fMRI-compatible EEG electrode cap
Category: Brain science research
Time：2024-12-21
Over the past several decades, the incidence of neurodegenerative and psychiatric disorders has been on the rise, necessitating more sophisticated and advanced tools—from electrophysiology to neuroimaging—to achieve reliable diagnostic accuracy. Electrophysiology, particularly electroencephalography (EEG), is a comprehensive and widely used tool that supports the diagnosis of neurological disorders. Unlike imaging techniques, EEG offers exceptional temporal resolution, recording brain electrical activity in milliseconds via electrodes placed on the scalp. Through EEG signal-processing techniques and specialized experimental setups, it is possible to obtain quantitative parameters such as frequency spectra, amplitude, and coherence.
On the contrary, computed tomography (CT), and especially magnetic resonance imaging (MRI), can provide morphological views of the brain with outstanding spatial resolution, enabling multi-parameter assessment of brain tissue properties—including both structural and functional information. In this context, similar to electroencephalography but at a different timescale (milliseconds versus seconds), functional MRI (fMRI) allows for non-invasive investigation of brain functional activation during both resting states and task performance, enriching the array of parameters that MRI can measure—for example, structural connectivity revealed by diffusion tensor imaging, metabolite concentrations detected via magnetic resonance spectroscopy, and perfusion assessed through arterial spin labeling. The complementary nature of this information is being leveraged more deeply by multimodal acquisition systems, which have been developed to overcome the limitations of single-modal approaches and enhance patient compliance. In neurological research, the simultaneous use of PET/MRI has already paved the way for a more comprehensive study of brain tissue and physiology, enabling the examination of brain connectivity across structural, functional, and metabolic domains in a single integrated scan. To fully investigate brain function in both health and disease, simultaneously acquiring both electroencephalographic and functional MRI signals while integrating the optimal temporal and spatial resolutions of these two techniques holds significant potential for both clinical applications and research.
EEG
Electroencephalography (EEG) is one of the most commonly used techniques for studying brain electrical activity. The discovery of EEG and the identification of brain electrical activity have undoubtedly transformed the way we investigate the structure and function of the brain, and over time, EEG has become an essential tool in both clinical practice and research. Brain electrical activity originates from the synchronized firing of cortical neurons, particularly pyramidal cells. These cells exhibit distinct electrical charges along their neuronal membranes: the dendrites carry a negative charge, while the rest of the cell maintains a positive charge. This charge difference determines the dipole moments that EEG electrodes can detect, which are represented as a series of positive and negative waves. However, the electric field generated by a single pyramidal cell is insufficient to produce a detectable EEG signal. Therefore, EEG electrodes record the activity of groups of cells arranged in parallel, generating both radial and tangential dipoles. EEG signals are acquired using the international 10-20 system, which involves placing electrodes on the scalp according to four primary reference points: the nasion, the inion, and two preauricular points (A1 and A2). The electrodes are secured to the scalp with conductive gel, and they record a wide range of brain oscillations, including delta rhythm (0.5–4 Hz), theta rhythm (4–8 Hz), alpha rhythm (8–13 Hz), beta rhythm (13–30 Hz), and gamma rhythm (above 30 Hz). Additionally, during task performance, evoked potentials can be recorded, enabling the study of various neuronal processes (16). Evoked potentials can be categorized based on their latency. Specifically, potentials occurring within 100 milliseconds after stimulation typically reflect the nature of the stimulus itself, whereas subsequent components reflect cognitive processes associated with stimulus perception. With ongoing technological advancements, high-density EEG systems equipped with multiple channels/electrodes have now been developed, allowing for quantitative EEG analysis and studies of brain connectivity. Currently, in clinical settings—typically using a 20-electrode configuration—EEG is employed to characterize a variety of conditions, including metabolic or drug-induced alterations, sleep disorders, epileptic syndromes, neurodegenerative diseases, traumatic brain injuries, tumor lesions, as well as to assess patients in comatose states and determine brain death.
