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High Density EEG (HD EEG)

Overview of HD EEG for Patients

An electroencephalogram (EEG) is a test that records brain waves, or the brain’s electrical activity, as you think, feel, or do things. EEG is most helpful to diagnose seizures and epilepsy, but it can also help to diagnose other brain diseases. Standard EEGs use approximately 30 electrodes (small metal or plastic discs attached painlessly to the scalp) to record brain activity. Abnormal EEG signals, such as spikes or sharp waves that are present in the EEG between seizures, can point to a diagnosis of epilepsy. They can also help your doctors identify where seizures will likely begin in the brain.

High-density EEG uses many more electrodes than standard EEG—usually 64, 128, or 256. This produces a very detailed view of brain activity across the scalp. It also records brainwave activity many times per second, typically more than 1,000 times per second. 

Steps of the High-Density EEG Procedure

High-density EEG is used mainly for patients undergoing evaluation for epilepsy surgery or devices, but neurologists sometimes use it for other reasons. Here is what you can expect during your high-density EEG procedure:

  1. To increase the chances of abnormal brain activity being recorded, you may be asked to undergo sleep deprivation (sleeping for only a few hours) the night before the test. If you are sleep deprived, you will need to bring someone with you to the test to ensure that you can travel safely to and from the test. 
  2. Once you arrive in the EEG lab, the electrodes are placed using a flexible cap that allows for the placement of many electrodes in a short amount of time (about 30 minutes). 
  3. High-density EEGs are typically recorded for 2 hours while you lie quietly. Your care team will ask you to try to fall asleep. 
  4. The EEG technician will remove the electrodes and clean your hair before you leave the EEG lab.

After the test, your neurologist will analyze it with electrical source localization to identify the location of specific brain activity. They will combine this output with the results of your MRI to create a detailed, 3D picture of the electrical activity in your brain. Using these tests, your doctors can get an estimate of where your seizures may begin in the brain and the best way to treat your seizures or epilepsy. The results of high-density EEG are often more accurate than those of a standard EEG because of the large number of electrodes used. The test takes two to three weeks to analyze, and your neurologist or epilepsy doctor will discuss the results with you when they are complete.

The results of the high-density EEG will be combined with the results of other epilepsy tests and discussed with your epilepsy care team. High-density EEG can help pinpoint which area will be removed with epilepsy surgery; it can also help your doctors decide where to put invasive electrodes into or onto your brain for epilepsy surgery evaluation.

medical team analyzing results on a laptop

Information for Physicians and Other Healthcare Providers

High-density EEG (electroencephalography) is an advanced neuroimaging technique that provides a highly detailed and precise measurement of the electrical activity in the brain. It uses multiple (64 to 256) electrodes on the scalp to noninvasively record the electrical signals generated by neurons in the cerebral cortex. The results of high-density EEG are analyzed using electrical source localization (ESI) algorithms and combined with MRI to show where specific brain activity originates. Our clinical high-density EEG program provides precise spatial localization for patients with epilepsy undergoing evaluation for epilepsy surgery or devices or when more accurate localization of seizures is necessary.

Some key characteristics and features of high-density EEG and ESI:

  • Increased Electrode Density: High-density EEG uses more electrodes (64, 128, or 256 electrodes) than standard EEG (19-32 electrodes). 
  • Enhanced Spatial Resolution: The more closely spaced electrodes in high-density EEG can record from smaller and more localized brain regions and produce a more detailed map of brain activity on the scalp. This can sometimes show activity that a standard EEG doesn’t show.
  • High Sampling Rate: High-density EEG samples are taken more than 1,000 times per second, providing exceptionally accurate timing of brain events.
  • Electrical Source Localization (ESI): This mathematical model determines the precise locations where electrical activity originates within the brain. ESI first combines the results of high-density EEG, the shape of the head, and the conductivity of brain tissues to analyze the brain activity across the scalp. Then, using the same models, ESI tries to solve the inverse problem by estimating the most likely source locations within the brain that could generate the observed signals on the scalp. Finally, the algorithms produce a 3D map showing where the electrical activity is likely originating in the brain. This 3D map is usually combined with a neuroimaging technique like MRI to display the source imaging results.
  • Research and Clinical Applications: High-density EEG is used in various applications, including epilepsy diagnosis and presurgical evaluation, cognitive neuroscience research, and brain-computer interfaces. Please call us at (602) 406-6262 to discuss potential applications of high-density EEG.

In summary, high-density EEG is a powerful tool for studying brain activity with excellent precision and sensitivity. It is invaluable for understanding brain function, diagnosing neurological disorders, and guiding treatments.

Medically Reviewed by Vladimir Shvarts, MD, FACNS on June 14, 2024