to generate a magnetic potential difference between the main poles and the adjacent auxiliary poles, whereby the magnetic potential of the auxiliary poles is increased with respect to the potential of the main pole and to the detriment thereof, the whole while keeping the distance between the pairs of opposite auxiliary poles at least equal to, or greater than the distance between the main poles.
The two solutions, which may be provided either singularly or in combination, allow for the generation of a peripheral magnetic field near the opening, creating such conditions as to bring the static magnetic field between the main poles back to a condition of sufficient intensity and homogeneity, within predetermined tolerances, substantially for the whole volume interposed between the main poles and defined thereby.
The two arrangements may be provided, either independently or in combination, for any type of magnet configuration, be it with one, two, three, four or more open sides, or with a peripheral completely open covering.
The first option consists in increasing the potential of the magnet
Biglieri Eugenio
Coscia Gianluca
Sanfilippo Carlo
Trequattrini Alessandro
Arana Louis
Burns Doane Swecker & Mathis L.L.P.
Esaote S.p.A.
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