Assessing the placement of a cochlear electrode array by multidimensional scaling

Correct placement of the electrode is crucial for cochlear implantation (CI) surgery. It determines the access to the auditory nerve and subsequent hearing performance. Here, we propose an objective measures tool that can partially verify the electrode position. The intracochlear spread of the electrical fields is measured and analyzed by means of multidimensional scaling resulting in an intuitive visual representation. The user can then detect major issues, such as electrode foldover or ossification. Other implantation issues, such as electrode migration into the scala vestibuli, may not significantly alter the electrical conduction pattern and remain undetected. Still, as the measurement is quick and readily available, it may be a valuable intraoperative verification tool.     

Proposal of a new method for narrowing and moving the stimulated region of cochlear implants: animal experiment and numerical analysis

We have proposed the tripolar electrode stimulation method (TESM) for narrowing the stimulation region and continuously moving the stimulation site for cochlear implants. The TESM stimulates the auditory nerve array using three adjacent electrodes which are selected among the electrodes of an electrode array within the lymphatic fluid. Current is emitted from each of the two lateral electrodes and received by the central electrode. The current received by the central electrode is made equal to the sum of the currents emitted from the lateral electrodes. In this paper, we evaluate whether or not TESM works according to a theory which is based on numerical analysis using an electrical equivalent circuit model of the auditory nerve fibers. In this simulation, the sums of the excited model fibers are compared to the compound action potentials (CAP's) which we obtained through animal experiments. To identify the main parameter while maintaining the amplitude of the CAP (the sum of the fired fibers), we assumed the presence of some parameters from the radial current density profile. In the case of the width value among the parameters being kept constant, the amplitude of the CAP was almost constant; thus, the number of the fired fibers was also almost constant. The width value equals the distance between the points at which the profile of the radial current density of the electrode array and the line of the radial threshold current density of the electrode array intersect. It is possible to determine the measure of the stimulation region or site by controlling the width value and the ratios of the currents emitted from the lateral electrodes. As a result, we succeeded in narrowing the stimulation region by controlling the sum of the currents emitted from the two lateral electrodes. Also we succeeded in continuously moving the stimulation site by modifying the currents emitted from the two lateral electrodes.     

A manually operated, advance off-stylet insertion tool for minimally invasive cochlear implantation surgery

The current technique for cochlear implantation (CI) surgery requires a mastoidectomy to gain access to the cochlea for electrode array insertion. It has been shown that microstereotactic frames can enable an image-guided, minimally invasive approach to CI surgery called percutaneous cochlear implantation (PCI) that uses a single drill hole for electrode array insertion, avoiding a more invasive mastoidectomy. Current clinical methods for electrode array insertion are not compatible with PCI surgery because they require a mastoidectomy to access the cochlea; thus, we have developed a manually operated electrode array insertion tool that can be deployed through a PCI drill hole. The tool can be adjusted using a preoperative CT scan for accurate execution of the advance off-stylet (AOS) insertion technique and requires less skill to operate than is currently required to implant electrode arrays. We performed three cadaver insertion experiments using the AOS technique and determined that all insertions were successful using CT and microdissection.     

Three-dimensional localization of cochlear implant electrodes using epipolar stereophotogrammetry

Three-dimensional (3-D) localization of individual cochlear implant electrodes within the inner ear is of importance for modeling the electrical field of the cochlea, designing the electrode array, and programming the associated speech processor. A 3-D reconstruction method of cochlear implant electrodes is proposed to localize individual electrodes from two X-ray views in combination with the spiral computed tomography technique. By adapting epipolar geometry to the configuration of an X-ray imaging system, we estimate individual electrode locations in the least square sense without using a patient attachment required by an existing stereophotogrammetry technique. Furthermore, our method does not require any knowledge of the intrinsic and extrinsic parameters of the imaging system. The performance of our method is studied in numerical simulation and with patient data and is found to be sufficiently accurate for clinical use. The maximum root mean-square errors measured are 0.0445 and 0.214 mm for numerical simulation and patient data, respectively.     

