Testing tuned circuits

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 energizing the tuned circuit then receiving a signal back from it. This signal is then analyzed 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.

BACKGROUND

Field of the Invention

The present technology relates generally to the testing of an aspect of, an implantable medical device. In one form, a parameter of a tuned circuit forming part of an implantable medical device is tested. In another, a location of a part of an implantable medical device is tested.

Related Art

Implantable medical devices are used to assist recipients with a wide range of conditions including sensory, motor or cognitive conditions. One group of implantable medical devices are auditory prostheses, such as middle ear implants, cochlear implants, brain stem implants, auditory mid-brain implant and other devices which provide acoustic, mechanical and/or electrical stimulation to a recipient to assist with hearing. Many implantable medical devices include an internal unit which is implanted in a recipient and an external unit which performs energy-consumption intensive processing or other functions that must be performed outside the body. In such systems there is a requirement for energy (power) and/or data to be delivered transcutaneously from the external unit to the implanted unit. This is generally performed wirelessly, as a physical link between the external and internal units may cause discomfort to the recipient and may be a potential source of infection.

To illustrate transcutaneous wireless inductive energy transmissionFIG. 1illustrates a schematic block diagram of an implantable cochlear implant system1. The system1includes an external unit3having a microphone7, a sound processor8and a radio frequency (RF) transmitter10. The system1is powered by a battery9that is located in the external unit3. Since the external unit3is typically worn behind the recipient's ear, there is a constraint on the size and weight of the external unit3and consequently there is a need for efficient usage of the power of the battery9.

In order to transmit energy to the internal unit5, the external unit3is provided with an external coil11that is coupled to a radio frequency transmitter10. The transmitter10generates a radio frequency alternating in current in coil11, which generates an oscillating magnetic field. That field extends through the recipient's skin13and interacts with an implanted coil17that is located so as to enable inductive coupling with the external coil11. The received signal is induces an EMF in the implanted coil17of the implanted unit5of the system1. A signal16may also be transmitted from the implanted unit5to the external unit3, for example to provide telemetry about the status of the implanted unit5.

Generally speaking, the implanted unit5includes: circuitry19that rectifies and regulates the received RF signal15; a data decoder21that extracts the data encoded in the received RF signal15; and an amplifier23that drives an actuator25based on the decoded data. The actuator may, for example, include an array of electrodes that stimulate the auditory nerve of the cochlea of the recipient to give a sensation of sound (also referred to as a ‘sound percept’).FIG. 2illustrates a simplified circuit diagram of components involved in the transfer of electrical energy from the external unit3to the implanted unit5.

The RF transmitter10is part of the external unit3and connects an RF source12(in commercially available cochlear implant systems, e.g., from Cochlear Limited, Sydney Australia, the RF source operates, e.g., at a frequency of 5 MHz) to the transmitter coil11.

The separation distance, d, between the coils11and17, which are typically positioned with their centres on a common axis, is determined by the depth at which the internal coil17is implanted in the recipient's body. This depth is sometimes known as the skin flap thickness (SFT), as the implantation depth is determined by the thickness of the flap of skin overlying the coil17. In practice, the SFT may be in the range, e.g., from about 0 mm to about 12 mm, but is preferably as low as safely possible in order to maximise efficiency of coupling between the external coil11and the internal coil17. The separation distance d affects the transfer of electrical energy to the implant (link efficiency) and modulates the electrical voltage that is generated in the implant electronics. However, typically the separation distance d is not accurately known.

The internal coil17forms part of a tuned (resonant) circuit101along with a tuning capacitor130. Electrical energy to sustain the device's functionality is extracted from the tuned circuit101by rectifying the received RF signal, using a transformer100and a rectifier diode102, and storing the received electrical energy in a storage capacitor104. The voltage across the storage capacitor is denoted is the implanted unit's supply voltage107. A primary voltage protection diode106is provided to shunt excess voltages, that might be generated by external sources, to a level that is considered safe for the operation of the implant electronics. Additional circuitry for protecting the electronics can also be provided.

The supply voltage107provides energy to an electrical load120, which includes a data decoder21, an amplifier23and an actuator25.

