ELECTROENCEPHALOGRAM

ELECTROENCEPHALOGRAH (EEG)

   Electroencephalogramh is an instrument for recording the electrical activity of BRAIN,by suitably placing surface electrodes on the scalp.

EEG, describing the general function of the brain activity, is the superimposed wave of neuron potential operating in a non- synchronized manner in the physical sense. It's stochastic major originates just from this, and the prominent signal groups can be empirically connected to diagnostic conclusion .

      Monitoring the electroencephalograh has proven to be an effective method of diagnosing many neurological illness and diseases, such as epilepsy, tumor, cerebrovascular lesions, and the problems associated with trauma. It is also effectively used in the operating room to faciliate anaesthetics and to established the integrity of the anaesthetic patient's nervous system. This has become possible but then advent of small, computer-based EEG analysers. consequently, routine EEG monitoring in the operating room and intensive care unit is becoming popular.

       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.

This type of recording is called 'monopollar' recording. However 'bipolar' recording is more popular wherein the voltage difference between two scalp electrodes is recorded. Such recording are done with multi-channel 

electroencephalographs.

     

        EEG signals picked up by the surface electrodes are usually small as compared with ECG signals. They 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 as much longer interval of time in order to be able to detect any kind of abnormalities.

       Selecting the proper filter band (band with must be at least 0.5Hz 70 Hz) is important to Acquire proper signal. This is important for digitizing and data storing. Sufficient and optimum sampling rate (140Hz)  should be adopted.

    Several types of electrodes may be used to record EEG. These include:peel and stick electrodes, Silver plated cup electrodes and Needle electrodes.

     EEG electrodes are smaller in size than ECG electrodes. They may be applied separately to the scalp or may be mounted in special bands, which can be placed on the patient's head. In either case, electrode jelly or paste is used to improve the electrical contact. If the electrodes are intented to be used under the skin of the scalp, needle electrodes are used. They offer the advantage of reducing movement artefacts.  EEG electrodes give high skin contact impedance as compared to ECG electrodes. Good electrode impedance should be 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 kilo ohms. EEG preamplifiers are generally designed to have a very high value of take care of high electrode impedance.

    

    Electrode contact impedance usually varies between 1K and 10k for classical electrodes. Less than 1K contact impedance indicates probable shortcut between the electrons. Greater than 10k contact impedance may disort EEG signals. Before measurement, contact impedance should be measured, and EEG should be observed while recording. Using today's technology,  high input impedance (1G) amplifier chips and active electrodes approaches decrease dependency of the contact impedance. To acquired proper signal, electrode should not be moved. Otherwise,  it causes fluctuation of the EEG signal, and spikes on it.

     Noise reduction techniques must be considered in electronics circuitry and printed circuit board design. Electronic cards and connection cables should be placed in a metal box to reduce electronic noise as much as possible. Using twisted, blended, and it driven signal cables gives good results. Because EEG signals are of low amplitude, they are very sensitive to electronic noise. Electronic noise should be less than 2uV(peak-to-peak).

 Block diagram of EEG 

 

MONTAGES

    A pattern of electrode on the head and the channels they are connected to is called a montage. Montages are always symmetrical.  The reference electrode is generally placed on non-active site such as the forehead or earlobe. EEG  electrodes or arranged on the scalp according to a standard known as the 10/20 system, adopted by American EEG Society (Barlow et al., 1974).

  

   Traditionally they are 21 electrode locations in that 10/20 system. This system involves placement of electrons at distances of 10% and 20% of measured coronal, sagittal and circumferential arcs between landmarks and the cranium. Electrodes are identified according to their position on the head:

         Fp for frontal-polar

          F  for frontal

          C  for central

          P  for parietal

          T  for temporal 

          O  for occipital

        Odd numbers refers to electrodes on the left side of the head and even numbers represent those on the right while Z donate midline electrodes. One electrode is labelled isoground and placed at a relatively neutral site on the head, usually the midline forehead. A new montage convention has been introduced in which electrodes are spaced at 5% is a distance along the cranium.  These electrodes are called closely spaced electrodes and have their own naming convention.

ELECTRODE MONTAGE SELECTOR

    

       EEG signals are transmitted from the electrodes to the head box, which labelled according to the 10 to 20 system, 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 signal subtracted from each other to create an array of channels of output called montage.  Each channel is created in the form of the input from one electrode minus the input from a second electrode.