Electrode impedance spectroscopy

Aspects of the present invention are generally directed to impedance spectroscopy in an active implantable medical device (AIMD) comprising a component with one or more electrodes. In an embodiment, the AIMD applies a signal at a plurality of frequencies using one or more of the electrodes. Measurements are then obtained for the applied signal to determine impedance(s) at the applied frequencies of the tissue in which the electrodes are located.

BACKGROUND

Field of the Invention

The present invention relates generally to implantable stimulator systems, and more particularly, to impedance measurements in an implantable stimulator system.

Related Art

Implantable medical devices have provided benefits to recipients over recent decades. Implantable medical devices are devices having one or more components or elements that are at least partially implantable in a recipient. One type of implantable medical device is an active implantable medical device (AIMDs), which are medical devices having one or more implantable components that rely for their functioning upon a source of power other than the human body or gravity, such as an electrical energy source. Exemplary AIMDs include devices configured to provide one or more of stimulation and sensing, such as implantable stimulator systems and implantable sensor systems.

Implantable stimulator systems provide stimulation to a recipient of the device. Exemplary implantable stimulator systems include, but are not limited, to cochlear implants, auditory brain stem implants, cardiac pacemakers, neurostimulators, functional electrical stimulation (FES) systems, etc.

Cochlear implants include an electrode assembly implanted in the cochlea and are used to treat sensorineural hearing loss. Electrical stimulation signals are delivered directly to the auditory nerve via the electrode assembly, thereby inducing a hearing sensation in the implant recipient.

An Auditory Brain Stem Implants (ABI) is another type of surgically implanted electronic device that provides a sense of sound to a recipient suffering from sensorineural hearing loss. ABIs are typically used in recipients suffering from sensorineural hearing loss that, due to damage to the recipient's cochlea or auditory nerve, are unable to use a cochlear implant.

A cardiac pacemaker is a medical device that uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of a heart. The primary purpose of a pacemaker is to maintain an adequate heart rate.

A neurostimulator, also sometimes referred to as an implanted pulse generator (IPG) is a battery powered device designed to deliver electrical stimulation to the brain. Neurostimulators are sometimes used for deep brain stimulation and vagus nerve stimulation to treat neurological disorders.

FES uses electrical currents to activate nerves innervating extremities affected by paralysis resulting from, for example, spinal cord injury, head injury, stroke, or other neurological disorders.

Other types of implantable stimulator systems include systems configured to provide electrical muscle stimulation (EMS), also known as neoromuscular stimulation (NMES) or electromyostimulation, which involves the application of electric impulses to elicit muscle contraction.

Exemplary implantable sensor systems include, but are not limited to, sensor systems configured to monitor cardiac, nerve and muscular activity.

SUMMARY

In one aspect of the invention, there is provided a method of operating an active implantable medical device (AIMD) comprising an electrode, the method comprising: applying a measurement signal at a plurality of frequencies to a recipient of the AIMD using the electrode; and performing, using the AIMD, a measurement, responsive to the measurement signal and indicative of an impedance of the electrode and tissue of the recipient, at each of the plurality of frequencies.

In another aspect, there is provided an active implantable medical device comprising: a component comprising an electrode; a signal generator configured to apply a signal using the electrode at a plurality of frequencies; and a control circuit configured to perform a measurement, responsive to the signal, indicative of an impedance of the electrode and tissue of the recipient for each of the plurality of frequencies.

In yet another aspect, there is provided a system for performing spectroscopy comprising: an active implantable medical device (AIMD) comprising: an electrode; means for applying a measurement signal to a recipient using the electrode at a plurality of frequencies; and means for performing, using the AIMD, a measurement, responsive to the measurement signal and indicative of an impedance of the electrode and tissue of the recipient for each of the plurality of frequencies.

DETAILED DESCRIPTION

Embodiments of the present invention are generally directed to impedance spectroscopy in an active implantable medical device (AIMD) comprising a component with one or more electrodes. In an embodiment, the AIMD applies a signal at a plurality of frequencies using one or more of the electrodes. Measurements are then taken for the applied signal. These measurements are used to measure impedance(s) at the applied frequencies of the tissue in which the electrodes are located. The measured impedances are then analyzed to determine, for example, the proximity of the electrodes to one or more biological structures of the recipient. This information may be used by a surgeon during implantation of the component of the AIMD in the recipient or by a clinician at any time to diagnose potential problems with or to confirm correct operation of one or more electrodes of the AIMD or the tissue surrounding them.

Embodiments of the present invention are described herein primarily in connection with one type of Active Implantable Medical Device (AIMD), namely a cochlear implant system (commonly referred to as cochlear prosthetic devices, cochlear prostheses, cochlear implants, cochlear devices, and the like; simply “cochlea implant systems” herein.) Cochlear implant systems generally refer to hearing prostheses that deliver electrical stimulation to the cochlea of a recipient. As used herein, cochlear implant systems also include hearing prostheses that deliver electrical stimulation in combination with other types of stimulation, such as acoustic or mechanical stimulation. It would be appreciated that embodiments of the present invention may be implemented in other types of AIMDs.

