Electroencephalography (EEG)

Transcription

1 Electroencephalography (EEG)

2 Electroencephalography (EEG) Electroencephalography (EEG) is a record of electric signals produced by synchronous action of brain cells. Electroencephalograph system diagram [4]

3 EEG with Special Electrodes Sphenoidal Electrodes Foramen Ovale Electrodes

4 EEG with Special Electrodes Epidural Electrodes Subdural Electrodes

5 EEG with Special Electrodes Depth Electrodes Nasopharyngeal Electrodes

6 EEG with Special Electrodes

7 Scalp EEG Recording Methods Electrodes properties and general technical requirements Silver/silver chloride (Ag/AgCl), gold, tin, platinum or other metals, which do not interact chemically with the scalp. Only high- purity metals are used. Low contact impedances (less than 5 k ). Very low impedances of less than 100 Ω usually relate to accidental short-circuits between electrodes caused by electrolytic gel or other conductive materials bridging the gap between sites. Use a fairly large contact area and using amplifiers with high input impedance (problematic when trying to record low frequency EEG), which keeps the current density low to reduce polarization and bias potentials (a result of the exchange of metal ions and electrolytes in the absence of current flow) (Fisch, 1991 and Webster 1999). A ground electrode should be added and connected properly All electrodes should be made of the same material. Care in storing and cleaning is necessary to prevent surface contamination. Electrode pastes and gels should be protected from contaminated from foreign metal ion. The sensitivity of amplifiers should initially be set at 5-10µV/mm and then adjusted as necessary.

8 Electrodes Positions Electrode positions in system. (Malmivuo and Plonsey, 1995) In the standard system, there are 19 EEG sites plus 2 ear references. F, T, C, P, and O = Frontal, Temporal, Central, Parietal and Occipital. Even numbers (2, 4, 6, and 8) = right hemisphere. Odd numbers (1, 3, 5, and 7) = left hemisphere. z = midline. (smaller the numbers are closer to the midline)

9 Electrodes Placements

10 Multichannel EEG Recordings Referential montages or monopolar montages: the reference electrode may be placed on the earlobe or the mandibular angle. Usually, this electrode is connected to the negative input of the amplifier. Bipolar montages: a pair of different electrodes are connected to the positive and negative poles of each amplifier, Laplacian montage. It references each electrode to the four closest neighbours. Common Average Reference (CAR) uses the average of signals from every electrode as a reference signal. Others Monitoring of extra cerebral activity

11 Brain Anatomy [1]

12 Hemispheric Lateralization [1]

13 Sources of EEG The sources of EEG are the summed extracellular synaptic potential fields generated by inhibitory and excitatory postsynaptic activity of nerve cells in the cerebral cortex and its underlying nuclei in response to various kinds of input. The amplitude of an EEG measured with the scalp electrodes is approximately 50 µv to 200 µv (Webster, 1999).

14 Neuron

15 Generation of Action Potential

16 Sources of EEG The action potential in the afferent neuron causes the release of a neurotransmitter from its nerve terminal that diffuses across the synaptic cleft. This causes a local change in the postsynaptic potential. The potential difference between the postsynaptic membrane and other parts of the neuronal membrane causes an electrical current to flow along the neuronal membrane and to change the membrane potential of the perikaryon.

17 Generation of EEG (Events at a Cholinergic Synapse) 1. An Action Potential Arrives and Depolarizes the Synaptic Terminal. 2. Extracellular Calcium Ions Enter the Synaptic Terminal, Triggering the Exocytosis of ACh.

18 Generation of EEG (Events at a Cholinergic Synapse) 3. ACh Binds to Receptors and Depolarizes the Postsynaptic Membrane. 4. ACh Is Removed by AChE

19 EEGs

20 The Rhythmic Activity Alpha waves - the most prevalent normal brainwave in the EEG of a person who is awake but relaxed. Frequency 8-12 Hz. Maximum amplitude over the posterior head region. Decrease in adult life. Disappear in drowsiness and sleep and can be blocked by eye opening. (Berger, 1929). Beta waves-the normal brainwave in the EEG, which is characteristic for alertness, focused attention, concentration or even stress and psychological tension. Frequency Hz. Very low amplitude. Disappear in drowsiness and sleep (Berger, 1929).

21 The Rhythmic Activity Theta waves-the normal brainwave in the encephalogram of a person who is awake but relaxed and drowsy. Frequency 4-8 Hz. Low amplitude. Children, stressed adults, during rapid-eye-movement sleep. The presence of theta waves under other circumstances may indicate an underlying brain disorder (Walter and Dovey, 1944). Delta waves are normally seen in the EEG of a person in deep dreamless sleep or an awake infant. It also presents during states of high conscious focused attention. Large amplitude (75µV to 200 µvp-p) Low frequency (0.5-4 Hz).

