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ELECTROENCEPHALOGRAPHY (EEG) Contents 1, Overview 2, Clinical signification 3, Mechanisms 4, Block diagram of EEG 5, Brain wave Classification 6, Applications & limitations 1, Overview Electroencephalography (EEG) is an electrophysiological monitoring method to record activity of the brain In clinical contexts, EEG refers to the recording of the brain’s spontaneous electrical activity over a period of time EEG is most often used to diagnose epilepsy, which cause abnormalities in EEG readings It’s also used to diagnose sleep disorders, coma, encephalopathies, and brain death EEG used to be a first-line method of diagnosis for tumor, stroke and other focal brain disorders, but this use has decreased with the advent of high resolution anatomical imaging techniques such as MRI and CT EEG is recorded by noninvasive method, with the electrodes placed along the scalp, although invasive electrodes are sometimes used in specific applications Diagnostic applications generally focus on the spectral content of EEG, that is, the type of neural oscillations (popularly called “brain waves”) that can be observed in EEG signals 2, Clinical signification A routin clinical EEG recording typically lasts 20-30 minutes (plus preparation time) and usually involves recording from scalp eletrodes Routin EEG is typically used in the following clinical circumstances: to distinguish epileptic seizures from other types of spells, such as psychogenic non-epileptic seizures, syncope (fainting), subcortical movement disorders and migraine variants to differentiate "organic" encephalopathy or delirium from primary psychiatric syndromes such as catatonia to serve as an adjunct test of brain death to prognosticate, in certain instances, in patients with coma to determine whether to wean anti-epileptic medications it can also be used to 'train' brains- particularily children's (or, I should say, find which areas of the brain require 'training' and then further EEGs are done, but only focusing on those certain areas) However this is not widely done 2, Clinical signification Additionally, EEG may be used to monitor certain procedures to monitor the depth of anesthesia EEG can also be used in intensive care units for brain function monitoring to monitor for non-convulsive seizures/non-convulsive status epilepticus to monitor for secondary brain damage in conditions such as subarachnoid hemorrhage as an indirect indicator of cerebral perfusion in carotid endarterectomy to monitor amobarbital effect during the Wada test to monitor the effect of sedative/anesthesia in patients in medically induced coma (for treatment of refractory seizures or increased intracranial pressure) 3, Mechanisms The brain’s electrical charge is maintained by billions of neurons Neurons pass signals via action potential created by exchange between sodium and potassium ions in and out of the cell- Volume condution When the wave of ions reaches the electrodes on the scalp, they can push or pull electrons on the metal on the electrodes, the difference in push, or voltage, between any electrodes can be measured by a votlmeter Recording these voltages over time gives us the EEG Scalp EEG activity shows oscillations at a variety of frequencies Several of these oscillations have characteristic frequency ranges, spatial distributions and are associated with different states of brain functioning 4, Block diagram of EEG 4, Block diagram of EEG Non- invasive and painless Major components: To study the brain organization of cognitive processes such as perception, memory, attention, language and emotion in normal adults and children Electrodes with conductive media Amplifier with filters A/D converted Recording device 4, Block diagram of EEG Electrodes read the signal from the head surface, amplifiers bring the microvolt signals into the range where they can be digitalized accurately, converter changes signals from analog to digital form and personal computer stores and displays obtained data Source: Power supply for function blocks 4, Block diagram of EEG a, Recording electrodes Types of electrodes Disposable (gel-less, and pre – gelled types) Reusable disc electrodes (gold, silver,s.s ortin Headbands and electrode caps Saline-based electrodes Needle electrodes 4, Block diagram of EEG Electrode caps are preferred, with certain number of electrodes installed on its surface Commonly used scalp electrodes consist of Ag-AgCl disk, to 3mm in diameter, with long flexible leads that can be plugged into an amplifier Needle electrodes are used for long recordings and are invasively inserted under the scalp Electrode locations and names are specifier by the international 10-20 system for most clinical and research applications 4, Block diagram of EEG Display of the EEG may be set up in one of several ways The representation of the EEG channels is referred to as a montage Bipolar montage: each channel represents the difference between two adjacent electrodes The entire montage consists of a series of these channels Referential montage: each channel represents the difference between a certain electrode and a designated reference electrode Average reference montage: the outputs of all of the amplifiers are summed and averaged, and this averaged signal is used as the common reference for each channel Laplacian montage: each channel represents the difference between an electrode and a weighted average of the surrounding electrodes 4, Block diagram of EEG b, Amplifiers and filters The input signal to the amplifier consists of five components: Desired biopotential Undersired biopotential A power line interference signal of 50/60 Hz and its harmonics Interference signals generated by the tissue/electrode interface Noise .The A/D converter is interfaced to a computer system so that channels of analog signal are converted into a digital representation .Analog low-pass filters prevent distortion of the signal by interference effects with sampling rate, called aliasing, which would occur if frequencies greater than one half of the sampling rate survive 5, Brain wave Classification Brain patterns form wave shapes that are commonly sinusoidal Measured from peak to peak and normally range from 0.5 to 100 uV in amplitude Signal is derived by means of Fourier transform power spectrum from the raw Brain waves have been categorized into basic groups: Beta (>13Hz) Alpha (8-13 Hz) Theta (4-8 Hz) Delta (0.5-4 Hz) 5, Brain wave Classification 6, Applications & limitations Applications Monitor alertness, coma and brain death Locate areas of damage following head injury, stroke, tumour, etc Test afferent pathways (by evoked potentials) Monitor cognitive engagement (alpha rhythm) Produce biofeedback situations, alpha, etc Control anaesthesia depth Investigate epilepsy and locate seizure origin Test epilepsy drug effects Assist in experimental cortical excision of epileptic focus Monitor human and animal brain development Test drugs for convulsive effects Investigate sleep disorder and physiology 6, Applications & limitations Limitations: Poor spatial resolution Most sensitive to a particular set of post- synaptic potentials, those generated in superficial layers of the cortex, on the crests of gyri, in dendrites and deep structures or producing currents that are tangential to the skull It is mathematically impossible to reconstruct a unique intracranial current source for a given EEG signal, as some currents produce potentials that cancel each other out This is referred to as the inverse problem
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