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Amplitrode ™ : A new approach to ASSR and ABR amplification

Amplitrode ™ : A new approach to ASSR and ABR amplification. Yuri Sokolov , PhD Isaac Kurtz , M.H.Sc, P.Eng Vivosonic Inc., Toronto, Canada Presentation at NHS 2004 – Villa Erba, Como, Italy May 28, 2004. Vivosonic goal is providing clinical efficiency through innovation.

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Amplitrode ™ : A new approach to ASSR and ABR amplification

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  1. Amplitrode™:A new approach to ASSRand ABR amplification Yuri Sokolov, PhD Isaac Kurtz, M.H.Sc, P.Eng Vivosonic Inc., Toronto, Canada Presentation at NHS 2004 – Villa Erba, Como, Italy May 28, 2004

  2. Vivosonic goal is providing clinical efficiencythrough innovation Vivosonic, through its innovative solutions makes objective audiometric tests practical - clinically and economically: • Easy to learn and administer • Reduced test time • High accuracy • Increased patient throughput and revenues • Reduced operating costs

  3. Having made OAE practical, Vivosonic is working to make practical ABR and ASSR Auditory Brainstem Response (ABR) and Auditory Steady State Response (ASSR) modalities are available from a number of manufacturers, but are quite difficult to administer for some practitioners: • Complicated user interfaces, risk of electrode mismatch, artefacts make them difficult to learn and administer • Long test time to achieve accuracy • Low patient throughput • High operating costs • Clinical systems are not portable • Significant electromagnetic interferences bind to shielded rooms • Stringent requirement of ≤5 kOhm electrode impedance is challenging to achieve, requires scrubbing the skin in many cases

  4. ABR has diagnostic, screening, and threshold-finding applications Auditory Brainstem Response (ABR) is a transient response, provides valuable information on hearing thresholds and useful for differential diagnostics: • Objective • Non-invasive • Site-specific (which is different from ASSR) • Well researched over several recent decades • Responses are looked for in the time domain in the form of characteristic waves • Recommended by many established NHS protocols • Has dedicated CPT codes in the US

  5. Click-ABR is used mostly for screening and differential diagnostics V III I Diagnostic application • Response is generated by Acoustic Nerve and Brainstem • Has characteristic wave structure • 70-80 dB nHL click typical • Looking for Waves I, III, V, and I-III, III-V, I-V intervals • Diagnostics of Acoustic Neuroma and Auditory Neuropathy • Bio-logic stacked ABR® detects smaller Acoustic Neuroma Screening application • 30 dB nHL click stimulus • Looking for Wave V • Typically automated (e.g. AABR®) 0 10 ms I-III III-V I-V V 0 10 ms AABR® is a registered trademark of Natus Medical Inc. Stacked ABR® is a trademark of Bio-logic Systems Corp.

  6. Tone-burst-ABR is used mostly for finding thresholds • Established and recommended protocol • Tone bursts instead of click stimuli: typically 500 (very difficult), 1000, 2000, 4000 Hz • Frequency-specific • Levels vary to find the threshold • Looking for Wave V • Technically similar to screening click-ABR • Detect thresholds up to 80 dB HL V 0 10 ms

  7. ASSR is a frequency-specific Evoked Potential Auditory Steady State Response (ASSR) is a tone-like response present as long as stimulus is presented. Elicited by amplitude (AM) or Frequency (FM) or combined AM+FM modulation of carrier frequencies. • Audiometric carrier frequencies: 500, 1000, 2000, 4000 Hz • Modulation • 40 Hz – sensitive to sleep • 80-110 Hz – insensitive to sleep • Responses are looked for in the frequency domain – at modulation frequencies • Thresholds – for carrier frequencies 82 84 86 88 92 96 Hz Source: Multiple publications by Terry Picton, Sasha John and others.

  8. ASSR is a promising tool for finding hearing thresholds Auditory Steady State Response (ASSR) has been scientifically proven to provide valuable information on hearing thresholds, particularly in babies • Objective • Non-invasive • Frequency-specific, as tone-burst-ABR • Not site-specific (generators are unknown) • Faster than tone-burst ABR • Accurate, particularly at higher HL, above 40 dB HL • Effective at severe and profound hearing loss, while tone-burst ABR is limited to 80 dB HL • The origin is unknown; hence, differential diagnostic value is not established • Does NOT have a dedicated CPT code in the US yet Source: Multiple publications by Terry Picton, Sasha John and others.

  9. ASSR is a very small Auditory EP signal Source: Cutmore, James (1999). Identifying and reducing noise in physiological recordings. Int. J. Physiol., V. 32, No. 2, pp. 129-150.

  10. ASSR testing is affected by many factors • Adequate stimulation, AM / FM modulation • Effective 70-110 Hz filtering to reduce noise • Physiological • EEG – increases in sleep • ECG – does NOT decrease in sleep • EOG, EMG – decrease in sleep • 50/60 Hz line noise • Radio-frequency (RF) interferences • Noise reduction techniques required • Gain, ≥ 10,000 required • Dynamic range of A/D, ≥ 16 bit required • Automated signal detection

  11. RF noise may strongly interfere with EP recording • RF noise comes from various sources: • Cell phones, pagers • Wireless networks used in many hospitals • PDAs (Personal Digital Assistants) • Medical equipment in ICUs, operating rooms, general offices • Interferes at EP (low) frequencies despite RF frequencies are much higher – in MHz and GHz ranges – because of amplifier non-linearity • There is no common-mode rejection (CMR) at frequencies ≥20 kHz • Concern for testing outside electromagnetically shielded environment Source: Kitchin et al. (2003). Input filter prevents instrumentation-amp RF-rectification errors. EDN, Nov 13, p. 101-102.

