Download eeg electrodes

EEG ELECTRODES
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Electroencephalography (EEG) is the recording of electrical activity along
the scalp produced by the firing of neurons within the brain. In clinical
contexts, EEG refers to the recording of the brain's spontaneous electrical
activity over a short period of time, usually 20–40 minutes, as recorded from
multiple electrodes placed on the scalp. In neurology, the main
diagnostic application of EEG is in the case of epilepsy, as epileptic
activity can create clear abnormalities on a standard EEG study. A
secondary clinical use of EEG is in the diagnosis of coma,
encephalopathies, and brain death. EEG used to be a first-line method for
the diagnosis of tumors, stroke and other focal brain disorders, but this use
has decreased with the advent of anatomical imaging techniques such as
MRI and CT.
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Chloride silver discs are one of the most commonly used electrodes for
EEG.
They have a diameter of approximately 6-8mm.
Contact with the scalp is made via electrolytic paste through a washer of
soft felt.
They have an ac resistance varying from 3-20kohms.
Small needle electrodes are sometimes used for carrying out special EEG
studies when they are inserted subcutaneously.
Silver ball or pellet electrodes covered with a small cloth pad are useful
when electrical activity is recorded from the exposed cortex, but they have a
high dc resistances.
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A pad electrode is made from a silver rod belled out at the end and padded
with sponge, or similar material, contained in gauze.
It is screwed into an insulated mount and held in place on the head with a
rubber cap.
Another type of EEG electrode consists of multiple fine chloride silver wires
fixed in a rigid plastic cup.
The plastic cup is fixed to the scalp with an adhesive. It is filled with jelly
through a hole in the top.
In this electrode, contact with the tissue is made via an electrolyte bridge so
that jelly in contact with the electrode metal is not disturbed by the scalp
movement.
To avoid metal junctions which may get corroded with electrolyte, the silver
wires are used as the output lead.
The large surface area and excess of silver chloride favor stability.
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