A factor in prolonging battery life in the external unit3is the efficiency of coupling between the external coil11and the tuned circuit101of the internal unit3. However, even if the initial coupling of the RF link in an implanted medical device is good at implantation, the properties of the circuit may drift over time, leading to reduced efficiency.

For example, over time the properties of the metal from which the implanted coil17is made can change. Moreover, the bodily environment in which the implanted coil17resides is harsh, and in some situations if the sealing of the implanted coil17breaks down, the implanted coil17may be exposed to bodily fluids and tissues, which can cause further performance degradation. In some instances, the implanted coil17or its connections to other components of the tuned circuit101can fail mechanically. This could be caused, e.g., by a bump or knock on the body part on which the coil17is mounted.

SUMMARY

Embodiments of the present technology are directed to 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. Such methods include 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. Such embodiments can be advantageously employed prior to implantation, e.g. during manufacture or testing.

Other embodiments of the present technology are directed to a method and system for determining the implantation depth, d, of a component of an implanted medical device. Such a method includes determining the position of a magnetic element which is mounted in a fixed physical relationship with the component of the medical device. For example, the component being analysed is a coil of a tuned circuit forming part of the wireless energy transmission system of the implanted medical device.

Such systems can be integrated into a common test unit, respectively.

Advantageously, these methods can be performed on an implanted medical device in vivo, i.e., without the need to explant the device.

DETAILED DESCRIPTION

In the course of developing embodiments of the present technology, the inventor: observed: from time to time, in a clinical setting, a recipient of an implanted medical device may report a problem with the implant for which the cause may be related to the RF link of a recipient's implant system; accordingly, knowledge of the properties of a recipient's implant, and particularly its energy transmission system, then is of particular interest to recipients, clinicians, implanted medical device manufacturers and designers; and to date, however, no mechanism has existed to test the properties of the implanted unit's tuned circuit such that a need exists for the same. At least one embodiment of the present technology, among other things, addresses the need recognized herein by the inventor.

A system for testing a tuned circuit of an implantable medical device will now be described. The tuned circuit to be tested has a coil for transferring energy between it and another inductively coupled coil of the medical device. The system includes: a field generator to induce an electromotive force (EMF) in the coil of the tuned circuit; a field detector to detect resonance of the tuned circuit; and an analysis system to analyse the detected resonance over a period of time to determine a property of the tuned circuit. The field generator can be a signal generator coupled to a radiating coil configured to emit an oscillating magnetic field when energised by a signal generated by the signal generator. The field detector may also include a coil arranged to interact with a field produced by the resonating tuned circuit. This may be the same coil that is a part of the field generator. The detector can also include a digitiser to convert an analog output of the coil to a digital signal for analysis.

FIG. 3is a schematic block diagram illustrating components of a tuned circuit testing system300according to an embodiment of the present technology. AndFIG. 4is a flow diagram illustrating a method400of operation for testing performance of tuned circuit, according to an embodiment of the present technology. The tuned circuit testing system300ofFIG. 3, and the associated flow diagram of,FIG. 4describe can be used for measuring one or more properties of a tuned circuit in an implanted (or implantable) medical device. The example will be described in the context of performing in vivo testing of the tuned circuit. However, in other embodiments a similar testing methodology and system could be used to test the tuned circuit of an implantable medical device either during manufacture or prior to implantation, to verify correct operation of the tuned circuit and/or to determine the baseline characteristics of the circuit prior to implantation.

InFIG. 3, the tuned circuit testing system300includes a probe302which houses a test coil304. The test coil304is used to inductively couple the testing system300with a tuned circuit of a device under test306. The coil304is arranged for selective connection to a signal generator308or a digitiser310via a switch312.

The signal generator308outputs an alternating current of a desired frequency. In the illustrative example, the generator outputs a signal at 5 MHz, although other frequencies may be used in other embodiments of the present technology, depending on the likely resonant frequency of the device under test306.

The digitiser310, e.g., includes a high speed analogue to digital converter that receives an analogue input from the coil304and outputs a digital signal for further analysis.

The selective connection of the coil304of the probe302to either the generator308or digitiser310is performed via the switch312, which may be selectively switched between connection to the generator and digitiser as required in the method to be described below.

