Presentation on theme: "1 In-situ AEP amplification and wireless recording of Auditory Evoked Potentials and Otoacoustic Emissions Yuri Sokolov, PhD Vivosonic Inc., Toronto, ON."— Presentation transcript:
1 In-situ AEP amplification and wireless recording of Auditory Evoked Potentials and Otoacoustic Emissions Yuri Sokolov, PhD Vivosonic Inc., Toronto, ON Early Hearing Detection and Intervention Conference Atlanta, March 3, 2005
2 Audiology clinicians experience significant frustrations with ABR and ASSR Auditory Brainstem Response (ABR) and Auditory Steady State Response (ASSR) are quite difficult to administer for many clinicians, particularly in post-screening assessment environments: Noise is FRUSTRATION # 1 reported by 84 % of U.S. clinics Noise leads to unclear results and long test times – up to min, typical min per test Long test time results in low patient throughput Requirement of 5 kOhm impedance is challenging to achieve, often by abrading the skin until bleeding Abrading the skin increases the risk of infection (Ferree et al., Scalp electrode impedance, infection risk, and EEG data quality. Clin. Neurophysiol., 112, p ) The above results in higher risks and operating costs Source: Tannenbaum, S: US infant post-screening market survey (The Hearing Review, Jan 2005).
3 ABR and ASSR have very low amplitudes relatively to noise, but largely coincide with noise frequency AEP signal recording, analysis, and detection is simply about three things: Signal, Noise, and Signal-to-Noise Ratio (SNR) The three major sources of noise in AEP are: Physiological Electric field, RF, and power-line Magnetic field The same signal can or cannot be detected depending on noise and SNR SNR N S
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 Known generators (on the opposite 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 Source: Multiple publications by J. Hall III, M. Hyde, C. Berlin, L. Hood, and others. Amplitude: μV (millionth of V) Frequency range: 50-3,000 Hz
5 Click-ABR is used mostly for screening and differential diagnostics Diagnostic application Response is generated by Acoustic Nerve and Brainstem Has characteristic wave structure 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 Stacked ABR ® may detect smaller Acoustic Neuroma Screening application dB nHL 100 μs click stimulus Looking for Wave V Typically automated detection (e.g. AABR ® ) 0 10 ms I III V 0 10 ms V I-IIIIII-V I-V 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 (difficult to record), 1000, 2000, 4000 Hz Frequency-specific Levels vary to find the threshold Looking for Wave V threshold Technically similar to screening click-ABR, but not automated Detect thresholds up to 80 dB HL 0 10 ms V
7 ASSR is a promising tool for finding hearing thresholds Auditory Steady State Response (ASSR) has been proven to provide valuable information on hearing thresholds, particularly in infants Objective Non-invasive Frequency-specific, as tone-burst-ABR Not site-specific (generators are unknown) Typically faster than tone-burst ABR Accurate, particularly at higher HL, above 40 dB HL Effective at severe and profound hearing loss, up to 110 dB HL, while tone-burst ABR is limited to 80 dB HL Source: Multiple publications by T. Picton, S. John, D. Stapells, and others.
