TESTING TUNED CIRCUITS

• US 20140024919A1
• Filed: 07/19/2012
• Published: 01/23/2014
• Est. Priority Date: 07/19/2012
• Status: Active Grant

• Current Assignee: Cochlear Limited
• Original Assignee: Cochlear Limited
• Inventors: Metzenthen, Bill
• Granted Patent: US 9,571,166 B2
• Time in Patent Office: Days
• US Class Current: 600/409
• CPC Class Codes: A61B 2560/0219 of externally powered implanted units
• CPC Class Codes: A61B 2560/0271 using a remote monitoring unit
• CPC Class Codes: A61B 2562/0223 Magnetic field sensors
• CPC Class Codes: A61B 2562/046 in a matrix array
• CPC Class Codes: A61B 5/0031 Implanted circuitry
• CPC Class Codes: A61B 5/1075 for measuring dimensions by non-invasive methods, e.g. for determining thickness of tissue layer A61B8/0858 takes precedence
• CPC Class Codes: A61N 1/36038 Cochlear stimulation
• CPC Class Codes: A61N 1/37223 Circuits for electromagnetic coupling
• CPC Class Codes: A61N 1/37252 Details of algorithms or data aspects of communication system, e.g. handshaking, transmitting specific data or segmenting data
• CPC Class Codes: H04B 5/24 Inductive coupling
• CPC Class Codes: H04R 25/606 acting directly on the eardrum, the ossicles or the skull, e.g. mastoid, tooth, maxillary or mandibular bone, or mechanically stimulating the cochlea, e.g. at the oval window

Abstract

Methods and systems for determining one or more parameters of a tuned circuit forming part of a wireless energy transmission system in an implanted (or implantable) medical device are described. The method involves energising the tuned circuit then receiving a signal back from it. This signal is then analysed to determine a property of the circuit such as its quality factor (Q) or resonant frequency. Also described herein is a method and system for determining the implantation depth of a component of an implanted medical device. The method involves determining the position of a magnetic element which is mounted in a fixed physical relationship with the component of the medical device. The methods can be performed on an implanted medical device without the need to explant the device.

Claims

1. A method of testing an implantable medical device, the implantable medical device including a tuned circuit having a coil for transferring energy between the implantable medical device and another device, the method comprising:

inducing an EMF in the coil of the tuned circuit;

detecting a resonance of the tuned circuit in response to the EMF induced in the coil of the tuned circuit; and

analysing the detected resonance over a period of time to determine a property of the tuned circuit.

2. The method of claim 1, wherein the inducing the EMF includes:

applying a time varying magnetic field to the coil of the tuned circuit.

3. The method of claim 2, wherein the inducing the EMF includes:

applying the time varying magnetic field for at least an amount of time sufficient for the circuit of the neurodevice to reach a threshold state, prior to detecting a resonance of the tuned circuit.

6. The method of claim 1, wherein the detecting resonance includes:

detecting a time varying magnetic field generated by the tuned circuit.

7. The method of claim 6, wherein the analysing includes:

generating a time varying voltage signal in response to the time varying magnetic field; and

sampling the time varying voltage signal over a sampling interval to generate instantaneous voltage samples at corresponding times.

8. The method of claim 7, wherein the analysing the detected resonance includes:

fitting a curve to the sampled instantaneous voltage values over at least the sampling interval.

9. The method of claim 8, wherein:

the curve fitted to the sampled instantaneous voltage values has a decaying sinusoidal form.

10. The method of claim 8, wherein the curve fitted to the sampled instantaneous voltage values is of the general form:

v=Ve^t/τ

sin(2π

ft+φ

)

where,

v is the voltage at time t;

V is an initial voltage amplitude at a start of the measurement window;

τ

is a decay constant describing the energy dissipated in a cycle of the received signal compared to the total signal energy;

f is the resonant frequency of the tuned circuit; and

φ

is a phase offset.

11. The method of claim 10, wherein the curve fitted to the sampled instantaneous voltage values further includes terms to account for at least one of more of the following:

a predictable unwanted component in the generated time varying voltage signal;

an artefact introduced into the instantaneous voltage samples by the sampling process; and

a distortion caused by the presence of a non-linear component in the tuned circuit.

