EEG Montages

Summary

This video script delves into the intricacies of EEG interpretation, emphasizing the importance of understanding EEG montages. It introduces various montage types, including bipolar, common reference, and average reference montages, each with its strengths and weaknesses. The script explains how these montages are used to detect focal or broad abnormalities in EEG readings, with a focus on the limitations of each method. The key takeaway is the necessity of using multiple montages for comprehensive EEG analysis, as no single montage is ideal for all scenarios.

Takeaways

• 🧠 Understanding EEG Montages is crucial for interpreting EEG data, as they represent different ways of viewing the EEG output.
• 🔌 The foundation of EEG is the differential amplifier, which emphasizes that EEG output is always relative.
• 📊 Bipolar Montages compare a single EEG electrode tracing to its adjacent neighbor, useful for detecting focal abnormalities.
• 🌐 Anterior Posterior Bipolar Montage arranges electrodes as if viewing the top of the patient's head, facilitating the identification of left and right hemisphere activities.
• 🛤 Transverse Bipolar Montage focuses on the center of the head, beneficial for observing sleep transients that are maximal at the head's center.
• 🔄 Common Reference Montages compare each electrode's signal to a single common reference, such as CZ, to detect broadly distributed abnormalities.
• 🎚️ Ipsilateral Mastoid Montage compares electrode positions to the contralateral mastoid, useful for distant reference comparisons.
• 📊 Common Average Reference Montages compare each position to the average of the rest of the head, but are prone to reference contamination.
• 🌀 LASSAN Montage compares each electrode to an average of its nearest neighbors, excellent for identifying focal discharges but less effective for broad fields.
• 🔍 Distant Reference Montages, like the Ipsilateral Mastoid, can provide high contrast for broad electrical fields, enhancing the visibility of certain abnormalities.
• 📚 The choice of Montage depends on the type of EEG abnormality being investigated, and experimenting with different montages can improve diagnostic accuracy.

Q & A

What is the fundamental principle behind EEG interpretation?

The fundamental principle behind EEG interpretation is the differential amplifier, which means that EEG output is always relative.

What are EEG montages and why are they important?

EEG montages are different ways of looking at EEG data, which are crucial for interpreting the EEG signals by comparing electrode tracings in various configurations.

How does a bipolar montage differ from a common reference montage?

A bipolar montage compares a single EEG electrode tracing to its adjacent neighbor, while a common reference montage compares the signal at every electrode position on the head to a single common reference point.

What is an anterior posterior bipolar montage and how is it arranged?

An anterior posterior bipolar montage is arranged as if looking at the top of the patient's head while they are facing rightward, starting with the left temporal chain and ending with the right temporal chain.

What is the purpose of a transverse bipolar montage?

A transverse bipolar montage is arranged to focus attention on the center of the head, which is particularly useful for observing sleep transients that are maximal at the center of the head.

How does the common reference montage with CZ as the reference work?

In a CZ common reference montage, every electrode on the head is compared to CZ, generating a chain that helps in identifying abnormalities that are broadly distributed across the head.

What is reference contamination and how can it affect EEG readings?

Reference contamination occurs when an electrode's potential is influenced by a strong electrical field far from it, leading to misleading findings that do not reflect true electrical phenomena.

What is the lassan montage and how does it differ from other montages?

The lassan montage compares one electrode position to an average of its nearest neighbors, which is excellent for focal discharges but not for broadly distributed abnormalities.

Why might a distant reference montage be chosen over a common average reference montage?

A distant reference montage might be chosen when dealing with broad electrical fields to maximize the contrast between the area of interest and the reference, making the abnormalities more apparent.

What are the limitations of the lassan montage when dealing with broad electrical fields?

The lassan montage, which compares one electrode to only its closest neighbors, may not show much difference in broad electrical fields, leading to a flattened appearance that makes it difficult to identify the abnormalities.

How can experimenting with different montages enhance EEG interpretation?

