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Montages and polarity by dr Raheel.pdf

  1. Montages and Polarity D R . R A H E E L A H M E D N E U R O L O G I S T & C L I N I C A L N E U R O P H Y S I O L O G IST
  2. Designations ➢Each electrode in EEG is designated by a letter & No ➢Letter: area of the underlying brain region ➢Number: right/left Even numbers: the right side Odd numbers: denote the left side “z” midline placement (i.e., Cz = central midline, Pz, Fz)
  3. ➢In EEG, an electrode pair in an amplifier is called a derivation ➢DERIVATIONS has input 1 and input 2 Input 1 Input 2 ➢Voltage difference between two electrodes is seen as EEG signal
  4. MONTAGE ➢The Combinations of derivations is called montage ➢Modern digital EEG machines provides the facility to redesign montages of prerecorded tracings Objectives: ➢To compare activity from homologous electrodes between two hemispheres ➢To localize the electrical activity/ epileptogenic focus
  5. Types of Montages There are two basic montages 1. Bipolar Montages: ➢ Adjacent electrodes are paired and that can be in a longitudinal, circumferential or transverse lines ➢Input 2 becomes input 1 of the next channel 2. Referential Montages/ monopolar: ➢Input 1 that is active and the input 2 electrode is “considered the reference” ➢Input 2 is common to all derivations/channels
  6. Bipolar Montages ➢Each channel has input 1 and 2 ➢Input 2 becomes input 1 of next channel ➢EEG signal is displayed as the potential difference b/w 2 inputs
  7. Types of Bipolar montage ➢Commonly used bipolar montages are: 1. Longitudinal Bipolar Montage 2. Transverse Bipolar Montage 3. Circumferential or hatband bipolar montage
  8. Longitudinal Bipolar Montage:
  9. Transverse Montage ➢Runs crosswise, across the head from L to R Fp1-Fp2 F7-F3 P3-Pz Pz-P4 F3-Fz Fz-F4 F4-F8 T3-C3 C3-Cz Cz-C4 C4-T4 T5-P3
  10. ➢The localization criteria for bipolar montages is “Phase Reversal” ➢If any activity shows phase reversal between two channels, the activity is maximum at the electrode that is commonto both channels
  11. Referential/monopolar Montage ➢ Input 1 is active electrode ➢ Input 2 is a reference electrode that is common to all derivations/channels ➢Common referential montages are: i. Average reference Montage ii. Ear reference montage(ipsilateral/contralateral) iii. Cz reference montage ➢ The criteria for localization is Amplitude in referential montages
  12. Localization criterion
  13. Average reference montage ➢In this activity from all the electrodes are measured, summedtogether and averaged ➢The resulting signal is than used as a reference electrode and connected to input 2 of each amplifier
  14. POLARITY ➢EEG is graphical representation of difference in voltage between two scalp locations i.e electrodes ➢Every deflection indicates a change of voltage ➢Negative potential displays upward deflection ➢Positive potentials shows downward deflection ➢All the epileptiform discharges are negative and has a field
  15. POLARITY ➢ Deflection will be up if input 1 is relatively more negative Or if input 2 is relatively more positive ➢ Deflection will be down if input 2 is relatively more negative Or if input 1 is relatively more positive
  16. End of chain phenomenon ➢When all deflections move in same direction and there is no phase reversal ➢When the potential is maximum at input 1 of the first channel or input 2 of the last channel in bipolar montage ➢To complete the phase reversal additional electrodes are needed or montagemay be changed
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