Polarity Montages

Localization

Bassel Abou-Khalil, M.D.

I have no financial relationships to disclose that are relative to the

content of my presentation

Learning Objectives

to predict the appearance of potentials of negative or positive polarity in different montages

to localize EEG potentials in referential and bipolar recordings

to build montages that are logical; identify the advantages and disadvantages of bipolar and referential montages; select the best montages to characterize particular potentials

What is the polarity of the transient demonstrated below

A. Positive

B. Negative

C. Either positive or

negative

D. Neither

Fp1-F3

Where is the most likely source of this transient?

A. P3 and O1

B. O1

C. Fp1, C3, & P3

Fp1-F3

F3-C3

C3-P3

P3-O1

What is the most likely polarity of this transient?

A. Positive

B. Negative

C. Either positive

or negative

Fp1-F3

F3-C3

C3-P3

P3-O1

Where is the center of the field?

A. C3 and P3

B. C3

C. F3

D. Fp1

Fp1-F3

F3-C3

C3-P3

P3-O1

The EEG

The EEG measures potential differences between electrode positions on the scalp.

Factors affecting scalp EEG potentials

Size, depth, orientation of generator/dipole Conductive properties of intervening tissues Duration of discharge Degree of synchronization

– Synchronous discharge over 10 cm2 cortex surface area needed for spikes to show on scalp

Propagation pattern Recording technique

-

-

Dipole orientation

EEG at scalp Amplitude depends on:

– intensity of electrical potential – distance of potential – spatial orientation of dipole – resistance and capacitance of structures between

source and electrodes Amplitude may decrease with:

– increased impedance – decreased impedance resulting in current shunt

EEG at scalp

Potential changes are favored if they: – occur near the recording electrodes – are generated in a large area of tissue – rise and fall at a slow speed

Potentials generated at a distant site are rarely recorded, but those of high amplitude and low frequency may be transmitted through volume conduction

T1

10-10 system Combinatorial Nomenclature

Frontopolar Frontopolar

Mid-frontal Mid-frontal Midline frontal

Vertex Central Central

Parietal Parietal Midline Parietal

Occipital Occipital

Mid-temporal Mid-temporal

Anterior temporal Inferior frontal Anterior sylvian Fronto-temporal

Posterior temporal Posterior temporal

Anterior temporal Inferior frontal Anterior sylvian Fronto-temporal

Fp1

F7

F3 F4

F8

O1 O2

T7

Fp2

C3 Cz C4

Fz

T8

P4

P8

P3

Pz

P7

Zg1

Sp1

Zg2

Sp2

Inferolateral temporal

Inferomesial temporal

Inferolateral temporal

Inferomesial temporal

T1 T2

True anterior temporal

True anterior temporal

POLARITY- LOCALIZATION

Negative Positive

↑ ↓ ↓ ↑

B

C

-

+

Lüders Method

B

C

-

+

A

B

+

Lüders Method

MONTAGES- BIPOLAR

or midline

Longitudinal Bipolar Montage Vanderbilt arrangement

9

10

11

12

1

2

3

4

5

6

7

8

13

14

15

16

17

18

Transverse Bipolar Montage Vanderbilt- Arcs replacing single derivation chains

2

4 5 6 7

8 9 10 11 12 13

14 15 16 17

19

1 3

20 18

END OF CHAIN POTENTIALS

Vanderbilt “Additional” Montage

1

2 3

4

5

6

7 8

9

10

11

12

13

14

15

16

MONTAGES- REFERENTIAL

Referential Montages Vanderbilt Organization

1 2

3 4

5 6

7 8

9 10

11 12

13 14

15 16

17

18

19

Referential Montages Alternate Arrangement 1

1 6

2 7

3 8

4 9

5 10

11 14

12 15

13 16

17

18

19

Referential Montages Alternate Arrangement 2

4 12

5 13

6 14

7 15

8 16

1 17

2 18

3 19

9

10

11

References

Ipsilateral ear Linked ears Average Laplacian Vertex or other midline reference Balanced noncephalic reference

Considerations in choosing ear reference

Ideal for generalized abnormalities, which usually predominate in the midline. The ears are the least involved and usually provide the best fidelity.

EKG artifact may be prominent- linking the ears helps cancel out EKG artifact.

Ear reference can be contaminated with temporal lobe abnormal activity (sharp waves or slow activity)

Fp1

F3

C3

F7

T7 A1

P3

O1

Cz

Pz

Fz

P7

Fp1-A1 F3-A1 C3-A1 P3-A1 O1-A1

F7-A1 T7-A1 P7-A1

Fp1-A2 F3-A2 C3-A2 P3-A2 O1-A2

F7-A2 T7-A2 P7-A2 A1-A2

Active ear reference

Considerations in choosing average reference

Average reference is ideal for focal activity, particularly from the temporal lobe

Average reference can be improved by excluding affected electrodes

Average reference tends to get contaminated with generalized abnormal activity

Considerations in choosing vertex reference

Can emphasize/clarify temporal activity (particularly when average reference is affected by artifact).

