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
1
Vivosonic goal is providing clinical efficiency
through 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
2
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
3
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
4
Click-ABR is used mostly for screening and
differential diagnostics
V
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, 0
III-V, I-V intervals
• Diagnostics of Acoustic Neuroma
and Auditory Neuropathy
• Bio-logic stacked ABR® detects
smaller Acoustic Neuroma
III
I
I-III
10 ms
III-V
I-V
V
Screening application
• 30 dB nHL click stimulus
• Looking for Wave V
• Typically automated (e.g. AABR®)
AABR® is a registered trademark of Natus Medical Inc.
Stacked ABR® is a trademark of Bio-logic Systems Corp.
0
10 ms
5
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
6
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
Source: Multiple publications by Terry Picton, Sasha John and others.
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 signal
Signal
Frequency, Hz
Amplitude, nV
EP Signals
ASSR
70-110
10 - 50
ABR
50-3,000
100-1,000
MLR
3-300
500-3,000
LLR
1.5-15
200-16,000
P300
1-15
5,000-20,000
Noises
Electrooculogram (EOG)
0.5-10
10,000-500,000
EEG awake
3-40
5,000-10,000
EEG sleep
3-16
2,000-400,000
Electrocardiogram (ECG)
0.5-50
80,000-2,000,000
Electromyogram (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.
9
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
10
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.
11
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
12
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
13
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
14
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
15
Long lead wires introduce large EMI in a
conventional amplifier
EP
EMI
Amp
Ground lead
Other leads
A/D
“Garbage” IN
DSP
“Garbage” OUT
Amp – amplifier
A/D – analog-to-digital conversion
DSP – digital signal processing
16
Amplitrode™ largely eliminates EMI
EMI
EP
A/D
DSP
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.
17
Optimizing gain is difficult in conventional EP
amplifiers
Noise
EP
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.
18
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
Noise
EP
<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.
19
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.
20
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 1010,000 Hz band
≤ 0.45
≤ 0.35
Shorter test time, better
accuracy
Filtering at the first stage
No
Yes
Reduces physiological noise
CMRR, dB
≥110
≥120
Reduces 50/60 Hz noise, ECG
Real-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
24
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)
3200 clicks
Stimulus: Click, 30 dB nHL, 21.1/sec, ipsi
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Amplitrode™ works with the VivoScan™ via VivoLink™
VivoLink™ EP is a wireless
control unit
• Generates ABR and ASSR stimuli
• Conditions stimuli for
•
•
Amplitrode™
ER-3A
ER-3A
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
Vivo
Link™
EP
Wireless
Bluetooth
communication
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Questions?
Thank you for your interest!
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