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Module 6: Limitations to EEG (ICETAP)

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ICETAP EEG

on 11 January 2015

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Transcript of Module 6: Limitations to EEG (ICETAP)

Module 6: Limitations to EEG




Pros and Cons of EEG
Advantages of EEG
Disadvantages of EEG
Safe
Non-invasive
No discomfort to the patient
Affordable
EEG has very good temporal resolution below the millisecond scale
EEG measures the electrical potential difference across your scalp arising from current flow within the head and brain
Reflects activity of only superficial layers of the cortex
Input creates a vertically orientated electrical activity of pyramidal cells; a dipole
This consist of only 1/3 of cortical neurons, a very small proportion of active neurons
It is not known whether anesthetics induce unconsciousness by subcortical or cortical mechanisms, therefore it is not known if EEG truly analyzes CNS structures essential for unconsciousness.

EEG amplitude is very low and signals must be amplified before they are analyzed.
Requires low impedence to prevent artifacts

EEG is a non-invasive means of determining the physiological or functional state of the brain.
EEG has a wide range of clinical applications that range from diagnosis of epilepsy and brain tumors to the analysis of sleep and depth of anesthesia.
However there are limitations in its diagnostic usefulness that a clinician must keep in mind.

Introduction
Although the temporal resolution of EEG is superior to other imaging modalities, spatial resolution is poor. Why?

Limitations of EEG
Limit #1: Poor Spatial Resolution
EEG electrodes are separated from your brain by cerebrospinal fluid, scalp, and skull which blur the EEG.
Blurring is caused by volume conductor effects
Volume conductor consists of the volume between the closed cortical surface and the closed cortical surface

Example of a volume conductor:
Inner Sc surface is defined as being above the cortical surface

The EEG signals also must pass through the CSF, skull, and scalp (different tissues have different effects on volume conduction)
The amplitude of signals from the cortex are attenuated when they are volume conducted to the surface of the scalp.
The amplitudes are also effected by the location of the electrodes and interelectrode distance

Resistivity is more important than shape

The resistivities are not straightforwardly defined and vary with temperature and field frequency
Skull has the highest resistivity and has the largest effect of electric fields
Skull also varies over different lobes and has different resistivities over sutures
Skull has different layers within certain areas with different resistivities



Important characteristics in a head volume conduction model are its shape and resistivities of the tissues

Common model used is the 3D model consisting of 3 concentric circles representing skull, scalp, and brain (not the most accurate)

Now there are head models constructed for each individual (thru MRI/CT)
Caveat: The resistivities of tissues depends on type of tissue as well as thickness



3D model and how it fits the real anatomy

Spatial sampling in routine scalp EEG is incomplete, as significant amounts of cortex, particularly in basal and mesial areas of the hemispheres, are not covered by standard electrode placement
Poor Spatial Resolution is also due to electrode placement
To relate the EEG signals with their source in the cortex, the EEG inverse problem is used
There are infinite numbers of intracranial source configurations that can generate the same type of scalp distribution, therefore constraints must be placed on the source model.


The EEG Inverse Problem
1. Source can be modeled as a single dipole, the source is relatively small and comes from deep brain structures such as the thalamus and brainstem.
A limitation to this is that the number of dipole sources must be decided first before solving the inverse problem
2. The source can be distributed called source imaging methods, where the source consists of the whole brain volume

Two types of source models:

Increase the number of measurement electrodes
Studies have shown that increase in number to at least 128 improve accuracy of results
Spatial enhancement methods reduce blurring
Cortical imaging methods solve the potential difference or current density on the cortical surface using an imaginary layer
This model provides a much higher spatial resolution than scalp potential resolution
Laplacian derivation: assumes skull thickness and resistance are constant in all areas

Methods to Improve Spatial Resolution
Remember from module 4?

Limit #2: Artifacts
Artifacts are also important factors one must take into account when interpreting EEGs
Answer: Electrical signals detected by the EEG that are not of cerebral origin


What are artifacts?

