Improving surgical outcome with electric source imaging and high field magnetic resonance imaging.

While most patients with focal epilepsy present with clear structural abnormalities on standard, 1.5 or 3 T MRI, some patients are MRI-negative. For those, quantitative MRI techniques, such as volumetry, voxel-based morphometry, and relaxation time measurements can aid in finding the epileptogenic focus. High-field MRI, just recently approved for clinical use by the FDA, increases the resolution and, in several publications, was shown to improve the detection of focal cortical dysplasias and mild cortical malformations. For those cases without any tissue abnormality in neuroimaging, even at 7 T, scalp EEG alone is insufficient to delimitate the epileptogenic zone. They may benefit from the use of high-density EEG, in which the increased number of electrodes helps improve spatial sampling. The spatial resolution of even low-density EEG can benefit from electric source imaging techniques, which map the source of the recorded abnormal activity, such as interictal epileptiform discharges, focal slowing, and ictal rhythm. These EEG techniques help localize the irritative, functional deficit, and seizure-onset zone, to better estimate the epileptogenic zone. Combining those technologies allows several drug-resistant cases to be submitted to surgery, increasing the odds of seizure freedom and providing a must needed hope for patients with epilepsy.

Validating Non-invasive EEG Source Imaging Using Optimal Electrode Configurations on a Representative Rat Head Model.

The curtain of technical limitations impeding rat multichannel non-invasive electroencephalography (EEG) has risen. Given the importance of this preclinical model, development and validation of EEG source imaging (ESI) is essential. We investigate the validity of well-known human ESI methodologies in rats which individual tissue geometries have been approximated by those extracted from an MRI template, leading also to imprecision in electrode localizations. With the half and fifth sensitivity volumes we determine both the theoretical minimum electrode separation for non-redundant scalp EEG measurements and the electrode sensitivity resolution, which vary over the scalp because of the head geometry. According to our results, electrodes should be at least ~3 to 3.5 mm apart for an optimal configuration. The sensitivity resolution is generally worse for electrodes at the boundaries of the scalp measured region, though, by analogy with human montages, concentrates the sensitivity enough to localize sources. Cramér-Rao lower bounds of source localization errors indicate it is theoretically possible to achieve ESI accuracy at the level of anatomical structures, such as the stimulus-specific somatosensory areas, using the template. More validation for this approximation is provided through the comparison between the template and the individual lead field matrices, for several rats. Finally, using well-accepted inverse methods, we demonstrate that somatosensory ESI is not only expected but also allows exploring unknown phenomena related to global sensory integration. Inheriting the advantages and pitfalls of human ESI, rat ESI will boost the understanding of brain pathophysiological mechanisms and the evaluation of ESI methodologies, new pharmacological treatments and ESI-based biomarkers.

Imagem por Fontes Eletroencefalográficas - Uma Revisão Sistemática Sobre Sua Acurácia na Determinação da Zona Epileptogênica em Pacientes com Epilepsia Focal Refratária / EEG source imaging - a systematic review about its accuracy in defining the epileptogenic zone in patients with refractory focal epilepsy

O objetivo da avaliação pré-cirúrgica em pacientes com epilepsia refratária é delimitar a zona epileptogênica (ZE), área do encéfalo capaz de gerar crises e cuja ressecção tem o potencial para abolir ou reduzir as crises do paciente. Neste sentido, há um grande esforço no desenvolvimento e aprimoramento de técnicas diagnósticas não invasivas que possam localizar a ZE com precisão, buscando evitar ou diminuir a utilização de métodos invasivos, de custo e risco elevados. Uma técnica diagnóstica que tem recebido renovada atenção é a Imagem de Fontes Eletroencefalográficas (IFE). O uso dessa técnica se baseia no fato de que a localização da área do encéfalo geradora das descargas interictais (zona irritativa) guarda próxima relação com a ZE. Estudos recentes têm sugerido que a IFE tem um potencial para determinar a localização da ZE similar à magnetoencefalografia. Nesta revisão, analisamos estudos recentes utilizando a técnica na localização da ZE de pacientes com epilepsia refratária. Encontramos evidências de que a acurácia média do teste foi de 79%, bastante similar à acurácia da Imagem por Fontes Magnéticas reportada na literatura, que é de aproximadamente 77%.

The main goal of presurgical evaluation in patients with refractory epilepsy is to define the localization and extension of epileptogenic zone (EZ), the brain area responsible for generating seizures and whose resection has the potential to reduce or abolish epileptic seizures. Therefore, there has been an effort to develop diagnostic tests that can accurately localize the EZ non-invasively, avoiding invasive investigations that are risky and expensive. A diagnostic technique that has received renewed interest is electroencephalographic source imaging (ESI). This technique is based on the assumption that the irritative zone, the brain area that generates interictal EEG spikes, is spatially related with the EZ. Recent studies have shown that EEG has the potential to determine the localization of EZ similar to magnetoencephalography. In this review, we searched for studies reporting the accuracy of ESI on presurgical evaluation of patients with refractory epilepsy. We found that the accuracy of the test was 79% overall, similar to the accuracy of magnetic source imaging reported in the literature (77%).