Experimenting with different montages allows for a better understanding of their strengths and weaknesses, helping to choose the most appropriate montage for specific abnormalities and improving the accuracy of EEG interpretation.

Outlines

00:00

🧠 Understanding EEG Montages

This paragraph introduces the concept of EEG montages, which are essential for interpreting electroencephalogram (EEG) readings. Montages are different ways of viewing EEG data, with the most common being bipolar montages. These compare a single EEG electrode's signal to its adjacent neighbor. The video will cover five montages, starting with the anterior-posterior bipolar montage, which involves comparing electrodes in a chain along the temporal aspect of the head. The montage is arranged to mimic the view of the patient's head from above, with left hemisphere electrodes shown above right hemisphere electrodes. Another discussed montage is the transverse bipolar montage, which focuses on the center of the head, useful for observing sleep transients. The paragraph emphasizes the importance of understanding the differential amplifier as the basis of EEG and the relativity of EEG output.

05:02

🔍 Dealing with Reference Contamination in EEG

The second paragraph delves into the concept of reference contamination in EEG readings, particularly when using average reference montages. It uses the analogy of comparing an average-sized man to a group where one outlier skews the perception of average height. This concept is applied to EEG, where a high-voltage field in a limited area can skew the average voltage of the rest of the head, leading to misleading interpretations. The paragraph introduces the LASSAN montage as a method to mitigate reference contamination by comparing each electrode to the average of its nearest neighbors, thus providing a more accurate representation of focal electrical activity. The LASSAN montage is contrasted with the common average reference montage, highlighting its advantages in identifying focal discharges without the distortion caused by reference contamination.

10:03

📊 Selecting the Right EEG Montage for Different Abnormalities

The final paragraph discusses the importance of choosing the appropriate EEG montage based on the type of abnormality being observed. It explains that while bipolar montages are versatile for general screening, they may not be the best for focal or diffuse findings. Common reference montages are effective for broad abnormalities, especially when using a distant reference, but they are less effective for focal discharges. The average reference montage is versatile but susceptible to reference contamination. The LASSAN montage excels at identifying focal discharges but struggles with broad abnormalities. The paragraph concludes by emphasizing the importance of experimenting with different montages to understand their strengths and weaknesses and to never rely solely on one montage for EEG interpretation.

Keywords

💡EEG

EEG stands for Electroencephalogram, which is a diagnostic tool used to measure the electrical activity of the brain. In the context of the video, EEG is central to understanding brain function and interpreting the brain's electrical signals through various montages. The video discusses how different montages can affect the interpretation of these signals.

💡Montage

In EEG interpretation, a montage refers to the arrangement of electrodes on the scalp and how their signals are displayed. The video explains that different montages can highlight different aspects of brain activity, with examples including bipolar, common reference, and average reference montages.

💡Differential Amplifier

A differential amplifier is an electronic device that amplifies the difference in voltage between two input signals. In the video, it is mentioned as the basis of EEG, emphasizing that EEG output is always relative, comparing the voltage difference between electrodes.

💡Bipolar Montage

A bipolar montage is a type of EEG display where each electrode's signal is compared to its adjacent neighbor. The video describes the anterior posterior bipolar montage, which is arranged as if looking at the top of the patient's head, and is used for systematic analysis of brain activity.

💡Common Reference Montage

This montage type compares the signal at each electrode to a single common reference point, such as CZ. The video explains how this montage can be useful for broadly distributed abnormalities but may not be as effective for focal findings.

💡Ipsilateral Mastoid Montage

An ipsilateral mastoid montage is a specific type of common reference montage where all electrodes on one side of the head are compared to a single electrode on the mastoid process of the same side. The video suggests that this montage can provide a high contrast for broad electrical fields.

💡Reference Contamination

Reference contamination occurs in EEG interpretation when the chosen reference electrode influences the appearance of the signal, potentially leading to misleading results. The video uses the example of an average reference montage to illustrate how this can affect the interpretation of vertex waves.