Not a good choice during sleep (a lot of high voltage sleep activity is maximal in the midline) or to display activity that is generalized or close to the midline.

What is this?

Focal left temporal sharp waves+ contaminated ear

Focal right temporal sharp waves- Average Ref

Focal right temporal sharp waves- Cz Ref

Focal right temporal sharp waves- Ipsi Ear Ref

Focal right temporal sharp waves- Linked Ear Ref

Focal right temporal sharp waves- Average Ref

Average reference Focal right temporal sharp waves- Average Ref

Average reference- ave cleaned Focal right temporal sharp waves- Average ref- cleaned

Focal left posterior activity- Average Ref

Focal left posterior activity- Average Ref- cleaned

Sleep spindles and vertex waves- Ipsi Ear Ref

Sleep spindles and vertex waves- Average Ref

Generalized spike-and-wave activity- Linked Ear Ref

Generalized spike-and-wave activity- Ave Ref

Bifrontal spike-and-wave activity- Ipsi Ear Ref

Bifrontal spike-and-wave activity- Ave Ref

Bifrontal spike-and-wave activity- Fz Ref

Laplacian reference

Each channel in a Laplacian montage is referenced to an average of the electrodes surrounding it

Filters out coherent widespread activity and emphasizes localized waveforms

The Laplacian reference performs well in environments where low-to-moderate artifact can be expected (e.g., any in-lab or inpatient scalp EEG recording)

Balanced Non-Cephalic Reference

Can be considered when it is not possible to find a cephalic reference that is not contaminated by the EEG signal of interest.

Commonly used for evoked potential recordings, where signal averaging effectively removes EKG artifact.

EKG contamination is problematic, but can be diminished by deriving the reference from a combination of two electrodes. – If the electrodes are placed to present R-waves of

opposite polarity they can be summed to cancel each other, removing the EKG artifact from the non-cephalic reference.

Bipolar Recordings

Advantages: sharp distinction Disadvantages

– Distort wave shape and amplitude – Widespread potentials may be canceled – Potentials maximal at the end of a chain or

equal in the last 2 electrodes in a chain can result in confusing potentials

Referential recordings

Advantages – Undistorted display of the shape of

potential changes – Especially useful for potential changes with

a wide distribution (not average reference) Disadvantages

– Reference electrode may be active – Small focal potentials may be less visible

Localization of potentials

Referential recordings with a neutral reference – First input in the channel with the highest

amplitude (usually greatest negativity, or rarely greatest positivity) is the center of the field

Bipolar recordings – Electrode(s) common to the channels

where reversal of polarity occurs is usually the center of the field

Fp1-F3 F3-C3 C3-P3 P3-O1 Fp2-F4 F4-C4 C4-P4 P4-O2 Fp1-F7 F7-T7 T7-P7 P7-O1 Fp2-F8 F8-T8 T8-P8 P8-O2 Fz-Cz Cz-Pz EKG

Fp1-Av Fp2-Av F3-Av F4-Av C3-Av C4-Av P3-Av P4-Av O1-Av O2-Av F7-Av F8-Av T7-Av T8-Av P7-Av P8-Av Fz-Av Cz-Av Pz-Av ECG

Fp1-Av Fp2-Av F3-Av F4-Av C3-Av C4-Av P3-Av P4-Av O1-Av O2-Av F7-Av F8-Av T7-Av T8-Av P7-Av P8-Av Fz-Av Cz-Av Pz-Av ECG Average reference with removal of Fz, Cz, F4, C4, T4 from the average

What is the polarity of the transient demonstrated below

A. Positive

B. Negative

C. Either positive or

negative

D. Neither

Fp1-F3

Where is the most likely source of this transient?

A. P3 and O1

B. O1

C. Fp1, C3, & P3

Fp1-F3

F3-C3

C3-P3

P3-O1

What is the most likely polarity of this transient?

A. Positive

B. Negative

C. Either positive

or negative

Fp1-F3

F3-C3

C3-P3

P3-O1

Where is the center of the field?

A. C3 and P3

B. C3

C. F3

D. Fp1

Fp1-F3

F3-C3

C3-P3

P3-O1

40 40

100

40

O1- X

EEG LOCALIZATION PROBLEMS

Most likely solutions

A-ref

B-ref

C-ref

D-ref

E-ref

A-B

B-C

C-D

D-E

1

5

5

4

A

B

C

D

E

A-B

B-C

C-D

D-E

5

2

3

2

A

B

C

D

E

A-B

B-C

C-D

D-E

5

2

3

2

A

B

C

D

E

A-B

B-C

C-D

D-E

5

5 5

A

B

C

D

E

A-B

B-C

C-D

D-E

5

5 5

A

B

C

D

E

A-B

B-C

C-D

D-E

5

5

5

5

Less likely solution

A

B

C

D

E

5

5

4

2

2

A-B

B-C

C-D

D-E

2

1

A

B

C

D

E

3

A-B

B-C

C-D

D-E

2

1 3

2

A

B

C

D

E

5

5

4

2

2

A-B

B-C

C-D

D-E

2

1

A

B

C

D

E

A-B

B-C

C-D

D-E

2

1

1

3

3