Geodesic EEG System 300

The GES 300 saves time, and increases your
confidence that all clinically relevant data are recorded.

 

Strengths of the GES 300:

• Complete Head Coverage: ensures that all clinically relevant information is captured.
• Quick Application Time: under 10 minutes using the patented Geodesic Sensor Net.
• Superior Patient Comfort: patients prefer the Geodesic Sensor Net to traditional electrode-application techniques.
• Superior Patient Safety: no scalp abrasion.
• Various Channel Counts: available in 256-, 128-, and 64-channel configurations.
• Clinical Applications: routine digital EEG, long-term monitoring, and pediatric EEG.
• No Sedation: rapid application times make routine sedation unnecessary.

More about the GES 300:

• Annotated Publication List
• FAQs

 

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Complete Head Coverage

Complete head coverage ensures that all clinically relevant info is captured. Guaranteeing complete head coverage requires attention to both appropriate intersensor distance and coverage of the underside of the head.

The first aspect of complete head coverage refers to the intersensor distance (that is, how far apart one sensor is from another sensor). Smaller intersensor distances translate to a more accurate measurement of the voltage field.

The second aspect refers to coverage of the underside of the head (for example, below the ears, including the face). It is common to believe that electrodes on the inferior surface of the head do not pick up EEG. However, voltage fields generated by the brain are volume conducted throughout the entire head. Moreover, certain brain regions, such as the inferior temporal lobes and ventral aspects of the frontal lobe, are oriented such that the voltage fields generated by these brain regions are best detected by electrodes on the inferior surface of the head.

The consequence of incomplete head coverage is that clinically significant EEG signals can be missed by sparse electrode placement and by lack of electrode placement on the underside of the head. For detailed information, see:

Luu, P., Tucker, D. M., Englander, R., Lockfeld, A., Lutsep, H., & Oken, B. (2001). Localizing acute stroke-related EEG changes: Assessing the effects of spatial undersampling. Journal of Clinical Neurophysiology, 18, 302-317.

Sperli, F., Spinelli, L., Seeck, M., Kurian, M., Michel, C. M., & Lantz, G. (2006). EEG source imaging in pediatric epilepsy surgery: A new perspective in presurgical workup. Epilepsia, 47, 1-10.

Srinivasan, R., Tucker, D. M., & Murias, M. (1998). Estimating the spatial Nyquest of the human EEG. Behavior Research Methods, Instruments, & Computers, 30, 8-19.

Quick Application Time

Application times for the sponge-based HydroCel Geodesic Sensor Net range between 5 minutes for 32 channels to 15 minutes for 256 channels. EEG staff rapidly learn the application technique, which is described in the HydroCel Geodesic Sensor Net Quick Start placard.

Superior Patient Comfort

In study after study, patients report that they prefer EEGs performed with Geodesic Sensor Nets over conventional EEG sensor arrays. Patients report that the GSN is more comfortable, and that the electrolyte formulation does not smell or mess up their hair. Children are far less likely to cry because the process is rapid and abrasion-free.

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Superior Patient Safety

The Geodesic Sensor Net uses abrasion-free application methods and the electrodes themselves are held in place by gentle radial compression afforded by the unique geodesic tension structure. Because no abrasion is required, the scalp on the skin is not compromised, reducing infection risk. Often, it is argued that mild abrasion during the course of skin preparation does not compromise the integrity of the skin. However, studies have shown that this is not an accurate assumption. For further information, see:

Bild, S. (1997). Detection of occult blood on EEG surface electrodes. American Journal of Electroneurodiagnostic Technology, 38, 251-257.

Various Channel Counts

The Geodesic EEG System 300 is available in 256-, 128-, and 64-channel configurations.  The 64- and 128-channel Nets are available for all patient populations, from infants to adults.  The 256-channel Nets are currently available for pediatric through adult.

Clinical Applications

The Geodesic EEG System is currently in use for the following clinical applications:

Routine Digital EEG: We recommend the use of 128-channel GES 300s using the sponge-based HydroCel Nets for this application. For a recent publication related to routine EEG, please see:

Holmes, M. D., Brown, M., & Tucker, D. M. (2004). Are "generalized" seizures truly generalized? Evidence of localized mesial frontal and frontopolar discharges in absence. Epilepsia, 45(12), 1568-1579.

Long-Term Monitoring: We recommend that LTM for epilepsy evaluation be done with the 256-channel GES 300 using the HydroCel Nets. Recordings can be conducted for 24+ hours with a minimum of electrode reapplication time. This configuration allows for complete coverage of the head, providing the best capture of spikes, seizures, and other clinically relevant information. Automated spike detection can be performed with the Persyst Reveal software.

Pediatric EEG: The Geodesic EEG systems work well with children and infants. Sedation is not required for electrode application. Geodesic Sensor Nets have been successfully used with hard-to-record populations such as autistic children and ADHD patients.