💡Lassan Montage

The Lassan montage is a type of montage where each electrode's signal is compared to the average of its nearest neighbors. The video highlights its effectiveness in identifying focal discharges without reference contamination but notes its limitation with broadly distributed electrical discharges.

💡Phase Reversal

Phase reversal is a phenomenon in EEG interpretation where the waveform changes polarity at a specific electrode, indicating the maximum point of an electrical field. The video uses this concept to explain how to identify the focus of an electrical field in a montage.

💡Electrode

Electrodes are the sensors placed on the scalp to record the brain's electrical activity. The video discusses how the arrangement and comparison of electrode signals in different montages can affect the visualization and interpretation of brain activity.

💡Focal Discharges

Focal discharges refer to localized areas of abnormal electrical activity in the brain. The video explains that certain montages, like the Lassan montage, are particularly effective at highlighting these focal points without the interference of reference contamination.

Highlights

Understanding EEG montages is key to interpreting EEG data.

EEG output is always relative due to the use of differential amplifiers.

Bipolar montages compare a single EEG electrode tracing to its adjacent neighbor.

The anterior posterior bipolar montage is a common method for systematic EEG analysis.

Montage chains are created by connecting electrodes along the head's temporal aspect.

Institutional variations exist in the arrangement of montage chains.

The transverse bipolar montage focuses on the center of the head, useful for sleep analysis.

Common reference montages compare each electrode to a single common reference.

CZ Montage is a common reference montage used to detect broadly distributed abnormalities.

Ipsilateral mastoid montage compares all electrodes on one side of the head to the mastoid.

EEG montages can be organized in various ways to suit different analytical needs.

Common average reference montages compare each electrode to the average of the rest of the head.

Reference contamination can occur in average reference montages, leading to misleading findings.

Lassan montage compares one electrode position to an average of its nearest neighbors.

Lassan montage is effective for focal discharges but not for broad electrical fields.

Distant reference montages can provide maximal contrast for broad electrical fields.

Different montages have strengths and weaknesses; it's important to use the appropriate one for the analysis.

Experimentation with montages is crucial for understanding their impact on EEG interpretation.

