Also called arrays, montages are combinations of multiple derivations (electrode recording combinations).
What are the 2 different types of montages that are used?
1) Monopolar (Referential) 2) Bipolar
What are some types of referential montages that are used?
1) Common average reference 2) Cz reference 3) Ear reference (contralateral or ipsilateral)
What are some types of bipolar montages that we use?
1) Longitudinal (anterior to posterior) 2) Transverse (left to right)
What is a referential montage?
A series of derivations in which the same electrode is used in input 2 of each amplifier.
How are potentials localized on a referential montage?
What is important about the placement of the reference?
The reference must be placed in an electrically quiet spot on the patient. If placed in an electrically busy area, there will be contamination of all the other leads.
What is reference contamination?
This occurs when the reference is no longer neutral, meaning it has been obscured with an overriding potential, whether mechanical or biological.
What are the disadvantages to a CAR montage?
1) Contamination 2) Distortion of brain signals - if the CAR becomes part of the electrical field, the electrodes nearest will cancel out and those farthest will give false localization.
What is the advantage of a CAR montage?
Equal interelectrode distances between the two hemispheres.
What is the advantage of a Cz reference montage?
Equal interelectrode distances.
What is the disadvantage of a Cz reference montage?
Can't be used when the patient is asleep. Sleep potentials come from the vertex and therefore would contaminate all other leads with sleep potentials.
What are the advantages to an ear reference montage?
1) Little to no cancellation when using a contralateral ear montage due to long interelectrode distances. 2) Long interelectrode distances allow potentials to appear higher amplitude than usual, making it easier to read.
What are the disadvantages to an ear reference montage?
1) Contamination with EKG potentials. 2) Cancellation (when ipsilateral) due to reference becoming a part of the electrical field.
What is a bipolar montage?
A montage that consists of an adjacent pair of electrodes of the 10-20 system of electrode placement.
How are potentials localized on a bipolar montage?
What is an advantage of a bipolar montage?
Best for analysing low to medium amplitude waveforms that are highly localized.
What are some disadvantages of a bipolar montage?
1) Cancellation 2) End of chain
What is cancellation?
Cancellation is when two inputs are of equipotential, therefore, producing a flat line on the EEG. This occurs because the differential amplifier filters out potentials that are common between two inputs.
What is the end of chain phenomenon?
This occurs when potentials appear with greatest voltage in the last electrode in the chain of electrodes producing waveform deflections that are all in the same direction (no phase reversal).
What is a transverse bipolar montage?
This montage has derivations going from left to right across the head. Channels are usually arranged from front to back.
What are the advantages of a transverse montage?
1) Easy comparison of left and right hemispheres. 2) Easy comparison of amplitude gradient between anterior and posterior head.
What is a longitudinal bipolar montage?
This montage has derivations arranged from anterior to posterior.
What is a laplacian montage?
This montage is similar to the common average reference, but instead on refers one electrode to the nearest neighbouring electrodes. Creates a local weighted average reference.
What is the disadvantage of a laplacian montage?
Edge electrodes are not symmetrically surrounded by other electrodes and therefore, cannot be uniform for all input 1 electrodes.
What is an average reference montage?
This montage adds all the activity of all electrodes together in input 2 at every amplifier to serve as a reference for each of the individual electrodes in input 1.
What is the disadvantage of an average reference montage?
If one electrode has a much higher potential than the rest of the electrodes, it will contribute much more to the reference causing contamination of the EEG.
What can prevent contamination in an average reference montage?
More electrodes contributing to the reference.
What is a weighted average montage?
This montage is similar to an average reference montage except the electrodes in input 2 do not contribute equally to the reference and input 1 is not included in the reference. Electrodes closer to input one contribute more the reference than those further away.
What is the advantage of a weighted average montage?
It reduces the possibility of a widespread activity contaminating the reference.
What are the 3 good montage requirements?
1) Left hemisphere derivations over the right. 2) Derivations are organized in an anatomical order (ant to post). 3) Common electrode reference montages should have long interelectrode distances.