Patent Publication Number: US-2023140633-A1

Title: MRI-Safety and Force Optimized Implant Magnet System

Description:
This application is a continuation of U.S. application Ser. No. 16/607,798, filed Oct. 24, 2019, which is a national phase entry of International Patent Application No. PCT/US2018/028785, filed Apr. 23, 2018, which claims priority from U.S. Provisional Patent Application 62/488,932, filed Apr. 24, 2017, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to implantable hearing devices such as cochlear implants, and specifically, to implantable magnets in such devices. 
     BACKGROUND ART 
     Some hearing implants such as Middle Ear Implants (MEI&#39;s) and Cochlear Implants (CI&#39;s) employ cooperating attachment magnets located in the implant and the external part to hold the external part in place over the implant. For example, as shown in  FIG.  1   , a typical hearing implant system may include an external transmitter housing  101  containing transmitting coils  107  and an external attachment magnet  105 . The external attachment magnet  105  has a conventional cylindrical disc-shape and a north-south magnetic dipole having an axis that is perpendicular to the skin of the patient as shown. Implanted under the patient&#39;s skin is a corresponding receiver assembly  102  having similar receiving coils  108  and an implant magnet  106 . The implant magnet  106  also has a cylindrical disc-shape and a north-south magnetic dipole having a magnetic axis that is perpendicular to the skin of the patient as shown. The internal receiver housing  102  is surgically implanted and fixed in place within the patient&#39;s body. The external transmitter housing  101  is placed in proper position over the skin covering the internal receiver assembly  102  and held in place by interaction between the magnets  105  and  106  thus, the internal magnetic field lines and the external magnetic field lines. Rf signals from the transmitter coils  107  couple data and/or power to the receiving coil  108  which is in communication with an implanted processor module (not shown). 
     One problem with the typical hearing implant, as shown in  FIG.  1   , arises when the patient undergoes Magnetic Resonance Imaging (MRI) examination. Interactions occur between the implant magnet and the applied external magnetic field for the MRI. As shown in  FIG.  2   , the direction of the magnetic dipole {right arrow over (m)} of the implant magnet  202  is essentially perpendicular to the skin of the patient. In this example, the strong static magnetic field B from the MRI creates a torque {right arrow over (T)}={right arrow over (m)}×{right arrow over (B)} on the internal magnet  202 , which may displace the internal magnet  202  or the whole implant housing  201  out of proper position. Among other things, this may damage the implant or the adjacent tissue of the patient. In addition, the external magnetic field {right arrow over (B)} from the MRI may reduce, remove or invert the magnetic dipole {right arrow over (m)} of the implant magnet  202  so that it may no longer be able or strong enough to hold the external transmitter housing in proper position. Torque and forces acting on the implant magnet and demagnetization of the implant magnet is especially an issue with MRI field strengths exceeding 1.5 Tesla. 
     Thus, for existing implant systems with magnet arrangements, it is common to either not permit MRI, or at most limit use of MRI to lower field strengths. Other existing solutions include use of a surgically removable magnets, spherical implant magnets (e.g. U.S. Pat. No. 7,566,296), and various ring magnet designs (e.g., U.S. Patent Publication 2011/0022120). 
     U.S. Pat. No. 8,634,909 describes an implant magnet having a diametrical magnetization, where the magnetic axis is parallel to the end surfaces of a disc shaped implant magnet—that is, perpendicular to the conventional magnetic axis of a disc-shaped implant magnet. The magnet is then held in a receptacle that allows the magnet to rotate about its center axis in response to an external magnetic field such as from an MRI to realign and avoid creating torque. But this rotation is only possible around a single axis, the central axis. 
       FIG.  3    shows the head of a patient with bilateral hearing implants  301  having such an implant magnet in the presence of a typical MRI scanning magnetic field B 0 , which is aligned along the longitudinal axis of the patient. The magnetization axis of the hearing implants  301  is angled with respect to the magnetic field B at some relative angle α B  as shown in  FIG.  3   , which can create an undesirable torque on the hearing implants  301 . This relative angle α B  is dependent on the individual patient&#39;s anatomy and the exact implant position, for example on the skull of the patient. 
