fMRI-compatible EEG electrode cap

Category: Brain science research

Time：2024-12-21

In recent decades, the prevalence of neurodegenerative and psychiatric disorders has been rising, driving demand for more sophisticated tools spanning electrophysiology to neuroimaging to deliver reliable diagnostic outcomes. Electroencephalography (EEG), a major branch of electrophysiology, serves as a robust, widely adopted auxiliary technique for neurological diagnosis. Unlike imaging modalities, EEG captures scalp electrical activity at millisecond resolution, delivering superior temporal precision. Via EEG signal processing pipelines and dedicated experimental hardware, researchers can extract quantitative metrics including frequency, amplitude and spectral coherence.

In contrast, Computed Tomography (CT) and especially Magnetic Resonance Imaging (MRI) generate anatomical brain scans with outstanding spatial resolution, enabling multiparametric evaluation of brain tissue covering structural and functional features. Analogous to EEG but operating on vastly different timescales (milliseconds versus seconds), functional Magnetic Resonance Imaging (fMRI) permits noninvasive mapping of brain activation during resting states and task execution. It expands the measurable parameters of conventional MRI techniques: structural connectivity traced via diffusion tensor imaging, metabolite concentrations quantified via magnetic resonance spectroscopy, and cerebral perfusion measured via arterial spin labeling. Multimodal acquisition systems fully leverage complementary multimodal signals to break through the bottlenecks of single-modality detection and improve patient compliance. Within neurological research, simultaneous PET-MRI enables holistic characterization of brain anatomy and physiology, allowing researchers to map structural, functional and metabolic connectomes in a single scanning session. Joint recording of EEG and fMRI to probe brain activity under healthy and pathological conditions carries great practical and research value, as it combines the millisecond-level temporal resolution of EEG and high spatial resolution of fMRI.

EEG

EEG is a dominant technique for recording cerebral electrical activity. The discovery of human brain waves has revolutionized research into brain structure and function, establishing EEG as a foundational tool for both clinical practice and basic neuroscience research. Cortical neural activity originates primarily from the synchronous firing of pyramidal neurons. Uneven charge distribution across these neurons creates negative charges on dendrites and positive charges on somas and axons. Such charge separation forms electric dipoles detectable by scalp electrodes, manifesting as oscillatory positive and negative waveforms. A single pyramidal cell generates an electric field far too weak to be picked up by EEG sensors; electrodes therefore record synchronized populations of parallel-aligned neurons that produce radial and tangential dipoles.

Scalp electrodes are placed following the international 10–20 system, which defines four core landmarks: nasion, inion, and bilateral preauricular points A1 and A2. Fixed with conductive electrode gel, these sensors capture canonical brain oscillations: delta rhythm (0.5–4 Hz), theta rhythm (4–8 Hz), alpha rhythm (8–13 Hz), beta rhythm (13–30 Hz) and gamma rhythm (>30 Hz). Task-evoked potentials can also be recorded to dissect distinct neural processing stages. Evoked potentials are categorized by latency: components occurring within 100 ms post-stimulus reflect exogenous sensory input, while later waveforms correspond to higher-order cognitive processing of stimuli. Technological advances have led to high-density multichannel EEG arrays designed for quantitative EEG analysis and brain network mapping. In routine clinical practice using standard 20-electrode montages, EEG aids diagnosis across a broad spectrum of conditions: metabolic or pharmacologically induced brain alterations, sleep disturbances, epilepsy, neurodegenerative disease, traumatic brain injury, intracranial tumors, coma and brain death assessment.

fMRI

Functional Magnetic Resonance Imaging (fMRI) is a mainstream noninvasive modality for mapping human brain function. Its signal contrast relies on the blood-oxygen-level-dependent (BOLD) effect, which describes magnetic property fluctuations of red blood cells determined by hemoglobin oxygen saturation. Deoxygenated hemoglobin is paramagnetic, whereas oxygenated hemoglobin exhibits diamagnetic properties. At rest, balanced concentrations of these two molecules yield signals indistinguishable from surrounding brain parenchyma. Upon sensory or cognitive stimulation, regional hemoglobin balance shifts transiently: initial accumulation of deoxyhemoglobin suppresses local MRI signal intensity, followed by elevated oxyhemoglobin levels that boost signal readout. These dynamic signal variations are reconstructed into brain activation maps to pinpoint task-responsive brain regions.

It is critical to note that the BOLD signal acts as an indirect proxy for neuronal firing, mediated by neurovascular coupling modulated by cerebral blood flow, cerebral blood volume and complex interactions between activated neurons, astrocytes and cerebral vasculature. Briefly, stimulus-triggered neuronal excitation releases neurotransmitters into synaptic clefts, which are taken up by astrocytic processes. Subsequent calcium transients within astrocytic endfeet trigger vasoactive peptide release, driving hemodynamic fluctuations captured by fMRI. This cascade underlies the inherent temporal lag between neural activity and BOLD signal variation, setting fMRI apart from direct electrophysiological recordings.

Since its inception, fMRI has been widely applied to characterize functional brain connectivity under physiological and pathological states, including brain tumors, multiple sclerosis, Alzheimer’s disease, epilepsy and mood disorders.

Combined fMRI-EEG

Simultaneous EEG-fMRI recording quantifies the correlation between neural electrical activity and hemodynamic responses. fMRI delivers excellent spatial resolution yet suffers from slow BOLD hemodynamic responses on the timescale of seconds, lacking high-frequency temporal sampling capacity. In contrast, EEG achieves millisecond temporal precision but suffers poor spatial source localization accuracy. Concurrent multimodal recording overcomes the inherent limitations of both standalone techniques and expands analytical dimensions to extract richer neural data. Simultaneous acquisition maintains identical mental states, task paradigms and recording environments for subjects — a condition impossible to guarantee when EEG and fMRI are recorded sequentially, especially for cognitively unstable patient cohorts tested in separate scanning environments.

From a technical perspective, the simultaneous EEG/fMRI acquisition involves using specialized EEG hardware that is safe, MR-compatible, and comfortable for the participant. Improper use of the equipment can pose significant risks. Regarding safety, the potential risks to participants arise from heating of the electrodes and conductive leads during MR radiofrequency transmission, which can lead to discomfort or even burns. To reduce the risk of discomfort or injury, precautions should be taken, such as using gradient echo-planar imaging (GE-EPI) sequences for fMRI; for anatomical reference scans, low specific absorption rate (SAR) sequences should be used, especially GE-T1 weighted sequences; and for all sequences in the EEG-fMRI protocol, SAR should not exceed that of GE-EPI sequences. Otherwise, extensive safety tests with temperature sensors are necessary. Staff conducting EEG-MRI research must be properly trained, as accidental misuse of equipment could result in injury from MR-compatible EEG devices, especially during body coil transmission. Following these guidelines is particularly important for participants with reduced alertness (e.g., those asleep or sedated) or those who cannot reliably communicate discomfort (e.g., children).

Greentek EEG-fMRI Compatible Electrode Cap Advantages:

C-type opening electrodes avoid induced currents that form electric fields, minimizing induction-type artifacts.
Non-ferromagnetic electrodes and wires used in conjunction with resistors to avoid forming current loops, ensuring safety.
It is available in multiple freely configurable multi-channel fMRI-EEG compatible electrode caps.
Available in different sizes, suitable for research across various age groups.
It can connect to any EEG system.
Six months warranty.

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EEG Electrode Cap for Combining EEG and fNIRS

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