FIG. 1is perspective view of a cochlear implant system, referred to as cochlear implant system100implanted in a recipient.FIG. 2is a functional block diagram of cochlear implant system100. The recipient has an outer ear101, a middle ear105and an inner ear107. Components of outer ear101, middle ear105and inner ear107are described below, followed by a description of cochlear implant system100.

In a fully functional ear, outer ear101comprises an auricle110and an ear canal102. An acoustic pressure or sound wave103is collected by auricle110and channeled into and through ear canal102. Disposed across the distal end of ear cannel102is a tympanic membrane104which vibrates in response to sound wave103. This vibration is coupled to oval window or fenestra ovalis112through three bones of middle ear105, collectively referred to as the ossicles106and comprising the malleus108, the incus109and the stapes111. Bones108,109and111of middle ear105serve to filter and amplify sound wave103, causing oval window112to articulate, or vibrate in response to vibration of tympanic membrane104. This vibration sets up waves of fluid motion of the perilymph within cochlea140. Such fluid motion, in turn, activates tiny hair cells (not shown) inside of cochlea140. Activation of the hair cells causes appropriate nerve impulses to be generated and transferred through the spiral ganglion cells (not shown) and auditory nerve114to the brain (also not shown) where they are perceived as sound.

Cochlear implant system100comprises an external component142which is directly or indirectly attached to the body of the recipient, and an internal component144which is temporarily or permanently implanted in the recipient. External component142typically comprises one or more sound input elements, such as microphone124for detecting sound, a sound processor126, a power circuit (not shown), and an external transmitter unit128. External transmitter unit128comprises an external coil130and, preferably, a magnet (not shown) secured directly or indirectly to external coil130. Sound processor126processes the output of microphone124that is positioned, in the depicted embodiment, by auricle110of the recipient. Sound processor126generates encoded signals, sometimes referred to herein as encoded data signals, which are provided to external transmitter unit128via a cable (not shown). Sound processor126may further comprise a data input interface (not shown) that may be used to connect sound processor126to a data source, such as a personal computer or musical player (e.g., an MP3 player).

Internal component144comprises an internal receiver unit132, a stimulator unit120, and a stimulating lead assembly118. Internal receiver unit132comprises an internal coil136, and preferably, a magnet (also not shown) fixed relative to the internal coil. Internal receiver unit132and stimulator unit120are hermetically sealed within a biocompatible housing, sometimes collectively referred to as a stimulator/receiver unit. The internal coil receives power and stimulation data from external coil130. Stimulating lead assembly118has a proximal end connected to stimulator unit120, and a distal end implanted in cochlea140. Stimulating lead assembly118extends from stimulator unit120to cochlea140through mastoid bone119. In some embodiments stimulating lead assembly118may be implanted at least in basal region116, and sometimes further. For example, stimulating lead assembly118may extend towards apical end of cochlea140, referred to as cochlea apex134. In certain circumstances, stimulating lead assembly118may be inserted into cochlea140via a cochleostomy122. In other circumstances, a cochleostomy may be formed through round window121, oval window112, promontory123or through an apical turn147of cochlea140.

Stimulating lead assembly118comprises a longitudinally aligned and distally extending array146of electrodes148(also referred to as electrode contacts), sometimes referred to as array of electrodes146or array of electrode contacts146herein. Although array of electrodes146may be disposed on stimulating lead assembly118, in most practical applications, array of electrodes146is integrated into stimulating lead assembly118. As such, array of electrodes146is referred to herein as being disposed in stimulating lead assembly118. Stimulator unit120generates stimulation signals which are applied by electrodes148to cochlea140, thereby stimulating auditory nerve114. Because, in cochlear implant system100, stimulating lead assembly118provides stimulation, stimulating lead assembly118is sometimes referred to as a stimulating lead assembly. Stimulator unit120may further be connected to an extra-cochlear electrode (not shown) located external to the recipient's cochlea140.

In cochlear implant system100, external coil130transmits electrical signals (that is, power and stimulation data) to internal coil136via a radio frequency (RF) link. Internal coil136is typically a wire antenna coil comprised of multiple turns of electrically insulated single-strand or multi-strand platinum or gold wire. The electrical insulation of internal coil136is provided by a flexible silicone molding (not shown). In use, implantable receiver unit132may be positioned in a recess of the temporal bone adjacent auricle110of the recipient.

