Introduction:

• Unwanted electrical activity arising from different sources, other than cerebral activity

• EEG -highly sensitive recording device, easily interrupted by other electrical activity of very high voltages

• Some readily distinguished, others closely resemble cerebral activity .

Principles used to discriminate artifacts from EEG signals:

• Physiological activity has a logical topographic field of distribution with an expected fall off of voltage potentials.

• Artifacts have an illogical distribution that defies the principles of localization

Classification:

• Physiological : from patient’s own physiological generator sources other than the brain

• Extraphysiological : Externally generated e.g. instrumental & environmental

Physiological artifacts:

• Cardiogenic-electrical mechanical

• Muscle activity– Eye movement – surface EMG – Movement

glossokinetic respiration photomyoclonic• Skin artifacts perspiration salt bridges

• Eye movements Blink flutter Lateral gaze Lateral rectus spike Slow eye movements Electroretinogram REM sleep

Eye movement artifacts:• most common, due to cornea-retinal potentials. The cornea is

positive to the retina by 50-100 mv

• Fp1, Fp2, F7, F8 , amplitude is approx 50- 200uv

• frontopolar rhythmic slow wave activity - simulating brain activity

• Types of eye movement artifacts – Eye open, eye closure and blink – Eyelid flutter – Eyeball movement (lateral, vertical and oblique eye movements

Eye opening and eye closure

Blink

Eye blink is a “U” shape transient potential which appears in frontopolar electrodes which are closer to the eyes.

Eyelid flutter

Fine eyelid movements may produce rhythmic 4-8 Hz activity in the frontal leads (disappear on fixation)

Lateral eyeball movements

Elimination:

• close the eyes or eye fixation

• close his eyes with his fingers or place a soft cloth or a cotton pads over the closed eyelids

• Monitoring eye movements by placing extraocular electrodes will easily help to distinguish from frontal slow waves

Muscle (EMG) artifacts:

• EMG artifacts are due to the muscle contraction

superimposed upon the EEG activity

• The motor unit potentials (MUPs) arising from

the scalp muscles may cause misinterpretation by

resembling spike or cortical β-activity

Lateral rectus spikes:

Frontalis:

Temporalis:

Clenching and grinding teeth

Swallowing:

Swallowing of saliva usually produces a short burst activity

Chewing:

Sniffling artifact:

Small EMG component with a slow wave

Facial myokymia:

Glossokinetic artifact:

Movement of the tongue during speaking may produce synchronous intermittent rhythmic EEG activity (2-6 Hz) because the mass of the tongue acts as a charged body with a negative tip

Glossokinetic artifact:

Tremor:

Differentiation between EMG artifacts & the cortical spikes:

• EMG potentials are of extremely short duration (2-20 msec) and spiky even on increasing paper speed to 60 mm/s

• On basis of morphology and frequency

• During sleep EMG activity is reduced whereas cortical spikes increase

• Document using extra muscle electrodes

Reduction/elimination:• Frontalis - close eyelids lightly and relax

• Temporalis - opening the mouth

• Use of relaxation techniques (reassuring, comforting the patient),Massaging the particular muscle groups

• Occipitalis - putting the pillow under the neck

• As a last resort change the high frequency filter to 35/15 Hz

Cardiogenic artifacts:

• EP arising in the cardiac muscles, high in amplitude

• Spread to scalp by volume conduction

• Prominent in babies, obese, short – neck, cardiomegaly

• Referential montage picks up ECG artifact due to large interelectrode distances and close proximity of ear electrodes to the heart

• Equipotential on the scalp - not picked up in bipolar montage

Types:

• ECG

• Pulse

• Pacemaker

ECG

Coming at regular intervals synchronising with ECG

ECG:

Coming at regular intervals synchronising with ECG

Pulse

Pulse:

Elimination:

• Changing the head position relative to the thorax

• Lower the electrode resistance (A1 and A2) or use body earth (A1+A2)

• Change reference to Cz

• Shift the electrode a little - eliminate pulse wave

Sweat artifact:

• Warm and humid recording room

• Anxiety and emotional tension

• Autonomic dysfunction

• Electrical potentials due to sweat glands

• Reduction in the skin resistance (GSR)

• NaCl and lactic acid of sweat may produce large base line sways

Sweat:

These are the slow waves of very low frequency (1/3-1/2 Hz)

Salt bridge:

Electrolyte in sweat interact with electrode gel / paste to produce a salt bridge

Elimination:

• Proper air conditioning

• Reducing emotional stress

• Sponging the patient’s face & forehead with alcohol/spirit

• Setting the short time constant (LFF) if all the efforts fail

Movement artifacts:

• Movement of head, body and limbs produce irregular high voltage potentials

• Movement of the head during HV or respiration can produce occipital electrode artifacts