Construction and validation of a plunge electrode array for three-dimensional determination of conductivity in the heart.

The heart's response to electrical shock, electrical propagation in sinus rhythm, and the spatiotemporal dynamics of ventricular fibrillation all depend critically on the electrical anisotropy of cardiac tissue. Analysis of the microstructure of the heart predicts that three unique intracellular electrical conductances can be defined at any point in the ventricular wall; however, to date, there has been no experimental confirmation of this concept. We report the design, fabrication, and validation of a novel plunge electrode array capable of addressing this issue. A new technique involving nylon coating of 24G hypodermic needles is performed to achieve nonconductive electrodes that can be combined to give moderate-density multisite intramural measurement of extracellular potential in the heart. Each needle houses 13 silver wires within a total diameter of 0.7 mm, and the combined electrode array gives 137 sites of recording. The ability of the electrode array to accurately assess conductances is validated by mapping the potential field induced by a point current source within baths of saline of varying concentration. A bidomain model of current injection in the heart is then used to test an approximate relationship between the monodomain conductivities measured by the array, and the full set of bidomain conductivities that describe cardiac tissue.     

Digital X-ray stereophotogrammetry for cochlear implantation

Multielectrode, intracochlear implant systems are effective treatment for profound sensorineural hearing loss. In some cases, these systems do not perform well, which may be partially due to variations in implant location within the cochlea. Determination of each electrode's position in a patient's inner ear provides an in vivo basis for both the cochlear modeling of electrical fields and the future design of electrode arrays that deliver electrical stimulation to surviving auditory neurons, and may improve speech processor programming for better speech recognition. We developed an X-ray stereophotogrammetric approach to localize implanted electrodes in three dimensions. Stereophotogrammetry of implanted electrodes is formulated in weak perspective geometry, with knowledge of a three-dimensional (3-D) reference structure and electrode positions in each of two digital stereo-images. The localization error is theoretically, numerically, and experimentally quantified. Both numerical and experimental results demonstrate the feasibility of the technique.     

Electrical stimulation of the auditory nerve: direct current measurement in vivo

Neural prostheses use charge recovery mechanisms to ensure the electrical stimulus is charge balanced. Nucleus cochlear implants short all stimulating electrodes between pulses in order to achieve charge balance, resulting in a small residual direct current (DC). In the present study we sought to characterize the variation of this residual DC with different charge recovery mechanisms, stimulation modes, and stimulation parameters, and by modeling, to gain insight into the underlying mechanisms. In an acute study with anaesthetised guinea pigs, DC was measured in four platinum intracochlear electrodes stimulated using a Nucleus C124M cochlear implant at moderate to high pulse rates (1200-14,500 pulses/s) and stimulus intensities (0.2-1.75 mA at 26-200 microseconds/phase). Both monopolar and bipolar stimulation modes were used, and the effects of shorting or combining a capacitor with shorting for charge recovery were investigated. Residual DC increased as a function of stimulus rate, stimulus intensity, and pulse width. DC was lower for monopolar than bipolar stimulation, and lower still with capacitively coupled monopolar stimulation. Our model suggests that residual DC is a consequence of Faradaic reactions which allow charge to leak through the electrode tissue interface. Such reactions and charge leakage are still present when capacitors are used to achieve charge recovery, but anodic and cathodic reactions are balanced in such a way that the net charge leakage is zero.     

