Montages
Montages:[edit | edit source]
A montage is the arrangement of channels on the EEG display or tracing, and each channel consists of paired signal sources. When channel derivations are arranged into montages their sequence provides information that identifies of polarity and location of the cortical activity of interest. Montages can be created in any array, though the most useful ones will be those that allow for rapid visual interpretation of the EEG signals. There are two types of montages: bipolar and referential. The rules for interpretation for each type of montage are different based on the way they display the information.
Bipolar montages are created by making chains of sequential channels where each channel consists of a pair of individual electrodes. In order to convey spatial resolution, the chains of sequential channels can be arranged in anatomically relevant directions. A new chain starts when the sequence begins in a new location. The chains can run in an anterior to posterior direction (AP Bipolar Montage also called the double banana), or in a transverse direction across the head; whatever manner that gives useful localizing information.
Rule for interpretation of a bipolar montage: A phase reversal in a chain identifies an area where the changes in electrical potential are maximal relative to the surrounding signals. Phase reversals can either be positive or negative in polarity. When there is a negative phase reversal, the waveforms appear to point toward each other; when there is a positive phase reversal the waveforms appear to point away from each other. Phase reversals often indicate a region of interest electrophysiologically, showing a particular spatial point on the EEG where voltage changes are maximal and/or the spatial extent of voltage changes that surround it, its field.
In this hypothetical bipolar example, systematically evaluating each channel will allow us to make an interpretation of the entire chain and by using the polarity conventions of EEG display, localize electrical events in space.
- In the first channel where a is compared to b, there is no large net difference in their electrical potentials so the display shows a relatively flat line.
- When b is compared to c, there is a small downward deflection. This finding suggests that either b is slightly more positive than c or c is more negative than b.
- When c is compared to d, there is another small deflection downwards also suggesting that c is slightly more positive than d or that d is more negative than c.
- There is a large deflection upwards when d is compared to e. In this case, d is either much more negative than e or e is much more positive than d.
- When e is compared to f, there is a small upward deflection suggesting e is either slightly more negative than f or f is slightly more positive than e.
- Finally, in the last channel f and g are equipotential and the tracing appears relatively flat.
The conclusion from this analysis suggests that either there are two positive generators on either side of the chain or electrode d is the region of maximum negativity and the potential of interest. Considering that most cortical events of interest are negative in potential, this finding would be the most logical conclusion. The deflections between channels c-d and d-e represent a negative phase reversal.
Referential montages give the same electrophysiological information but present the information in a slightly different format. The voltage at each electrode is compared to a common or neutral electrode or to mathematical average of a group of electrodes. In a referential montage, the area of highest amplitude is of the most interest because it represents the region most negative or positive in polarity. If there is an apparent “phase reversal” there are two potential interpretations: 1) The reference electrode is active or is involved in the area of interest, or 2) there is a horizontal dipole and that both ends of the dipole are apparent on the montage.
Common referential montages:
Ipsilateral ear reference – Each electrode over the left hemisphere is compared to the left ear electrode; each electrode over the right hemisphere is compared to the right ear electrode.
Contralateral ear reference – Each electrode over one hemisphere is compared to the contralateral ear electrode.
A1-A2 reference – Each electrode is compared to an average of the A1 and A2 electrodes
Cz Reference – each electrode is compared to Cz
Average reference – Each electrode is compared to the average signal of a group of electrodes (usually those least affected by eye movement or muscle artifact
Laplacian anatomic average reference – Each electrode is compared to a weighted average of an immediately surrounding set of electrodes.
In this hypothetical referential example (which shows the same electrical event as the bipolar example), systematically evaluating each channel will allow us to make an interpretation of the entire EEG.
- In the first channel when a is compared with the reference electrode, there is no net difference between the electrical potentials so the display shows a relatively flat line.
- When b is compared to the reference electrode, there is also little net difference between the electrical potentials so the display also appears as a flat line. These findings suggest that neither a nor b is involved in the discharge of interest.
- When c is compared to the reference, there is a small deflection upwards, thus suggesting that c is slightly more negative than the neutral reference.
- There is a large deflection upwards when d is compared to the reference. This finding suggests that d is much more negative than the reference.
- When e is compared to the reference, there is a small upward deflection suggesting that e is slightly more negative that the reference.
- Finally, the last channel f is again equipotential to the reference electrode.
The conclusion from this analysis suggests that electrode d is the maximally negative electrode with a field that extends to the surrounding electrodes c and to a lesser extent e. This analysis of the same set of electrodes draws us to the same conclusion as when we the bipolar montage was analyzed systematically. A referential display also allows quantification of the events recorded since the voltage of each channel is being compared to a common reference.