FIG. 3Ais a simplified diagram of an exemplary stimulating lead assembly318, in accordance with an embodiment of the present invention.FIG. 3Billustrates stimulating lead assembly318inserted in cochlea140. As illustrated, stimulating assembly318is configured to adopt a curved configuration during and or after implantation into the recipient's cochlea140. To achieve this, in certain embodiments, stimulating assembly318is pre-curved to the same general curvature of a recipient's cochlea140. In such embodiments, stimulating assembly318is sometimes referred to as perimodiolar stimulating assembly where stimulating assembly318adopts its curved configuration in cochlea140. When implanted, the surface of stimulating lead assembly318that faces the interior of cochlea140is referred to herein as the medial surface of stimulating lead assembly318. As illustrated, electrodes348are located on the medial side of stimulating lead assembly318. Further, as shown, when implanted the tip350of stimulating lead assembly318is located near the cochlear apex134.

AlthoughFIGS. 3A-3Billustrate a perimodiolar stimulating assembly. In other embodiments, stimulating assembly may be a non-perimodiolar stimulating assembly which does not adopt a curved configuration. For example, stimulating assembly318may comprise a straight stimulating assembly or a mid-scala assembly which assumes a mid-scala position during or following implantation. In further embodiments, cochlear implant could include a stimulating assembly implantable into a natural crevice in the cochlea that allows for the hydrodynamic nature of the cochlea to be maintained, or an assembly positioned adjacent to the cochlea.

The state of the tissue and anatomical structures surrounding the electrodes of a stimulating lead assembly is typically an important factor with regard to the effectiveness of stimulation delivered to the recipient. Present stimulator systems measure the impedance of each electrode, which provides a useful measure that assists clinicians in diagnosing faults, assessing the position of the electrode and determining anatomical anomalies in the cochlea. This impedance is currently reported as a single number and thus provides limited information about the state of the tissue surrounding the electrode. For example, this impedance is typically determined at what is essentially a single frequency and used for the limited purpose of fault detection in cochlear implants.

As will be discussed further below, an embodiment of the present invention, measures the impedance of an electrode and its surrounding tissue over a range of frequencies (referred to herein as impedance spectroscopy) to obtain a more detailed picture of the state of the electrode and its surrounding tissue (including the surrounding anatomical structures).

These impedance measurements may be used in a plurality of applications. For example, these impedance measurements may be used to determine the proximity of an electrode to different physical structures (e.g., the modiolus, the lateral wall, etc.) within the cochlea since different physical structures may present different spectroscopic signatures. This information may be used during surgical implantation, for example, to provide the surgeon with information regarding the position of the stimulating lead assembly in the recipient's cochlea. For example, this information may be used to determine the instantaneous insertion depth of the stimulating lead assembly318. Further, post surgery, this information (e.g., the proximity of the electrodes to the modiolus) may be useful in predicting performance of the cochlear implant.

Impedance spectroscopy in accordance with embodiments of the present invention may also be useful in detecting issues with the tissue surrounding the stimulating lead assembly, such as assessing the extent of scar tissue around the electrodes or the presence of an infection near the electrode. Additionally, during surgical implantation impedance spectroscopy in accordance with embodiments may be useful in detecting tip fold-over (i.e., when tip350folds back on stimulating lead assembly318during the insertion process). Impedance spectroscopy in accordance with embodiments may also be useful in detecting other stimulating lead assembly faults (e.g., faults more complex than simple open circuits or short circuits) as will be discussed in more detail below.

As noted above, an embodiment measures the impedance of an electrode and its surrounding tissue over a range of frequencies to obtain a more detailed picture of the state of the electrode and its surrounding tissue (including the surrounding anatomical structures). In an embodiment, these impedances are measured by measuring the impedance between a pair of electrodes (e.g., an electrode of the stimulating lead assembly and an extra-cochlear electrode) of a stimulating lead assembly over the frequency range. The measurements may be used to generate impedance spectroscopy plots. The characteristics of the tissue close to the electrodes can have a significant impact on the measured impedances, and thus the shape of generated impedance spectroscopy plots. For example, the close proximity of the modiolar wall to the electrode may have a profound effect on the shape of the plot compared to the plot's shape when the electrode is surrounded by perilymph, the fluid which occupies the space inside the cochlea140. Thus, a plot of measurements when the electrode is near the modiolus will look quite different from a plot of measurements taken when the electrode is distant from it and surrounded by perilymph. As will be discussed further below, an embodiment of the present invention uses impedance spectroscopy to obtain an impedance spectroscopy plot and then compares the obtained plot with different characteristic plot shapes to obtain information regarding the stimulating lead assembly, such as, for example, the proximity of the electrode(s) to various cochlear structures, characteristics of the tissue surrounding the electrode(s), issues with the stimulating lead assembly (e.g., tip fold-over, faults, etc.).

FIG. 4provides a simplified diagram of exemplary circuitry for impedance spectroscopy in a cochlear implant, in accordance with an embodiment of the present invention. For ease of explanation only the components discussed below are illustrated inFIG. 4. As illustrated, stimulator unit420is connected to internal receiver unit432, electrode448and extra-cochlear electrode450. Stimulator unit420, internal receiver unit432, electrode448, extra-cochlear electrode450may be, for example, components such as the correspondingly named stimulator unit120, internal receiver unit132, electrode148and extra-cochlear electrode (not shown) discussed above with reference toFIGS. 1-2.