The nerve-electrode interface of the cochlear implant: currentspread

One of the fundamental facets of the cochlear implant that must be understood to predict accurately the effect of an electrical stimulus on the auditory nerve is the nerve-electrode interface. One aspect of this interface is the degree to which current delivered by an electrode spreads to neurons distant from it. This paper reports a direct mapping of this current spread using recordings from single units from the cat auditory nerve. Large variations were seen in the degree to which the different units are selective in responding to electrodes at different positions within the scala tympani. Three types of units could be identified based on the selectiveness of their response to the different electrodes in a linear array. The first type of unit exhibited a gradual increase in threshold as the stimulating site was moved from more apical to more basal locations within the scala tympani. The second type of unit exhibited a sharp local minimum, with rapid increases in threshold in excess of 6 dB/mm in the vicinity of the minimum. At electrode sites distant from the local minima the rate of change of the threshold approached that of the first type of units. The final type of unit also demonstrated a gradual change in threshold with changing electrode position, however, two local minima, one apical and one basal, could be identified. These three types are hypothesized to correspond to units which originate apical to the electrode array, along the electrode array and basal to the electrode array     

The effects of interpulse interval on stochastic properties of electrical stimulation: models and measurements

It is known that some cochlear implant users have improved speech perception using higher rates of interleaved pulsatile stimulation. There are, however, significant limitations on their performance presumably due in part to temporal and spatial interactions. To address these limitations, we have examined refractory characteristics of the auditory nerve using experimental animal models and computational simulations. A stochastic model of the node of Ranvier modified for mammalian sodium channel kinetics has been developed to calculate the masked input-output (I/O) functions for different interpulse intervals (IPI) [26]. The model is based upon 1000 voltage-gated sodium channels and incorporates parameters such as nodal resistance and capacitance. The relative spread (RS) [35] calculated from the I/O functions was typically 0.03 for 17 different IPIs between 450 micros and 6 ms for cathodal stimuli. For IPI = 830 and 870 micros, the RS was ten times greater than those for other IPIs. Although it is not fully understood how the electrically evoked compound action potential (EAP) data are related to single fiber data, the RS of single fibers is a partial contributor [19]. We have measured the EAP using a monopolar intracochlear stimulating electrode and a recording electrode placed directly on the nerve and have observed changes in slope of EAP growth functions consistent with the theoretical RS values. These results have significant implications for speech coding in a cochlear implant since they suggest an increased membrane noise for pulse trains of specific rates.     

A Speech Processor with Lateral Inhibition for an Eight Channel Cochlear Implant and its Evaluation

A speech processor including the function of lateral inhibition was proposed to compensate for the wide current spread for an eight channel cochlear implant. Our cochlear implant system is composed of a speech processor, percutaneous connector and an eight channel electrode array made of Pt?Ir wires.     

Lumped-parameter model for in vivo cochlear stimulation

A lumped-parameter model that simulates the in vivo electrical properties of a guinea pig cochlea implanted with a multielectrode stimulating array is presented. A basic model of the low-frequency electroanatomy in a normally functioning guinea pig cochlea is developed by adding critical membrane capacitances to D. Strelioff's resistive network model (1973). The basic model of normal cochlear tissues is modified to account for anatomical and physiological differences between a normal and implanted cochlea, resulting in an impedance model of an implanted cochlea. Simulating the results of in vivo cochlear stimulation verifies the accuracy with which the modified cochlear model represents electrical properties within an electrically stimulated cochlea. Generalized simulations using this model suggest a straightforward phasing scheme capable of achieving sharply focused, channel-independent multielectrode cochlear stimulation     

Discrimination of synthetic vowels by using tactile vocoder and acomparison to that of an eight-channel cochlear implant

A vowel discrimination test using a tactual vocoder was administered and the results were compared to that of an eight-channel cochlear implant. Both the tactile vocoder and the cochlear implant divided the speech signals into 16 frequency components using band-pass filters and lateral inhibition circuits. In the tactile vocoder, these 16 components were converted into a vibration with 200 Hz frequency and applied to a 3 x 16 element vibrator array using bimorph piezoelectric elements. The vibratory patterns were sensed on the fingertip. In the cochlear implant, the 16 components were reduced to eight current stimulation signals, consisting of biphasic pulses with 200 Hz frequency, which were applied to an eight-channel electrode array implanted in the scala tympani. The electrode array passed through the round window into the scala tympani to a depth of 23 mm. These psychophysical experiments investigate the ability of human subjects to discriminate synthetic vowels as a function of the number of channels employed. The results suggested that an eight-channel and a 16-channel tactile vocoder provided essentially the same discrimination scores. However, the ability to discriminate synthetic vowels decreased rapidly when less than eight channels were employed. The ability of an eight-channel tactile vocoder is expected to be better than that of the eight-channel cochlear implant because it is supposed that vowel discrimination is degraded by a phenomenon known as "current spreading" in the case of cochlear stimulation. However, the comparison between the two devices was not done on the cochlear implant subject.     