8 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 Hz – insensitive to sleep Responses are looked for in the frequency domain – at modulation frequencies, not carrier frequencies Thresholds – for carrier frequencies Source: Multiple publications by T. Picton, S. John, D. Stapells, and others Hz Amplitudes: nV (billionth of V) Frequencies – AM and/or FM Multiple-frequency ASSR responses
9 Noises are introduced by multiple sources in most clinical environments Physiological EEG – increases in sleep ECG – does not decrease in sleep EOG, EMG – decrease in sleep Electric and magnetic Power line noise: 50 or 60 Hz and their harmonics Electric field noise Magnetic field noise Radio-frequency (RF) interferences
10 Multiple sources introduce physiological noises in AEP recording Noise on the scalp Frequency range, Hz Amplitude EEG awake μV EEG sleep3-162 – 400 μV Electrooculogram (EOG) μV Electrocardiogram (ECG) μV – 2 mV Electromyogram (EMG) μV - 2 mV Source: Cutmore, James (1999). Identifying and reducing noise in physiological recordings. Int. J. Physiol., V. 32, No. 2, pp
11 ECG noises may be stronger in infants than in adults The heart is positioned more centrally – aligned with the sagittal plain The heart is much larger relatively to the body The heart is closer to the head The heart-beat rate is twice higher than in adults Temporary post-natal heart conditions may increase ECG noise frequency - up to 100 Hz
12 Filtering after the first stage of amplification introduces distortion in conventional AEP amplifiers Noise EP Amp 1 BPF Amp 2 High gain in the 1 st stage results in saturation by the unfiltered, often EEG noise, i.e. reaching the maximum voltage of the 1 st stages dynamic range. Saturation distorts the signal: The 1 st 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. Saturation Distorted signal
13 Electric field noises are introduced through wires and cables acting like antennas Introduced by: Electronic equipment Electric wiring Improper grounding Typical strength of electric fields in North American clinics:, average 5.5 V/m, range V/m (5 Hz – 2 kHz band)* Noise amplitude: up to 10 mV (1 mV = a thousandth of V) Can be reduced by: Shielding of input-circuit wires Shielding of wires and circuits Proper grounding * Source: - Web site of Environmental Health Science, NIH, U.S. Government.www.niehs.nih.gov/emfrapid/html/Q&A-Workplace.html Electric fields Unshielded lead wires Differential AEP amplifier
14 Magnetic field noises are introduced through wires and cables acting like antennas Introduced by: Transformers Electric motors and wiring Looped wires and cables Typical strength of magnetic fields in North American clinics: average 1.7 mG, range mG (milliGauss) (5 Hz – 2 kHz band)* Noise amplitude: up to 10 mV Can be reduced by: Reducing wire/cable length Positioning, NOT moving Reducing loop area Twisting wires Very thick shielding (steel) * Source: - Web site of Environmental Health Science, NIH, U.S. Government.www.niehs.nih.gov/emfrapid/html/Q&A-Workplace.html Magnetic fields Looped lead wires and cables Loop area Differential AEP amplifier
15 Long lead wires and cables introduce large electro- magnetic field noises in a conventional amplifier EP Amp EMI A/D DSP Ground lead Other leads Garbage IN Garbage OUT Amp – amplifier A/D – analog-to-digital conversion DSP – digital signal processing
16 RF noise may strongly interfere with EP recording Radio-frequency (RF) noise comes from various sources: Cell phones, pagers, Blackberry, wireless intercom FM-systems, FM-radio Wireless computer networks used in many hospitals PDAs (Personal Digital Assistants), Palmtops Medical equipment (ICUs, operating rooms, general offices) Office equipment: copiers, fax-machines, computers Introduce mostly electrical noise 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 Amplitude: up to 10 mV (thousandth of V) Source: Kitchin et al. (2003). Input filter prevents instrumentation-amp RF-rectification errors. EDN, Nov 13, p
17 Power line noise is not only 60 Hz and comes from both electric field and AC power lines Power Line noise comes from Electric field – picked up by electrode wires & cables AC power outlets when plugged into the wall – introduced through electronic circuits, power supplies Through USB computer ports (5 V) – introduced through electronic circuits Interferes with EP at a number of frequencies – mostly 50 / 60 Hz & harmonics: 60 Hz, 120 Hz, 180 Hz, 240 Hz … due to amplifier non-linearity Amplitude: up to 10 mV and higher Hz AMPPC USB AC outlet Power-line noise in AEP amplifiers
18 Low A/D resolution can significantly affect AEP recording due to insufficient dynamic range Typical Integrity Low
19 Putting all things together: ABR and especially ASSR are very small signals as compared to noises in AEP recording SignalFrequency, HzAmplitude, nV (dB) AEP Signals ASSR – 50 (0) ABR50 - 3, ,000 (10-20) MLR ,000 (15-25) LLR ,000 (16-60) P , ,000 (15-65) Noises in AEP recording Electrooculogram (EOG) , ,000 (60-85) EEG awake3-405, ,000 (55-60) EEG sleep3-162, ,000 (65-90) Electrocardiogram (ECG) (up to 100)80, ,000,000 (70-110) Electromyogram (EMG) , ,000,000 (70-110) Electric, magnetic, RF50/60 Hz, MHz, GHzUp to 10,000,000 (up to 120)
20 Clinical ABR/ASSR testing is challenging in practice Long testing time Best reported: 19 minutes (Luts, Wooters, unpublished), 21 minute (Perez-Abalo et al., 2001) Typical minutes (John et al., 2003), up to 90 – 120 min (Tannenbaum, 2004) Sensitivity to electromagnetic interferences Electromagnetically shielded booth required Sensitivity to electrode impedance Requires rubbing the skin Need for sedation in many cases Difficult to administer in electro-magnetically shielded booth
21 In-situ AEP amplification and filtering is a novel method of noise reduction in Auditory Evoked Potentials Amplifier is mounted in-situ – directly on the ground electrode pad, with no lead Lead length to non-inverting (+) and inverting (-) electrodes minimized to the distance between electrodes Filtering prior to amplification Gains optimized for ASSR and ABR Impedance mismatch monitored in real time Risk of wrong electrode connection minimized
22 In-situ amplification largely eliminates electro- magnetic field-induced noises A/D DSP EMI EP In-situ pre-amplifier, the Amplitrode, is mounted directly on the ground electrode eliminating ground lead. The other leads are very short and shielded. This significantly reduces electric and magnetic field-induced and allows for a clearer EP signal at the amplifier output.
