Surface electrodes are used for almost all routine EEG applications. They are disks which are fixed to the skin in a variety of ways. Electrode gel forms a malleable connections between the rigid disk and the skin. The electrode is secured on the scalp usually by pressing a gauze pad onto the gel-electrode combo. This can hold for a short time
providing the patient is cooperative so that the head does not move. This electrode system does not work well if the patient moves or if the patient has stiff hair or other impediment to fixation of the electrode. Collodion fixation is a much more secure method of securing the electrodes on the scalp, although it does require special equipment, adequate
ventilation, and more technician time. We usually use collodion preferentially in our laboratory even for routine outpatient studies.
Application of surface electrodes using gel consists of the following steps:
1. Locate the positions for electrodes using the 10-20 Electrode Placement System (explained later)
2. Separate strands of hair over the electrode positions using the wooden end of a cotton-tipped applicator.
3. Clean dead skin and dirt from the region with an agent such as Omni-Prep using the cotton-tipped applicator.
4. Scoop some gel into the electrode.
5. Place the electrode in position over the skin.
6. Put a 2"x2" gauze pad over the electrode and push it firmly onto the head, providing a seal which prevents the electrode from falling off the scalp. Application of electrodes with collodion involves the following steps:
1. Prepare the head at the electrode positions as mentioned for electrode gel. 2. Place the electrode on the scalp
3. Place a piece of gauze soaked with collodion over the electrode 4. Use compressed air top dry the collodion
5. Insert a blunt-tipped needle into the up and scrape the skin to lower electrode impedance
6. Inject electrolyte into the cup of the electrode using the blunt-tipped needle. Removal of electrodes is easy for the gel fixation. The gauze pads are pulled off then the electrodes gently pulled off, tilting them to release the vacuum effect which holds them on. Then, the gel left on the scalp can be largely removed by rubbing with a warm, wet
wash cloth. After the patient washes the hair that evening, all traces of the recording are gone.
Collodion is more difficult to remove. First, the collodion is softened by use of acetone, then the areas cleaned as above. The degree of washing required in greater both
immediately by the technician and later by he patient. Some patients object to the acetone smell more than any other part of the procedure.
Each method has its advantages. Collodion provides a more secure attachment and is more suitable for long-term recordings. Electrode gel is easier to apply and remove and is suitable for most routine office and hospital recordings.
Needle electrodes
Needle electrodes offer no advantages over conventional surface electrodes and should not be used for routine studies unless recording cannot be accomplished any other way. The risk of infection to the patient and technician is unacceptably high.
Sphenoidal electrodes
Sphenoidal electrodes are used to record activity from the temporal lobe which would not show on scalp recordings. The electrodes are inserted percutaneously adjacent to the zygoma until they reach the base of the skull. Sphenoidal electrodes should only be used by physicians trained in their insertion and experienced in interpretation of the recorded potentials.
Subdural strip electrodes
Figure 3-6: 10-20 Electrode Placement System.
A: The 10-20 electrode placement system – superior view. B: The 10-20 electrode placement system – left side view.
Subdural strip electrodes are used to evaluate patients for epilepsy surgery. The strips are placed during surgery through burr holes. The strips allow for a detailed map of the recorded electrical activity. Subdural strip electrodes should only be used by physicians trained and experienced in placement and interpretation, and only as part of a
comprehensive epilepsy intervention program. Depth electrodes
Depth electrodes are used to localize seizure foci for surgery. A depth electrode consists of an array of electrodes on a single barrel which is inserted into the brain, usually in the temporal lobe. Only trained and experienced epileptologists should use depth electrodes. Electrode position
Electrodes should be placed according to the 10-20 Electrode Placement System, as recommended by the International Federation of Societies for EEG and Clinical
Neurophysiology. This system uses 21 electrodes placed as positions that are measured a 10% and 20% of head circumference.
Electrodes are named according to their position within the 10-20 system, with the first part of the name referring to the region and the second part referring to the area within the region. For regions, the following assignments have been made:
• F = frontal • C = Central • P = Parietal • T = Temporal • O = Occipital • A = Auricular (ear) • Fp = Frontopolar
The second part of the electrode name indicates the exact location. Numbers refer to pre- arranged location, and the reader has to know the 10-20 system to know the difference between T3 and T5, for example. Odd numbers are on the left side of the brain, even numbers are on the right. Within a region, lower numbers are more anterior and medial to higher numbers. Midline electrodes are designated with "z" rather than a number.
Some examples of this numbering: • Fz = frontal at the vertex • Cz = central at the vertex. • T3 = left temporal
• T5 = left temporal, posterior to T3.
1. Measure the distance from the nasion to inion across the vertex. Mark a line 50% of this distance a the top of the head.