A 3-D SAR model for current source interstitial hyperthermia

A three-dimensional (3-D) model is presented for the calculation of the specific absorption rate (SAR) in human tissue during current source interstitial hyperthermia. The model is capable of millimeter resolution and can cope with irregular implants in heterogeneous tissue. The SAR distribution is calculated from the electrical potential. The potential distribution is determined by the dielectric properties of the tissue and by the electrode configuration. The dielectric properties and the current injection of the electrodes are represented on a 3-D uniform grid. The calculated potential at an electrode current injection point is not the actual electrode potential at that point. To estimate this potential a grid independent representation of an electrode together with an analytical solution in the neighborhood of the electrode are used. The calculated potential on the electrode surface is used to estimate the electrode impedance. The tissue implementation is validated by comparing calculated distributions with analytical solutions. The electrode implementation is verified by comparing different discretizations of an electrode configuration and by comparing numerically calculated electrode impedances with analytically calculated impedances     

Electrical impedance tomography

Electrical impedance tomography (EIT) is an imaging modality that estimates the electrical properties at the interior of an object from measurements made on its surface. Typically, currents are injected into the object through electrodes placed on its surface, and the resulting electrode voltages are measured. An appropriate set of current patterns, with each pattern specifying the value of the current for each electrode, is applied to the object, and a reconstruction algorithm uses knowledge of the applied current patterns and the measured electrode voltages to solve the inverse problem, computing the electrical conductivity and permittivity distributions in the object. This article focuses on the type of EIT called adaptive current tomography (ACT) in which currents are applied simultaneously to all the electrodes. A number of current patterns are applied, where each pattern defines the current for each electrode, and the subsequent electrode voltages are measured to generate the data required for image reconstruction. A ring of electrodes may be placed in a single plane around the object, to define a two-dimensional problem, or in several layers of such rings, to define a three-dimensional problem. The reconstruction problem is described and two algorithms are discussed, a one-step, two-dimensional (2-D) Newton-Raphson algorithm and a one-step, full three-dimensional (3-D) reconstructor. Results from experimental data are presented which illustrate the performance of the algorithms     

Two-dimensional numerical model of memory devices with a corrugated capacitor cell structure

A mathematical model of a corrugated capacitor cell (CCC) structure for megabit-class dRAM's is proposed. This model is constructed using a numerical analysis of time-dependent current continuity equations and Poisson's equation. An electrically floating electrode and nonplanar geometries are modeled by zero-current boundary conditions and by Gauss's theorem for Poisson's equation. Transient analyses by a full two-dimensional simulation reveal a complicated device operation mechanism. The storage charge in a floating capacitor is strongly affected by the nearest neighboring capacitor potential, if two storage capacitors are laid out with small pitches. The potential in the floating capacitor significantly shifts when the drive capacitor is charged-up, compared with the reverse case, i.e., a discharge operation for the drive capacitor. A leakage current between two cells was generated due to the electric coupling between them. The current decreases exponentially with respect to the spacing between cells. That is, the electrical coupling strength rapidly decreases with an increase in the spacing. The present model can be highly applicable for designing dRAM devices with complicated and highly integrated structures. Design criteria are clarified.     