As illustrated, stimulator unit420, includes a control circuit402, a signal generator404, a resistor406, and a voltage measurement circuit408. Control circuit402may be a circuit (e.g., an Application Specific Integrated Circuit (ASIC)) configured for exercising control over the stimulator unit420. For example, control circuit402may be configured for receiving, from the internal receiver unit432, the encoded data signals regarding the sound and generating the stimulating signals for applying stimulation via electrodes448and450.

Signal generator404generates a voltage for application via the stimulating lead assembly. For ease of explanation, signal generator404is a separate voltage generator distinct from the electronics used for applying stimulation to cause a hearing percept by the recipient. It should be noted thatFIG. 4provides but one simple example of an embodiment for performing impedance spectroscopy in accordance with the present invention and that in other embodiments other types of systems and components may be used. For example, as will be discussed further below, another embodiment uses the existing current generator present in typical cochlear implants for generating the signal in place of the voltage generator used in the embodiment ofFIG. 4.

As illustrated, a voltage measurement circuit408is connected to opposite ends of resistor406. Resistor406may be a standard resistor, such as, for example, a 100 ohm resistor. Voltage measurement circuit408may include any type of circuitry configured to output a signal indicative of the voltage across resistor406. For example, in an embodiment, voltage measurement circuit408may comprise a differential amplifier that takes as inputs the signals on opposite sides of resistor406and then amplifies the difference in the voltage between the two sides. Voltage measurement circuit408provides the measured voltage to control circuit402. Further, in embodiments, voltage measurement circuit408may comprise an analog to digital converter (ADC) that digitizes the measured voltage before providing the measured voltage to the control circuit402.

FIG. 5provides a flow chart of an exemplary method500for obtaining impedance measurements, in accordance with an embodiment of the present invention. Control circuit402, at block502, initiates the process for measuring impedances. In an embodiment, the process is performed during surgical implantation to obtain information regarding the location of the stimulating lead assembly in the recipient's cochlea during the surgical implantation procedure. In such an embodiment, the electrode(s) of the stimulating lead assembly may be connected to an external device configured to aid the surgeon during the implantation process. In the illustrated embodiment ofFIG. 4, the external device may be, for example, connected to the sound processor126(FIG. 1), connected to the internal receiver unit132, or connected to the stimulator unit120. In such examples, the connection between the external device and the component of the cochlear implant may be via a wired or wireless connection. In such embodiments, the process may be initiated by a command (e.g., initiated by the surgeon) being transmitted from the external device to the control circuit402to initiate the process. A further description of exemplary external devices is provided below.

In another embodiment, the external device may be directly connected to the leads connected to the electrode(s), such as, for example, in embodiments in which the stimulating lead assembly is implanted prior to connecting the stimulating lead assembly to the stimulator unit. In such embodiments, the control circuit402, signal generator404, resistor406, and voltage measurement circuit408may be implemented in the external device.

Or, for example, in embodiments in which the process is performed after surgical implantation of the stimulating lead assembly, a clinician may connect to the sound processing unit126and direct sound processing unit126to send a command to the stimulator unit420to initiate the process. In another embodiment, control circuit402may, for example, determine to initiate the process based on an amount of time that has elapsed since the last measurement (e.g., the control circuit402performs measurements once a day, week, month, etc.). Or, for example, control circuit402may monitor performance of the stimulator unit420and initiate the process if a particular event occurs (e.g., a fault is detected).

In the presently discussed embodiment, at block504, control circuit402selects the first pair of electrodes448and450for which impedance measurements are to be taken. The selected pair of electrodes448and450may be a pair of electrodes that is used for application of stimulation via monopolar stimulation, where current flows from an electrode448of the stimulating lead assembly to an extra-cochlear electrode450. Or, for example, (e.g., in systems that use bipolar stimulation) both electrodes of the pair may be electrodes of the stimulating lead assembly.

In an embodiment, the stimulating lead assembly comprises 22 electrodes, where each electrode is paired with the extra-cochlear electrode to provide 22 separate stimulation channels. In one such embodiment, control circuit402performs impedance measurements for each of the 22 separate stimulation channels using the electrode pairs corresponding to each stimulation channel. Control circuit402may select, for example, a pair of electrodes corresponding to one of these stimulation channels as the first selected pair of electrodes and then in subsequent passes, control circuit402may select the electrode pairs for the other stimulation channels.

For each selected electrode pair, control circuit402takes impedance measurements for a plurality of frequencies. In an embodiment, these frequencies are spaced across the operational frequency range of the device. However, in other embodiments, the frequencies may include frequencies outside the normal operation range of the device.