Operation of the tuned circuit testing system300is controlled by a data processing system314. The data processing system314for example can be a general purpose computer, including one or more processors that are programmed to perform a method, e.g., as described in connection withFIG. 4. The processing system314includes an input/output port316and output port318in communication with external devices. In this example, the input/output port316may be connected to one or more user input devices319, e.g., a keyboard and/or mouse, in order to allow the operator of the system300to enter details about the device under test306, its recipient or other details which may be needed by the system or desirable to track in the testing process. A display, printer or other output device320is connected to the input/output port318in order to provide an indication of operation of the device to its user. As will be appreciated, in some instances the input and display components may be integrated into single device, such a touch screen device which performs both input and output functions.

The data processing system314is also connected to a memory system322which will include both program data and working data which is used by the processor to execute the methods described herein. The memory322can also include data storage, e.g. a hard drive or solid state disk or the like, which is used to store test data and test outputs for later use or further processing.

Each of the components is supplied with energy (power) from a power supply324. It will be appreciated that in the event that the system300is used for in vivo testing, and if the system is powered from the electrical mains, then the power supply will need to comply with safety requirements for power supplies used for medical devices in order to ensure safety for the recipient whose device is being tested and the operator of the system. Alternatively, a battery can be used to supply the system.

One possible embodiment of the system300is to incorporate all of the components of the system300into the casing of the probe302.

In the flow diagram ofFIG. 4, the left hand column401illustrates the steps performed by the test system300and the right column402illustrates the operation (the intended induced response) of the device under test306.

Prior to beginning the process400, the user of the system300is required to position the probe302such that its coil304is substantially aligned with the induction coil329of the device under test306. As described below, an initial alignment process can also be performed.

The process begins in block404by connecting the test coil304to the generator308via switch312. At block406, the device under test306energises the testing coil304with a high frequency AC voltage. As noted above, e.g., a frequency of about 5 MHz is suitable in some applications. When the testing coil304is energised with the radio frequency signal, the coil304of the testing system300generates a magnetic field oscillating at the generator's output frequency. In this example, a 5 MHz oscillating magnetic field is produced. The magnetic field408intersects the coil329of the device under test306and in block410induces a time varying EMF in the coil329. The induced EMF in the coil329charges the tuning capacitor130(as well as the storage capacitor104) of the device under test306to store energy in the tuned circuit at block412.

Next, at block414, the generator308is deactivated and the switch312configured such that the coil304of the probe302is connected to the digitiser310. In this mode, no additional energy is coupled into the coil329of the device under test306. However, the tuned circuit of the device306will “ring” and expend its stored energy, radiating it via the coil329. This is indicated in block416. During this “ring down” phase, the amount of energy in the output signal from the coil329diminishes over time. The oscillating electrical signal in the coil329will generate an oscillating magnetic field having a frequency, f, which will interact with the coil304of the test system300and induce an alternating EMF therein. In block418, the induced EMF in the coil304is detected and digitised by the digitiser310. The digitiser310samples the time varying output of the coil304and the digitised output signal is then passed to the processing system314for analysis in block420. For example, the method400is iterative, and so can be repeated a number of times and the results averaged to cancel noise before the analysis in block420.

According to one embodiment of the present technology, in block420, the received digitised signals are analysed to determine the frequency of the oscillation of the tuned circuit of the device under test306and the quality factor (Q) of the tuned circuit of the device under test306.

In an initial phase, an alignment process can be performed to assist the user of the testing system to achieve correct alignment of the test coil304and the induction coil329of the device under test306. The alignment process essentially involves repeating the process400while monitoring the received signal amplitude. This allows the signal level received by the digitiser310to be maximised (or at least substantially maximised). The system can facilitate the identification of the correct alignment by cycling through the process400and providing a real-time display of the voltage amplitude of the signal received by the digitiser310to the user, while the user attempts to align the coils304and329. In this initial alignment phase, a reduced level of signal analysis may be used, since only the signal amplitude is necessary for determining the quality of alignment.

FIG. 5illustrates an exemplary output signal produced by the digitiser310of the system300. The plot450illustrates the digitiser output signal over a 5 microsecond period beginning at time to 0, when the input from the coil304is switched into the digitiser310.

