By: Andrew Ehrenberg, BS, R. EEG T., CNIM
Special Interest Section Leader: Innovative Technologies & Practices
The September 2019 edition of the Journal of Clinical Neurophysiology, the official publication of the ACNS, focuses on research around different aspects of montages. While not covered here, I highly recommend you read some of the original research, covering topics such as prognostics post cardiac arrest in children and recovery prediction in adults, as well as an article on reporting of EEG in refractory status epilepticus. This edition, in particular, is one I think has some incredibly practical information for technologists, and such is the focus of this Tech Tips article for the ASET Spring Newsletter. While I will present materials referencing each, I highly recommend this issue of JCN to be one that technologists look up, include in their library and read in depth. At the conclusion of the discussions of the montage articles, hyperlinks to the relevant ACNS guidelines for electrode names and montages are included.
When asking technologists about their use of varying montages during EEG recording, often the response, “these are used per lab protocol” is fairly common. However, knowledge and understanding of the different types of montages and why they are used is incredibly beneficial for technologists. The first article in this issue of JCN, “Overview of EEG Montages and Principles of Localization” presents a very nice review of montage basics as well as an easy to use three-step guide to localization.
Montaging, the configuration of electrode derivations, should not be a random selection, but should be configured based on a logical arrangement of international 10–20 electrode locations (Acharya & Acharya, 2019) where channels are intuitively aligned and contributory. Many times, one montage alone is insufficient for adequate evaluation of activity, or at the least does not give the full picture. Knowledge of the different montages, with each having various strengths and limitations (Acharya & Acharya, 2019), allows for a more comprehensive evaluation of activity when seen. Channels are usually arranged front to back, and in the US, left sided are displayed above right (Acharya & Acharya, 2019). As stated in this article, “In general, the goal of montages for routine scalp EEG are to (1) display activity over the entire head, so that no activity is missed, (2) compare activity on the two sides, to provide lateralizing information, and (3) to localize activity to a specific brain region, if possible.”
Montages, in general, can be classified in accordance with consideration of electro–anatomical arrangement in unpaired, paired channel (i.e., FP1–Cz, Fp2–Cz), and paired group (LRLR or LLRR) (Acharya & Acharya, 2019). Each of these has its own strengths and limitations related to voltage and appearance of asymmetry. These can be spatially arranged in longitudinal or transverse, in “bipolar, referential, common average, or Laplacian.” (Acharya & Acharya, 2019).
- Bipolar montages are arranged in chains, where input 2 of one channel is input 1 of the next (i.e., Fp1–F3, F3–C3) where both electrodes are considered active. Localization is good in highly focal phenomenon, where the classic phase reversal is used, and to minimize artifact, but is relatively limited in widely distributed fields or to determine voltage (Acharya & Acharya, 2019).
- Referential montages use a non-active input 2 (as non-active as possible) as a common to all electrodes or to one side of the head. A limitation is inherent in the relatively inactive nature of the reference as some references may contain artifact (EKG A1/A2) or cerebral activity (Cz, seizure focus). Referential montages are strong in determination of “shape and amplitude of the waveforms” (Acharya & Acharya, 2019).
To overcome some of the limitations in referential montages (artifact, close to focality of activity) Average References are sometimes used. In these cases, multiple electrodes are averaged together to minimize interfering activity common to only a few electrodes. In addition, Laplacian montages, where the nearby electrodes only are averaged (i.e., Hjorth methodology), can provide good field potential and highly localized activity, however, limitations of the Laplacian model are assumptions of equidistance and those without four equidistant neighbors (lateral circumference) (Acharya & Acharya, 2019).
Montages are arranged in such logical and contributory fashion to aid in localization of activity. In general the assumption is made that activity is from a single dipole oriented perpendicular to the surface, however this is an overgeneralization in many cases (Acharya & Acharya, 2019). Acharya and Acharya present a 3-step approach to localizing activity. These steps are, (1) determine the montage as bipolar or referential, (2) identify the negative phase reversal, and (3) make an educated guess about the location based on physiology (or pathophysiology). The authors also mention that it is important to note the non-occurrence of phase reversal, due to of “end of chain” in a bipolar montage or in referential montages where phase reversals likely indicate involvement of the ‘inactive’ reference.