The frequency of the applied sinusoidal voltage may be swept from low to high or high to low in a number of steps and measurements of the sinusoidal current amplitude and phase taken at each frequency step. For example, in a system where the operation frequency range is between 50 and 20 kHz (i.e., a frequency sweep ranging from 50-20 kHz), control circuit402may take measurements at 200 logarithmic steps along the frequency range. The frequency range for which measurements are to be taken may vary depending on the specifics of the embodiment but is typically over many orders of magnitude (e.g. 50-20 kHz, 10 mHz to 1 MHz, etc.)

In another embodiment the current is applied at the selected frequency and the voltage is measured. In this case block508inFIG. 5would read “Apply current at selected frequency” and block510inFIG. 5would read “Measure voltage”.

In another embodiment the applied voltage signal is not of a single frequency but comprises the sum of a plurality of frequencies. Since the response properties of tissue for small voltage perturbations can be considered linear the recorded current will then contain signals at the same frequencies as those in the stimulating voltage waveform. These frequencies can be separated with appropriate filtering to yield the same information as if individual frequencies had been applied and measured sequentially as illustrated inFIG. 5. This method has the advantage that it may be quicker to perform than the method described inFIG. 5since all frequencies are applied simultaneously. It has the disadvantage that it may be less accurate than the method described inFIG. 5due to the additional filtering step.

Blocks506-512illustrate a simplified method of applying a frequency sweep and performing measurements for a selected electrode pair. It should, however, be understood that other mechanisms for applying a frequency sweep and obtaining measurements may be used. Further, the voltages, number of measurements and frequency range of the sweep are exemplary only, and in other embodiments different values may be used.

At block506, control circuit402selects the starting frequency (e.g., 50 Hz) and voltage for the sweep (e.g., 50 mV). At block508, control circuit402directs signal generator404to begin the frequency sweep. In response, signal generator404applies a signal to the electrodes at the specified frequency and voltage. The voltage selected at block506is preferably fairly small (e.g. 50 mV) so that the Voltage/Current (V/I) characteristic for the medium to be measured can be considered linear over the voltage range of the applied sinusoidal signal. Further, in an embodiment, the selected voltage may be a sub-threshold voltage, so that a hearing percept is not caused by the applied signal and the recipient may be unaware that the measurements are taking place. The signal applied by signal generator404to electrodes448and450may be a fixed sinusoidal signal at the specified frequency and voltage.

As noted above, resistor406is in series with signal generator404, electrode448, the recipient's tissue452, and electrode450. Thus, the current through resistor406corresponds to the current passing through electrodes448and450. Control circuit402measures the current through the electrodes448and450at block510. This measurement may include both the amplitude and phase of the current. In the illustrated embodiment, voltage measurement circuit408measures the voltage drop across resistor406and provides the measured voltage to control circuit402, which converts the measured voltage to current using the formula: I=V/R, where I is the measured current, V is the measured voltage, and R is the resistance of resistor406. Further, the measured currents (or voltages) can be converted to a total impedance for the electrodes448and450and tissue452using the following formula: Vtot=VR+I*Rtot, where Vtotis the voltage supplied by signal generator404, VRis the voltage drop across resistor406, I is the measured current through resistor406, and Rtotis the total impedance of electrodes448and450and tissue452. This formula can be rewritten as Rtot=(Vtot−Vr)/I. Further, the measured voltages, currents, and impedances may be complex comprising real and imaginary parts based on the measured amplitudes and phases.

Control circuit402determines if the frequency sweep is completed or not at decision512. If not, control circuit402increases the frequency of signal generator404at block514. As noted above, in an embodiment, the frequency sweep may range from 50 Hz to 20 kHz, with the control circuit taking 200 measurements logarithmically spaced between 50 Hz and 20 kHz. Thus, in an embodiment, control circuit402may direct the signal generator404to apply a signal at the next frequency (e.g., 51.5 Hz, 53.1 Hz, . . . 19409.8 Hz, 20 kHz) for which the control circuit402is to obtain a measurement.

Once the frequency sweep is completed and the measurements obtained, the control circuit402determines, at decision516, if measurements are to be obtained for other electrode pairs. For example, in an embodiment, control circuit402obtains measurements for each electrode (paired with the extra-cochlear electrode) of the cochlear implant. However, in embodiments, control circuit402only obtains measurements for a subset of the electrodes.

If measurements are to be obtained for other electrode pairs, the process returns to block504and measurements are obtained for the next electrode pair. Once measurements are obtained for each electrode pair to be measured, the process proceeds to block518and measurements are analyzed. The particulars of this analysis may vary depending on the particular information sought. Exemplary mechanisms for analyzing and using this data (e.g., presenting location information to a surgeon) are discussed below.

