Metal working – Means to assemble or disassemble – Means to assemble electrical device
Reexamination Certificate
2000-01-26
2002-10-22
Vo, Peter (Department: 3729)
Metal working
Means to assemble or disassemble
Means to assemble electrical device
C029S607000, C029S744000, C029S762000, C029S419200, C029S602100, C029S719000, C029S759000, C029S760000, C029SDIG009, C029SDIG001, C269S013000, C269S014000
Reexamination Certificate
active
06467157
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magnet construction. More particularly it relates to a method for the construction of annular permanent magnets, especially but not solely for use in MRI systems, and an apparatus for constructing such magnets.
BACKGROUND OF THE INVENTION
Magnetic resonance imaging systems are known in the art. The remarkable soft tissue contrast resolution associated with these techniques is invaluable and renders these techniques high appreciation among the medical community.
Basically MRI techniques exploit nuclear magnetism induced on the patient's tissues, eliciting Radio-Frequency induced signal response which is picked up, analyzed and processed to obtain an image of the imaged region of the patient's tissue (a very clear explanation of the MRI principles is provided by Joseph P Homak, of the Rochester Institute of Technology, on the World Wide Web, http:/www.cis.rit.edu/htbooks/mri/mri-main.htm, and see also U.S. Pat. No. 5,304,933 (Vavrek et al.), titled SURGICAL LOCAL GRADIENT COIL).
The first stage of MRI involves the aligning of the patient's tissue nucleons magnetic spins. This is achieved by placing the patient (or the patient's organ to be imaged) in a strong magnetic field generated by a strong permanent magnet or a super-conductive magnet. It is imperative that the magnetic field in the field of view of the system be homogeneous, as distorted field may result in the distortion of the image and the appearance of artifacts in the image.
Super-conductive magnets can produce extremely strong and substantially homogeneous magnetic fields (typical magnet strength in known MRI systems may be as high as 2 Tesla). See for example U.S. Pat. No. 4,924,186 (Matsutani) titled MAGNET APPARATUS FOR USE IN MAGNETIC RESONANCE IMAGING SYSTEM. These are large superconductor magnets, which take up a large space (sometimes as large as a room), are expensive and require high operating and maintenance costs. The large size of these magnets prevents any access to the patient.
However, recently it was realized that whole body imaging is not necessary for the performance of an interventional medical procedure on a patient in an MRI system. It has been realized that, in fact, a machine with a restricted field of view performs satisfactorily in such a setting and can be built in a more efficient and economical fashion than one built for accommodating a whole body. Furthermore, in order to leave an open access to reach conveniently the part of the body on which the intervention is performed, compact magnet assemblies were introduced.
Israel Pat. Appl. No. 119558 (Katznelson et al.) filed Nov. 4, 1996, discloses a compact, mobile, intra-operative MRI System, which includes a host computer coupled to a central electronics system which may be coupled to different MRI probes.
In U.S. Pat. No. 5,428,292 (Dorri et al.), filed Apr. 29, 1994, A pancake-like MRI magnet was disclosed, presenting a relatively narrow lateral cross-section.
Usually permanent magnet assemblies for MRI systems incorporate ferromagnetic structures for the creation of return paths of the magnetic flux. Attaching ferromagnetic (usually iron) annular plates on the surface of the magnet facing the patient act as magnetic field uniformity enhancement.
U.S. Pat. No. 5,900,793 (Katznelson et al.), filed Jul. 23, 1997, titled PERMANENT MAGNET ASSEMBLIES FOR USE IN MEDICAL APPLICATIONS, described, inter alia, compact permanent magnet assemblies for use in medical applications, including MRI and/or MRT (Magnetic Resonance Therapy). It consists of a plurality of annular concentric magnets, spaced apart along their axis of symmetry. The magnet assemblies disclosed in that patent are not provided with a ferromagnetic structure.
Permanent magnets are made of non-rare or rare earth magnetic materials. Non-rare earth magnets include Alnico (Aluminum-Nickel-Cobalt) magnets and Ceramic (Strontium and Barium Ferrite) magnets. Rare earth magnets include Sm—Co (Samarium-Cobalt) magnets and Nd—Fe—B (Neodymium-Iron-Boron) magnets.
The conventional manufacturing method of a permanent magnet involves compressing the magnet material, which is available in the form of powder, shaping it into a predetermined shape, and then magnetizing it by placing it in an extremely strong electromagnetic field (in the order of 3 Tesla). This powerful electromagnetic field, which permanently aligns the magnetic dipoles of the matter is generated by passing powerful electric current produced by discharging a large number of capacitors in a predetermined switching sequence through a copper or super-conductive coil within which the item to be magnetized is placed. See, for example, U.S. Pat. No. 5,250,255 (Segawa et al.), titled METHOD FOR PRODUCING PERMANENT MAGNET AND SINTERED COMPACT AND PRODUCTION APPARATUS FOR MAKING GREEN COMPACTS.
MRI magnets may be cylindrical in their shape, suitable for reception of a patient within the internal space of the cylinder. as illustrated in U.S. Pat. No. 5,659,250 (Domigan et al.). In interventional MRI systems, which require that the patient be accessible to the medical staff, when positioned within the magnetic field, the magnetic field is usually produced by two magnet assemblies that are positioned opposite each other, allowing reception of the patient in between. MRI systems can also utilize a single magnet to produce an image.
Permanent magnets used in MRI systems that require accessibility of the patient are usually annular in their shape. In order to prevent substantial eddy currents induced by the strong magnetic field, MRI magnets are usually segmented, formed from magnetic segments, and glued to each other using a non-conductive adhesive. Thus the generation of eddy currents over the whole magnet is prevented, limiting possible eddy current induction to the segments.
The size of magnets produced in this manner is limited by the size of the coil used for magnetization, which itself is limited by the minimal electromagnetic filed strength required for effective magnetization. It is noted that in order to effectively magnetize the magnet material it has to be subjected to a strong magnetic field that generates magnetic flux within the material exceeding the saturation flux (the flux at which al the magnetic dipoles within the material align). For Nd—Fe—B the saturation magnetic flux is about 1.35 Tesla, and for practical reasons it is customary to subject the magnetic material to magnetic fields of twice or more the stated flux.
In view of the above mentioned considerations it is therefore why small magnetic rings are produced in the following manner first, the magnet segments are shaped roughly to form sectors of the ring. Then their sides, designated to be glued to other segments, are ground and the segments are bonded together (using an insulating adhesive) to form the ring. Then the ring undergoes further mechanical processing to bring it to its final shape and dimensions, and finally the whole construction is magnetized.
The manufacturing procedure described above is followed sequentially in the provided order, for if the segments were to be magnetized prior to their joining together, it would be very difficult to adhere the magnetized segments together. As the magnetized segments are drawn closer strong magnetic repulsion forces act on the magnetized segments attempting to flip the segments over, so as to bring opposite polarities to face each other. Therefore enormous forces are required to position the segments adjacent each other and hold them firmly while the adhesive hardens.
The problem with the method discussed above is that it is impossible to control or predict the level of homogeneity of the magnetic field of the constructed magnet that would be attained in this process. In MRI applications it is imperative that the produced magnet possessed a substantially homogeneous magnetic field. However irregularities in the magnetic material, as well as flaws in the magnetization process could render the manufactured magnet non-homogeneous and therefore u
Katz Yoav
Livni Avinoam
Kim Paul D
Odin Technologies Ltd.
Pennie & Edmonds LLP
Vo Peter
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