fMRI
Functional magnetic resonance imaging (fMRI) is one of the primary non-invasive techniques used to measure brain function. The mechanism underlying fMRI signals is known as the blood-oxygen-level-dependent (BOLD) effect, which describes changes in the magnetic properties of red blood cells associated with hemoglobin oxygenation. Specifically, deoxygenated hemoglobin—known as deoxyhemoglobin—exhibits paramagnetic properties, whereas oxygenated hemoglobin—known as oxyhemoglobin—exhibits diamagnetic properties. Under resting conditions, the balance between these two forms of hemoglobin in the cerebral vasculature generates a signal that is indistinguishable from that of the surrounding brain tissue. When a stimulus is applied, the hemoglobin balance in specific brain regions shifts, initially favoring an increase in deoxyhemoglobin concentration and thus reducing the fMRI signal. Subsequently, this shift reverses, favoring oxyhemoglobin concentration and leading to an increase in the fMRI signal. Detection of these signal changes is translated into a series of images, which can then be analyzed to reveal the activation of specific brain regions following the performance of a particular task. It is crucial to understand that the BOLD effect represents an indirect measure of neuronal activation, dependent on neurovascular coupling and various interplays involving alterations in blood flow and volume, as well as complex interactions between activated neural circuits and astrocytes and vascular targets. In brief, stimulus-induced neuronal activation triggers the release of neurotransmitters into the synaptic cleft, which are subsequently taken up by astrocytic processes. Secondary astrocytic activation initiates intracellular calcium ([Ca²⁺]) waves in the endfeet of astrocytic processes, thereby eliciting vascular-active peptide release and triggering cellular-molecular and hemodynamic changes that are recorded by fMRI. This intricate cascade of events involving neurovascular coupling and the BOLD effect also accounts for the temporal delay between neuronal activation and fluctuations in the BOLD signal, distinguishing fMRI from direct electrophysiological measurements.
Since its development, functional magnetic resonance imaging (fMRI) technology has been applied to characterize brain functional connectivity under various physiological conditions and in numerous diseases, including brain tumors, multiple sclerosis, Alzheimer’s disease, epilepsy, and mental disorders.
fMRI-EEG combined use
Simultaneous electroencephalography-functional magnetic resonance imaging is used to assess the correlation between brain electrical activity and hemodynamic fluctuations. While functional MRI, with its high spatial resolution, cannot provide sufficient temporal sampling due to the slow BOLD response (measured in seconds), electroencephalography offers high temporal resolution (measured in milliseconds) but suffers from poor source localization. The integration of these two tools in a combined, synchronized acquisition can overcome the inherent limitations of each technique and enable a wide range of additional analyses, thereby increasing the amount of information that can be obtained. Simultaneous recording also ensures that the subject’s mental state, task performance, and recording environment are identical across both modalities. This would not be possible if the two methods were recorded separately—especially when recordings take place in different environments or in patients with cognitive instability.
From a technical standpoint, the simultaneous acquisition of EEG and functional MRI involves the use of specialized EEG hardware that is safe, MR-compatible, and comfortable for participants. Improper use of such equipment could pose significant risks. In terms of safety, potential risks to subjects arise from the heating of electrodes and conductive leads during MR radiofrequency transmission, which may cause discomfort or even burns in the subject. To minimize the risk of discomfort or injury to subjects, several preventive measures should be taken: For instance, functional MRI sequences should be based on gradient-echo echo-planar imaging (GE-EPI); for anatomical reference scans, sequences with low specific absorption rate (SAR) should be employed, particularly GE-T1-weighted sequences; and for all sequences used in EEG-fMRI protocols, their SAR levels must be confirmed to be no higher than those of the GE-EPI sequence. Otherwise, extensive safety testing using temperature sensors will be required. Personnel conducting EEG-MRI studies must receive appropriate training, as injuries caused by MR-compatible EEG devices cannot be ruled out if the equipment is inadvertently operated beyond its specified parameters—especially when body coils are involved in signal transmission. Adherence to these guidelines is particularly crucial for subjects with reduced alertness (such as those who are asleep or sedated) or for subjects who typically cannot reliably report any discomfort (such as children).
Advantages of the Grintech EEG-fMRI-compatible electrode cap:
1. The C-shaped open electrode prevents the formation of an electric field due to induced currents, thereby minimizing induction-induced artifacts to the greatest extent possible.
2. Use non-ferromagnetic electrodes and wires, and in combination with resistors, to prevent the formation of current loops and ensure safety.
3. A variety of freely configurable, multi-channel fMRI-EEG compatible electrode caps are available.
4. Different sizes are available, making it suitable for studies involving people of various age groups.
5. Can be connected to any EEG system.
6. 1-year warranty.
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