FIG. 6Aillustrates a platinum electrode surrounded by a cellular medium (i.e. tissue).FIG. 6Ais provided to illustrate how impedance spectroscopy can be used to determine the properties of the medium surrounding the electrode.FIG. 6Billustrates an equivalent circuit model600of the system illustrated inFIG. 6A. As shown inFIG. 6Bthe equivalent circuit model600includes Constant Phase Elements (CPEs)652and654that are used to describe the circuit properties of biological interfaces, which are typically rough or non-uniform at the microscopic level. A description of the CPE circuit model will be discussed below with reference toFIGS. 7A-7B.

As shown inFIG. 6A, current from the electrode648(e.g., an electrode148(FIG. 1)) travels from the electrode's surface650through the tissue via two paths: an intra-cellular path where the current passes through cells632of the tissue and an extra-cellular current path where the current travels around the cells632.

As noted the electrode's surface650is non-uniform at the microscopic level and can be modeled as a CPE652. Similarly, the walls of the cells632may be modeled as a CPE654. The resistance of the intra-cellular current path is modeled as resistive element, Rint656. The resistance of the extra-cellular path is modeled as resistive element, Rext658.

FIG. 7Aprovides a diagram for illustrating how the uneven surface of an electrode at the microscopic level can be modeled as a CPE. As shown, the electrode surface702may have one or more microscopic pores704and the surface can be modeled as an imperfect capacitor.

FIG. 7Bprovides a diagram of a circuit model for a CPE in accordance withFIG. 7A. The circuit model uses conventional electrical components and comprises a plurality of arms, each with a resistor712and a capacitor714. The resistance and capacitance of each arm increase by a factor of N (e.g., N=2) for each arm of the circuit. The circuit model includes enough arms so that the RC time constants for the circuit span the frequency range being modeled. The CPE can be viewed as a circuit element whose phase angle (angular difference between the phase of the voltage sinusoid and the current sinusoid) remains the same, regardless of the frequency applied to it. In an embodiment, the phase angle is such that the current leads the voltage by around 45 degrees.

As noted above with reference toFIGS. 4-5, the data recorded for each electrode pair is in the form of the amplitude and phase of a sinusoid resulting from the applied signal. This data can also be represented as real and imaginary parts of the complex impedance of the electrodes448and450and tissue452(FIG. 4). This data can be plotted in a number of ways, each highlighting different features of the medium being tested. For example, the data can be plotted with the magnitude of the impedance on the y-axis against frequency on the x-axis.

FIG. 8is an exemplary plot for an electrode surrounded by a cellular medium, such as shown inFIG. 6A.FIG. 8provides a curve800of the measured impedance amplitudes versus frequency for one of the measured electrodes458of the stimulating lead assembly. In the model ofFIGS. 6A-6B, at extremely high frequencies the CPEs652and654all have very low impedance so the total impedance of the circuit becomes the parallel combination of Rintand Rext. Hence, above a certain frequency, the system looks completely resistive. At very low frequencies the CPEs652and654(representing the platinum/tissue interfaces) dominate the system and the magnitude of the interface CPEs652and654can be determined. The bumps in the curve800at intermediate frequencies are caused by the presence of the cells and cell walls in the medium. Different cell types cause different inflections and plateaus in this intermediate region of the plot. Thus, obtaining impedance measurements for plurality of frequencies, as shown, helps provide a mechanism for distinguishing between impedances due to the electrode and those due to the tissue.

In embodiments, the selected set of frequencies to be measured can be tailored to the particular application. For example, the typical time taken to run a full impedance sweep can be several minutes or hours since typically the frequencies in the milli-Hertz region require long data collection times. Since these low frequency measurements provide information mostly about the electrode/tissue interface and not its surrounding structures, in certain embodiments (e.g., when used during cochlear implant implantation), the method can omit many of the low frequency measurements. For example, when used during cochlear implantation, the method may only take measurements for a subset of key frequencies that can be quickly obtained and provide sufficient information for identifying the location of the stimulating lead assembly.

In the above discussed embodiment ofFIG. 4, a dedicated signal generator404(FIG. 4) was used for applying the signal to the electrode pair. This dedicated signal generator circuitry may be useful in forcing a sinusoidal voltage or current and measuring a resulting sinusoidal current or voltage, respectively.

In other embodiments, the stimulating signal generation circuitry used for applying stimulation to a recipient in an implantable stimulator system may be used to generate the signal(s) used to measure impedance. For example, cochlear implants typically include circuitry designed to deliver a square wave current pulse as part of their neural stimulator function. Existing cochlear implants also typically include circuitry for measuring a voltage drop across the electrodes (e.g., the voltage drop across a particular electrode and the extra-cochlear electrode or the voltage drop across two electrodes of the stimulating lead assembly). Embodiments of the present invention use the square wave current pulse of the signal generation circuitry to obtain impedance spectroscopy data by measuring and storing the voltage across the electrodes (e.g., the electrode of the stimulating lead assembly that applied the pulse and the extra-cochlear electrode) at various times during the pulse.

