EEG - Montages, Equipment and Basic Physics
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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
Transcript of EEG - Montages, Equipment and Basic Physics
Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
Recording technique !!
Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
EEG measurement setup
• 10-20 Lead system is most widely clinically accepted
• Certain physiological featuresare used as reference points
• Brain research utilizes even 256 or 512 channel EEG hats
CORTICAL CONFIGURATION
A TYPICAL RESEARCH CAP CONFIGURATION
HAS 64-256 ELECTRODES
Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
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
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
A silver/silver chloride electrode, shown in cross section.
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
Metal or carbon electrodes• Other metals are seldom used as high-quality noble
metal electrodes or low-cost carbon or polymericelectrodes 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
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-.
Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
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.
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
Road Map for the Session
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
Why do we need Filtering
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
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%
High-pass
Low-pass
Band-pass and Notch
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
Filtering Artefacts
• “Precision in the time domain is inversely related to precision in the frequency domain.”
Time constant of Low pass Filter
o Want the majority of voltage of signal to be measured across the Co It takes time to charge the C ()o It takes longer to charge a capacitor with a big capacitance
(can hold a lot of chargeo It takes longer to charge if R is big b/c it slows the currento We only have until the peak of the half cycleo Want the time to peak of half cycle (or longer) to charge the capacitor
Summary
• Introduction
• 10 – 20 System
• Electrodes
• Montages
• Post Processing