2. Measure the distance between the preauricular points, just in front of the ear. Mark a line at 50% of this distance, at the top of the head. The intersection of this line with that of step 1 is Cz.
3. Lay the measuring tape from nasion to inion through Cz. Mark 10% of this distance above the nasion for Fpz and above the inion for Oz. Fz is 20% of this distance above Fpz. Pz is 20% of this distance above Oz.
4. Lay the tape between the preauricular points through Cz. T3 is 10% of this
distance above the left preauricular point and T4 is 10% of this distance above the right preauricular point. C3 is 20% of this distance above T3, and C4 is 20% of this distance above T4.
5. Lay the tape from Fpz to Oz through T3. FP1 is 10% of this distance from Fpz, F7 is 20% of this distance posterior to Fp1. O1 is 10% of this distance anterior to Oz, and T5 is 20% of this distance anterior to O1. Measure in the same manner for Fp2, F8, O2, and T6 over the right hemisphere.
6. Lay the tape from Fp1 to O1 through C3. F3 is half the distance between Fp1 and C3. P3 is half the distance between C3 and O1. Repeat for the right side, with the tape from Fp2 to O2 through C4. F4 is half the distance between Fp2 and C4, and P4 is half the distance between C4 and O2.
7. Lay the tape from F7 to F8 through Fz., F3 and F4 to ensure that the distance between the electrodes is equal. Then lay the tape from T5 to T6 through Pz, P3, and P4 to ensure equal interelectrode distances.
Abbreviations for special electrodes and less standardized, and may differ between laboratories. Sp usually indicates sphenoidal electrode and Naso usually indicates
nasopharyngeal electrode. Odd numbers are left-sided and even numbers are right-sided. Subdural strip and depth electrodes are also named using letters and numbers where the letter indicates the array and the number indicates which electrode in he array.
Montages
The sequence of electrodes being recorded at one time is called a montage. All montages fall into one of two categories: bipolar or referential. Referential means that the reference for each electrode is in common with the other electrodes. The reference could be a single non-cephalic electrode, ipsilateral ear, or perhaps even Cz, though this is not a useful reference because of it’s electrical activity. Bipolar montage means that the reference electrode for one channel is the active electrode for the next channel. Bipolar montages are particularly useful for visual analysis of focal cerebral activity such as spikes and sharp waves. For example, in the longitudinal bipolar (LB) montage, the first four channels are shown in table 3-2.
Table 3-2: Left parasagittal portion of the longitudinal bipolar montage.
Channel number Active electrode Reference electrode Output value
1 Fp1 F3 Fp1-F3
2 F3 C3 F3-C3
3 C3 P3 C3-P3
For all channels,, negativity at the active electrode produces an upward deflection of the
pen on the paper. Negativity at the reference produces a downward deflection. The Guidelines recommends the following principles in designing montages:
• Record at least 8 channels.
• Use the full 21 electrode placement of the 10-20 System.
• Every routine recording session should include at least one montage from each of the following groups: referential, longitudinal bipolar, and transverse bipolar.
• Label each montage on the recording.
• Use simple montage that allow for easy visualization of the spatial orientation of waveforms - for example, bipolar montages should be in straight lines with equal interelectrode distances.
• Have the anterior and left-sided channels above the posterior and right-sided channels.
• Use at least some montages which are commonly used in other laboratories.
• Remember that negativity in the active electrode of each channel produces an upward deflection of the pen.
The recommended montages for routine use in adults are shown in Table 3-3, and diagrammed in Figure 3-7. Additional channels, when available, are used for monitoring other biological functions, such as ECG, eye movements, respirations, and EMG.
Table 3-3: Recommended EEG montages.
Channel LB TB Ave Ref
1 Fp1-F3 Fp1-Fp2 Fp1-Ave Fp1-A1
2 F3-C3 F7-F3 Fp2-Ave Fp2-A2
3 C3-P3 F3-Fz F3-Ave F3-A1
4 P3-O1 Fz-F4 F4-Ave F4-A2
5 Fp2-F4 F4-F8 C3-Ave C3-A1
6 F4-C4 A1-T3 C4-Ave C4-A2
7 C4-P4 T3-C3 P3-Ave P3-A1
8 P4-O2 C3-Cz P4-Ave P4-A2
9 Fp1-F7 Cz-C4 O1-Ave O1-A1
10 F7-T3 C4-T4 O2-Ave O2-A2
11 T3-T5 T4-A2 F7-Ave F7-A1
12 T5-O1 T5-P3 F8-Ave F8-A2
13 Fp2-F8 P3-Pz T3-Ave T3-A1
14 F8-T4 Pz-P4 T4-Ave T4-A2
15 T4-T6 P4-T6 T5-Ave T5-A1
16 T6-O2 O1-O2 T6-Ave T6-A2
17 Fz-Cz Fz-Cz Fz-Ave Fz-A1