EEG - Montages, Equipment and Basic Physics

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

2. Road Map for the Session
•Introduction
•10 – 20 System
•Electrodes
•Montages
•Post Processing

3. Road Map for the Session
•Introduction
•10 – 20 System
•Electrodes
•Montages
•Post Processing

4. Recording technique !!

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

8. CORTICAL CONFIGURATION

9. A TYPICAL RESEARCH CAP
CONFIGURATION
HAS 64-256 ELECTRODES

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:
–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

13. A silver/silver chloride
electrode, shown in cross
section.

14. 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
-
.

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. 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
Unipolar and Bipolar EEG measurement

22. Road Map for the Session
•Introduction
•10 – 20 System
•Electrodes
•Montages
•Post Processing

23. Why do we need Filtering

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%

26. High-pass

27. Low-pass

28. Band-pass and Notch

29. Problems with Filters
•Original waveform, band pass of .01
– 80Hz
•Low-pass filtered, half-amp cutofff =
~40Hz
•Low-pass filtered, half-amp cutofff =
~20Hz
•Low-pass filtered, half-amp cutofff =
~10Hz

30. Filtering Artefacts
•“Precision in the time domain is inversely related to precision in the
frequency domain.”

31. Time constant of Low pass Filter
oWant the majority of voltage of signal to be measured across the C
oIt takes time to charge the C (t)
oIt takes longer to charge a capacitor with a big capacitance
(can hold a lot of charge
oIt takes longer to charge if R is big b/c it slows the current
oWe only have until the peak of the half cycle
oWant the time to peak of half cycle (or longer) to charge the capacitor

32. Summary
•Introduction
•10 – 20 System
•Electrodes
•Montages
•Post Processing