       FIG.  4    shows in greater detail the geometry of an implant magnet  401  with a magnetic dipole {right arrow over (m)} that is parallel to the skin, and an MRI scanning magnetic field B aligned along the longitudinal symmetry axis. The cylindrical disc shape of the implant magnet  401  has a height h and a diameter Ød. Depending on the specific orientation of the implant within the patient, there will be a relative angle α B  between the direction of the magnetic dipole {right arrow over (m)} of the implant magnet  401  and the static magnetic field {right arrow over (B)}. The relative angle α B  also remains when implant magnet  401  is rotatable about its cylindrical axis  402 , as for example described in U.S. Pat. No. 8,634,909. This relative angle α B  leads to a torque force on the implant magnet  401 , where the torque {right arrow over (T)}={right arrow over (m)}×{right arrow over (B)}, and the force at the circumference of the stiff structure is {right arrow over (F)}={right arrow over (T)}/{right arrow over (D)}, where D is the distance or diameter of the stiff structure surrounding the implant magnet  401 . 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are directed to a magnet arrangement for a hearing implant device. An implantable magnet has a modified disc shape with a primary center rotation axis, a cylindrical height, a diameter, an outer circumference and opposing end surfaces. The implant magnet shape has at least one cross section view in which the primary center rotating axis is defined where the height of the magnet system is greatest and an axis normal to the cross section view is defining the secondary deflection axis. This magnet shape is capable of responding to an external magnetic field by rotating about the primary center rotation axis. The implant magnet shape has at least one cross-sectional view in which the cylindrical diameter corresponds to a horizontal coordinate axis, the primary center rotation axis corresponds to a vertical coordinate axis, and the height between the end surfaces is greatest. The height between the end surfaces progressively decreases from the primary center rotation axis along the cylindrical diameter towards the outer circumference to define a secondary deflection angle with respect to the horizontal coordinate axis so that the implant magnet is capable of responding to the external magnetic field by deflecting within the secondary deflection angle about a secondary deflection axis defined by a cylinder diameter normal to the at least one cross-sectional view. 
     In further specific embodiments, there may also be a magnet housing enclosing a cylindrical shaped interior volume that contains the implant magnet. The implant magnet then is configured to securely fit within the interior volume so as to allow free alignment to an external magnetic field about the primary rotating axis as is limited partial rotation about the secondary deflection axis. In such embodiments, the interior volume may contain a damper oil which surrounds the implant magnet and/or at least one ferromagnetic domain which enabled a magnetic fixation of the implant magnet inside the embodiment. The implant magnet may include one or more low-friction contact surfaces configured to connect the implant magnet to the magnet housing. 
     The at least one cross-sectional view may be exactly one cross-sectional view, or it may be every cross-sectional view in which the cylindrical diameter corresponds to a horizontal coordinate axis and the primary center rotation axis corresponds to a vertical coordinate axis. 
     Embodiments of the present invention also include a hearing implant system containing a magnet arrangement according to any of the foregoing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows portions of a typical cochlear implant system and the magnetic interaction between the implant magnet and the external holding magnet. 
         FIG.  2    illustrates the force interactions that can occur between an implant magnet and the applied external magnetic field for an MM system. 
         FIG.  3    the head of a patient with bilateral cochlear implants in the presence of a typical MRI scanning magnetic field. 
         FIG.  4    shows geometry of an implant magnet with a magnetic dipole parallel to the skin and an MRI scanning magnetic field. 
         FIG.  5    shows cross-sectional view geometry of a modified disc-shaped implant magnet according to an embodiment of the present invention. 
         FIG.  6    shows a cross-sectional view of an implant magnet enclosed within a magnet housing. 
         FIG.  7    shows geometry of an implant magnet arrangement according to  FIG.  6    in an MRI scanning magnetic field. 
         FIGS.  8 A- 8 B  show elevated perspective views of a rotationally symmetric and a non-rotationally symmetric implant magnet according to embodiments of the present invention. 
