1. EEG Artifacts
&
How to Resolve
Lalit Bansal M.D.
Director of Epilepsy Surgery Medical Program
Pediatric Epilepsy & Clinical Neurophysiology
Children’s Mercy Hospital
05/15/2018
2. EEG records cerebral activity and electrical activities from sites other than the brain
Anything that is NOT of cerebral origin is termed as ARTIFACT
Types of Artifact:
1. Physiological artifact - generated other than brain ie. body
2. Extraphysiological artifact - arise outside the body, eg: equipment, enviornment
Introduction
3. Physiological activity has a logical topographic field of distribution with an
excepted fall of the voltage potentials
Artifact have an illogical distribution that defies the principles of localization
Principles to discriminate artifacts from EEG signals
4. Good, clean prep
Balanced impedances
Good hook-up, neatly bundled electrodes
Place jack-box close to patients head
Keep patient cool, not cold
Unplug all electrical items close to patient, i.e. bed, radio, fan, etc.
KEYTO AN ARTIFACT FREE RECORDING
6. Heart produce 2 types of artifact
1. Electrical
2. Mechanical
Timed lock to cardiac contractions and synchronized with EKG complexes
P/T wave are usually not visible on EEG (distance from the heart and the suboptimal axis)
Artifact is a poorly formed QRS complex
Prominent in patients with short neck
Most prominent over temporal region
R wave - A1 Negative and A2 Positive
Cardiac Artifact
11. Generalized across the scalp, comprises high frequency, polyphasic potentials
with a duration that is shorter than EKG artifact
Pacemaker artifact
5 Year old
male with
h/o
arrhythmia
13. Mechanical artifact from the heart arise through the circulatory pulse
Electrode artifact - occurs when an electrode rests over a vessel
Periodic slow wave with a regular interval - follows EKG artifact’s peak by about 200msec
Most common over frontal and temporal, less common over occipital
How to Identify:
Applying pressure on electrode alters its appearance on the EEG
Pulse Artifact
17. Mechanical cardiac artifact
Results from slight movement of the head or body that occurs with cardiac contraction
Similar to pulse artifact but is more widespread
May involve one or few electrodes – due to electrode lead movement
Biposterior electrodes if movement of the head on the pillow
Occasionally can be generalized
How to resolve:
Reposition the head
Ballistocardiographic Artifact
20. Types:
1. Electrode pop
2. Electrode contact
3. Electrode/lead movement
4. Perspiration
5. Salt bridge
6. Movement artifact
21. Usually manifest as one of two disparate waveforms, brief transients that are limited to
one electrode and low frequency rhythms across a scalp region
Due to spontaneous discharging of electrical potential present between the electrode or
its lead
Electrode pops - reflect the ability of the electrode and skin interface to function as a
capacitor and store electrical charge across the electrolyte paste or gel that holds the
electrode in place
With the release of the charge there is a change in impedance, and a sudden potential
appears in all channels that include the electrode
Sometimes more than one pop occurs within a few seconds
Characteristic morphology - very steep rise and a more shallow fall
Electrode Pop
23. Produces artifact with a less conserved morphology than electrode pop
Poor contact produces instability in the impedance, which leads to sharp or slow waves
of varying morphology and amplitude
These waves may be rhythmic if the poor contact occurs in the context of rhythmic
movement, such as from a tremor.
Poor electrode contact or lead movement
27. Lead movement has more disorganized morphology that does not resemble true EEG activity
Often includes double phase reversal (without consistency in polarity that indicates a cerebrally
generated electrical field)
Lead Movement
Artifact
29. Seen due to smearing of the electrode paste between electrodes or presence of
perspiration across the scalp
Forms an unwanted electrical connection between the electrodes forming a channel
Perspiration artifact
- manifests as low amplitude
- undulating (smooth) waves
- duration is typically greater than 2 sec
Slat bridge artifact
- lower in amplitude
- not wavering with low frequency oscillation
- typically include only one channel
It may appear flat and close to isoelectric
Salt Bridge and Perspiration
Artifact
34. External devices produce EEG artifact through the electrical fields they generate or through
mechanical effects on the body
Commonly due to the alternating current present in the electrical power supply
Medium to low amplitude and has the monomorphic frequency - 60 Hz in North America and
50 Hz in much of the rest of the world
May be present in all channels or in isolated electrode with poorly matched impedances
36. Electrical noise may also result from falling electrostatically charged droplets in an IV drip
In-phase activity
A spike like EEG potential results, which has the regularity of the drip
EEG
37. Ventilators and circulatory pumps produce artifacts with slower components than other
electrical devices
Resemble ballistocardiographic or other electrode artifact
Monomorphic frequency with fixed interval
Slow wave or a complex including a mixture of frequencies superimposed on a slow wave
Exceptions to typical pattern – High frequency oscillator
Mechanical devices
42. Frequency is higher than that of clinical EEG and too fast to be visually estimated
Without filtering, EMG artifact usually has a more disorganized appearance because the
individual myogenic potentials overlap with each other
Occasionally, individual potentials are discernible.
Duration of EMG artifact varies according to the duration of the muscle activity; a second to an
entire EEG recording.
Artifact occurs most commonly in frontal and temporal electrodes
Muscle Artifact
66. 1
2
1
1 1
2
Conjugate EM:
Vertical
Horizontal
Out of phase
EEG
In-phase
Out of phase
In-phase
Detect all EM
Out of phase
In-phase
EEG = EM
EEG = vertical EMMisses low
amplitude EM
Good because:
Bad because:
82. Sympathetic Skin Response
(Galvanic Skin Response)
Skin potential medicated by unmyelinated cholinergic sympathetic fibers
Changes in electrical properties of skin during sweating, sensory stimulation or
emotional stress
Appear as long lasting potential of abrupt onset with a monophasic, biphasic or
triphasic morphology.
Most commonly from frontal but may be diffuse
Confirmation: extracranial recording from palm of the hand