ELECTRODES PLACEMENTS 10/20 INTERNATIONAL SYSTEM
Introduction to the International 10-20 System of Measurement
In order to perform consistent testing for electroencephalographic (EEG) recordings , a system was developed which would describe the locations for electrodes on the human skull.
A great deal of research was conducted in the creation of the system. Important features which needed to be considered in the development include:
· A standard format for measured sites
· The use of common terminology
· Anatomically correct locations
As a result of this joint effort of neurological societies worldwide, the " International 10-20 Electrode Placement System" for measurement and placement of electrodes was developed.
The system used for sleep monitoring is essentially the same, with one exception; The number of sites is customarily fewer than when performing a complete EEG.
Known skull landmarks were used to develop reference points.
Interestingly enough , cadavers were used for testing to ensure proper and consistent recommendations for EEG locations. Therefore, the International 10-20 electrode placement is founded on the relationship of a "skull measured" electrode location and the corresponding cortical anatomy.
The distinction between skull landmarks and electrode placement sites should be clearly marked, and is easily mistaken. When viewing the skull it is important to double check.
Anatomic landmarks are used for measurement purposes only.
Why is the system termed 10-20?
EEG electrodes are placed on the scalp at 10 and 20 percent of a measured distance.
If a measurement was made around your skull, (i.e - a hat size measurement) a circumference distance would be determined. This measurement would be approximately 55 cm. Therefore, 10% of this measurement or 5.5 cm would be used to determine precise locations around the skull. An anatomic landmark is needed to know where to start with the measurements.
Why are percentages used?
The skull may be different from patient to patient. A pediatric or adolescent may be smaller than an adult. A traumatic accident to the skull may have occured in the patients history creating an out of proportion condition. How about congenital abnormalities to the skull and scalp.
The percentage relationship remains the same for the location of the internal brain lobes.
Skull dimensions are measured accordingly in centimeters and then site distances or spacings are converted with the 10 and 20% factors. Fifty percent (50%) is used frequently, but is a composite of 10, 20, & 20%. Remember, once a landmark is located, it should be used consistently and regularly for a site locations.
The farther the site location from the landmark , the greater the chance of error. Consequently the use of the 50% marks or locations.
What does the "International 10-20 Electrode Placement System" help to create?
1. It creates a method of labeling electrode locations which may be used worldwide.
2. Electrode sites have been proven to reflect accurate voltage recordings of the correct anatomicstructures, regardless of skull size and shape.
3. The system may be modified to include additional electrodes. (5% increments)
An important feature of the international 10-20 system is that a patient tested in one faciltiy, will be recorded accurately and precisely, the same as completed in another totally different facilty, providing the 10-20 system is used.
The following measurement requirements should be used religiously. Practice, practice, practice.
Short cuts should be avoided, due to the variance of patients.
Clinically, a practitioner would need to do 10-50 patients to feel comfortable with the measurement system. Each electrode position and anatomic location and the rationale behind the measurements will then be evident. A short period of time or absence from measuring locations will send you back to the books.
Success requires patience, regular practice, manual dexterity, memorization and some math calculations. By the way, it's always better to write down the distances and percentages when first starting.
Good Luck !!
Location of the electrodes
The utilization of this system is to determine the location of electrodes, in relationship to the cortical areas, in order to reliably and precisely maintain locations from individual to individual.
You may want to use something 'roundish' like a styrofoam wig head or balloon to practice. If your lucky you may get a real person!!
Also, it will be a good idea to print some of the pictures for reference while you practice.
Use a china marker and measuring tape. Use the centimeter side.
Be sure to make your marks perpendicular to the direction of your tape measure.
If using one side of the tape measure, be sure to be consistent, and use the same side.
Anterior to Posterior Measurement
· (A) Measure from the Nasion (the bridge of the nose) to the Inion (the occipital pretuberance) Record your total (i.e. 36 cm)
(B) Calculate and make a horizontal mark at 50% of the total. This will be the first mark of Cz. (i.e. this example = 18 cm)
(C) Measure 20% forward from the mark of Cz to mark a
Frontal site ( F ) ; {What is 20% of the original example of 36 cm ??)
Check point: 10% of the Nasion to Inion measurement of 36 cm = 3.6cm. therefore 20% = 7.2 cm.