An advanced multiple channel cochlear implant

An advanced multiple channel cochlear implant hearing prosthesis is described. Stimulation is presented through an array of 20 electrodes located in the scala tympani. Any two electrodes can be configured as a bipolar pair to conduct a symmetrical, biphasic, constant-current pulsatile stimulus. Up to three stimuli can be presented in rapid succession or effectively simultaneously. For simultaneous stimulation, a novel time-division current multiplexing technique has been developed to obviate electrode interactions that may compromise safety. The stimuli are independently controllable in current amplitude, duration, and onset time. Groups of three stimuli can be generated at a rate of typically 500 Hz. Stimulus control data and power are conveyed to the implant through a single transcutaneous inductive link. The device also incorporates a telemetry system that enables electrode voltage waveforms to be monitored externally in real time. The electronics of the implant are contained almost entirely on a custom designed integrated circuit. Preliminary results obtained with the first patient to receive the advanced implant are included.     

Model-based design of artificial zero power cochlear implant

This paper deals with a model-based design of an autonomous biomechatronic device for sensing and analog signal processing of acoustic signals. The aim is to develop a biomechatronic artificial cochlear implant for people with hearing loss due to damage or disease of their cochlea. The unique artificial electronic cochlear implant is based on an array of microelectromechanical piezoelectric membranes. Oscillations of membranes detect and filter acoustic signals in individual acoustic frequencies. The proposed biomechatronic device of the artificial cochlear implant consists of an active filters array, signal processing electronics, stimulation nerves electrodes and energy harvesting system for autonomous powering of the device. This solution differs from current cochlear implants solutions, which are bulky electronic systems limited by their high power consumption. The multidisciplinary models of the artificial cochlea implant concept are presented. The mechatronic approach based on model seems to be very useful for development of the full implantable cochlear implant which is designed for the sensing and processing of acoustic signals without external energy source.     

Thin-film multielectrode arrays for a cochlear prosthesis

The design and fabrication of flexible thin-film microelectrode arrays for use in a cochlear prosthesis are described. The electrode array is designed to be inserted through the round window of the cochlea into the spiral scala tympani chamber of the cochlea. A lifetime of decades under stimulation is sought. The electrode array is comprised of photolithographically defined platinum-on-tantalum conductors sandwiched between polyimide layers. A liquid polyimide is used, which polymerizes in two stages. After the first stage of curing, the polyimide is susceptible to photolithographic etching, allowing patterned access holes to be cut into the top layer of the insulating sandwich. After the second cure, the polymer becomes inert biocompatible Kapton. The processing techniques and the electrode test results are presented.     

Force application during cochlear implant insertion: an analysis for improvement of surgeon technique

Highly invasive surgical procedures, such as the implantation of a prosthetic device, require correct force delivery to achieve desirable outcomes and minimize trauma induced during the operation. Improvement in surgeon technique can reduce the chances of excessive force application and lead to optimal placement of the electrode array. The fundamental factors that affect the degree of success for cochlear implant recipients are identified through empirical methods. Insertion studies are performed to assess force administration and electrode trajectories during implantations of the Nucleus 24 Contour and Nucleus 24 Contour Advance electrodes into a synthetic model of the human Scala Tympani, using associated methods. Results confirm that the Advance Off- Stylet insertion of the soft-tipped Contour Advance electrode gives an overall reduction in insertion force. Analysis of force delivery and electrode positioning during cochlear implantation can help identify and control key factors for improvement of insertion method. Based on the findings, suggestions are made to enhance surgeon technique.     

激光器阵列的非本征理想因子

研究了激光器阵列的电导数表征方法,建立了激光器阵列的等效电路模型,导出了理想情况下阵列的电导数公式.理论、PSPICE仿真和实验结果表明:一致性良好的阵列的非本征理想因子等于每个阵列单元的非本征理想因子,该结论可用于对阵列单元非本征理想因子检测与阵列可靠性研究.导致单元本征理想因子变大,电流泄漏的可靠性问题均可导致阵列非本征理想因子变大.