22 The Rhythmic Activity Gamma waves is associated with perception and consciousness (Keiser and Lutzenberger, 2003). Occur as high frequency bursts with a frequency more than 35 Hz ( Hz) Sometimes, it is included in beta waves classification (Jasper and Andrews, 1938). µ (mu) waves are EEG waves that show a shape suggestive of a wicket fence with sharp tips and rounded bases. Frequency is generally half of the fast activity present. Younger adults (the central part of the head over the motor cortex) Can be blocked by movement, by intention to move, or by tactile stimuli. The EEG is not abnormal if it shows only a few trains of µ waves in one side (Sterman et al, 1974).

23 The Rhythmic Activity

24 The Rhythmic Activity a b c a) Normal, awake EEG; similar features between hemispheres; and no epileptiform activity. b) Abnormal discharge called a generalized spike and wave. This EEG pattern is typical for absence seizures. c) Abnormal discharge called focal spike. This examples occurs over the right temporal region of the brain.

25 ERP and ERSP ERP (Event-related Potential) and ERSP (Event-related Perturbation) ERP and ERSP of the EEG related to the real movement transition from 0, Subject No.4, electrode C3, CAR montage. The time at 0 s was the movement onset time. ERP and ERSP of the Independent Components representing a typical auditory response

26 Artefacts Blinking and other eye movements. Eye movement artefacts usually are identified by their frontal distribution, symmetry, amplitude, and their characteristic shape. Muscle artefact. Muscle activity causes very brisk potentials which usually reappear. Movement artefact. Movement artefacts are often erratic and not repetitive unless the movement is rhythmical or is triggered. ECG. Potential changes generated by the heart are picked up in the EEG mainly in recordings with wide inter-electrode distances, especially in linkages across the head and to the left ear, and in subjects with short necks. It is rarely a problem in bipolar montages. Pulse wave artefacts. An electrode may pick up periodic waves of smooth or triangular shape, on or near a scalp artery due to pulse wave producing slight changes of the electrical contact between electrode and scalp.

27 Artefacts Perspiration artefact consists of slow waveforms that are normally longer than 2 seconds. Galvanic skin response consists of slow waves, each with the period of second Movement of the tongue and other oropharyngeal structures may produce irregular or repetitive slow waves in a wide distribution, often with a maximum in the middle of the head. Dental restorations with dissimilar metals may produce spike-like artefacts whenever the metal pieces are moved against each other. External electrical interference from the other power sources such as power lines or electrical equipment. Normally, this form of noise is observed as a 50 Hz or 60 Hz signal. Internal electrical malfunctioning Skin temperature difference at electrode sites can cause residual potentials (depends on electrode s temperature coefficient).

28 Deep Brain stimulation for movement disorders

29 Deep Brain Stimulation Parkinson s Disease Midbrain neurons project to the putamen and caudate, where they release dopamine. When more than half of the dopaminergic nerve terminals are affected, the motor impairments of Parkinson s disease arise. (resting tremor, rigidity, slowed movement, decreased dexterity, small handwriting, flexed posture, gait disorder, and imbalance.dementia can develop over several years. Essential Tremor begins in the arms and then spreads to these other regions. not present at rest. Dystonia

30 Deep Brain Stimulator System Diagram Deep brain stimulation (DBS) It is now hypothesized that DBS increases output from the stimulated structure, in addition to suppressing local neuronal activity. The exact mechanism of action remains a matter of debate (Hiner et al., 2009). It is nondestructive, reversible, and adjustable.

31 Deep Brain Stimulator Patient Programmer: on-off, battery status, stimulator status, change therapy settings (basic). Clinician Programmer: advance therapy settings. Implantable pulse generator (IPG) and electrodes. Battery: Primary cell (based on lithium thionyl chloride (Li/SOCl 2 ) chemistry), lasted 3-5 yrs (bilateral stimulation). Or, Rechargeable secondary Li ion (9 yrs).

32 Electrodes Conductor wire Platinum-iridium, Conductor wire insulation, Electrodes Platinum-iridium. Diameter 1.27 mm; lead length cm. Four electrodes at the end of a lead are spaced either 0.5 or 1.5 mm apart (Medtronic, 2008) and span a distance of 7.5 to 10.5 mm of brain tissue. Some DBS centers outside the United States commonly use 5-contact electrodes. (Hardesty, 2006)

33 Immunity from Electromagnetic Interference must not be susceptible to electrical influences due to external electromagnetic fields in the range of 10 Hz-30 MHz. ANSI/AAMI/ISO :2008 assess protection from static magnetic fields, electromagnetic fields in the range of MHz, and electromagnetic fields in the range of 450 MHz3 GHz (AAMI, 2009).

34 Waveforms A rectangular waveform of adjustable duration (pulse width: μs) and amplitude (voltage difference between anode and cathode: V). Typical settings are V, μs pulse width, and pulses/s (Medtronic, 2008). Reductions in tremor are typically observed only when the frequency of stimulation is > 90Hz; conversely, low-frequency DBS (<50Hz) often worsens symptoms [b] Average rate of 130 Hz was more effective at reducing tremor when pulses were evenly spaced (Birdno et. al, 2007). Waveforms must be biphasic to prevent tissue damage. The cathodic (negative) phase of the waveform has the greatest effect on neural activation.

