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Amplitrode™:A new approach to ASSRand ABR amplification Yuri Sokolov, PhDIsaac Kurtz, M.H.Sc, P.EngVivosonic Inc., Toronto, CanadaPresentation at NHS 2004 – Villa Erba, Como, ItalyMay 28, 2004

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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

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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

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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

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Click-ABR is used mostly for screening and differential diagnostics

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

IIII

V

0 10 ms

V

I-III III-VI-V

AABR® is a registered trademark of Natus Medical Inc.Stacked ABR® is a trademark of Bio-logic Systems Corp.

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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

0 10 ms

V

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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

Source: Multiple publications by Terry Picton, Sasha John and others.

82 84 86 88 92 96 Hz

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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.

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ASSR is a very small Auditory EP signalSignal Frequency, Hz Amplitude, nV

EP SignalsASSR 70-110 10 - 50ABR 50-3,000 100-1,000MLR 3-300 500-3,000LLR 1.5-15 200-16,000

P3001-15 5,000-20,000

NoisesElectrooculogram (EOG) 0.5-10 10,000-500,000

EEG awake 3-40 5,000-10,000EEG sleep 3-16 2,000-400,000

Electrocardiogram (ECG) 0.5-50 80,000-2,000,000Electromyogram (EMG) 30-500 10,000-2,000,000

Line, Radio frequency (RF) 50/60 Hz, MHz, GHz Up to 10,000,000

Source: Cutmore, James (1999). Identifying and reducing noise in physiological recordings. Int. J. Physiol., V. 32, No. 2, pp. 129-150.

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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

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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.

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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

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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

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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

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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

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Long lead wires introduce large EMI in a conventional amplifier

EP

Amp

EMI

A/D DSP

Ground leadOther leads

“Garbage” IN “Garbage” OUT

Amp – amplifier

A/D – analog-to-digital conversion

DSP – digital signal processing

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Amplitrode™ largely eliminates EMI

A/D DSP

EMIEP

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.

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Optimizing gain is difficult in conventional EP amplifiers

NoiseEP

Amp 1 BPF Amp 2

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.

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Amplitrode™ filters EP signal at “source” and thus reduces both physiological and RF noise at higher gain

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 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.

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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.

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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.

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Amplitrode™ meets or exceeds specifications of conventional EP amplifiers

Parameter Conventional EP amplifiers

Amplitrode™ Benefit

Gain 15,000 150,000 (ASSR)15,000 (ABR)

Larger gain yields larger signal prior to A/D

System noise, µV in 10-10,000 Hz band

≤ 0.45 ≤ 0.35 Shorter test time, better accuracy

Filtering at the first stage No Yes Reduces physiological noiseCMRR, dB ≥110 ≥120 Reduces 50/60 Hz noise, ECGReal-time impedance mismatch monitoring

No Yes Reduced operator’s anxiety and stress; less training required

Montage control Color leads & dots on cable

Clip cable length No risk of electrode misconnection; less training

Susceptibility to extraneous EM noise

Unpublished TBD Testing in office environment, shorter test time, better accuracy

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Amplitrode™ speeds up EP recording at low stimulus levels

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

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Amplitrode™

VivoLink™

EP

ER-3A ER-3A

Amplitrode™ works with the VivoScan™ via VivoLink™

Wireless Bluetooth communication

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

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Questions?

Thank you for your interest!