12. The method of claim 1, wherein:

the method is performed prior to implantation of the implantable medical device.

13. The method of claim 1, wherein:

the method is performed on an implantable medical device in vivo.

4. The method of 1, wherein the analysing includes:

determining a quality factor (Q) of the tuned circuit as the property thereof.

5. The method of 1, wherein the analysing includes:

determining at least one resonant frequency of the tuned circuit as the property thereof.

14. A method of determining a property of a prosthetic device implanted in a device recipient, the implantable medical device including a tuned circuit having a coil for providing electrical coupling between a circuit of the prosthetic device and another device, the method including:

inducing an EMF in the coil of the tuned circuit;

detecting a resonance of the tuned circuit in response to the EMF induced in the coil of the tuned circuit; and

analysing the detected resonance over a period of time to determine a property of the tuned circuit.

15. The method of claim 14, wherein the prosthetic device is an implanted auditory prosthesis.

16. A method of determining a position of a component of an implanted medical device, said component including a magnetic element positioned in a fixed spatial arrangement with respect to said component;

the method comprising;

measuring a localised magnetic field produced by the magnetic element at a plurality of positions remote from the component; and

analysing quantities representing the measured localised magnetic field at the plurality of said positions according to a model of the magnetic field produced by the magnetic element in order to determine the position of the magnetic element with respect to the plurality of positions.

17. The method of claim 16, wherein the analysing includes:

comparing the quantities against the model.

18. The method of claim 16, wherein the analysing includes:

processing the quantities using an equation modelling the localised magnetic field produced by the magnetic element.

19. The method of claim 16, further comprising:

determining the position of the component on the basis of the determined position of the magnetic element.

20. The method as claimed in claim 19, wherein:

the plurality of positions comprise a set of positions in a known spatial relationship with each other.

21. The method of claim 18, wherein the method further comprises:

aligning the set of positions with respect to the magnetic element, the aligning including;

determining an initial position of the magnetic element with respect to the plurality of positions; and

indicating, in the event that the set of positions is not sufficiently aligned with respect to the magnetic element;

that the set of positions should be re-aligned with respect to the magnetic element.

22. The method of claim 21, further comprising:

determining a direction of relative movement by which to change the set of positions so as to better align the set of positions with the magnetic element; and

visually indicating how the set of positions should be realigned with respect to the magnetic element.

23. The method of claim 20 wherein:

the component is an induction coil and the implanted medical device is an auditory prosthesis, said coil having a magnetic element held in fixed relationship therewith, said coil being implanted at a depth below the surface of an overlying skin layer of a recipient;

the set of positions is a set of positions located at a known separation above the surface of the overlying skin layer;

the method further comprises;

determining a depth of implantation of the induction coil below the surface of the overlying skin layer on the basis of the measured position of the magnetic element relative to the set of positions, and the known separation of the set of positions above the surface of the overlying skin layer.

24. The method as claimed in claim 20, further comprising:

providing a plurality of magnetic field sensors arranged in a known spatial arrangement with respect to each other, each sensor outputting a value representing the magnetic field strength sensed at its respective position;

processing the set of outputs from the plurality of sensors according to a model of the magnetic field of the magnetic element; and

determining a separation between a reference point having a known location with respect to the plurality of magnetic field sensors and the magnetic element.

25. A system for testing an implantable medical device, the implantable medical device including a tuned circuit having a coil for transferring energy between the implantable medical device and another device, the system comprising:

means for inducing an EMF in the coil of the tuned circuit;

means for detecting a resonance of the tuned circuit in response to the EMF induced in the coil of the tuned circuit; and

means for analysing the detected resonance over a period of time to determine a property of the tuned circuit.

26. A system for determining a position of a component of an implanted medical device, said component including a magnetic element positioned in a fixed spatial arrangement with respect to said component;

the system comprising;

means for measuring a localised magnetic field produced by the magnetic element at a plurality of positions remote from the component; and

means for analysing quantities representing the measured localised magnetic field at the plurality of said positions according to a model of the magnetic field produced by the magnetic element in order to determine the position of the magnetic element with respect to the plurality of positions.