EEG Montages
Summary
TLDRThis 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
🧠 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.
🔍 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.
📊 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.
Mindmap
Keywords
💡EEG
💡Montage
💡Differential Amplifier
💡Bipolar Montage
💡Common Reference Montage
💡Ipsilateral Mastoid Montage
💡Reference Contamination
💡Lassan Montage
💡Phase Reversal
💡Electrode
💡Focal Discharges
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
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