Transcripts

one of the keys to a deeper

understanding of the interpretation of

EEG is understanding the basis of EEG

montages one of the things we have to

remember is that the basis of EEG is the

differential amplifier we have to keep

at the top of our minds the fact that

EEG output is always relative for this

reason we have a variety of ways of

looking at EEG which we call

montages we will go through four of

these five montages in this video video

the most commonly used montages in EEG

interpretation are bipolar montages a

bipolar montage is based on the

principle of comparing a single EEG

electrode tracing to its adjacent

neighbor one of the commonest bipolar

montages is the anterior posterior

bipolar Montage going through this

systematically we may start by comparing

fp2 to f8 this will generate a tracing

which we call a channel this can also be

called a derivation we will then

continue posteriorly starting at f8 and

going to T8 generating another Channel

or derivation we will continue

posteriorly to generate a chain of

electrodes along the temporal aspect of

the head eventually we put a number of

these chains together to generate a

montage this is an example of an

anterior posterior bipolar Montage going

through this systematically we can see

that this particular montage is AR

arranged as if we are looking at the top

of this patient's head while they are

facing rightward first we have the left

temporal

chain then the left parasagittal chain

then a midline chain in the middle then

a right parasagittal chain and finally a

right temporal chain there are

institutional variations in the

arrangement of these chains within a

montage however in general left

hemisphere electrodes are shown on top

of right hemisphere electrodes another

type of bipolar montage is the

transverse bipolar Montage this

transverse bipolar montage is arranged

as if we are looking at the top of the

head and the patient is facing

upward first we have a short chain over

the forehead then a longer chain

extending from left to right over the

front of the head then a very long chain

extending from ear to ear left to right

then a parietal chain and finally a

short Pari occipital chain the

transverse bipolar montage is

particularly good at focusing one's

attention to the center of the head many

sleep transients are maximal at the

center of the head and so transverse

bipolar Montage can be useful at looking

at sleep the second type of Montage we

will discuss is the common reference

Montage in this situation we compare the

signal at every electrode position on

the head to a single common reference

one of the most common common reference

montages is the CZ Montage in this

situation we compare every electrode on

the head to CZ the first channel or

derivation we will look at is f8 to ZZ

we then move posteriorly comparing T8 to

ZZ and then finally p8 to CZ in this way

we generate a chain much in the same way

as we generated a chain in the anterior

posterior bipolar Montage this

particular CZ reference montage is

arranged in the same way as the anterior

posterior bipolar Montage first we start

with a left temporal chain then a left

parasagittal chain then a midline chain

then a right parasagittal chain and then

a right temporal chain there are many

other types of common reference montages

and in fact we can compare signals on

the head to any reference we like for a

reference that is not very close to the

middle of the head another option is the

ipsilateral mastoid Montage you can see

here that all of the electrodes on the

left side of the head are being compared

to a single electrode attached to the

left mastoid we can also compare Electro

positions to the contralateral Mard

there is a tremendous amount of freedom

in organizing EEG montages and we will

discuss a little bit about why we might

choose looking at one Montage compared

to another the next type of montage is a

common average reference Montage in this

situation we compare the signal at each

position in the head to the average of

the rest of the head let's first look at

F7 what we would do in this situation is

compare F7 to the signal everywhere else

in the head divided by the number of

electrodes however because fp1 and fp2

are very susceptible to IM movement

artifact and because o1 and O2 are very

susceptible to head movement artifact

they are generally excluded from the

average reference Montage therefore we

are actually comparing each electrode

position to a smaller sample of of the

electrodes on the head one of the main

considerations with average reference

montage is a concept called reference

contamination one way to conceptualize

reference contamination is to look at

how we deal with averages in this

situation let's take an averaged sized

man if we compare him to a group of

other average sized men we will say that

he is of average height however if one

of the men in the comparison group is

larger than the other men then we would

come to the conclusion that our

gentleman is of below average height

this is actually the effect of the

outlier in the comparison group and has

nothing to do with our man who as we

said before is of average height in fact

this will make all other men within the

group appear to be of below average

height which can be very misleading now

let's look at how this might affect

findings on

EEG this is an anterior posterior

bipolar Montage during sleep in a young

person here we can see a number of

vertex waves maximal along the center of

the head if we look at these vertex

waves more carefully we see that the

maximum is at C3 CZ and

C4 therefore we have an electrical field

that looks something like this because

this is a very high voltage field in a

very limited area it acts like our very

tall man changing the average of the

rest of the head so that the average

over the entire head is is negative now

when we take an electro position that is

not involved in this field and compare

it to the average of the rest of the

head we can say that 01 in this example

is neutral and the average as we said

before is relatively negative for the

rest of the head therefore it will

appear like o1 is positive with respect

to the average of the rest of the head

o1 appearing electropositive is very

similar to the average men in our

previous example appearing small now if

we change to an average reference

Montage we can see that while it is true

that C3 C4 and CZ are relatively

electronegative there is also the false

impression that all other waveforms are