FIG. 9illustrates a square wave current pulse902and a resulting measured voltage904, in accordance with an embodiment of the present invention. In this embodiment, the stimulating signal generation circuitry of the cochlear implant is used to apply the current pulse. Further, circuitry included in the stimulator unit is used to measure the voltage across the electrode pair. Although data obtained in this manner may not be exactly translated into the data obtained through conventional impedance spectroscopy, it is closely related to it and is likely to provide useful information for deriving information about the structures near the electrodes.

In an embodiment, the time during the pulse at which the voltage across the electrodes is measured is closely related to the inverse of the frequency in a conventional impedance spectroscopy system. For example, the voltage measured 10 μs after the start of the current pulse is closely related to the amplitude data that would be obtained at a frequency of 100 kHz (= 1/10 μs) with a conventional impedance spectroscopy system. Similarly, voltage measured 100 μs after the start of a pulse is closely related to an impedance spectroscopy amplitude measured at a frequency of 10 kHz= 1/100 μs.

The Laplace transform can be used to relate the measured voltage during the stimulation pulse to the frequency spectrogram as follows: During (phase 1 of) a constant current stimulation pulse, assume the time domain current is I(t) and the time domain voltage is V(t). I(t) and V(t) can be transformed using the Laplace operator to yield the frequency domain current, I(s), and frequency domain voltage, V(s) as follows.

For a constant current stimulator I(t)=I (a constant) so the frequency domain current, I(s) is the Laplace transform of the constant I so: I(s)=I/s. The signal V(t) is measured numerically by the AIMD. The Laplace transform, V(s), of the time domain voltage waveform, V(t), can be calculated numerically from the measured values of time domain voltage, V(t). i.e.: V(s)=L[V(t)] where L is the Laplace operator.

From the frequency domain current, I(s), and the frequency domain voltage, V(s), the frequency domain impedance, also known as the impedance spectrogram, can be calculated as: Z(s)=V(s)/I(s). As noted above, the measured impedance data may be analyzed in various manners depending on the particular implementation. For example, in an embodiment, the raw current measurements may be provided by control circuit402(FIG. 4) to an external device that analyzes the measurements to display information to a surgeon regarding the location of the stimulating lead assembly. This external device may be, for example, a computer or specialized piece of hardware and/or software. Or, for example, in an embodiment, the data is analyzed by cochlear implant100, such as by the sound processor126or control unit402. The cochlear implant100may analyze the data to, for example, determine if a fault occurred or the stimulating lead assembly has shifted position from its initial position. The cochlear implant (e.g., sound processor126) may then take appropriate action such as, for example, modifying the MAP used in generating electrical stimulation, providing an indication to the external speech processor that can be used to notify the recipient or surgeon of an issue with the cochlear implant, or, for example, terminate the application of stimulation to the recipient.

As noted, in an embodiment, control circuit402provides the raw data to an external device, such as a computer or specialized piece of hardware that analyzes the data. In such an embodiment, the external device may be connected to the sound processor126(FIG. 1) by a wired or wireless connection. For example, in an embodiment, control circuit402provides the data to internal receiver unit132(FIG. 1), which transmits the data to external transmitter unit128. External transmitter unit128then provides the data to sound processor126, which then provides the data to the external device.

The external device may compute a curve, such as curve800, from the raw data and compare the computed curve to known curve patterns representing different characteristics to identify the proximity of the electrode to a particular structure. Various techniques may be employed for performing this comparison, such as, for example, by using a neural network to compare the measured curve to known curve patterns to determine the proximity of the electrode to different structures. For example, the curve shape for an electrode close to the modiolus will be different than the curve shape for an electrode that is located away from the modiolus and surrounded by perilymph.

The following provides an exemplary description of an embodiment in which the measured information is provided to an external device so that information on the location of the stimulating lead assembly may be presented to a surgeon during implantation of the stimulation lead assembly. In an embodiment, this information is provided in real time so that a surgeon implanting a stimulating lead assembly can obtain real time data about the cochlear structures near the stimulating lead assembly. This information may be displayed to the surgeon in a visual form (for example a head up display of one or more of the impedance spectroscopy plots). Or, for example, this visual data may be presented through the operating microscope as a head-up display. Or, for example, the data may be presented aurally as a single frequency continuous tone (e.g. the tone frequency or amplitude could be related to the impedance spectroscopy amplitude data measured at a particular frequency). Or the sound could be a continuous, complex sound (e.g. consisting of a summation of multiple tones, each tone related to the impedance spectroscopy amplitude at a range of frequencies). Or the sound could be a series of tone bursts where the tone burst frequency is related in some way to the salient impedance spectroscopy data.

As noted, in embodiments the data is presented in real time (so the tone or visual data changes more or less instantaneously as the stimulating lead assembly is moved) or it may be presented after the fact. The advantage of real time is that it allows the surgeon to respond instantly to any detected changes or problems associated with the proximity to neural structures. The advantages of “after the fact” presentation is that it allows more time for any post processing of data that may be necessary in order to extract the most useful data to present to the surgeon.