It can be advantageous to collect several waveforms, for example one hundred, and process them, e.g., take their average, in order to reduce the effects of unrelated noise upon the measurement. Since the measurement time of the method400is short, this has only a small impact upon the total time taken to perform a measurement.

Over an initial period/window451, e.g., typically less than 1 microsecond, the output from the coil304to the digitiser310includes large amounts of unwanted signals. In the time period/window451, the output of the digitiser310is discarded. Analysis of the digitiser310output begins at the start of a measurement window452that follows period/window451. In this example, the measurement window452is begun at about 1 microsecond, although in some embodiments the measurement window may start at an earlier time, e.g., 400 nanoseconds after measurement begins.

As can be seen, the output of the digitiser in the measurement window452is a decaying sinusoidal form. The digitised signal in the measurement window452is analysed by the processing system314to find a best fit function. In its simplest form, the curve fitted to the digitised signal is of the form:

v=V⁢⁢ⅇ-tτ⁢sin⁡(2⁢⁢π⁢⁢f⁢⁢t+ϕ)
where: v is the voltage of the output signal output 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.

Well known methods such as those using least squares and conjugate directions may be used as the basis of a method for finding the fitted curve. On such method, e.g., is described by Powell (“An efficient method for finding the minimum of a function of several variables without calculating derivatives”, The Computer Journal, Vol 7, p. 155).

Once the curve is fitted, the resonant frequency of the tuned circuit of the device under test306is then given by f (as per the equation above) and the quality factor (Q) is determined using:
Q=πfτ.

In practice, a more complex function may need to be used in order to compensate for defects in the equipment and/or measure more complex behaviour of the tuned circuit.

For example the curved fitted to the output voltage values can include additional terms as follows:

v=V⁢⁢ⅇ-tτ⁢sin⁡(2⁢⁢π⁢⁢f⁢⁢t+ϕ)+a+bt
where a and b represent noise and imperfections in the digitiser310. The term bt introduces a dc shift, so the simple form bt can be replaced by the alternative expression:

b⁡(1-2⁢⁢tT)
where T is the measurement window.

In some cases, non-linearity in the tuned circuit can cause the frequency f to vary with time (i.e., to chirp). This can be approximated by replacing f with a function of time, such as:
f0+tf1+t2f2+ . . . .

Similarly, non linearity in the elements of the tuned circuit can cause the decay of the resonant signal to depart from a strictly exponential form. This can be approximated by replacing

V⁢⁢ⅇ-tτ
with a function such as:

Moreover, some tuned circuits have more than one resonance. A similar process can be used to find the properties of the multiple resonances, for example, by replacing the decaying sinusoid with the sum of two (or more) decaying sinusoids. Alternatively, an iterative approach to fitting the curve can be adopted where the residue from an earlier curve fitting process is used as the raw data for another round of fitting. This can be repeated more than once if necessary.

Advantageously, using the method and apparatus ofFIGS. 3 and 4, interrogation and testing of an implanted tuned circuit forming part of the energy transmission link in an implanted medical device can be performed, without the need for physical connection between the circuit under test and the testing equipment. As will be appreciated, the technique described herein only requires a short period of time to perform. In principle, the technique only requires, e.g., microseconds to perform. However, the computational analysis may take, e.g., several hundred milliseconds on an ordinary desktop computer. The time taken to perform the test may be further increased by the need to charge the storage capacitor104to at least some minimum level prior to moving into the detection phase of the measurement. But overall, the process may take less than, e.g., one second to test the function of the resonant circuit. This speed of testing also makes such a process viable for integration into a manufacturing process, or quality checking process, for the coil or implantable medical device.

A system for determining a position of a component of an implanted medical device will now be described. The component for which the position is to be located includes a magnetic element positioned in a fixed spatial arrangement with respect thereto. The system includes: one or more sensors for detecting a localised magnetic field produced by the magnetic element at a plurality of positions remote from the component; a processor to compare the measured localised magnetic field at a plurality of said remote positions with a model of the magnetic field produced by the magnetic element; and to thereby determine the position of the magnetic element with respect to the plurality of locations. The model of the magnetic field produced by the magnetic element can be stored in a memory, e.g., in a look up table, that can be interrogated by the processor to determine a relative position of the one or more sensors and the magnetic element at a point in time. The one or more sensors can include a plurality of Hall effect sensors. If a plurality of Hall effect sensors is used, these can be mounted in a fixed spatial relationship with one another, e.g. in a hand held probe, and used to determine a set of magnetic field readings in a set of positions in a known spatial relationship with each other. In an embodiment with a probe of this type, the probe can include indicia to selectively indicate correct alignment of the probe relative to the device being tested.

