This presentation discusses the 10-20 system of electrode placement, with its modifications. Also discussed are the Equipment Specifications, basic Physics and sources of interference
5. Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
6. EEG measurement setup
• 10-20 Lead system is most
widely clinically accepted
• Certain physiological
features
are used as reference points
• Brain research utilizes even
256 or 512 channel EEG
hats
10. Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
11. Electrodes – Basics
• High-quality biopotential measurements require
– Good amplifier design
– Use of good electrodes and their proper placement on the
patient
– Good laboratory and clinical practices
• Electrodes should be chosen according to the application
• Basic electrode structure includes:
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The body and casing
Electrode made of high-conductivity material
Wire connector
Cavity or similar for electrolytic gel
Adhesive rim
12. Ag-AgCl, Silver-Silver Chloride
Electrodes
• The most commonly used electrode type
• Silver is interfaced with its salt silver-chloride
• Choice of materials helps to reduce junction
potentials
• Electrolytic gel enhances conductivity and also
reduces junction potentials
• The gel is typically soaked into a foam pad or
applied directly in a pocket produced by electrode
housing
• Relatively low-cost and general purpose electrode
• Particularly suited for ambulatory or long term use
14. •
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Gold Electrodes
Very high conductivity suitable for low-noise meas.
Inertness suitable for reusable electrodes
Body forms cavity which is filled with electrolytic gel
Compared to Ag-AgCL: greater expense, higher
junction potentials and motion artifacts
• Often used in EEG, sometimes in EMG
Conductive polymer electrodes
• Made out of material that is simultaneously conductive and adhesive
• Polymer is made conductive by adding monovalent metallic ions
• Aluminum foil allows contact to external instrumentation
• No need for gel or other adhesive substance
• High resistivity makes unsuitable for low-noise meas.
• Not as good connection as with traditional electrodes
15. Metal or carbon electrodes
•
Other metals are seldom used as high-quality noble
metal electrodes or low-cost carbon or polymeric
electrodes are so readily available
• Historical value. Bulky and awkward to use
• Carbon electrodes have high resistivity and are noisier
but they are also flexibleand reusable
• Applications in electrical stimulation and impedance
plethysmography
Needle electrodes
• Obviously invasive electrodes
• Used when measurements have to be taken from the organ itself
• Small signals such as motor unit potentials can be measured
• Needle is often a steel wire with hooked tip
16. Electrode-electrolyte interface The current crosses it from
left to right. The electrode consists of metallic atoms C. The
electrolyte is an aqueous solution containing cations of the
electrode metal C+ and anions A-.
17.
18. Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
19. In clinical EEG recording, the
waveforms are recorded in a
series of Montages .
What is a montage?
Each EEG trace is generated
from an active and a reference
electrode. Different patterns of
electrodes are selected and the
traces grouped to provide data
from different areas of the scalp.
20. Unipolar and Bipolar EEG measurement
Bipolar or unipolar electrodes can be used in the EEG measurement.
In the unipolar method the potential difference between a pair of electrodes is measured.
In the bipolar method the potential of each electrode is compared either to a neutral
electrode or to the average of all electrodes
21.
22. Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
24. Types of Filter
1.
Low-pass – attenuate high frequencies
2.
High-pass – attenuate low frequencies
3.
Band-pass – attenuate both
4.
Notch – attenuate a narrow band
25. Properties of Filters
•
“Transfer function”
1. Effect on amplitude at each frequency
2. Effect on phase at each frequency
•
“Half Amp. Cutoff”
1. Frequency at which amp is reduced by 50%
31. Time constant of Low pass Filter
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o
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Want the majority of voltage of signal to be measured across the C
It takes time to charge the C (τ)
It takes longer to charge a capacitor with a big capacitance
(can hold a lot of charge
It takes longer to charge if R is big b/c it slows the current
We only have until the peak of the half cycle
Want the time to peak of half cycle (or longer) to charge the capacitor
Half amp. Cutoff, frequency at which amplitude is reduced by 50%, or in terms of power; power = 50%, when amp = 71%
Diminishes the larger peak due to filtering out the lower frequency components
Artefactual peaks introduced into the higher frequency components.
Removes high frequency 30Hz + noise, gamma etc
Band pass useful for selecting a band of frequencies, e.g. if you wanted to purely examine Beta or Theta oscillations.
Notch useful for removing a specific frequency e.g. 50Hz mains supply, or local interference source.
Although the waveform at the bottom looks the smoothest, and perhaps nicest, it now contains very little information and doesn’t resemble the original very much.
The sharp cut-off in the filter leads to distortion in the waveform, a change in the onset time, and extra oscillations which were not previously present.
A sharp cut-off would seem ideal, and specific, but in reality they cause more problems than they solve.