EEG

 

What is EEG?

  • Electroencephalography(EEG) is a technique that records the electrical activity of the brain.
  • A complex physiological signal whose resulting waveform represents the sum of all brain activity produced by the cerebral cortex along the scalp. 
  • During an EEG test, small electrodes like cup or disc type are placed on the scalp. 
  • It records the signals as wavy lines on to a computer screen or paper in order of microvolt.
  • Normal EEG: – Small amplitude (20-200 microvolts) – Variable frequency (0-50 Hz) 

 

Source of EEG activity

  • Neurons are electrically charged by membrane transport proteins that pump ions across their membranes.
  • When the wave of ions reach the electrodes on the scalp, they can push or pull electrons on the metal of the electrodes.
  • Push or pull difference measured as voltage across time is referred as EEG.

EEG Frequency bands: 

EEG Frequency bands

Why EEG is used?

  • An EEG is mainly used when there is a need to diagnose and manage epilepsy 
  • It can also be used to investigate other conditions such as encephalitis, dementia, head injuries, brain tumors, hemorrhage 
  • An EEG can identify areas of the brain that are not working properly 
  • EEGs are also used to determine the level of brain function in people who are in a coma
  • Test epilepsy drug effects
  • Test afferent pathways (by evoked potentials) 
  • Investigate sleep disorder and physiology 
  • Control anesthesia depth

 

Types of EEG

Routine EEG

  • Around 20 electrodes are stuck to the scalp using a special paste and EEG signals are recorded

Sleep EEG

  • The EEG tracing will be recorded along with the heart rate, airflow, respiration, oxygen saturation and limb movement

Ambulatory EEG

  • It involves recording the brain activity throughout the day and night 
  • A small portable EEG recorder is clipped on to the clothing

Types of EEG

EEG electrodes

Types of electrode placement

  • EEG electrodes placed separately on scalp
  • EEG electrodes mounted as special band on head

 

Types of EEG electrodes

  • Disk electrodes
  • Ear clip electrode
  • Intracortical electrodes

 

Type of electrode connections

  1. Between each member of a pair (bipolar)
  2. Between one monopolar lead and a distant reference 
  3. Between one monopolar lead and the average of all

 

EEG electrodes

International 10-20 system

It’s a method to describe and apply the location of scalp electrodes. This system is based on the relationship between the electrode and underlying cerebral cortex.

The 10 and 20 refer the actual distance between the adjacent electrodes .

 

In general 25 electrodes are used in general EEG test but number of electrodes may vary as per the EEG requirement and area of investigation.

In 25 electrode EEG system:

  • 23 electrode are active electrodes
  • 1 is ground electrode
  • 1 is ref electrode 

 

  • A1-left ear 
  • A2-right ear 
  • Fp-frontal pole leads 
  • F-frontal leads 
  • P-parietal leads 
  • C-central leads 
  • T-temporal leads 
  • O-occipital leads 

International 10-20 system

Montages

Sequential montage

  • Each channel represents the difference between two adjacent electrodes 

Referential montage

  • Each channel represents the difference between a certain electrode and a designated reference electrode

Average reference montage

  • The outputs of all of the amplifiers are summed and averaged

Laplacian montage

  • Each channel represents the difference between an electrode and a weighted average of the surrounding electrodes

 

EEG Artifacts

Mains Interference

  • Mains voltage of 110/230 volts, exceeds the EEG's 50 to 100 microvolts by 126dB
  • Amplifier notch filters are designed to suppress a certain amount of mains interference

Biological Artifacts

  • Eye-induced artifacts - eye blinks, eye movements
  • ECG and EMG induced artifacts
  • Glossokinetic artifacts

Environmental Artifacts

  • Movement by the patient, or even settling of the electrodes
  • Presence of an IV drip that can cause rhythmic, fast, low-voltage bursts, which may be confused for spikes