The second article in the September 2019 issue of JCN, “Montages for non-invasive recordings” goes in depth on many of the aspects presented in the first overview article. Examples and illustrative graphics are well presented by Kutluay and Kalamangalam (Kutluay & Kalamangalam, 2019) and also go into greater depth regarding the underlying physiology and pathophysiology. One interesting and important point they make is that to truly appreciate and use different montages, “demands basic understanding of the properties of electrical fields” and “the behavior of neurophysiological potential generators” (Kutluay & Kalamangalam, 2019). Some general principles are presented: One important point is that referential montages are in fact a type of bipolar recording as the reference electrode is not quiet nor “zero”; no location on the head is truly non-active, and therefore, the choice of an appropriate reference electrode for the activity being examined is essential (Kutluay & Kalamangalam, 2019). A second point, well made, is about the actual anatomical and physiological basis of scalp EEG activity. While we know that a large group of neurons (six to 10 square centimeters) are required to generate recordable activity at the scalp, the activity is likely being generated at the part of the sulcus nearest the scalp (crown), and activity in the sulci or from tangential dipoles (versus radial to surface) is usually not as recordable (Kutluay & Kalamangalam, 2019).
Illustrations are provided within the text for some epileptic activity, namely temporal lobe (TLE), frontal lobe (FLE), centrotemporal, occipital and generalized. While I will discuss some points related to each, I would strongly recommend these sections for full review, as the graphics contribute significantly to the material.
- With TLE, a lateral temporal focus can present well with standard electrodes, but additional electrodes have been found to be helpful with mesial structures, namely foramen ovale, nasopharyngeal and sphenoidal electrodes, with less invasive T9/T10 of some benefit (Kutluay & Kalamangalam, 2019).
- With FLE, the authors note the benefit from categorizing activity into “(1) the frontal pole, including the orbitofrontal area, (2) the lateral frontal lobe, and (3) the medial frontal lobe” (Kutluay & Kalamangalam, 2019). With FLE, traditionally longitudinal bipolar and referential montages might need the addition of transverse, with ear reference versus Cz recommended, as well as the possible addition of 10–10 electrodes to better identify and differentiate field and localization (Kutluay & Kalamangalam, 2019).
- For centrotemporal and occipital seizures, appropriate reference selection as well as the benefit of a circumferential montage in occipital cases (Kutluay & Kalamangalam, 2019).
- In generalized epilepsies, longitudinal bipolar montages are recommended with referential limited to linked ear references for field distribution (Kutluay & Kalamangalam, 2019).
In “Montages for Invasive Monitoring,” Pickard and Skidmore discuss both subdural grids and strips, as well as the increasingly common stereo EEG (sEEG). In general, they note intracranial montages are usually patient–specific, and electrode naming (while the paradigm might be variable between facilities) should be consistent within an institution and included in the EEG report for outside review (Pickard & Skidmore, 2019). They present a naming convention, with anatomical naming of three to four letters. First letter denoting the hemisphere (L or R) and the next two to three denoting the anatomical structure. For grids and strips, letters A, B, C, etc., denote anterior to posterior relative location, with a possible addition of G or S to denote grid or strip. With sEEG, they recommend naming based on terminal (or contact 1) location for the electrode (Pickard & Skidmore, 2019). Graphics and examples of both grids and strips, as well as sEEG are presented. They present a sEEG naming convention of commonly implanted electrodes used at their facility (as well as many European centers) including alphabetic identification (A=amygdala, B=hippocampal head, etc.) and two letter abbreviations for others (OF = orbitofrontal, Ba–Bc for Brocas area structures, etc.) (Pickard & Skidmore, 2019).
Invasive montages in referential and bipolar are also discussed. In particular, for intracranial EEG they present the comparison of different reference selections and the importance of understanding limitations of each, such as artifact with scalp leads, and possible cerebral activity using intracranial leads (Pickard & Skidmore, 2019). Bipolar configurations allow highly localized examination of focal sources (~1 square centimeter) but it is noted that widely distributed activity can be attenuated as common mode.
For montage creation, the authors recommend beginning with the most anatomically anterior and progressing to the most posterior. An example of an implanted patient is given with the subsequent resulting montage. Important recommendations are that it is usually numerically linear, but end of chain must be carefully avoided. In addition, the consideration of multiple anatomical areas covered by one (large grid, for example) electrode are examined, as well as the importance of verification and integration of the post–operative imaging to ensure anatomical location in montage creation (Pickard & Skidmore, 2019).