FIG. 6Aillustrates an implantable unit500of an implantable medical device, e.g., a cochlear implant, according to another embodiment of the present technology. The implantable unit500includes an energy transmission coil502which is connected to device electronics504. The device electronics504comprise, e.g., the RF rectification regulation sub-system19, data decoder21, amplifier23and actuator25of the internal unit5of the device1described above in connection withFIG. 1. The actuator506, in this example, is a lead housing an array of one or more electrodes for applying stimulation to the auditory nerve of the recipient. Concentrically mounted with the energy transmission coil502is a magnetic element508. The implantable unit500is encased in a suitable housing material510to provide protection once implanted. The magnetic element508is held by the casing510in a fixed physical relationship with the coil502. In use, the magnetic element508is used to hold the external coil of the wireless energy transmission link, e.g., coil11inFIGS. 1 and 2, in alignment with the coil502.

FIG. 6Billustrates a cross-sectional view of the implanted unit500taken along line6-6ofFIG. 6A. As can be seen, the centrally located magnetic element508sits in a generally planar relationship with the coil502and is encased in the housing material510.FIG. 7illustrates the device500implanted in a recipient. The implanted device500is mounted between a bone, e.g., skull,700and a skin surface702. The depth of implantation of the device500beneath the skin surface702is denoted by d. This depth is also known as the skin flap thickness.

FIG. 8illustrates a system800for measuring the skin flap thickness, d, of an implanted medical device, according to another embodiment of the present technology. In other words, the system determines the depth of the coil502of the implanted device500. This can be done, e.g., by measuring the distance to the magnetic element that is in a fixed physical relationship with the coil.

The testing system800generally comprises a testing wand802carrying an array of Hall effect sensors804to812and an array of indicators814to820. The array of indicators and Hall effect sensors are connected via respective wiring to other components of the analysis system, as described below. The analysis system800additionally includes a processor822which controls the operation of the system800and runs suitable software for analysing the output of the Hall effect sensors of the testing wand802. Memory system828is also provided. The memory system828includes memory for carrying machine readable instructions for controlling the operation of the processor822and additionally includes a data storage system for storing results from testing and other data as required. The memory system828also stores a model of the expected magnetic field produced by the magnetic element of a device under test. This can be, e.g., in the form of a look up table of expected Hall effect sensor outputs induced by the modelled magnetic field at a plurality of locations. These location specific magnetic field data can be arranged in groups corresponding to the specific separation and relative spatial positioning of the hall effect sensors in the testing wand802. The model may be simplified if it is assumed that the orientation of the wand will be aligned in a particular orientation with respect to the magnetic element being detected. In this example, as illustrated inFIGS. 10 and 11, the wand802is assumed to be parallel to the skin surface and consequently parallel to the plane of the magnetic element508.

The system800also includes one or more user input devices824for entering data into the system and display, printer or other output devices826for providing feedback and data back to a user of the system.

The system800additionally includes a digitiser830, e.g., an analog-to-digital converter. The digitiser830samples the time varying output of the Hall effect sensors804to812.

Each of the components of the system is powered by a power supply832. As described in connection withFIG. 3the power supply832, e.g., is a battery, or if supplied from the electrical mains is arranged to comply with power supply requirements for medical devices to ensure patient and operator safety. Similarly, one possible embodiment of the system800is to incorporate all of the components of the system800into the casing of the wand802.

In use, the testing wand802is placed adjacent the site of the implantation of an implantable device500and the Hall effect sensors804to812interact with, and sense, the magnetic field produced by the magnetic element508of the implantable unit500. The digitised outputs from the Hall effect sensors are provided to the processor822which compares the plurality of Hall effect sensor values to its stored model of the expected magnetic field of the magnetic element508. The model of the expected magnetic field can be expressed, e.g., as a lookup table of expected Hall effect sensor readings for the plurality of sets of positions around the magnetic element508.