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No Sedation

Sedating children to obtain useful EEG is often done in clinical settings. This is not required when you use the Geodesic Sensor Net; rapid application time and the comfortable fit make sedation unnecessary.

Annotated Publication List

Articles by EGI customers:

• clinical
• all topics

FAQs

What is the quality of the data recorded with the GES 300?

Three questions are usually asked with regards to data quality:

• How does high electrode-scalp impedance affect data quality?
• What are the effects of impedance mismatch on data quality?
• What are the risks of electrolyte bridges?

The issues related to recording with high electrode-scalp impedance are covered in a paper by Ferree et al. (Ferree, Luu, Russell, & Tucker, 2001). The take-home message is that high electrode-skin impedance is acceptable and does not result in any signal loss if the amplifier's input impedance is high. Of course, 50/60 Hz noise will couple more easily into high electrode-scalp impedance recordings, but this noise can be easily handled by a good digital filter.

Lowering the electrode-skin impedance to the conventional 5 KOhms is required for low input impedance amplifiers. However, abrasion of the scalp surface is usually required to achieve such low levels.

Impedance mismatch also introduces 50/60 Hz noise into the recordings. However, modern amplifiers attenuate this effect by the use of an isolated common reference (see Ferree et al., 2001).

According to Pellouchoud and colleagues (1997), the use of dense electrode arrays can increase the risk of electrolyte bridge formations. However, this increased risk is most commonly associated with electrode caps and EEG gel-based systems.

Our own in-house testing shows that electrolyte bridges are extremely difficult to obtain using our standard application methods, which ensure that the electrodes make good contact with the scalp. Recent published studies have been conducted with the GSN confirming our own observations that electrolyte bridges will not form if EGI's application procedures are followed (Tenke & Kayser, 2001; Greischar, et al., 2004).

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For which patient populations is the GES 300 suitable?

Suitable for all patient ages and hair types (with the exception of corn rows and dreadlocks, where hair density prevents seating of electrodes), Geodesic Nets have been used successfully with difficult-to-record populations such as autistic children, elderly patients with dementia, and ADHD teens. For more information concerning specific populations, please contact info@egi.com.

Is sedation required?

The Geodesic Sensor Net is well tolerated by all patient groups. We do not recommend the use of sedation during EEG recording unless it is required to achieve some other clinical purpose.

Can you perform artifact detection and removal with the GES 300?

Many sources of artifacts interfere with data review and analysis. The interested person is referred to a good EEG atlas (for example, Goldensohn et al., 1999) for a complete description of artifacts.

In general, three types of artifacts, excluding 50/60 Hz noise, are prominent in most EEG recordings:

• ocular artifacts (such as blinks)
• ECG
• EMG

Detection of these artifacts by manual review of the EEG is quite simple.

However, automated detection requires a few more steps. For example, although automated detection of blinks is quite simple, automated detection of ECG artifacts is much more complex because of their similarity to EEG spikes. Net Station currently has the ability to detect ocular artifacts (that is, blinks and lateral eye movements).

For artifacts with known frequencies that do not overlap substantially with EEG frequencies, they can simply be removed with a digital filter.

For other types of artifacts, removal is a bit more involved. A common strategy involves the use of spatial filters (such as component decomposition). These approaches assume that the artifact is spatially stationary (its spatial distribution does not vary from occurrence to occurrence).

Net Station currently uses both digital (for 50/60 Hz noise) and spatial (for ECG and blinks) filters to remove common artifacts. EMG artifacts are difficult to remove because of their broad-band nature as well as complicated spatial configuration (due to multiple EMG sources).

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How do the GES 300 electrode positions compare to the 10-20 system?

The HydroCel GSN uses an electrode placement system that is inherently different from the 10-20 international system.

The HydroCel GSN emphasizes even placement of the electrodes across the entire head surface. The best method for doing this is the geodesic structure. Placement of the entire Net is based on identification of the Cz position. Therefore, the HydroCel GSN will not have exact equivalence to the 10-20 system, with the exception of the Cz position.

However, because the HydroCel GSN has many electrodes (depending of channel count) there are sensor positions that are approximately equivalent to the 10-20 positions (defined as being less than 1 cm from the actual 10-20 position, which is within the acceptable error bounds for electrode applications according to ABRET standard).

Can I view the data in standard EEG montages?

All EEG data can be viewed in standard clinical montages, such as bipolar pairs, double banana, etc., using the approximate 10-20 equivalent positions.

Which kinds of research tools are available with the GES 300?

All research tools that are compatible with Net Station can be used with the GES 300. These include functions such as stimulus presentation, ERP analysis, Joint Time Frequency analysis, and so forth. Our Data Analysis webpage provides more details.

For general Geodesic EEG technology questions, visit our Information for Patients page.