And now measure an additional 20% forward to mark Frontal Pole "zero" ( Fpz )
Measure 20% back from the mark of Cz to locate Parietal ( P),
then 20% back to mark Occipital "O"
Check point:
Horizontal china marks will be on the forehead at 10% up from the bridge of the nose, and at 10% up from t the skull
The International 10-20 System of Measurement
Pre Auricular Measurement
The next set of markings will be made across the head, from ear to ear. As the tape is held, markings will be perpendicular to the tape measure, as before.
(A) Measure from the tragus of ear - (the little protrusion at the openning of the ear )
and follow forward of the ear openning , until you feel the dent between the skull and jaw bone. (Condylar process)
Measure >>Pre-auricular point across the head to Pre-auricular point on the other side; and record your total.
(This sample, i.e. 40 cm).
Make sure the tape measure goes through, and lies on top of the Cz mark from the previous measurement.
(B) Make an anterior to posterior mark at 50% of the total. (i.e., 20 cm).
Notice that this is the second mark of Cz and you should visualize an "X" or " + ".
When the first mark of Cz and the second mark of Cz intersect, the true Cz is displayed
(C) From Cz; Measure 20% (8 cm in this example) in each direction towards the original pre auricular point(s).
Make an "anterior - posterior mark" on each hemisphere of the skull.
(remember perpendicular to the tape measure)
This will be the first mark of C3 (left side), and first mark of C4 (right side) of the skull.
(D) From this point make an additional mark 20% towards the pre auricular point.
T3 (left side), and T4 (right side)
Note: Normally these markings or measurements will be identical on each hemisphere.
On occasion anatomical structure of the head and skull may prove otherwise.
Take care to measure.
Check point: Five marks are now on the pre-auricular axis.
True Cz; two anterior - posterior marks on each hemisphere,
one each at 10% up from the tragus(temporal); one each at the central lobe level.
Other Marks
Make a cross ( + ) at the 10% line, up from the Nasion. (on the forehead)
From behind the patient, tilt the head backward, and place a vertical mark,
in line with the center vertical axis of the nose, to visulize a "+" on the forehead.
This will be your true Fpz. {The only non measured mark}
Circumferential Measurement
We will measure around the head in order to divide sites into 10 - 10% increments. (100%)
The sites which are located around the skull are off centered. In other words, we do not use the center of the forehead (Fp) and start measuring around in 10% increments. We split the difference at Fp and move 5% to each side. Markings will be made around the head at frontal poles, frontals, temporals, and occipitals.
Note: to make an accurate "hat size" measurement, wrap the tape measure, horizontally
around the head, assuring that the tape is aligned with the previous horizontal marks.
{Fpz; T; O; and T }
If these landmarks are not used, the circumferential measurement may be incorrect.
Remember to use a consistent marking technique, either above or below the tape measure.
(A) Record your total circumference. (i.e., 60 cm)
(B) Use one half of the total to mark the center of the back of skull. (i.e. 30 cm)
Measure from Fpz to "O" and make a vertical mark through
the horizontal mark at "O"
This is your true Oz and a "+" will be complete.
Hold the tape at Fpz, wrap it around to "O", at the one half level, mark a
vertical line to " + " the existing horizontal line at " O ".
Final Frontal pole and Occipital Locations
At this point you will note that Fpz and Oz are marked with vertical lines and represent the front and rear center of the skull. The electrodes around the circumference are placed 10% apart.
Ten sites are used for EEG.
Therefore the distance between FP1 and Fp2 would be 10%.
The measurement is 5% to either side of Fp
The distance between O1 and O2 = 10%.
These four (4) sites and landmarks are used for sleep studies.
If the skull circumference is 60 cm, then 10% = 6.0cm.
Fpz is the center of the frontal bone, therefore a mark 5% on either side would = 10%.
(A) Make a vertical mark on the forehead 5% (3 cm) on either side of Fpz.
Extend the horizontal mark through the vertical marks to show three " + " marks.
(B) Repeat this procedure for occipital sites O1 and O2. Make vertical marks 5% (3 cm) on each side of Oz. Extend the horizontal mark to show three "+" marks.
Note: the following "+" marks indicating exact locations are prevalent on the head....
Cz, Fpz, Fp1, Fp2, Oz, O1, and O2.
Final "Central" Measurements.
To mark the final cross sections ( "+" ) for C3 (and C4)......
(A) Hold the measuring tape and measure from FP1,
through the first central mark on the left side,
then back to O1 and record your total. (i.e., 25 cm).
(You are measuring the distance of 10% up from the front to 10% up from the inion.)
(B) Half of the total (i.e., 12.5 cm) is where the first mark of C...
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