Artifacts can be divided into two categories
Physiologic Artifacts:
Generated from the patient and arise from a source other than the brain

Extraphysiologic artifacts:
Arise from outside the body

1. Eye lid movement
2. Muscle activity: swallowing, talking, chewing
3. Sweating, movements of arms and legs
4. Myocardial activity: ECG cables running across EEG leads


Movement of eyelids or eyeballs causes a change in electric field that can be picked up from nearby electrodes
The eyeball can be considered a dipole with the positive pole in front (cornea) and negative pole in back (retina)
The movement of eyeballs or eyelids result in different artifacts
a. Lateral eye movements
b. Blink
c. Vertical eye movements


1. Eye movements
1a. Eye movements: Lateral movements
1b. Blink
Best seen in lateral frontal electrodes (F7=left F8=right)
Lateral eye movements show opposite polarity in electrodes F7 and F8



During Left lateral eye movement, the positive pole of the eye moves towards F7 and away from F8
one would see positive deflection in F7 and negative deflection in F8.
During Right lateral eye movement the positive pole of the eye moves towards F8 and away from F7
one would see positive deflection in F8 and negative deflection in
Ans: Right Lateral eye movement
F7 has a negative deflection and F8 has a postive deflection indicating the postive pole of the eye is moving towards F8 which is on the right

What lateral eye movement is represented here?

A blink causes the positive pole (cornea) to move closer to frontopolar (Fp1-Fp2) electrodes
Produces downward deflections
A blink artifact is characterized by a distinct positive plateau which then sharply drops off
Ans: The artifact is a blink artifact.  Note that the artifacts correlate with the rapid eye movement activity seen in the E1 and E2 leads.

What is the artifact noted in the F4 EEG lead?

During downward eye movement the positive pole (ie, cornea) of the globe moves away from frontopolar electrodes
Produces an upward deflection

The other source of artifacts comes from EMG potentials from muscles in and around the orbit.

1c. Vertical eye movements
Most monitors measure EEG from frontal electrodes. Frontal electrodes capture EMG too.
These artifacts are caused by jaw or facial movements associated with swallowing, chewing and talking
Signals most commonly picked up from temporalis and frontalis muscles
Muscle activity usually looks like short-lasting bursts with high frequency and amplitude
Parkinson’s and essential tremor disease can produce rhythmic 4- to 6-Hz sinusoidal artifacts that may mimic cerebral activity.

2. Muscle Activity
Muscle relaxants decrease the amount of muscle artifact

chewing artifact
The sodium and lactic acid released from sweat reacts with the metal electrodes producing slow baseline sways

3. Sweat Artifacts
Occur when the heart’s conduction system discharges are picked up by the EEG.
More common in patients with short and wide necks.
ECG artifact is recognized easily as recurrent sharp waves that synchronize with each QRS complex of the ECG.
Cardiac activity may also express itself as a pulse artifact, which occurs when an EEG electrode is placed over a pulsating vessel.

4. ECG Artifacts
1. Electrodes and Connections
2. Interference from surrounding electrical appliances
3. Interference from other people

1. Electrodes
Due to the following:
1. Artifact due to cable movement
2. Improperly fixed reference electrode during bipolar recording
3. High impedance >5K
4. Faulty insulation of electrode

Characteristic sharp, narrow waveforms due to abrupt impedance change.
Usually limited to a single electrode

2. Surrounding electrical appliances
Cautery Artifacts:
-Electrocautery uses electric current to cut or burn tissue
-Causes impressive EEG artifact (similar to electrocardiogram cautery artifact)
-Saturates EEG and produces a flat line in tracings

Interference from high-frequency radiation from radio, television, hospital paging systems, and other electronic devices can overload EEG amplifiers. The pens may deflect upward or downward to full excursion, and no EEG can be recorded.

The artifact produced by respirators varies widely in morphology and frequency. Monitoring the ventilator rate in a separate channel helps to identify this type of artifact.