The processor822compares the Hall effect sensor outputs with data in the model and determines a spatial position that corresponds to the Hall effect sensor outputs. Thereby the processor822is able to determine the location of the wand802relative to the magnetic element508. It will be appreciated that, because the magnetic field of the magnetic element508drops away rapidly with distance between the Hall effect sensor and the magnetic element, it will generally be advantageous to accurately position the wand802with respect to the magnetic element508prior to attempting to determine the implantation depth d of the magnetic element508.

In order to do assist in alignment of the wand802, the Hall effect sensor readings can initially be used to determine an approximate position of the wand with respect to the magnetic element508by comparing the Hall effect sensor readings to the stored model of the magnetic field of the magnetic element508. The wand802, as noted above, is provided with a set of indicators814to820, for example which may be LEDs or similar indicia that can be selectively activated by the processor822to indicate to the user of the system800a direction to move the wand802, to more closely align the Hall effect sensors with the magnetic element508.

As can be seen inFIG. 8, the Hall effect sensors804to812are arranged, e.g., in a cross pattern with four outer Hall effect sensors804to810located at the ends of the arms of the cross and a central Hall effect sensor812at the centre of the cross. In use, the processor822can be programmed to either use the central Hall effect sensor812solely for the distance calculation to the magnetic element, once correct alignment has been achieved using the external Hall effect sensors804to810. Alternatively, the depth measurement can be biased to more heavily weight a distance d determined according to the central Hall effect sensor812. Other schemes, e.g. which equally use all sensors, can be used. Which approach is optimal will depend on the configuration of the magnetic field produced by the magnetic element being located.

The present example includes 5 Hall effect sensors804to812, but this number is not fixed. Other examples may include more or fewer sensors, or sensors of a different type. A single sensor could be used, but more precise alignment becomes more important. Alternatively, a single sensor could be swept across a region to determine the magnetic field at a plurality of known positions along the swept path.

Turning now toFIG. 9, it illustrates a method of using a system of the type illustrated inFIG. 8, according to another embodiment of the present technology. The method900begins after a user has activated the system800and placed the wand802such that its Hall effect sensors804to812are roughly aligned with the magnetic element508of the recipient's implanted device.

In block902, the outputs of the Hall effect sensors804to812are read and passed to the digitiser830at block904which outputs a digital signal indicating the Hall effect voltage at each Hall effect sensor804to812at a particular point in time. The set of Hall effect sensor voltages is compared to the voltages from the magnetic field model and a relative position of the wand802and magnetic element508is determined at block906. Details of block906are provided, e.g., viaFIGS. 13 and 14.

FIGS. 13 and 14describe an exemplary process for using a magnetic model to determine the position of magnetic field sensing device relative to a magnetic element of an implanted medical device. This process is based on the knowledge that given the shape of a magnetic element and the magnetic properties of the material of the magnetic element, the magnetic field in the space surrounding the magnetic element can be computed. Therefore it is possible to determine what magnetic field should be experienced by a magnetic field sensor when it is placed in the vicinity of the magnet. Consequently the outputs produced by a magnetic field sensor can be predicted for any position of the sensor.

FIG. 13illustrates an example of a magnetic element1300that might be used in an implantable device. InFIG. 13, magnetic element1300is illustrated as a cylindrical magnet, the poles of magnet which are aligned along the Z axis. Using the known properties of magnetic fields, it is possible to compute the magnetic field at any point P.

FIG. 14illustrates a method1400, according to another embodiment of the present technology, for determining a position of a magnetic element, e.g.,1300, of an implanted device relative to the magnetic field testing wand802ofFIG. 8.

Once the wand802is brought into relatively close proximity to the magnetic element1300, a set of suitable outputs from the Hall effect sensors804to812within the wand802are produced, and the method1400ofFIG. 14is used to determine the relative position of the wand802with respect to the magnetic element1300as follows. Initially at block1402, an initial starting position (x,y,z) for the wand802relative to the magnetic element1300is nominated. This can be, e.g., a random selection, a selection at the origin of the coordinate system, an ideal position or some other selection. Next, in block1404, the expected Hall effect voltages at position (x,y,z) and an array of nearby positions are computed. In this example, a Cartesian grid of 3×3×3 positions, centred on (x,y,z) is used, resulting in 27 positions, with 27 corresponding expected hall effect voltages.