The data processing, salient feature extraction and presentation means will vary depending on the application required. For example, if a surgeon wishes to know the proximity of all the electrodes of a cochlear implant stimulating lead assembly to the lateral wall of the cochlea, then, in an embodiment, data from all the electrodes of the array are measured, processed and presented to he surgeon, in more or less real time.

In another embodiment, a clinician may wish to determine, postoperatively, the proximity of the electrodes of a stimulating lead assembly to the modiolus. This may provide useful data that affects the way the cochlear implant is programmed (e.g. there is evidence to show that close modiolar positioning of the electrodes reduces current spread and potentially allows the use of programming strategies requiring greater spatial selectivity). In this case there is no need for real time processing of the data since the clinician has ample to time to consider the best way to program the cochlear implant. In this case it may be advantageous to do more complex processing and presentation of the impedance spectroscopy data, perhaps presenting the data in a range of presentation styles and methods, since additional information relevant to modiolar proximity may be obtained through greater processing of the raw data.

In addition to plotting the measured impedance amplitudes versus frequency and comparing the plot to known patterns, embodiments of the present invention may also analyze the data in different ways depending on the particular physical characteristics of the cells or electrode that are to be determined. These other mechanism include, for example, plotting (a) the phase angle of the complex impedance vs. frequency; (b) the real component of the complex impedance vs. frequency; (c) the imaginary component of the complex impedance vs. frequency; and (d) the real (e.g., x-axis) vs. imaginary (y-axis) impedance components for each frequency measured. Further, in another embodiment, rather than simply using the raw data, the data may be further processed. For example, in an embodiment, the derivative of the measured impedance (or voltage or current) vs. time is obtained and the resulting derivative is used, for example, to obtain information regarding the stimulating lead assembly, such as information regarding the proximity of the electrode to the structure of the medium surrounding the tissue. In one such embodiment, the resulting derivative values are plotted versus frequency and displayed to the surgeon, or for example, used as inputs to a classification algorithm (e.g., neural network) such as discussed above.

In an embodiment, the impedance spectroscopy data may be combined (e.g., by an external device) with other data relating to structures near the electrode (e.g., x-rays and other imaging data, optical measurements, force measurements, etc.).

It should be noted that the above description provides one example for obtaining impedance spectroscopy measurements, and in other embodiments, more complex impedance spectroscopy measurements may be obtained. For example, in the embodiment the current is measured on the same electrode as the one that applies the voltage. It is also possible to apply current or voltage on one electrode and measure voltage or current respectively on one or more other electrodes in the array of electrodes. Applying voltage or current on one electrode and measuring the voltage or current on another electrode is known to those of skill in the art and is not discussed further herein. The system of forcing current on one electrode and then measuring voltage on other electrodes in an array of electrodes is sometimes referred to as electrode field imaging.

As noted above, impedance spectroscopy data may be used for determining proximity information. Additionally, as noted above, the impedance spectroscopy data may be used for obtaining other types of information. For example, the impedance spectroscopy data may be used to determine if tip fold over occurs during surgical implantation of a stimulating lead assembly. When tip fold-over occurs, the electrode spacing is changed, such that one or more electrodes may be located very near another electrode. In embodiments in which impedance spectroscopy data is used to detect tip fold over, a current or voltage may be applied on one or more electrodes and measured on one or more different electrodes. If tip fold over occurs the current or voltage applied on the electrode(s) may be received on the other electrode(s) with a resulting distinctive profile that may be used to detect tip fold over. In such an embodiment, a classification algorithm (e.g., a neural network) may be used to determine if the measured values indicate that tip fold over has occurred.

In another embodiment, the impedance spectroscopy data is used to detect whether there is a fault or whether the tissue includes diseased or damaged cells. In such an embodiment, these faults, as well as issues with the tissue may each have distinctive impedance spectroscopy characteristics. In an embodiment, a classification algorithm (e.g., neural network) may be used to analyze the measured impedance spectroscopy data to determine if any of these issues has occurred.

In another embodiment, impedance spectroscopy measurements are obtained using a four point impedance method. In this method the voltage between two nearby electrodes is measured while passing current between two other electrodes flanking the measurement electrodes. A further description of an exemplary four point impedance method is provided in U.S. Patent Publication No. 2011/0087085 entitled “Method and Device for Intracochlea Impedance Measurements.”

Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, operation, or other characteristic described in connection with the embodiment may be included in at least one implementation of the invention. However, the appearance of the phrase “in one embodiment” or “in an embodiment” in various places in the specification does not necessarily refer to the same embodiment. It is further envisioned that a skilled person could use any or all of the above embodiments in any compatible combination or permutation.

It is to be understood that the detailed description and specific examples, while indicating embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.