ELECTROENCEPHALOGRAPH (EEG)

Electroencephalograph is an instrument used for recording electrical activity of the brain, by suitably placing surface electrodes on the scalp. It can be used to detect any abnormalities related to electrical activity of the brain. This procedures tracks and records brain wave patterns. This actually gives a superimposed wave of neuron potentials operating in a non synchronized manner in a physical sense. Monitoring EEG has proven to be an effective method of diagnosing many neurological illness and diseases, such as epilepsy, tumour, and problems associated with trauma. It is also used in operating rooms to facilitate anesthetics and to establish integrity of the anaesthetized patient’s nervous system.

EEG RECORDING

There are two types of recording - monopolar and bipolar. EEG may be recorded by picking up the voltage difference between active electrode on the scalp with respect to reference electrode placed on the ear lobe or other part. This type of recording is known as monopolar recording. In bipolar recording both electrodes are placed on the scalp voltage difference between them is recorded, this is done using multichannel electroencephalographs. EEG signals picked up by electrodes may be small compared to ECG, they can be several 100 micro volts while 50 micro volts is the most typical. Unlike ECG, brain waves do not follow a pattern over and over again so they are recorded over a long interval of time so to detect any abnormalities. Selecting the proper filter band is essential to acquire proper signal. Band width must be atleast .5Hz to 70Hz. Optimum sampling rate must be adopted-14 Hz. Several type of electrode can be used to record EEG, like stick and peel electrodes, needle electrodes (if they are intended to place under skin) and silver plated cup electrode. EEG electrodes are smaller in size than ECG electrodes. They must be applied separately to scalp or may be mounted in special bands, which is then placed on patient’s body. Electrical jelly or paste used to improve electrical contact. The also got advantage of reducing movement artefacts. EEG electrodes give high contact impedance as compared to ECG electrodes. Good electrode impedance must be less than 5 kilo ohms. Impedance between two pair of electrodes must be balanced or their difference must be less than 2 kilo ohms.EEG preamplifiers are designed to have a very high value of input impedance to take care of high electrode impedance. Electrode contact impedance usually varies between 1k and 10k for classical electrodes. Greater than 10 k contact impedance may distort EEG signals. Now high input impedance (1G) amplifier chips and active electrode approaches decrease dependency of the contact impedance. To acquire proper signal electrodes should not be moved because it may cause spikes. Noise should be reduced as possible by placing electronic cards and connection cables in metallic boxes. Since these are of low amplitude EEG signals are sensitive to noise. Electronic noise should be less than 2micro volts (peak to peak).

BASIC PARTS DESCRIPTION

MONTAGE

A pattern of electrodes on the head and the channels they are connected to is called a montage. They are symmetrical. The EEG electrodes are placed on the scalp according to a standard known as 10-20 elecrode system. Reference electrodes are placed on ear lobes or other non active sites.10-20 electrode system They are a total of 21 electrode locations in this system. 10 and 20 refer the actual distance between the adjacent electrodes. The electrodes are classified based on their arrangements on head

  • Fp – Frontal polar
  • F – Frontal
  • C – Central
  • P – Parietal
  • O –Occipital
  • T – Temporal
  • Odd numbers – electrodes on left side
    Even numbers – electrodes on right side
    Z - mid-line electrodes
    One electrode is placed is on ground and placed at relatively neutral site on head.

    ELECTRODE MONTAGE SELECTOR

    This is a large panel containing switches that allow the user to select which electrode will have signals subtracted from each other to create an array of channels of output called a montage. They can be either bipolar or monopolar.

    PREAMPLIFIER

    Pre amplifier in electro electroencephalograph must have high gain, low noise , high CMRR, must be free from drift.

    SENSITIVITY CONTROL

    Overall sensitivity of the EEG machine is the gain of amplifier multiplied by the sensitivity of the writer. An EEG machine got two types of gain control one is continuously variable and other one operates in steps.

    FILTERS

    EEG may contain muscle artefacts due to the contraction of scalps and neck muscles, these artefacts are generally removed by using low pass filters. Some EEG machines have notch filters sharply tuned at 50 Hz so as to eliminate mains frequency interference.

    CHANNELS

    An electro encephalogram is recorded simultaneously from an array of many electrodes. The record can be made from bipolar or monopolar leads. The electrodes are connected to separate amplifiers and writing systems. Commercial EEG machines have up to 32 channels, although 8 or 16 channels are almost common.

    TYPES OF WAVES

  • DELTA WAVE

    Its frequency range is 1- 4 Hz. It can be recorded with an electro encephalogram and usually associated with slow sleep. Location: frontally in adults, posteriorly in children; high amplitude waves

  • THETA WAVE

    Two types of theta waves: hippocampal (observed in hippo campus) and cortical theta wave. Frequency range from 4-7 Hz.

  • ALPHA WAVE

    They are detected by electroencephalography and magneto electroencephalography. It’s mainly originated from occipital lobe during relaxation with closed eyes. Frequency range is from 8- 12 Hz. Location: posterior regions of head, both sides higher in amplitude on dominant side.

  • BETA WAVE

    Its frequency range is 12 – 30 Hz. Beta wave have three sections: high beta waves (19 Hz), beta waves (15 – 18 Hz), low beta waves ( 15 – 12 Hz). Location: both sides, symmetrical distribution, most evident frontally, low amplitude waves.

  • GAMMA WAVE

    Its frequency range is 25 – 100 Hz. Location: somatosensory cortex.