  12. Clinical ASSR testing is challenging in practice • Long testing time • Best reported: • 19 minutes (Luts, Wooters, unpublished) • 21 minute (Perez-Abalo et al., 2001) • Typical: 45-60 minutes (John et al., 2003) • Sensitivity to electromagnetic interferences • Electromagnetically shielded booth required • Sensitivity to electrode impedance • Requires rubbing baby’s skin • Needs a lot of operator’s attention • Need for sedation in many cases • Difficult to administer in magnetically shielded booth • Complicated user interfaces in existing devices • Longer learning time • More difficult to read and interpret results than other EPs

  13. Vivosonic took a new approach to recording ASSR and ABR • Optimizing stimulus (patent pending) • Optimizing amplification (patent pending) • 150,000 gain for ASSR – larger ASSR signal at the output • 15,000 gain for ABR • Optimizing filtration (patent pending) • At the first stage of amplification – less noise • Optimizing signal estimation (US patent) • Kalman Filter – less processing time • Optimizing signal detection

  14. Amplitrode™ is a new approach to EP recording • Amplifier is mounted in-situ – directly on the ground electrode pad, with no lead • Lead length to inverting and non-inverting electrodes minimized to the distance between electrodes • Filtering at the first stage of amplification • Gains optimized for ASSR and ABR • Impedance mismatch monitored during recording • Risk of wrong electrode wiring eliminated

  15. Amplitrode™ makes ABR, ASSR testing practical • No big “boxes” • Decreased electromagnetic interferences • Less attention to electrode impedance • Reduced noise • Easy mounting on electrode pads • No need to achieve ≤5 kOhm impedance • No hassles with long lead wires and cables • No risk of electrode lead mis-connection

  16. EP EMI Amp A/D DSP Ground lead “Garbage” IN “Garbage” OUT Other leads Long lead wires introduce large EMI in a conventional amplifier Amp – amplifier A/D – analog-to-digital conversion DSP – digital signal processing

  17. EMI EP A/D DSP Amplitrode™ largely eliminates EMI Amplitrode™ is mounted directly on the ground electrode pad eliminating ground lead. Other leads are very short and shielded. This significantly reduces EMI and allows for a clearer EP signal at the amplifier output. Line noise is significantly reduced in any environment.

  18. Noise EP Amp 1 BPF Amp 2 Optimizing gain is difficult in conventional EP amplifiers High gain in the 1st stage results in saturation by the unfiltered noise, i.e. reaching the maximum voltage of the 1st stage’s dynamic range. Saturation distorts the signal: The 1st stage output contains periods of the maximum voltage, and these periods become interruptions in EP signal after band-pass filtering (BPF). Low gain reduces EP amplitude and signal-to-noise ratio (SNR) at the amplifier output. Both saturation and low gain complicate signal detection.

  19. Noise EP Amplitrode™ filters EP signal at “source” and thus reduces both physiological and RF noise at higher gain Higher gain: 150,000 for ASSR 15,000 for ABR Exceptionally low intrinsic noise: < 350 nV in 10-10,000 Hz <10 nV in 0.05 Hz bands in 70-110 Hz EP signals at the Amplitrode™ output have large amplitude, contain little noise, have high SNR, and therefore, can be easier recorded and detected.

  20. Amplitrode™ monitors electrode mismatch in real time • Electrode impedance mismatch (EIMM) is more relevant than electrode impedance*. • Amplitrode™ measures EIMM in real time during testing, not only prior to it. • Operator is notified of EIMM immediately. • Reduces set up time. • Measuring EIMM and very high input impedance of the Amplitrode™ eliminates the need for skin abrasion – no need to achieve impedance below 5 kOhm. *) Ferree et al. (2001). Scalp electrode impedance, infection risk, and EEG data quality. Clin. Neurophysiol., 112, p. 536-544.

  21. Amplitrode™ eliminates the risk of improper mounting Amplifier is mounted on the ground electrode pad. The other two leads have different length. Electrode button release makes easy mounting and dismounting amplifier and clips on electrode pads. It is much easier to use even for less experienced practitioners.

  22. Amplitrode™ meets or exceeds specifications of conventional EP amplifiers

  23. Amplitrode™ speeds up EP recording at low stimulus levels ABR 800 clicks 100 clicks 400 clicks 1600 clicks Subject: Normal hearing female, 24 yrs, R ear Place: Vivosonic office, EMI ≥ 0.5 mGauss Phone: ER-3A (correction for 0.9 ms) Stimulus: Click, 30 dB nHL, 21.1/sec, ipsi 3200 clicks

  24. Amplitrode™ ER-3A ER-3A Vivo Link™EP Amplitrode™ works with the VivoScan™ via VivoLink™ VivoLink™ EP is a wireless control unit • Generates ABR and ASSR stimuli • Conditions stimuli for • ER-3A Insert Phones • B-71 Bone Conductor • Converts EP signals from the Amplitrode into digital form, 16 bit • Processes signals and communicates to the VivoScan software through wireless Bluetooth link VivoScan™ control the ASSR and ABR functions • Protocol setting • Test control • Data management Wireless Bluetooth communication

  25. Questions? Thank you for your interest!

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