UP Bioengineering Our people

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1 UP Bioengineering Our people Design and application of user-specific models of cochlear implants Tania Hanekom Tiaan K Malherbe, Liezl Gross, Rene Baron, Riaze Asvat, Werner Badenhorst & Johan J Hanekom Larry Schmidt, Tiaan Malherbe, Johannes Myburgh, Pieter Venter, Rene Baron, Dirk Oosthuizen, Johanie Roux, Werner Badenhorst, Liza Blignaut, Johan Hanekom, Liezl Gross, Heinrich Crous, Tania Hanekom, Alex Oloo. Insert: Riaze Asvat. CIAP UP Bioengineering Our place UP Bioengineering is part of Electrical, Electronic and Computer Engineering CIAP CIAP

2 Agenda The development of our user-specific models Human and guinea pig model generations What the models can do What the models show Progression of UP Bioengineering's volume conduction models Human Generation 1 Translating models into tools Research tools Model-predicted mapping (MPM) Model-based diagnostics (MBD) CIAP HG1: Generalised human cochlea extruded on analytical spiral from 2D section ( ). CIAP User-specificity in cochlear implants Realised that models need to migrate to include user-specific characteristics Started to work on user-specific models in 2007 Micro-CT data from guinea-pig subject plus neural data from same subject was made available through Russ Snyder/ Ben Bonham from Pat Leake s lab (Epstein Labs) Micro-CT of subject s cochlea Acoustic frequency response areas of the 16 electrode contacts implanted in the inferior colliculus What qualifies subject-specificity in animals? Subject-specificity characterised by, among others: duration of cochlear implantation and duration of deafness age of implantation stimulation mode (BP, MONO, CG) electrode insertion depth design of the electrode array position of the electrode array neural survival patterns cochlear morphometry CIAP CIAP

3 Progression of UP Bioengineering's volume conduction models Guinea pig Generation 1 Progression of UP Bioengineering's volume conduction models Guinea pig Generation 2 GPG1: From CT to FEM. Subject-specific cochlear morphometry and electrode location included ( ). CIAP GPG2: Adding subject-specificity: bone capsule and hook area (cochlear morphometry), return electrode location ( ). CIAP What qualifies user-specificity in humans? From dead guinea pig to live human Speech perception variability caused by, among others: duration of cochlear implantation and duration of deafness age of implantation stimulation mode (BP, MONO, CG) electrode insertion depth design of the electrode array position of the electrode array neural survival patterns cochlear morphometry speech perception before implantation speech processing algorithm used unilateral or bilateral implantation Challenge 1: Resolution of image data Challenge 2: Access to neural response data VC model ANF model CIAP CIAP

4 Human Generation 2 Human Generation 3 HG2: Realistic generalised human cochlea extruded on spiral derived from mid-modiolar section of cochlea (2009). TEMPLATE MODEL USED AS BASE FOR SUBSEQUENT GENERATIONS CIAP HG3. Person-specific models based on CT data from live implantees (2010>>). CIAP Extra-cochlear volume: infinite homogeneous Human Generation 4 HG1-3 & GPG1. Cochlea embedded into infinite bone volume (outer surface of sphere/cylinder modelled at infinity). CIAP HG4. Cochlea embedded into head-sized ellipsoid bone volume with accurate description of return electrode. CIAP

5 Human Generation 5 Human Generation 5 HG5. Cochlea embedded into skull with brain and scalp and accurate description of return electrode. CIAP HG5. Detail of return electrode placement. CIAP Current status of VC model User-specific cochlear morphometry (macro characteristics) User-specific electrode location Correct return electrode location Description of skull, brain and scalp NEXT: Human Generation : Initiated project to create morphometric library of inner structure templates collaborate with Dept Anatomy, Faculty of Health Sciences address low-res/soft tissue problem Improve user-specific model representation of morphometry 60 dry skulls imaged and digitized to date (micro-ct) CIAP CIAP