• Movement associated with seizure may obscure cerebral activity

• Artifacts may occur in essential tremors, Parkinson disease and other movement disorders

Head movement:

Movement between the head and the pillow

Movement associated with seizure:

Contraction of skeletal muscles before the seizure activity starts

Movement associated with seizure:

Difficult to determine the origin of the seizure activity

Patting artifacts:

Mother patting baby

Respiration artifacts:

More commonly seen during HV as slow wave activity

Reduction/elimination:

• Solicit the co-operation of the patient to be calm and still

• Changing the position or turning the patient over will diminish tremor artifacts

• Placing a roll of towel or firm material under the neck may do away with respiratory artifacts

Extra physiological artifacts:

– Electrode –electrode pop, contact– Technical-lead movement, placement– Instrumental –AC 60hz artifact

ventilator, circulatory pumps– Iv fluids– Environmental-mobile, TV, radio,

electromagnetic

• The scalp-electrode junction is an important and delicate link in the whole system

• Electrode artifacts occur due to the change in the resistance or electrode potential between the scalp and the electrode

• Poor contact, improper electrolyte application, broken lead, poor contact at the junction box, dry out of electrode paste

Electrode artifacts:

Electrode pop:

A sudden change in the electrical potential between the electrode and scalp gives rise to the ‘pop’

Electrode artifact:

Loose contact of the electrode with the scalp

Electrode movement artifact:

This is movement of electrode on the scalp, because of excessive jelly

Recognition & differentiation from cerebral activity:

• Electrode artifact is strictly confined to one lead or channels having input from this electrode, where as cerebral activity would usually spread to the adjacent electrodes

• In a bipolar montage, the artifacts show mirror images (like phase reversals) in two adjacent channels

• Wave morphology may change from time to time

Electrode artifacts:

Elimination

• Applying proper electrolyte

• Check electrode impedance & continuity

• Change the electrode

• If it still present change the position at a distance of 0.5-1cm

Electrode artifacts:

• due to the malfunctioning of certain parts of the machine

-Pen deflection -Amplifier

-Galvanometer-Faulty ground-Photic stimulator

Instrumental:

• Non-symmetrical electrode placement

• Erroneous settings of different channels

• Unequal electrode impedances result in loss of common mode rejection and recording of AC artifact

Technical artifacts:

Pen deflection:

A1- disconnected:

Photic:

Interchanging electrode placement:

Interchanging the Fp2 electrode with O2

AC artifact:60 hz artifact

Very high electrode impedance of more than 25 k

• troublesome as they cannot be easily controlled by the technologist

• Movement of other persons • Types of environmental artifacts:

– Electromagnetic Interference – Electrostatic Induction – Radio frequency Interference

• As technology expands – new artifacts appear– Mobile phone artifacts

Environmental artifacts:

Electromagnetic interference:

• Current flowing through an electrical conductor (main power supply), produces a magnetic lines of force around the conductor

• The greater the current, the stronger the field• Occurs between mains and patient or electrode attached to

the scalp or junction box• Common sources – power transformers

– Electric motors in lifts, fans, air-conditioners

• AC interference (result of capacitance or inductive effects on wires leading from the scalp to the junction box)

Electrostatic Induction:

• Opposite polarities of two charged bodies produces electrostatic induction when negative electric charge is applied

• Presence of several instruments adjacent to the EEG machine (like fluorescent light, ventilators etc.)

• Movement of any charged body near the patient (like plastic, rubber, synthetic fibres etc)

• Often the technologist himself is the recipient of this discharge

• Low electrical resistance between patient and the environment

Static artifact:

Produced by placing a charged synthetic paper near the patient

Static artifact:

walking in to collect a report

Electrostatic artifact:

Startle or clap response can also cause static artifact

AC Interference

Cell phone artifact:

Minimisation/removal of environmental artifacts:

• Keeping electrode leads close together.• Move away from source (at least 10 feet) (since electrostatic induction is inversely proportional to

the distance between patient and source)• Changing the orientation of the patient/source parallel to

the electromagnetic lines of force• Locate the EEG lab at least 50 feet away from AC

feeders, transformers etc.• One earthing point common to all equipment• Shielding the source of artifact and machine with earthed

metal shield

Artifacts are also useful:• Slow lateral eye movements and disappearance or

reduction of EMG are a valuable sign of drowsiness/sleep

• Eye open and eye closure artifacts record the precise point where the patient opened or closed his eyes

• Short bursts of EMG activity may mark the occurrence of myoclonic jerks

Environment for artifact free recording

• Quiet atmosphere • Comfortable bed• No synthetic carpeting• Convenient control of lighting• Proper earthing / shielding• Different power supply to equipment and air

conditioner• Comfortable temperature and humidity levels to

avoid sweating, tension, restlessness

Thank you