         FIG.  9    shows a cross-sectional view of an implant magnet arrangement with friction-reducing surfaces according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Embodiments of the present invention are directed to an improved implant magnet that can achieve a lower mechanical force during an MRI for a given magnetization or magnet strength. The inventive implant magnet has a limited deflection rotation about a secondary deflection axis to reduce the torque created by the static magnetic field {right arrow over (B)} in the MRI-scanner. This, in turn, allows use of a stronger implant magnet with the same mechanical torque during Mill. 
       FIG.  5    shows the cross-sectional view geometry of an implant magnet  501  according to one embodiment of the present invention, with a center rotation axis  502 , a cylindrical height  507  and diameter  503 , an outer circumference  504 , and opposing end surfaces  505 . The implant magnet  501  is capable of responding to an external magnetic field {right arrow over (B)} by rotating about the center rotation axis  502 . And the shape of the implant magnet  501  has at least one cross-sectional view as shown in  FIG.  5    where the cylindrical diameter  503  corresponds to a horizontal coordinate axis, the primary center rotation axis  502  corresponds to a vertical coordinate axis. The height  507  of the implant magnet  501  between the end surfaces  505  is greatest at the primary center rotation axis  502  and progressively decreases from the primary center rotation axis  502  along the cylindrical diameter  503  towards the outer circumference  504 . 
       FIG.  6    shows a cross-sectional view of a further specific embodiment with a magnet housing  601  that encloses a cylindrical shaped interior volume  602  that contains the implant magnet  501 . The implant magnet  501  is configured to securely fit within the interior volume  602  so as to be freely rotatable about the primary center rotation axis  502  and the secondary deflection axis  506 . In such embodiments, the interior volume  602  may contain a damper oil (to reduce rattler noise) which surrounds the implant magnet  501 . 
     The geometry of the implant magnet  501  defines a secondary deflection angle α B  with respect to the horizontal coordinate axis so that the implant magnet  501  is capable of responding to the external magnetic field {right arrow over (B)}, as shown in  FIG.  7   , by deflecting within the secondary deflection angle α B  about a secondary deflection axis  506  which is normal to the at least one cross-sectional view, up until further secondary rotation is prevented by the end surfaces  505  pressing against the inner surface of the magnet housing  601  as shown in  FIG.  7   . 
       FIGS.  8 A- 8 B  show elevated perspective views of two different shape approaches to an implant magnet  801  according to an embodiment of the present invention. The implant magnet  801  shown in  FIG.  8 A  is rotationally symmetric. The end surfaces on the top and bottom of the disc-shaped implant magnet  801  form two rounded cones centered around the primary center rotation axis  802  with a chamfer radius of half the magnet height. Every cross-sectional view through the end surfaces will be such that the height is greatest at the center of the primary center rotation axis  802  and progressively decreases radially outward towards the outer circumference. To enable a secondary deflection around a secondary deflection axis  806 , the edges of the cylindrical diameter are chamfered with the radius of the half diameter. In such a rotationally symmetric implant magnet  801  the diametrical magnetization in every direction is normal to the primary rotation axis  802 . 
     The implant magnet  801  shown in  FIG.  8 B  is non-rotationally symmetric design with a rounded dam-shaped design on the top and bottom of the cylindrical implant magnet  801  with the radius of the chamfers the same as in the symmetric design in  FIG.  8 A . For such a non-rotationally symmetric shape, the direction of the magnetic dipole {right arrow over (m)} has to align normal to the secondary deflection axis  806 , which is in turn parallel to the top and bottom line of the dam-shape. It will be appreciated in this embodiment, there is just a single cross-sectional view where the magnet height is greatest at the primary center rotation axis  802  and progressively decreases radially outward towards the outer circumference. 
       FIG.  9    shows a cross-sectional view of a further specific embodiment where the implant magnet  901  includes one or more low-friction contact surfaces  902 , e.g. made of titanium, that are configured to connect the implant magnet  901  to the magnet housing; for example, at the center axis of symmetry and/or at the outer circumference. 
     Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention.