Why High Density EEG?

Adequate Spatial Sampling

For many years, biophysical analyses have shown that information is lost unless an intersensor distance of one to two cm is achieved with EEG sampling.
Achieving a one to two cm sampling density would require 500 EEG channels distributed evenly over the surface of the head.  With 256-channel sampling, HD EEG now approximates adequate spatial sampling.  With EGI's medical grade EEG systems, this accuracy is available to clinicians wherever brain monitoring is required.

Malmivuo, J. & Plonsey, R. (1995). Bioelectromagnetism. New York: Oxford University Press.

Ryynanen, O. R., Hyttinen, J. A., Laarne, P. H., & Malmivuo, J. A. (2004). Effect of electrode density and measurement noise on the spatial resolution of cortical potential distribution. IEEE Trans Biomed Eng, 51(9), 1547-1554.

Ryynanen, O. R., Hyttinen, J. A., & Malmivuo, J. A. (2006). Effect of measurement noise and electrode density on the spatial resolution of cortical potential distribution with different resistivity values for the skull. IEEE Trans Biomed Eng, 53(9), 1851-1858.

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

Detecting Clinical Signs

The spatial sampling of HD EEG may be required not only to localize brain pathology, such as an epileptic spike, but to detect it.  
Comparisons of 256-channel HD EEG recordings have been made with subsampling that shows what would be seen by conventional (19-channel) EEG.  These studies confirm that not only accurate localization but in some cases the detection of neuropathology may require the channel densities of 128 or 256 provided by HD EEG (Holmes, 2008; Holmes et al., 2004; Lantz, et al., 2003).  Neuropathology that is limited to a small area of brain near the skull (such as a gyrus of the cortex) may project an EEG field that is invisible to conventional EEG.  With the 10-20 system placement of conventional EEG, the electrodes are as far as 7 cm apart.

Holmes, M. D. (2008). High Density EEG: Methodology and new hypothesis on epilepsy syndromes. Epilepsia, 49, 3-14.

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.

Lantz, G., Grave de Peralta, R., Spinelli, L., Seeck, M., & Michel, C. M. (2003). Epileptic source localization with high density EEG: how many electrodes are needed? Clin Neurophysiol, 114(1), 63-69.

Neurophysiology in the Brain

Whereas conventional EEG could only show the brain's electrical activity as it was propagated to the head surface, computational methods of source localization registered to the MRI image of head anatomy are now being validated for clinical use, and refined for implementation within the workflow of a busy clinical laboratory.  
Physicians are using the combined accuracy of MRI with EGI's High Density EEG technology. The result is localization of pathological physiological events with exact registration with the MRI that provides visualization of pathological anatomy (Seeck et al., 2001). 

Lantz, G., Spinelli, L., Seeck, M., de Peralta Menendez, R. G., Sottas, C. C., & Michel, C. M. (2003). Propagation of interictal epileptiform activity can lead to erroneous source localizations: a 128-channel EEG mapping study. Journal of Clinical Neurophysiology, 20, 311-319.

Michel, C. M., Grave de Peralta, R., Lantz, G., Gonzalez Andino, S., Spinelli, L., Blanke, O. et al. (1999). Spatiotemporal EEG analysis and distributed source estimation in presurgical epilepsy evaluation. J Clin Neurophysiol, 16(3), 239-66.

Michel, C. M., Murray, M. M., Lantz, G., Gonzalez, S., Spinelli, L., & Grave de Peralta, R. (2004). EEG source imaging. Clin Neurophysiol, 115(10), 2195-2222.

Seeck, M., Michel, C. M., Spinelli, L., & Lazeyras, F. (2001). EEG mapping and functional MRI in presurgical epilepsy evaluation. Rev Neurol (Paris), 157(8-9 Pt 1), 747-51.

Tucker, D. M., Brown, M., Luu, P., & Holmes, M. D. (2007). Discharges in ventromedial frontal cortex during absence spells. Epilepsy and Behavior, 11, 546-557.

Cost Savings in the EEG Laboratory

Because the Geodesic Sensor Net is fast and easy to apply, it improves EEG laboratory efficiency for even lower channel counts (32 and 64). 
Saving technician time and increasing patient throughput have caused routine EEG laboratories to see considerable cost savings that quickly overshadow the capital investment in HD EEG.  EGI's clinical sales representatives can provide representative economic analyses for the capital investment, patient throughput, and cost savings for a full range of EEG laboratories, clinics, and hospitals.

Marketing the EEG Laboratory

With the increasing popular press on EGI's Geodesic EEG System, patients are now requesting EGI's HD EEG not only because of its improved accuracy, but also because it is painless.

Parents particularly are concerned that their children have not only the best care, but are spared the pain and discomfort of outmoded EEG methods that require scraping the scalp, uncomfortable elastic caps, or toxic glues.  

In communities where HD EEG is available, physicians and administrators are seeing an important competitive advantage of the increased visibility brought to their hospital or clinic.  EGI's clinical sales representatives can provide examples of case stories in which the appearance of an EGI laboratory in a community changed the competitive medical climate.