EEG Basics monish.pptx

EEG Basics
DR.MOHINISH.S
DM PED NEURO RESIDENT
ICH & HC

•EEG
•Recording of
spontaneous electrical
activity generated by
cortical neurons

Recording the brain activity
Source:
https://www.epilepsy.
com/learn/profession
als/diagnosis-
treatment/basic-
science-eeg

The EEG machine
■ 8 – 64 identical channels recording
simultaneously from as many
different pairs of electrodes
■
■
■
■
Electrodes & electrode board
Amplifiers
Filters
Pen & chartdrive (screen)

EEG Machine
Electroencephalography (EEG) device technical features can be divided into three main areas:
1. Sensor or headset area (sensor layer).
 Sensor-body interface
 Signal transmission to the amplifier
2. The amplifier (acquisition layer)
 Analog and digital subareas
3. The amplifier connectivity area (connectivity layer)
 Other features like dimensions, power, weight, etc.

EEG Machine – Sensor Layer
3 types of electrodes
1. Recording electrodes
2. Reference electrode
3. Ground electrode
• To place both the amplifier and the body at the same potential
• To reduce common-mode interference
Electrode placement
19ch: the standard 10-20 system used in clinical research and practice.
Two anatomical landmarks are used for the essential positioning of the
EEG electrodes:
• nasion which is the point between the forehead and the nose;
• inion which is the lowest point of the skull from the back of the head and is normally
indicated by a prominent bump.
"10" and "20" refer to the fact that the actual distances between
adjacent electrodes are either 10% or 20% of the total front-back or
right-left distance of the skull.

Electrode in10 -20 system: Broadmann areas
Source:
https://www.researchgate.net/figure/A-semi-
schematic-diagram-depicting-dorsal-and-
sagittal-views-of-the-human-skull-and-
brain_fig1_272479756

Electrode placement: 10 -20 system
Source:
https://www.
medicine.mcgil
l.ca/physio/vla
b/biomed_sig
nals/eeg_n.ht
m

Electrode placement: 10 -10 system
• Source:
Source:
https://www.researchgate.
net/figure/International-
10-20-system-of-electrode-
placement-
7_fig2_256491418

EEG Machine – Acquisition Layer
EEG amplifier
Part of the data acquisition system responsible for accommodating, amplifying and
converting the analog electrical signals from the sensor into a digital signal that
can be processed by the computer
Sampling Rate
• number of times that the signal is measured per unit of time, usually given in
Hertz (Hz) = 1/second
• at least twice the maximum frequency of the signal being measured
• Higher sampling frequencies will give higher resolution in the EEG bandwidth (0
to 80 Hz) but no more information.
• Standard 256 Hz
Bandwidth
Effective frequency band that EEG system can measure according to sample rate
and internal filters
• Low pass / High frequency filter. 70 Hz
• High pass / Low frequency filter. 1 Hz
• Notch Filter: Designed to remove specific frequency from EEG tracing, typically
50 or 60 Hz activity (AC current field of nearby electrical equipment)

Resolution
Digitization process is carried out by an Analog to Digital Converter (ADC), which
codes each voltage value using a certain number of bits. This number of bits is
the resolution of the amplifier.
Resolution can also be used to refer to the smallest part of a signal that can be
resolved, which comes defined by the ratio between the input signal range of the
amplifier and the number of quantization levels (2n0 bits). E.g., 12-bit resolution
is 212 which is +/-2048
24 bits
Input Range
Maximum amplitude signal that can be recorded before saturation.
Voutput (output range) = G (Gain) X Vinput (input range)
Input Referred Noise
Noise voltage or current generated by the amplifier's internal circuitry even
when there is no signal at the input. <1 miicroVRMS