35 Waveforms Indication Stimulation Target Site Dystonia Unilateral or bilateral STN or GPi Essential Tremor Unilateral VIM Parkinson s Disease Bilateral STN or GPi In vivo stimulus waveform recordings. A) Voltagecontrolled (IPG) and current-controlled stimulus waveforms at 20 Hz and 185 Hz recorded across a 1 kω resistor (top) and corresponding voltage responses recorded in vivo (bottom). B) The peak voltage recorded in vivo was proportional to the applied stimulus amplitude

36 Surgery Complications Death is exceedingly rare. Symptomatic hemorrhage, infection, and seizure each occur in about 1% to 4% of cases. Complication rates increase with longer operations and multiple electrode insertions. Keeping leads externalized for more during assessment of stimulation effects increases the risk of infection. Repeat surgery may be necessary for subsequent complications, such as electrode migration, skin erosion, lead fracture, or battery failure. Batteries usually must be replaced every 3 to 5 years.

37 Results In patients with advanced PD, bilateral stimulation of STN or GPi over 3 months was associated with significant improvement in motor function [a]. Trial of 255 patients over 6 months of bilateral stimulation showed the improvement in motor function. However, this study demonstrated that DBS is associated with increased risks of falls and dystonia events (Weaver et al., 2009). In 21 explanted leads, recovered after 331 months from patients, foreign body giant cells were observed (Moss et al., 2004). Increased tissue impedance over time may contribute to the required increase in stimulation current.

38 Contraindications Patients who will be exposed to MRI using a full body radiofrequency (RF) coil or a head transmit coil that extends over the chest area. Patients who are unable to properly operate the neurostimulator. Patients for whom test stimulation is unsuccessful. Diathermy (e.g., shortwave diathermy, microwave diathermy or therapeutic ultrasound diathermy), and MRI (sometimes) can cause tissue damage resulted from heating of DBS electrodes due to excessive energy deposition and can result in severe injury or death (Dommerholt and Issa 2001). Transcranial Magnetic Stimulation (TMS).

39 Warnings, Precautions There is a potential risk of tissue damage using stimulation parameter settings of high amplitudes and wide pulse widths. Extreme care should be used with lead implantation in patients with a heightened risk of intracranial hemorrhage. The lead-extension connector should not be placed in the soft tissues of the neck due to an increased incidence of lead fracture. Theft detectors and security screening devices may cause stimulation to switch ON or OFF, and may cause some patients to experience a momentary increase in perceived stimulation. For patients with Dystonia, age of implant is suggested to be that at which brain growth is approximately 90% complete or above. Abrupt cessation of stimulation should be avoided as it may cause a return of disease symptoms, in some cases with an greater intensity ( rebound effect).

40 Adverse Events Depression, suicidal ideations and suicide have been reported, although no direct cause and effect relationship has been established. Stimulation not effective, cognitive disorders, pain, dyskinesia, dystonia, speech disorders including dysarthria, infection, paresthesia, intracranial hemorrhage, electromagnetic interference, cardiovascular events, visual disturbances, sensory disturbances, device migration, paresis/asthenia, abnormal gait, incoordination, headaches, lead repositioning, thinking abnormal, device explant, hemiplegia, lead fracture, seizures, respiratory events, and shocking or jolting stimulation.

41 Target Localization 1. The patient s head is fit with a rigid frame that provides a threedimensional coordinate system for target localization. The patient s brain is imaged using magnetic resonance imaging (MRI). 2. After the brain is exposed, a platinum-iridium alloy, insulated with glass, microelectrode recording (MER) is advanced from the cortical surface to the target. As the electrode tip passes, voltage characteristics of the signal enable structures to be identified.

42 Target Localization 3. Associated movement, is used to fine-tune the microelectrode location. The clinician listens for the modulation of action potential discharge in relation to passive movements of opposite side limbs. (Sillay & Starr, 2009).

43 Programming 4. The leads are then secured in a lead anchoring device on the skull, before the pulse generator is implanted several weeks later. Many programmers wait 2 to 4 weeks to allow for resorption of edema near the lead. Settings are programmed on a handheld computer that communicates to the IPG via a magnetic interface.

44 References 1. Fundamental of Anatomy and Physiology, Frederic H. Martini 2. Biomedical Instrumentation: Application and Design, John G. Webster 3. Introduction to Medical Electronics Applications, D. Jennings 4. Medical Device Technologies: A Systems Based Overview Using Engineering Standards, Gail D. Baura 5. Bioimpedance and Bioelectricity Basics, Orjan G. Martinsen 6. The Biomedical Engineering Handbook, Joseph D. Bronzino 7.