23 Filtering prior to amplification allows optimizing gain and reducing physiological and RF noises Noise EP 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 Hz EP signals at the Amplitrode output have large amplitude, contain little noise, have high SNR, and therefore, can be easier converted from analog to digital form, recorded, and detected.
24 In-situ amplification and wireless communications make AEP testing efficient Reduced physiological noise Largely reduced electromagnetic noise No big boxes Less attention to electrode impedance Easy mounting on electrode pads No need to achieve 5 kOhm impedance No hassles with long lead wires and cables Less risk of electrode lead misconnection
25 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
26 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.
27 In-situ AEP recording speeds up testing 100 clicks 400 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 1600 clicks 800 clicks 3200 clicks ABR
28 In ideal electro-magnetically shielded room, the benefit of in-situ amplification and filtering is less pronounced Recording: Montage Fz/A ms window Band-pass filter: Hz for Bio- Logic Navigator Pro Hz for Amplitrode ABR in a shielded room ( < 1 V/m, 0.1 mG) Subject: T.V., 44, normal hearing Stimulus: 1000 clicks 21.1 clicks per second Artifact Rejection disabled ER-3A Insert Headphones Source: I. Kurtz, T. Venema, 2004 (unpublished).
29 Outside a shielded room, the benefit of in-situ amplification and filtering is very significant Recording: Montage Fz/A ms window Band-pass filter: Hz for Bio- Logic Navigator Pro Hz for Amplitrode ABR in moderate electric (12 V/m) and magnetic ( 5.5 mG) fields Subject: T.V., 44, normal hearing Stimulus: 1000 clicks 21.1 clicks per second Artifact Rejection disabled ER-3A Insert Headphones Source: I. Kurtz, 2004 (unpublished). Correlation coefficient = 0.43 Correlation coefficient = 0.81
30 Wireless recording of OAE and AEP provides mobility and additional noise reduction Wireless communication with PC No cable to the PC oNo noise coming back into the EP and OAE amplifiers from AC power supply oNo cable-related hassles Mobility oTesting can be controlled form anywhere within the reach of Bluetooth ® oThe patient or a babys mother can move around – without the need to disconnect electrodes, connectors, or transducers oAdult and senior patients can take a relieving break. oIn the Operating Room, testing can be done from a distance, without cables getting in the way. Battery operation oNo AC-power-related noise in the amplifier circuits
31 Bluetooth ® is a wireless communications protocol Bluetooth ® Wireless communications protocol Digital signal in GHz range Noise-like, broadband (no fixed carrier frequency – unlike FM- radio) Low energy – below 0.1 mG Limited area – 30 feet (10m) Encoded – secure for medical information FDA-approved for various medical applications
32 Amplitrode Vivo Link ER-3A Integrity is the worlds first and only wireless OAE, ABR, and ASSR system Wireless Bluetooth communication VivoLink interface module Generates DPOAE, TEOAE, 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 computer software through Bluetooth ® Integrity computer program controls the OAE, ABR, and ASSR, functions Protocol setting – modular Test control – modular Data management - integrated B-71 Integrity