2015/07/15 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. 2 CIAP 2015 UP Bioengineering UP Bioengineering is part of Our place Electrical, Electronic and Computer Engineering www.up.ac.za/eece http://www.ee.up.ac.za/main/emk310/index 3 4 CIAP 2015 CIAP 2015 1
2015/07/15 Progression of UP Bioengineering's volume conduction models Agenda Human Generation 1 • The development of our user-specific models – Human and guinea pig model generations – What the models can do – What the models show • Translating models into tools – Research tools – Model-predicted mapping (MPM) – Model-based diagnostics (MBD) HG1 : Generalised human cochlea extruded on analytical spiral from 2D section (1996-2001). 5 6 CIAP 2015 CIAP 2015 User-specificity in cochlear implants… What qualifies subject-specificity in animals? • • Realised that models need to migrate to include user-specific Subject-specificity characterised by, among others: characteristics – duration of cochlear implantation and duration of deafness • Started to work on user-specific models in 2007 – age of implantation – stimulation mode (BP, MONO, CG) • 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 – electrode insertion depth lab (Epstein Labs) – design of the electrode array – position of the electrode array Micro-CT of Acoustic frequency – neural survival patterns subject’s cochlea response areas of the 16 electrode contacts – cochlear morphometry implanted in the inferior colliculus 7 8 CIAP 2015 CIAP 2015 2
2015/07/15 Progression of UP Bioengineering's volume conduction models Progression of UP Bioengineering's volume conduction models Guinea pig Generation 1 Guinea pig Generation 2 GPG2 : Adding subject-specificity: bone capsule and hook area GPG1 : From CT to FEM. Subject-specific cochlear morphometry (cochlear morphometry), return electrode location (2007-2009). and electrode location included (2007-2009). 9 10 CIAP 2015 CIAP 2015 From dead guinea pig to live human What qualifies user-specificity in humans? • 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 Challenge 1: Resolution of image data VC model – cochlear morphometry – speech perception before implantation – speech processing algorithm used Challenge 2: Access to neural response data ANF model – unilateral or bilateral implantation – … 11 12 CIAP 2015 CIAP 2015 3
2015/07/15 Progression of UP Bioengineering's volume conduction cochlear models Progression of UP Bioengineering's volume conduction cochlear models Human Generation 2 Human Generation 3 HG2 : Realistic generalised human cochlea extruded on spiral derived from mid-modiolar section of cochlea (2009). HG3 . Person-specific models based on CT data from live TEMPLATE MODEL USED AS BASE FOR SUBSEQUENT GENERATIONS implantees (2010>>). 13 14 CIAP 2015 CIAP 2015 Progression of UP Bioengineering's volume conduction cochlear models Progression of UP Bioengineering's volume conduction cochlear models Extra-cochlear volume: infinite homogeneous Human Generation 4 HG1-3 & GPG1 . Cochlea embedded into infinite bone volume HG4 . Cochlea embedded into head-sized ellipsoid bone volume (outer surface of sphere/cylinder modelled at infinity). with accurate description of return electrode. 15 16 CIAP 2015 CIAP 2015 4
2015/07/15 Progression of UP Bioengineering's volume conduction cochlear models Progression of UP Bioengineering's volume conduction cochlear models Human Generation 5 Human Generation 5 HG5 . Cochlea embedded into skull with brain and scalp and accurate description of return electrode. HG5 . Detail of return electrode placement. 17 18 CIAP 2015 CIAP 2015 Progression of UP Bioengineering's volume conduction cochlear models Current status of VC model NEXT: Human Generation 6 2013: Initiated project to create morphometric library of inner • User-specific cochlear structure templates morphometry (macro • collaborate with Dept Anatomy, Faculty of Health Sciences characteristics) • address low-res/soft tissue problem • User-specific electrode • Improve user-specific model representation of morphometry location • 60 dry skulls imaged and digitized to date (micro-CT) • Correct return electrode location • Description of skull, brain and scalp 19 20 CIAP 2015 CIAP 2015 5
2015/07/15 Progression of UP Bioengineering's Progression of UP Bioengineering's auditory nerve fibre (ANF) models auditory nerve fibre (ANF) models • Working to create physiologically-based neural models that ANF models integrate with VC models to predict neural can predict responses to electrical stimulation excitation from spread of electrical activity. – Computationally INTENSIVE! We have used / are using • GSEF / Hodgkin-Huxley / Rattay (literature) POSTER W26 We have worked on Development of a voltage dependent current noise algorithm for conductance based stochastic modelling of auditory nerve fibre • Smith-2·Hanekom model (own) populations in compound models Problems Werner Badenhorst • Single fibre instead of population • Electrical stimulation • Bottom-line: can't predict absolute thresholds; trends okay 21 22 CIAP 2015 CIAP 2015 a) Non-degenerate Neurons 85 Lateral Array S13R How big is the influence of person- S13L But are the models useful? S3R S3L specificity on the periphery? S25R 80 Istim [dB re 1 A] • Neural threshold profiles of – duration of cochlear implantation and duration of deafness e.g. Scar tissue 75 – five cochlear models – age of implantation e.g. Bone impedance – inserted with identical medial and lateral arrays Medial Array – speech perception before implantation ?? S13R – stimulated on electrode 4 70 S13L S3R – speech processing algorithm used Predict characteristics of peripheral neural – inserted at the same angle in all the models. S3L S25R excitation using a specific speech algorithm and a – stimulation mode used • Mean medial-lateral difference in specific stimulation mode 65 85 – unilateral or bilateral implantation (e.g. frequency matching between ears) b) Degenerate Neurons thresholds : 6.4 dB – electrode insertion depth Inter-user variability of 3.93 dB (both) 80 – design of the electrode array Integral part of the VC models; affect neural Istim [dB re 1 A] excitation. – position of the electrode array • Mean medial-lateral difference in CFs : 75 – cochlear morphometry 2988 Hz. Probe the effect of neural survival on excitation – neural survival patterns Inter-user variability of 2535 Hz characteristics 70 – complications (medial) and 1992 Hz (lateral). e.g. FNS apex base 65 4000 8000 12000 16000 Frequency along Organ of Corti [Hz] 23 24 CIAP 2015 CIAP 2015 6
2015/07/15 Translating models into tools • Research tools 1. Find ways to build models quickly 2. Augment images to improve representation of – Underlying functioning of auditory system inner structures (HG6) • Clinical tools 3. Define what we need from clinicians to enable – Visualization us to do this as a routine procedure – Model-predicted mapping (MPM) – Pre-op & post-op scans, neurophysiological data, psychoacoustic data, etc. – Models-based diagnostics (MBD) – 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. 25 26 CIAP 2015 CIAP 2015 Research Tools Research Tools • From our models we know bone impedance affect • Probe the fundamentals that underpin the spread of electrical activity, i.e. neural excitation functioning of the auditory system • Model parameters Equipotential surfaces at 0.5 dB below the electrode potential. – Quantify characteristics of the system so that we can describe it with mathematics – Example: bone impedance 27 28 CIAP 2015 CIAP 2015 7
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