Elelctroencephelogram notes

1. UEEG002
MEDICAL
INSTRUMENTATION
Electroencephalogram(EEG)
D.Poornima, AP(Sr.Gr)/EEE,
Sri Ramakrishna Institute of Technology,
Coimbatore

2. Electroencephalogram(EEG)
• The brain generates rhythmical potentials which originate in the individual neurons of the brain.
• These potentials get summated as millions of cell discharge synchronously and appear as a surface
waveform, the recording of which is known as the electroencephalogram
• The EEG signal can be picked up with electrodes either from the scalp or directly from the cerebral
cortex.
• The peak-to-peak amplitude of the waves that can be
picked up from the scalp is normally 100 mV or less
while that on the exposed brain, is about 1 mV.
• The frequency varies greatly with different behavioural
states.
• The normal EEG frequency content ranges from 0.5
to 50 Hz.
• The nature of the wave varies over the different parts of
the scalp

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• The variations in EEG signals both in terms of amplitude and frequency are of diagnostic value.
• Basic frequency of the EEG range is classified into the following five bands:
 Delta (d) 0.5–4 Hz
 Theta (q) 4–8 Hz
 Alpha (a) 8–13 Hz
 Beta (b) 13–22 Hz
 Gamma (g) 22–30 Hz
• EEG has wide variation among individuals and not
repeatable in a given person from one occasion to
another.
• Analysis is difficult.
• Certain characteristic EEG waveforms can be related to
gross abnormalities like epileptic seizure and sleep disorders
• The alpha rhythm is one of the principal components of EEG
• Indicator of the state of ‘alertness’ of the brain.
• It serves as an indicator of the depth of anaesthesia in the
operating room.

11. Electrodes for Electroencephalogram(EEG)
• Most commonly used electrodes for EEG (electroencephalogram)
recording are the chlorided silver discs
• Approximately 6–8 mm diameters
• Contact with the scalp is made via an electrolytic paste through
a washer of soft felt
• Small needle electrodes are sometimes used for carrying out
special EEG studies when they are inserted subcutaneously
• EEG electrodes are smaller in size
than ECG electrodes.
• May be applied separately to the scalp
or may be mounted in special bands,
which can be placed on the patient’s
head

12. • Silver ball or pellet electrodes covered with a small cloth pad are used for
exposed cortex
• They have high dc resistances.
• Pad electrode - made from a silver rod belled out at the end and padded
with a sponge, contained in gauze.
• Screwed into an insulated mount and held in place on the head with a rubb
er cap.
• Adjustable tripod mount is employed to hold them.
• Multiple fine chlorided silver wires fixed in a rigid plastic cup is also used
as EEG electrode.
• Plastic cup is fixed to the scalp with an adhesive, filled with jelly through a
hole in the top.
• Contact with the tissue is made via an electrolyte bridge so that jelly in
contact with the electrode metal is not disturbed by scalp movement.
• Silver wires are used as the output lead.
• Large surface area and excess of silver chloride favour stability

13. 10-20 Electrodes placement system for EEG
• Is a method used to describe the location of scalp electrodes for recording EEG
• 10-20 system is based on the relationship between the location of an electrode and the underlying area
of cerebral cortex.
• Each point on the figure indicates a possible electrode position.
• Each site has a letter (to identify the lobe) and a number or another letter to identify the hemisphere
location.
• The letters F, T, C, P, and O stand for Frontal, Temporal, Central, Parietal and Occipital. (Note that there
is no "central lobe", but this is just used for identification purposes.)
• Even numbers (2,4,6,8) refer to the right hemisphere and odd numbers
(1,3,5,7) refer to the left hemisphere.
• The z refers to an electrode placed on the midline.
• Smaller the number, the closer the position is to the midline.

14. • Nasion - point between the forehead and nose.
• Inion - Bump at back of skull
• The "10" and "20" refer to the 10% or 20% interelectrode distance.
• Measure the distance from Nasion to Inion
• Marks for the Z electrodes are made between these points along the mi
dline, at intervals of 10%, 20%, 20%, 20%, 20% and 10%.
• The T3, C3, Cz, C4, and T4 electrodes are placed at marks made at int
ervals of 10%, 20%, 20%, 20%, 20% and 10%, respectively, measured
across the top of the head.