Lastly, the article “Pediatric Montages in Clinical Practices” by San Juan, et al., reviews and summarizes montage considerations in neonates, infants and children. Montages of these patient populations follow many of the same basics of adult EEG, however “special considerations are needed to obtain optimal diagnostic yield in pediatric patients” (San Juan, Avila Ordonez, Munoz Montufar, Hortiales, & Anschel, 2019). In electrode selection and montaging, it is important to consider the reduced size of an infant’s head, and the possible need for physiologic recording channels.
For neonates and young infants, a recording that emphasizes polygraphic variables is recommended, with 12 EEG and mastoidal references, and ECG, respiration, EOG, and EMG (San Juan, Avila Ordonez, Munoz Montufar, Hortiales, & Anschel, 2019). The authors explore various techniques and equipment options for recording respiratory activity, and eye movements (EOG) in this article. There is discussion of using full 10–20 versus reduced arrays (double distance), with a comparative sensitivity and specificity in generalized seizures (96.8% and 100%) and background abnormalities (87% and 80%) (San Juan, Avila Ordonez, Munoz Montufar, Hortiales, & Anschel, 2019).
The authors, in line with ACNS recommendations, recommend using the full international 10–20 electrode positions for children, but suggest that other montages useful for younger children can be beneficial in examining EEG (i.e., transverse and double–distance based longitudinal bipolar). Example montages provided are (San Juan, Avila Ordonez, Munoz Montufar, Hortiales, & Anschel, 2019):
- Bipolar transverse montage as: Fp1–Fp2, F7–Fz, F3–F4, Fz–F8, T3–Cz, C3–C4, T5–Pz, P3–P4, Pz–T6, and O1–O2; or
- Bipolar longitudinal montage as: F7–T5, Fp1–C3, F3–P3, C3vO1, Fz–Pz, F8–T6, Fp2–C4, C4–O2, and F4–P4.
Lastly, special situations are presented in this article, such as emergent EEG, ambulatory EEG, and a type of quantitative trending amplitude-integrated EEG. In emergency EEG, it is recommended that while some use reduced 10–electrode applications, validation is needed before its use can be recommended (San Juan, Avila Ordonez, Munoz Montufar, Hortiales, & Anschel, 2019). In ambulatory EEG, while there are several montages that can be used, the authors provided one suggestion of: F3–F7, F7–T3, T3–T5, P3–C3, F4–F8, T4–T6, and P4–C4. One limitation of this montage, they present, is its diagnostic and prognostic yield (San Juan, Avila Ordonez, Munoz Montufar, Hortiales, & Anschel, 2019). Amplitude-integrated EEG is presented in a single (P3–P4) or a two–channel configuration (C3–P3, C4–P4; or F3–P3, F4–P4) with differing sensitivity. In closing, the authors made a very important point for technologists:
“…in the age of digital EEG recording, it is electrode placement rather than recording montage, that is most important.” (San Juan, Avila Ordonez, Munoz Montufar, Hortiales, & Anschel, 2019).
Lastly, I will leave you with references to the ACNS guidelines with a recommendation to review the ones related to electrode names and montages:
Guideline 1 “Minimum Technical Requirements for Performing Clinical EEG”
Guideline 2 “Guidelines for Standard Electrode Position Nomenclature”
Guideline 3 “Proposal for Standard Montages to be Used in Clinical EEG”
Guideline 5 “Minimum Technical Standards for Pediatric EEG”
Acharya JN & Acharya VJ. (2019). Overview of Montages and Principles of Localization. Journal of Clinical Neurophysiology, 325–9.
Kutluay E & Kalamangalam GP. (2019). Montages for Noninvasive EEG Recording. Journal of Clinical Neurophysiology, 330–6.
Pickard AA & Skidmore CT. (2019). Montages for Invasive Monitoring. Journal of Clinical Neurophysiology, 337–44.
San Juan DO, Avila Ordonez MU, Munoz Montufar JP, Hortiales SS, Anschel DJ. (2019). Pediatric Montages in Clinical Practice. Journal of Clinical Neurophysiology, 345–8.