Common Mode Rejection Ratio
Ability of a differential amplifier to eliminate or attenuate the common-mode
voltage VCM, (i.e., the voltage that is constant for both the positive and negative
inputs of the amplifier), while amplifying the differential mode voltage VDM (i.e.,
the voltage difference between the positive and the negative inputs)
Higher the CMRR, better the amplifier performance, as it will be able to
attenuate common-mode signals (unwanted signals) by a higher factor.
At least 80 dB at 50 / 60 Hz. 100-110 dB in commercial amplifiers.
Input Impedance
Electrode impedance & Input impedance.
Diminish the CMRR and increase the noise.
Solution – amplifier with high input impedance. At least 100m (i.e., 100 times
the electrode impedance), so that signal attenuation is < 1 %
Impedance Monitor
Monitoring signal quality loss due to sensor or skin-electrode impedance.
Monitoring during EEG set up and EEG recording
Because differential amplifiers record only differences
between inputs, anything which affects voltage equally
at both inputs will not be “seen” but “rejected”.
Potentials or activity in phase and same amplitude at
inputs 1 & 2 will have flat line as output.

EEG Terminology
EEG detected seizures
Ictal: EEG activity during a seizure event
Inter-ictal & Post-ictal: EEG activity captured between and after seizures
respectively.
Inter-ictal Epileptiform Discharges (IED): Specific pattern of pathological activity ,
which is clearly distinguished from the activity observed during the seizure itself.
Spike: 30-70 msec
Sharp: 70-200 msec
Types of EEG procedure
Routine EEG: carried out in a specialized clinic or in the hospital and it should last
less than 1.5 hours
Sleep EEG: obtain sleep record. Usually with HR, airflow, respiration, SpO2 and
limb movement.
Prolonged EEG: epilepsy monitoring unit for continuous EEG monitoring for 1 or 2
hours or inpatient over several days. A video camera may used.
Ambulatory EEG

EEG Technicality
Filters: Low frequency / High pass

EEG Technicality
Filters: High frequency / Low pass

EEG Technicality
Filters: Notch

EEG Technicality
Types of Recording
10-20 system: 19 electrodes
Montage: Montages are logical, orderly arrangements of
electroencephalographic derivations or channels that are
created to display activity over the entire head and to provide
lateralizing and localizing information.
• Bipolar: Each channel represents the difference between two
adjacent electrodes
• Longitudinal
• Transverse
• Referential: Each channel difference between a certain
electrode and a designated reference electrode

EEG Technicality
Polarity Convention
The amplifier cannot tell the difference between a negative
event at input 1 and a positive event at input 2 “negative into
input 1 with respect to input 2 is up going”
L1 NEGATIVE UP; “LINE UP”

EEG Technicality
Creation of Bipolar Montage
(Fp1 – R ) – (F3 – R) = Fp1 – F3
(F3 – R) – (C3- R) = F3 – C3
(C3 – R) – (P3 – R) = C3 – P3
(P3 – R) – O1 – R) = P3 – O1

EEG Technicality
Creation of Referential Montage
Ideally – Reference electrode should be inactive
Inactive reference is practically not feasible
If reference is active, can confound the field
• Ear referential (A1 & A2)
• Cz referential
• Common average referential: Outputs of all
the amplifiers are summed & averaged. This
averaged signal is used as the common
reference for each channel.

EEG Concepts
Localization clues in bipolar montage
• PHASE REVERSAL

EEG Concepts
• PHASE CANCELLATION

EEG Concepts
• END OF CHAIN

EEG Concepts
• BEGINNING OF CHAIN

EEG Concepts
Localization clues in referential average montage
• AMPLITUDE MAXIMA
• LEADING SPIKE

Reading EEG
• Background activity
• Symmetry (right Vs left)
• Abnormal Electrical Discharges
• Artefacts

Reading EEG
• Artefacts
γ, β, α, θ, δ

Reading EEG
• Artefacts
Asymmetry :
Lateralization (Focal
abnormality)

Reading EEG
• Artefacts
Focal Vs Generalized
Chaotic
Periodic (Patterned)
High Vs Low amplitude

Reading EEG
• Artefacts Eye movement (vertical Vs lateral)
Wet skin
Misplacement/displacement of lead
Abnormal position of reference lead

EEG Basics monish.pptx

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