15. EEG Machine Block Diagram and working
• EEG electrodes give high skin contact impedance as compared to ECG electrodes and are generally below 5 kilo
ohms.
• Impedance between a pair of electrodes must also be balanced or the difference between them should be less
than 2 kilohms.
• EEG preamplifiers are generally designed to have a very high value of input impedance to take care of high
electrode impedance.
• EEG may be recorded by picking up the voltage difference between an active electrode on the scalp with respect
to a reference electrode on the ear lobe or any other part of the body ----- called ‘monopolar’ recording.
• ‘Bipolar’ recording ------- the voltage difference between two scalp electrodes is recorded.
• Bipolar recordings are done with multi-channel electroencephalographs.
• EEG signals picked up by the surface electrodes are usually small as compared with the ECG signals.
• May be several hundred microvolts, but 50 microvolts peak-to-peak is the most typical.
• The brain waves, unlike the electrical activity of the heart, do not represent the same pattern over and over again
• Therefore, brain recordings are made over a much longer interval of time in order to be able to detect any kind
of abnormalities.

16. Montages: A pattern of electrodes on the head and
the channels they are connected to is called a
montage.
• The reference electrode is generally placed on a
nonactive site such as the forehead or earlobe.
• EEG electrodes are arranged on the scalp
according to standard 10/20 system

17. Electrode Montage Selector:
• EEG signals are transmitted from the electrodes to the head box and then to the montage
selector.
• The montage selector on analog EEG machine is a large panel containing switches that allow
the user to select which electrode pair will have signals subtracted from each other to create
an array of channels of output called a montage.
• Each channel is created in the form of the input from one electrode minus the input from a
second electrode
• Montages are either bipolar (made by the subtraction of signals from adjacent electrode pairs)
or referential (made by subtracting the potential of a common reference electrode from each el
ectrode on the head).
• Bipolar and referential montages contain the same basic information that is transformable
into either format
• The advantage of recording EEG in several montages is that each montage displays different sp
atial characteristics of the same data.

18. Preamplifier:
• Every channel has an individual, multistage, ac coupled, very sensitive amplifier with differential
input and adjustable gain in a wide range.
• The preamplifier used in electroencephalographs must have high gain and low noise
characteristics because the EEG potentials are small in amplitude.
• In addition, the amplifier must have very high common-mode rejection to minimise stray
interference signals from power lines and other electrical equipments.
• The amplifier must be free from drift so as to prevent the slow movement of the recording pen
from its centre position as a result of changes in temperature, etc.
• Noise performance is crucial in EEG work because skin electrodes couple brain waves of only a
few microvolts to the amplifier.
• Each individual EEG signal should be preferably amplified at the bedside.
• Therefore, a specially designed connector box, which can be mounted near the patient, is
generally employed with EEG machines.
• This ensures the avoidance of cable or switching artefacts.
• The use of electrode amplifiers at the site also eliminates undesirable cross-talk effects of the
individual electrode potentials.

19. Sensitivity Control:
• The overall sensitivity of an EEG machine is the gain of the amplifier multiplied by the sensitivity of the writer.
• Thus, if the writer sensitivity is 1 cm/V, the amplifier must have an overall gain of 20,000 for a 50 mV signal.
• Artefacts, several times greater than this, can cause excessive deflections of the pen by charging the coupling
capacitors to large voltages.
• Makes the system unusable over a period depending upon the value of the coupling capacitors
• Most modern EEG machines have de-blocking circuits to overcome this problem
Filters:
• When recorded by surface electrodes, an EEG may contain muscle artefacts due to contraction of the scalp
and neck muscles, which overlie the brain and skull.
• The artefacts are large and sharp, causing great difficulty in both clinical and automated EEG interpretation.
• The most effective way to eliminate muscle artefact is to advise the subject to relax, but it is not always success
ful.
• These artefacts are generally removed using lowpass filters.

20. Writing Part:
• The writing part of an EEG machine is usually of the ink type direct writing recorder.
• The best types of pen motors used in EEG machines have a frequency response of about 90 Hz.
• Most of the machines have a response lower than this, and some of them have it even as low as 45 Hz.
Paper Drive
• This is provided by a synchronous motor.
• An accurate and stable paper drive mechanism is necessary and it is normal practice to have several pa
per speeds available for selection.
• Speeds of 15, 30 and 60 mm/s are essential.
Microprocessors are now employed in most of the commercially available EEG machines. These machines p
ermit customer programmable montage selection; for example, up to eight electrode combinations can be
selected with a keyboard switch. In fact, any desired combination of electrodes can be selected with push b
uttons and can be memorized.

21. THANK YOU