Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2000-11-27
2002-06-25
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S298000, C335S299000, C335S306000, C324S319000, C324S320000
Reexamination Certificate
active
06411187
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to permanent magnet assemblies for use in medical applications and particularly to permanent magnet assemblies for use in Magnetic Resonance Imaging (MRI) and/or Magnetic Resonance Therapy (MRT) and/or interventional MRI (iMRI) which produce a predetermined volume of substantially uniform magnetic field extending in a first direction beyond the surface of the permanent magnet assemblies.
BACKGROUND OF THE INVENTION
The principles of MRI are set forth in several patents such as U.S. Pat. No. 5,304,933, which is incorporated herein by reference. Interventional MRI (iMRI) which is also referred to as intraoperative MRI, is the performance of an interventional medical procedure on a patient in an MRI system. During the procedure, a surgical instrument is inserted into a patient in order to perform the procedure at a predetermined site in the body. The iMRI system is used in this case to monitor in quasi real-time the correct placement of the instrument and also to observe the nature and the extent of the effect of the intervention on the tissue.
In an MRI and/or iMRI system a strong uniform magnetic field is required in order to align an objects nuclear spins along the z-axis of a Cartesian coordinate system having mutually orthogonal x-y-z axes. The required strong uniform magnetic field, used for full body imaging, is normally in the order of 0.1 to 2 Tesla. The image quality and the accuracy of an MRI and/or iMRI system is dependent on the degree of uniformity of the strong uniform magnetic field. Uniformity is critical in MRI and/or iMRI applications because if the strong uniform magnetic field is not properly uniform within the volume of interest, the desired discrimination between different elements, due to the finely controlled magnetic field gradient, will be subject to misinterpretation. Typically, the uniformity required for the strong uniform magnetic field is within the order of 10 ppm within the volume of interest. It is essential for iMRI systems used in interventional procedures to be based on an open structure, so as to provide the physician easy access to the intervention site. Presently, most MRI systems employ a large magnet, which effectively surrounds the whole body of the patient, to produce the strong uniform magnetic field. Such magnets are usually large superconductor resistive or permanent magnets, each of which is expensive and heavy. Further, the access to the patient in these cases is obstructed.
Attempts have been made to provide open magnets for interventional procedures by employing two spaced-apart Helmholtz superconductive coil assemblies. They provide only limited space between the assemblies allowing for constricted access by only one person, such as a surgeon. Moreover, they are large, massive, immobile and expensive. See U.S. Pat. No. 5,410,287 (Laskaris et al.) and U.S. Pat. No. 5,428,292 (Dorri et al.).
U.S. Pat. No. 5,574,417 (Dorri et al.) discloses an open MRI magnet having two opposing superconducting coils and two annular shaped permanent magnet arrays. The radially separated inward and radially apart from the superconducting coil. Each of the permanent magnet arrays has a magnetic field direction generally opposite to the magnetic field direction of the corresponding superconducting coil. The permanent magnet arrays are stationary.
U.S. Pat. No. 5,696,476 (Havens et al.) discloses a passively shimmed open architecture magnetic resonance imaging magnet utilizing separated superconducting coil assemblies with open space between including shaped ferromagnetic bands in the bore of the magnets, the shaping including an axial notch and circumferential holes for attaching passive shims, the ferromagnetic bands are stationary.
U.S. Pat. No. 4,710,741 (McGinley et al.) discloses a coil arrangement for producing a homogenous magnetic field, comprising a single pair of identical annular coils disposed coaxially in spaced relationship and a pair of annular members of ferromagnetic material disposed coaxially with the coils. The annular members are stationary.
U.S. Pat. No. 5,463,364 (Muller) discloses an open magnetic apparatus having two opposing electromagnet coils and one or two pairs of opposing ferromagnetic rings. The rings of one pair of ferromagnetic rings are at least partially integrated into the electromagnet coils. The pairs of rings are stationary relative to the coils.
British patent GB 2219406A (Warner) discloses an electromagnet for providing a volume of homogenous magnetic field, comprising a pair of annular coils disposed coaxially in spaced relationship and ferromagnetic means, such as ferromagnetic annuli, disposed coaxially with the coils. The ferromagnetic means are stationary means.
U.S. Pat. No. 4,875,485 (Matsutani) discloses an apparently more compact configuration, based on a pair of spaced-apart superconductive Helmholtz coil assemblies, arranged for movement relative to a platform carrying the patient. The access to the patient remains restricted in this case as well, due to the additional space occupied by the cryostat. Additionally, the movement of the coils independently of one another is impractical, because the superconducting properties of the coils require extreme precision in positioning of the two poles, in the absence of which the magnetic system quenches.
In comparison to superconductive systems, permanent magnets are less expensive, generate only a minimal unwanted fringe field and are not involved with liquefied gas handling or vacuum requirements. Open access MRI systems based on permanent magnets have been disclosed by U.S. Pat. No. 4,829,252 (Kaufman) and U.S. Pat. No. 5,134,374 (Breneman). Both are using a pair of opposing magnetic flat circular poles, employed one above the other, with the patient lying down between the magnets. The poles are mounted on end plates, supported by connecting members, which provide return paths for the magnetic flux. These systems are massive and immobile and the access to the patient is encumbered by the supporting structure.
A pair of opposing permanent magnet assemblies for use in MRI, each made of concentric magnetic rings, composed of a set of magnetic polygonal blocks, is disclosed in U.S. Pat. No. 5,332,971 (Aubert). Aubert teaches that the opposing concentric rings within each of the pairs of permanent magnets are to be spaced apart from each other the same distance. The magnet is massive, weighing about 3 tons and is therefore not amenable to movement relative to a patient's body.
In each of the above prior art magnets, used for providing the large uniform magnetic field for MRI and/or iMRI applications, the magnetic field is generated in a first stage as uniformly as possible. More uniformity is achieved subsequently by shimming.
Co-pending U.S. patent application Ser. No. 09/161,336, to Zuk et al., entitled “MAGNETIC APPARATUS FOR MRI”, filed Sep. 25, 1998, assigned to the assignee of the present invention, and the corresponding International Patent Application PCT/IL98/00463 published as International Publication No. WO 99/15914, the entire specification of which is incorporated herein by reference, disclose, inter alia, magnetic apparatus including an opposing pair of permanent magnetic assemblies defining an open region therebetween in which an organ or body part is positioned for imaging. The magnetic apparatus includes a plurality of gradient coils at least one of which is positioned outside of the open region.
Co-pending U.S. patent application Ser. No. 09/405,835 to Katznelson et al., filed Sep. 27, 1999, entitled “YOKED PERMANENT MAGNET ASSEMBLIES FOR USE IN MEDICAL APPLICATIONS”, assigned to the assignee of the present invention and incorporated herein by reference in its entirety discloses, inter alia, an MRI system having an open magnet including two spaced apart permanent magnet assemblies attached to an open ferromagnetic yoke.
U.S. Pat. No. 6,147,578 to Panfil et al. entitled “A METHOD FOR DESIGNING OPEN MAGNETS AND OPEN MAGNETIC APPARATUS FOR USE IN MRI/MRT PROBES”, assigned to the assignee of t
Katznelson Ehud
Rotem Haim
Barrera Ramon M.
Eitan Pearl Latzer & Cohen-Zedek
Odin Medical Technologies Ltd.
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