electropositive this is not a reflection

of any true electrical phenomenon but

rather an example of reference

contamination one way to get around

reference contamination is to use

something called the lassan Montage the

llan Montage is probably used least

often among eegs mainly because it is

very difficult to conceptualize this

Montage in general terms we are always

comparing one electrode position to an

average of its nearest Neighbors in this

example we would compare CZ to its four

closest neighbors we express this as a

first input which is CZ and a second

input which we call CZ Prime

CZ Prime is an average of the four

adjacent neighbors special consideration

has to be given to electrode positions

on the edge of the head where there are

fewer adjacent Neighbors when looking at

t7 for example we would derive t7 Prime

as an average of its three most adjacent

neighbors here is an example of how a

llan montage might be laid out again it

is laid out in a very similar way to our

other montages left over right with

temporal chains then parasagittal chains

then the midline let's take the example

of a right temporal sharp wave as seen

in this example on an anterior posterior

bipolar

Montage the sharp wave can be seen best

in the right temporal regions with some

extension into the right frontal regions

if we mapped out the field it might look

something like

this the phase reversal is at f8 and so

we would say that this field is likely

maximal at f8 and so we might include a

maximal negative Contour at f8 when we

look at this on average Montage we see

that indeed we do have maximal electr

negativity at f8 we also have a field

that extends to fp2 F4 and T8 which are

all involved in our theoretical field

but then we see a lot of electr

positivities in electrodes that are not

anywhere close to our field because of

reference contamination if we instead

change to the lassan Montage where we

are only considering most adjacent

neighbors we can see that the field is

very focal involving f8 with some

involvement of fp2 and T8 as we had

hypothesized before but no reference

contamination you can see here on the

left the lassan Montage and on the right

the common average reference Montage

with reference contamination seen best

in the left hemisphere the lassan

Montage has some limitations especially

with very broadly distributed electrical

discharges one example would be this

normal K complex during sleep again I

have shown it in an anterior posterior

bipolar Montage you can see that this K

complex has a very broad field extending

throughout much of the anterior aspect

of the head let's say that the field

involves this general area and is

approximately maximal in the frontal

Central region of the head because the

lassan Montage Compares One electrode to

only its closest Neighbors in an

electrical field that is very broad

there might not be much difference

between one electrode and its most

adjacent neighbors and we can see that

when we switch to the llan Montage for

this K complex things flatten out and it

is very difficult to see to summarize

llan montages are not good for broad

electrical Fields so we can ask the

question what if we choose a distant

reference a distant reference might have

the advantage of being very far from our

broad electrical field so that the

comparison between the activity within

our field and at the distance electrode

will be greatest one type of Montage

that we might want to consider is a

common reference Montage to the

ipsilateral mastoid process this is far

away from the area of interest and so we

would hypothesize that this would

generate the maximal amount of contrast

between our large area of interest and

the reference and we can see that when

we do apply the ipsilateral mastoid

Montage we get very high voltage

activity accentuating the K complex

again if we compare the ipsilateral

mastoid a distant reference Montage to

the lassan which Compares only adjacent

neighbors we can see a dramatic

difference in the electrical activity we

can also look at how different types of

montages can look at a broad area of

slowing in one hemisphere in this

example I have included A short segment

of a recording of a patient patient who

just had a seizure arising in the right

temporal region it is difficult to see

but there is some polymorphic Theta and

Delta activity in the right temporal

region again this has a very broad field

and if we were going to hypothesize the

field we might show it something like

this again when we use the lassan

Montage where we are comparing only to

closest adjacent neighbors it's very

difficult to see the slowing but we may

apply a distant reference Montage such

as the CZ montage and we can see a

dramatic change and the focal slowing is

much better seen in a short video it is

very difficult to give a comprehensive

view of the strengths and weaknesses of

different montages your ability to

recognize the relative strengths and

weaknesses of these montages will come

with experience and with

experimentation however we can make some

broad statements about the strengths and

weaknesses based on what we have looked

at so far a bipolar montage in

particular the anterior posterior

bipolar montage is used most frequently

for screening of EEG recordings because

it is very

versatile however it is not the best

Montage for either focal or diffus

findings and for this reason you have to

consider using other montages when you

read the common reference Montage as we

have shown can be very good for broadly

distributed abnormalities particularly

if you use an appropriate reference

distant to the area of Interest however

a common reference montage is not

particularly good at looking at focal

discharges and this is the main

limitation in using this as a screening

Montage the average reference montage is

generally very versatile and can also be

used as a screening Montage however it

is susceptible to reference

contamination as we have shown which can

lead to some misleading findings the

lassan Montage is extremely good at

looking at focal discharges but not very

good for broadly distributed

abnormalities

as I have said already I think it is

important to experiment with different

montages looking at different

abnormalities and comparing how they

look with one Montage compared to

another now that you have a good

understanding of the strength and

weaknesses of each of the montages I

hope that you understand that it is

important to never read EEG with just

one montage