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

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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%
  • 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.
  • EEG - Montages, Equipment and Basic Physics

    1. 1. Road Map for the Session • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing
    2. 2. Road Map for the Session • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing
    3. 3. Recording technique !!
    4. 4. Road Map for the Session • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing
    5. 5. 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
    6. 6. CORTICAL CONFIGURATION
    7. 7. A TYPICAL RESEARCH CAP CONFIGURATION HAS 64-256 ELECTRODES
    8. 8. Road Map for the Session • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing
    9. 9. 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
    10. 10. 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
    11. 11. A silver/silver chloride electrode, shown in cross section.
    12. 12. • • • • 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
    13. 13. 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
    14. 14. 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-.
    15. 15. Road Map for the Session • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing
    16. 16. 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.
    17. 17. 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
    18. 18. Road Map for the Session • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing
    19. 19. Why do we need Filtering
    20. 20. 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
    21. 21. 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%
    22. 22. High-pass
    23. 23. Low-pass
    24. 24. Band-pass and Notch
    25. 25. 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
    26. 26. Filtering Artefacts • “Precision in the time domain is inversely related to precision in the frequency domain.”
    27. 27. 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
    28. 28. Summary • Introduction • 10 – 20 System • Electrodes • Montages • Post Processing

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