Next in block406, the error between each of the 27 computed Hall effect voltages and each of the 5 sensed hall effect voltages from the Hall effect sensors804to812is computed. For each position in the array of positions, a total error is calculated, e.g., using the sum of the squared error for each sensed Hall effect voltage, i.e., using 27 total error values. Once the total error is calculated for all 27 positions, the position within the array of positions with the best (minimum) error is identified.

Next in block1408, the best position in the array of positions is compared to the current value of (x,y,z).

If the position amongst the array of 27 positions with the smallest error is the current (x,y,z), then (x,y,z) is determined to be the position of the wand802with respect to the magnetic element1300, in block1412. Otherwise, in block1410, the position which gave the smallest error is used as a new (x,y,z). This process is then iterated until the current position estimate (x,y,z) provides the best position estimate. The process can also be iterated starting with a relatively coarsely spaced array of points in block1404, and subsequently using a successively finer array at each iteration until a desired degree of spatial resolution is attained.

The process of searching for the estimated position of the magnetic element can be made faster by pre-computing the expected Hall effect voltages for every position in a pre-selected grid around the magnet element1300and storing these voltages in a lookup table. For example, a grid could contain expected hall effect voltages for each position at every x and y coordinate between −10 mm and +10 mm in 0.25 mm increments and for every z coordinate between 2 mm and 20 mm in 0.1 mm blocks. This would result in a table having 1187541 sets of expected Hall effect voltages.

Other methods of finding an estimate of the magnetic element position are possible. The process described above could be thought of as finding the value of a function (x,y,z)=P({V}) where {V} is the measured set of Hall effect voltages. A suitably precise mathematical expression for this function can be used to model the magnetic field of the magnetic element, e.g., by fitting an algebraic function to the values predicted by the model. This fitted function can then be used to directly estimate the magnet position given a set of Hall voltages.

Returning now toFIG. 9, if the comparison at block906reveals that the determined position (x,y,z) (e.g. see block1412ofFIG. 14) indicates a separation greater than a particular threshold, then at block908, it is determined that the wand802is not sufficiently well aligned with the magnetic element of the recipient's implanted device. In this case, at block910, the processor822makes a determination of a preferred direction of movement of the wand802with respect to the magnetic element, and one or more of the indicators814to820on the wand802is illuminated, in block912, to tell the user of the device which direction to move the wand802to better align it with the magnetic element of the implanted medical device.

In the event that the wand802is determined to be sufficiently aligned, e.g. the x and y positions indicate that the wand806is aligned with the z axis of the magnetic element508, a final distance between the magnetic element and wand802is determined at block914. The distance determined in block914, may need to be corrected for any offset in implant depth between the magnetic element and the coil of the implanted medical device (given that the position of the coil is the important factor for coupling efficiency of the wireless transmission link), or any offset between the Hall effect sensor positions and the external surface of the wand802which contacts the recipient's skin during test. If correction for these two dimensions is required, this is performed in block916and the skin thickness d is outputted in block918.

As will be appreciated the alignment process described herein can also tell the user the lateral position of the magnetic element1006and by extension the lateral position of the device1008to which it is attached; seeFIGS. 10-11.

FIGS. 10 and 11together serve to illustrate a concept of aligning the probe with the magnetic element prior to final determination of skin flap thickness, according to another embodiment of the present technology. In this regardFIG. 10illustrates a similar view to that ofFIG. 7and illustrates an implanted medical device1000positioned between a bone1002and skin surface1004. The medical device1000includes a magnetic element1006which is located in a fixed physical relationship with a coil1008of the wireless electrical transmission system of the medical device. In order to test the skin flap thickness, the probe1010is brought such that its lower side1012contacts the skin surface1004.

In this example, a Cartesian coordinate system is used, having its origin in the centre of the outermost surface of the magnetic element1006. The outermost surface of the magnetic element lies in the x-y plane of the coordinate system and the z axis extends outward, in a direction normal to the outermost surface of the magnetic element1006. The illustration is a cross section along the y-z plane.

In this cross-sectional view, three Hall effect sensors1014,1016and1018can be seen. As will be appreciated, the distance between the magnetic element1006and Hall effect sensor1014is substantially shorter than the distance between the magnetic element1006and Hall effect sensor1016and accordingly, the magnetic field perceived by the Hall effect sensor1014will be far greater than that perceived by the Hall effect sensor1016. When the outputs of the Hall effect sensors1014-1018are compared with the stored model, the model will indicate that the final determined position (x,y,z) of the set of Hall effect sensors1014,1016and1018is not sufficiently aligned with the centre of the magnetic element1006and therefore that the probe1010needs to be moved upward to more correctly align the central Hall effect sensor1018with the centre of the magnetic element1006. The alignment of the probe1010with the magnetic element1006is determined by comparing the x and y positions of the probe with those expected for correct alignment, e.g., ((0,0) with this presently defined origin of the coordinate system). In the event that they are not the same (or not sufficiently similar), the offset direction can be determined. In order to convey the necessary direction of movement to reduce the computed offset to the user of the probe1010, indicator light1019is illuminated whereas indicator light1020is left un-illuminated—indicating that the user should move the probe in the y direction, towards the direction of the illuminated indicator light1019. Thus, the user is told which direction to move the sensor probe1010.

Due to the speed of processing and comparing the magnetic fields with the model, this alignment process can be performed repeatedly, and can be used to steer the alignment of the probe1010in real time.FIG. 11illustrates the arrangement ofFIG. 10but with the probe1010correctly aligned such that the central Hall effect sensor1018is aligned with the centre of the magnetic element1006. Once correct alignment has been established such as inFIG. 11, the z coordinate determined by use of the magnetic field model is used to determine the distance d, being the implantation depth of the coil1008. Correct alignment is indicated to the user, e.g. by illuminating all indicia on the wand. The user can then choose to accept the current distance d, e.g. by pressing a button on the wand or through some other input.

As will be appreciated, this embodiment of the present technology can be used to measure the implantation depth to any implanted component of a medical device that is in a known physical relationship with a magnetic element.

In the context of measuring the implantation depth of a coil forming part of a wireless transmission link in the implanted component of a medical device, knowledge of implantation depth can be very useful. For surgeons, a knowledge of implantation depth can be used to hone their implantation technique so that optimum implantation dept can be achieved. Moreover, regular checking of implantation depth following implantation can be used to check swelling around the implantation site.

FIG. 12illustrates a further system made in accordance with an embodiment of the present technology. The system atFIG. 12performs the functions of the two systems ofFIGS. 3 and 8. The system1200includes a pair of probes1202and1204which are electrically connected to a test unit1206. The probe1202is used for testing the parameters of the tuned circuit of the device under test1206as described in connection withFIGS. 3 and 4. The probe1204is similar to the probe illustrated inFIG. 8and is used to determine the skin flap thickness. The measurement unit1206includes a processing system1210which runs suitable software to control the operation of the device and communicate its output with a display1212. As will be appreciated the circuit testing functionality requires digitisation of a signal oscillating at around 5 MHz whereas the outputs from the Hall effect sensors of the probe1204will produce a much lower frequency signal and thus only require a lower speed A/D converter. Accordingly, inputs from the probes1204and1206provided to the processor via a high speed and low speed analogue digital converters1216and1218respectively. The high speed A/D converter is connected to the circuit testing probe1202and the low speed A/D converter1218is connected to the skin flap thickness testing probe1204.

The system1206additionally includes a signal generator1220for providing a radio frequency signal to the circuit testing probe1202. All components of the system1206are powered by a power supply1222and memory requirements for the processing system are handled by a memory system1224.

The system also includes a power supply and input and output devices, analogous to those described in earlier embodiments.

In use the system ofFIG. 12can be used to determine the location of an implanted component of an implantable medical device in the manner described above. Similarly the system ofFIG. 12can be used to test a property of a tuned circuit, e.g. a tuned circuit forming part of a wireless energy or data transmission system of the device, in the manner described above.

The invention described and claimed herein is not to be limited in scope by the specific example embodiments herein disclosed, since these embodiments are intended as illustrations, and not limitations, of several aspects of the present invention. Any equivalent embodiments are intended to be within the scope of the present invention. Indeed, various modifications of the present invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.