Simultaneous EEG-fMRI for Functional Neurological Assessment

The increasing incidence of neurodegenerative and psychiatric diseases requires increasingly sophisticated tools for their diagnosis and monitoring. This review will focus on simultaneous EEG-fMRI technology and related early studies.

EEG:

EEG is one of the most used techniques for studying brain electrical activity. Brain electrical activity is derived from the synchronizations of a pool of cortical neurons, in particular of pyramidal cells. These cells present a different electrical charge along the neuron, resulting negative on dendrites and positive in the rest of cell. This difference determines an electric dipole that can be acquired by EEG electrodes, and represented as a series of positive and negative waves. However, the electric field derived by a single pyramidal cell is not enough to obtain a detectable EEG signal. For this reason, the electrodes record a pool of cells arranged parallel to each other, and producing radial and tangential dipoles.

fMRI:

The fMRI is one of the main non-invasive techniques that allow to measure brain function. The mechanism that subtends the signal of fMRI is called blood oxygen level dependent (BOLD) effect, that describes the variation in the magnetic status of the red blood cells linked to the hemoglobin oxygenation. Indeed, the form of hemoglobin without oxygen is deoxyhemoglobin, which has paramagnetic property, while oxyhemoglobin has diamagnetic property. In resting conditions, the balance between these two elements concentrations in the vascular brain system, provides a signal indistinguishable from the surrounding parenchyma. When a stimulus was applied, the hemoglobin balance in specific brain areas changes, initially in favor to deoxyhemoglobin concentration and so determining a decrease of signal, and following switching in favor to oxyhemoglobin concentration and a signal increase.The detection of these signal changes translates into a series of images, that can be analyzed to show the activations of specific brain areas.

Simultaneous EEG-fMRI

Unlike imaging techniques, EEG offers an excellent temporal resolution, recording the electric brain activity in the order of milliseconds through electrodes placed on the scalp. fMRI, provide a morphological view of brain, with an excellent spatial resolution, allowing a multiparametric assessment of the brain tissue properties, both in terms of structural and functional information. In this context, similarly to EEG but at different temporal scales (milliseconds vs. seconds). The integration of these two tools in a hybrid simultaneous acquisition allows to overcome the intrinsic limitations of both the techniques and to increase the plethora of analyses that can be performed, and in turns, of the information that can be achieved.

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