6 Progression of UP Bioengineering's auditory nerve fibre (ANF) models ANF models integrate with VC models to predict neural excitation from spread of electrical activity. We have used / are using GSEF / Hodgkin-Huxley / Rattay (literature) We have worked on Smith-2 Hanekom model (own) Problems Single fibre instead of population Electrical stimulation Bottom-line: can't predict absolute thresholds; trends okay Progression of UP Bioengineering's auditory nerve fibre (ANF) models Working to create physiologically-based neural models that can predict responses to electrical stimulation Computationally INTENSIVE! POSTER W26 Development of a voltage dependent current noise algorithm for conductance based stochastic modelling of auditory nerve fibre populations in compound models Werner Badenhorst CIAP CIAP How big is the influence of personspecificity on the periphery? Neural threshold profiles of five cochlear models inserted with identical medial and lateral arrays stimulated on electrode 4 inserted at the same angle in all the models. Mean medial-lateral difference in thresholds: 6.4 db Inter-user variability of 3.93 db (both) Mean medial-lateral difference in CFs: 2988 Hz. Inter-user variability of 2535 Hz (medial) and 1992 Hz (lateral). CIAP Istim [db re 1 A] Istim [db re 1 A] 85 Lateral Array S13R S13L S3R S3L 80 S25R apex a) Non-degenerate Neurons b) Degenerate Neurons Medial Array S13R S13L S3R S3L S25R base Frequency along Organ of Corti [Hz] But are the models useful? duration of cochlear implantation and duration of deafness e.g. Scar tissue age of implantation e.g. Bone impedance speech perception before implantation?? speech processing algorithm used Predict characteristics of peripheral neural excitation using a specific speech algorithm and a stimulation mode used specific stimulation mode unilateral or bilateral implantation (e.g. frequency matching between ears) electrode insertion depth design of the electrode array position of the electrode array Integral part of the VC models; affect neural excitation. cochlear morphometry neural survival patterns Probe the effect of neural survival on excitation characteristics complications e.g. FNS CIAP

7 Translating models into tools Research tools Underlying functioning of auditory system Clinical tools Visualization Model-predicted mapping (MPM) Models-based diagnostics (MBD) 1. Find ways to build models quickly 2. Augment images to improve representation of inner structures (HG6) 3. Define what we need from clinicians to enable us to do this as a routine procedure Pre-op & post-op scans, neurophysiological data, psychoacoustic data, etc. Challenge in SA: User records very difficult to find, e.g. can t find user records of imaging data older than a couple of months. CIAP CIAP Research Tools Probe the fundamentals that underpin the functioning of the auditory system Model parameters Quantify characteristics of the system so that we can describe it with mathematics Example: bone impedance Research Tools From our models we know bone impedance affect spread of electrical activity, i.e. neural excitation Equipotential surfaces at 0.5 db below the electrode potential. CIAP CIAP

8 Original CT image Image derived from model Clinical Tools Visualization Scala Tympani insertion Scala Vestibule insertion Clinical Tools Model-Predicted Mapping (MPM) Example: tool to estimate spread of excitation Commonly accepted 3 db/mm decay for monopolar stimulation does not hold. Depends on o location in cochlea (basal vs apical) o intrascaler location Derive equation to give a quick-and-dirty estimation of real current decay. o Useful for mapping o Also useful as a research tool in models that use current decay, e.g. acoustic models. Scala Vestibuli Possible damage to cochlear wall CIAP Source positions Reissner s membrane Scala Media Organ of Corti Basilar membrane Stria Vascularis Spiral ligament Scala tympani Buffer area Axonal nerve CIAP Clinical Tools Model-Based Diagnostics (MBD) For details about model-based tools in development Example: A case of facial nerve stimulation Potential of MBD Investigate the factors that may cause FNS o e.g. current paths Investigate the potential effectiveness of interventions based on the current implanted system o e.g. optimal electrode configuration Investigate the potential effectiveness of customized interventions designed for the individual o e.g. alternative stimulation strategies POSTER W28 Model-based interventions in cochlear implants Scala vestibule insertion Electrode array Spiral ganglion Red area indicates location of electrode contact Electrode array Nerve fibre plane Auditory nerve Potential distributions as a result of stimulation with different electrodes CIAP CIAP

9 Other work Models of perception 1. Model of central processing Neurophysiology 2. Acoustic Models Computational models Parameters Perception Models of the physical situation 3. Volume conduction model 4. Nerve fibre model 2. Speech perception 1. Psychoacoustics Poster R13 NEW APPROACHES TO FEATURE INFORMATION TRANSMISSION ANALYSIS (FITA) Dirk JJ Oosthuizen, Johan J Hanekom Poster R58 RATE PITCH WITH MULTI-ELECTRODE STIMULATION PATTERNS: CONFOUNDING CUES Pieter J Venter, Johan J Hanekom CIAP For postdoc positions in our group, speak to Prof Johan Hanekom Head of UP Bioengineering Johan.hanekom@up.ac.za or Prof Tania Hanekom tania.hanekom@up.ac.za Thank you CIAP