MRI-Safety and Force Optimized Implant Magnet System

Abstract

A magnet arrangement for an implantable medical device is described. An implant magnet has a modified disc shape and is capable of responding to an external magnetic field by rotating about a 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 center symmetry axis corresponds to a vertical coordinate axis, the height between the end surfaces is greatest at the center symmetry axis, and the height between the end surfaces progressively decreases from the center symmetry 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 cross-sectional view.

Claims

1. A magnet arrangement for an implantable hearing device, the magnet arrangement comprising: an implantable holding magnet having a modified disc shape with a primary center rotation axis, a cylindrical height and diameter, an outer circumference, and opposing end surfaces; wherein the implantable holding magnet is capable of responding to an external magnetic field by rotating about the primary center rotation axis, and wherein the implantable holding magnet shape has at least one cross-sectional view in which: i. the cylindrical diameter corresponds to a horizontal coordinate axis, ii. the primary center rotation axis corresponds to a vertical coordinate axis, iii. height between the end surfaces is greatest at the primary center rotation axis, and iv. 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 implantable holding 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.

2. The magnet arrangement according to claim 1, further comprising: a magnet housing enclosing a cylindrical shaped interior volume containing the implantable holding magnet, wherein the implantable holding magnet is configured to securely fit within the interior volume so as to be freely rotatable about the primary center rotating axis and about the secondary deflection axis.

3. The magnet arrangement according to claim 2, wherein the interior volume contains a damper oil which surrounds the implantable holding magnet.

4. The magnet arrangement according to claim 2, wherein the interior volume contains at least one ferromagnetic domain which surrounds the implantable holding magnet.

5. The magnet arrangement according to claim 2, wherein the implantable holding magnet includes one or more low-friction contact surfaces configured to connect the implantable holding magnet to the magnet housing.

6. The magnet arrangement according to claim 5, wherein the one or more low-friction contact surfaces are located at the center axis of symmetry.

7. The magnet arrangement according to claim 5, wherein the one or more low-friction contact surfaces are located at the outer circumference.

8. The magnet arrangement according to claim 1, wherein the at least one cross-sectional view is one cross-sectional view, which is a geometric non-rotationally symmetric design.

9. The magnet arrangement according to claim 1, wherein the at least one cross-sectional view is 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, which is a geometric rotationally symmetric design.

10. A hearing implant system comprising: an implantable hearing device having a magnet arrangement, the magnet arrangement comprising: an implantable holding magnet having a modified disc shape with a primary center rotation axis, a cylindrical height and diameter, an outer circumference, and opposing end surfaces; wherein the implantable holding magnet is capable of responding to an external magnetic field by rotating about the primary center rotation axis, and wherein the implantable holding magnet shape has at least one cross-sectional view in which: i. the cylindrical diameter corresponds to a horizontal coordinate axis, ii. the primary center rotation axis corresponds to a vertical coordinate axis, iii. height between the end surfaces is greatest at the primary center rotation axis, and iv. 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 implantable holding 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; and an external device comprising: an external transmitter housing containing an external attachment magnet configured to interact with the implantable holding magnet to hold the external device in place over the implantable hearing device on a patient's skin.

11. The hearing implant system of claim 10, wherein the magnetic arrangement further comprises: a magnet housing enclosing a cylindrical shaped interior volume containing the implantable holding magnet, wherein the implantable holding magnet is configured to securely fit within the interior volume so as to be freely rotatable about the primary center rotating axis and about the secondary deflection axis.

12. The hearing implant system of claim 11, wherein the interior volume contains a damper oil which surrounds the implantable holding magnet.

13. The hearing implant system of claim 11, wherein the interior volume contains at least one ferromagnetic domain which surrounds the implantable holding magnet.

14. The hearing implant system of claim 11, wherein the implantable holding magnet includes one or more low-friction contact surfaces configured to connect the implantable holding magnet to the magnet housing.

15. The hearing implant system of claim 14, wherein the one or more low-friction contact surfaces are located at the center axis of symmetry.

16. The hearing implant system of claim 14, wherein the one or more low-friction contact surfaces are located at the outer circumference.

17. The hearing implant system of claim 10, wherein the at least one cross-sectional view is one cross-sectional view, which is a geometric non-rotationally symmetric design.

18. The hearing implant system of claim 10, wherein the at least one cross-sectional view is 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, which is a geometric rotationally symmetric design.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 shows portions of a typical cochlear implant system and the magnetic interaction between the implant magnet and the external holding magnet.

[0014] FIG. 2 illustrates the force interactions that can occur between an implant magnet and the applied external magnetic field for an MM system.

[0015] FIG. 3 the head of a patient with bilateral cochlear implants in the presence of a typical MRI scanning magnetic field.

[0016] FIG. 4 shows geometry of an implant magnet with a magnetic dipole parallel to the skin and an MRI scanning magnetic field.

[0017] FIG. 5 shows cross-sectional view geometry of a modified disc-shaped implant magnet according to an embodiment of the present invention.

[0018] FIG. 6 shows a cross-sectional view of an implant magnet enclosed within a magnet housing.

[0019] FIG. 7 shows geometry of an implant magnet arrangement according to FIG. 6 in an MRI scanning magnetic field.

[0020] FIGS. 8A-8B show elevated perspective views of a rotationally symmetric and a non-rotationally symmetric implant magnet according to embodiments of the present invention.

[0021] 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

[0022] 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.

[0023] 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.

[0024] 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.

[0025] The geometry of the implant magnet 501 defines a secondary deflection angle α.sub.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.sub.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.

[0026] FIGS. 8A-8B 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. 8A 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.

[0027] The implant magnet 801 shown in FIG. 8B 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. 8A. 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.

[0028] 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.

[0029] 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.