Movement of other persons around the patient can generate artifacts, usually of capacitive or electrostatic origin. Avoid this type of artifact as much as possible.

3. Interference from other people
EEG has poor diagnostic specificity
can aid in localization, but not pathological disease
EEG can be normal in patients with obvious cerebral dysfunction, especially if only single recording is relied on.
Normal EEG never excludes any clinical condition.
EEG is sensitive to physiological variables such as level of awareness, blood-sugar level, acid-base equilibrium.
Such physiological changes are indistinguishable from certain pathological states and therefore can be misinterpreted.
Some normal patients have abnormal EEGs


Limit #3 : Specificity and Sensitivity
Specificity and Sensitivity
In psychiatric patients
Mental disorders of non-organic origin are associated with increases in abnormalities in the EEG. This makes it difficult in differentiating between organic and non-organic abnormalities.
Difficulty with psychiatric patients is that many drugs can effect EEG
Barbituates produce small amount of theta and delta activity
Chlorpromazine in low dosages disturb alpha activity
Electroconvulsive therapy makes it difficult to interpret EEG for a period of time as it gives persistent abnormalities.

Psychiatric patients
Epilepsy
A normal EEG does not exclude epilepsy, as around 10% of patients with epilepsy never show epileptiform discharges.
Secondly, an abnormal EEG demonstrating interictal epileptiform discharge (IED) does not in itself indicate that an individual has a seizure disorder, as IED are seen in a small percentage of normal subjects who never develop epilepsy, and IED may also be found in patients with neurological disorders which are not complicated by epilepsy.
Epileptiform activity is specific, but not sensitive, for diagnosis of epilepsy as the cause of a transient loss of consciousness or other paroxysmal event that is clinically likely to be epilepsy
EEG has relatively low sensitivity in epilepsy, ranging between 25–56%. Specificity is better at 78–98%.

A recent study asked if EEG can detect awareness in seemingly unaware patients with brain damage.
Patients in vegetative states were compared to those that are normal
-awareness evaluated by asking to follow commands
In normal patients, 1 out of 4 were not able to generate appropriate EEG responses.
Only three fourths of normal subjects showed signals resulting in a poor sensitivity in evaluating consciousness in vegetative patients.

Unconcious State
EEG has many limitations that one must be aware of.
Poor spatial resolution
Artifacts
Limitations in specificity and sensitivity

Summary

1. Cruse, D., Chennu, S., Chatelle,C., Beckinschtein, TA., Fernandez-Espejo, D., Pickard, JD., Laureys, S., Owen, AM., 2011. Bedside detection of awareness in the vegetative state; a cohort study, The Lancet. V. 378, p. 2088-94.
2. Srinivasan, R., 2006. Anatomical constraints on source models for high-resolution EEG and MEG dervived from MRI, Technol Cancer Res Treat. V. 5, p. 389-99.
3. Smith, S., 2005. EEG in the diagnosis, classification and management of patients with epilepsy, IJ Neurol Neurosurg Psychiatry. V. 76, p. ii2-ii7 .
4. Mashour, GA. 2010. Conciousness, Awareness, and Anesthesia. Cambridge University Press. New York, NY, 272p.
5. Fisch, B. J., 2009. Epilepsy and Intensive Care Monitoring: Principles and Practice. Demos Medical Publishing. New York, NY, 440 p.
6. Freye, E., Levy, J.V., 2005. Cerebral monitoring in the operating room and the intensive care unit: an introductory for the clinician and a guide for the novice wanting to open a window to the brain, Journal of Clinical Monitoring an Computing. V. 19, p. 1-76.
7. Srinivasan, R., 1999. Methods to improve the spatial resolution of EEG. IJBEM. V. 1, p. 102-111
8. Väisänen, O., 2008. Multichannel EEG Methods to Improve the Spatial Resolution of Cortical Potential Distribution and Signal Quality of Deep Brain Sources. Julkaisu, 741.

References
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