EEG - Montages, Equipment and Basic Physics
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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

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

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  • Half amp. Cutoff, frequency at which amplitude is reduced by 50%, or in terms of power; power = 50%, when amp = 71% <br />
  • Diminishes the larger peak due to filtering out the lower frequency components <br /> Artefactual peaks introduced into the higher frequency components. <br />
  • Removes high frequency 30Hz + noise, gamma etc <br />
  • Band pass useful for selecting a band of frequencies, e.g. if you wanted to purely examine Beta or Theta oscillations. <br /> Notch useful for removing a specific frequency e.g. 50Hz mains supply, or local interference source. <br />
  • 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. <br />
  • 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. <br /> A sharp cut-off would seem ideal, and specific, but in reality they cause more problems than they solve. <br />

EEG - Montages, Equipment and Basic Physics EEG - Montages, Equipment and Basic Physics Presentation Transcript

  • 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 !! View slide
  • Road Map for the Session • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing View slide
  • 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
  • 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 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
  • 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.
  • 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
  • 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 o o o o o 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
  • Summary • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing