EEG imaging offers a window into the brain’s dynamic electrical activity. The brain constantly generates electrical signals as neurons communicate, even during sleep. EEG imaging helps visualize these intricate patterns, providing insights into how the brain functions in real time.
What is EEG Imaging?
Electroencephalography, or EEG, is a technique that measures the brain’s electrical impulses through electrodes placed on the scalp. Unlike imaging methods such as Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans, which show the brain’s physical structure, EEG focuses on its electrical activity. This distinction means EEG provides a dynamic assessment of brain function rather than a static image of its anatomy.
The “imaging” aspect of EEG refers to mapping or visualizing these electrical patterns rather than creating a physical picture of the brain itself. EEG detects tiny electrical charges produced by brain cells, which are then amplified and displayed as wavy lines or maps on a computer screen. These representations allow healthcare professionals to observe brainwave patterns and fluctuations.
How an EEG Scan is Performed
Before an EEG scan, patients are advised to wash their hair and avoid styling products like gels or sprays, as these can interfere with electrode adhesion. It is also recommended to eat and take regular medications unless otherwise instructed. In some cases, sleep deprivation may be suggested the night before to increase the likelihood of recording specific brain activity, particularly in epilepsy evaluations.
During the procedure, an EEG technician measures the patient’s head to accurately place electrodes. Between 16 and 25 electrodes are attached to the scalp using a special paste or gel, or a cap containing electrodes may be used. Each electrode site on the scalp is gently scrubbed with a mildly abrasive cream to ensure good electrical contact.
The patient relaxes in a reclining chair or lies on a bed in a quiet, dimly lit room. Once the electrodes are in place and connected to the EEG recording machine, the patient is asked to remain still with eyes closed. During the 20 to 40-minute recording period, the technician may ask the patient to perform simple actions like opening and closing their eyes, breathing deeply and rapidly (hyperventilation) for a few minutes, or observing a flashing light. These actions can help elicit brainwave activity that might not be apparent during rest, aiding in diagnosis. Some EEGs also involve video recording to correlate physical movements with brainwave patterns, which can be particularly useful for diagnosing conditions like seizures.
Key Applications of EEG Imaging
EEG imaging is used in both diagnostic and research settings for various neurological conditions. It is employed for diagnosing and monitoring epilepsy, a condition characterized by repeated seizures. EEG helps identify abnormal electrical discharges in the brain that are indicative of seizure activity, to classify seizure types and guide treatment plans.
Beyond epilepsy, EEG plays a role in assessing sleep disorders, including sleep apnea and insomnia, by monitoring brain activity during different sleep stages. It is also used to evaluate brain injury, stroke, and conditions like encephalitis (brain inflammation), brain tumors, and encephalopathy, which causes brain dysfunction. In severe cases, EEG can help determine brain function in comatose patients or confirm brain death.
EEG also contributes to cognitive neuroscience research. Its ability to track rapid changes in brain activity with millisecond precision makes it valuable for studying cognitive processes such as attention, perception, and memory. For instance, researchers use EEG to investigate brain oscillations and their role in cognitive functions, and it can help identify atypical electrical activity in neurodevelopmental disorders.
Understanding What EEG Images Show
The electrical signals recorded by EEG are translated into wavy lines that represent different brainwave patterns. These patterns are classified by their frequency, measured in Hertz (Hz), and are associated with various states of consciousness or brain activity.
Delta waves, ranging from 0.5 to 4 Hz, are observed during deep sleep and are prominent in the frontocentral brain regions. An increase in delta activity in an awake individual can indicate brain tissue damage or dysfunction. Theta waves, occurring at 4 to 7 Hz, are linked with drowsiness, early stages of sleep (N1 and N2), and meditative states. Alpha waves, with frequencies between 8 and 12 Hz, are characteristic of a relaxed, awake state, seen when a person has their eyes closed.
Beta waves, ranging from 13 to 30 Hz, are associated with an alert, active, and thinking state, while gamma waves (30-70 Hz or higher) are involved in higher processing tasks, learning, and memory. Abnormal patterns or changes in these brainwave frequencies can indicate underlying neurological issues. EEG’s high temporal resolution, meaning its ability to detect rapid changes in brain activity within milliseconds, allows observation of these dynamic shifts.
Benefits and Limitations of EEG Imaging
EEG imaging offers several advantages that make it a widely used tool in neurology. It is a non-invasive procedure, meaning it does not involve incisions or injections, and it is considered safe with minimal risks. The equipment is relatively inexpensive compared to other neuroimaging techniques like MRI or PET scans, making it more accessible. Furthermore, some EEG systems are portable, allowing for long-term monitoring outside of a hospital setting, such as with ambulatory EEGs.
Despite its benefits, EEG imaging also has limitations. A significant drawback is its poor spatial resolution, meaning it struggles to pinpoint the exact location of brain activity. This limitation arises because electrical signals from the brain must travel through several layers, including brain tissue, cerebrospinal fluid, skull, and scalp, before reaching the electrodes, which can cause the signals to spread out and blur. Consequently, while EEG excels at showing when brain activity occurs, it is less precise about where it originates, especially for deep brain structures.
EEG signals are also susceptible to artifacts, which are unwanted electrical signals that can interfere with the recording. These can include physiological artifacts from muscle movements, eye blinks, or heart activity, as well as non-physiological artifacts from poor electrode contact or environmental noise. While technicians are trained to manage these, they can sometimes obscure genuine brain activity.