Biomedical subjects

Federica Vatta

Publications and source records attributed to Federica Vatta.

4 recordsLinked to original sources

PubMed2003

Head model extension for the study of bioelectric phenomena.

Bioelectrical phenomena spread within the whole body (the conductor medium) independently of electrical source position within the body. However, under certain circumstances, it is possible to limit the volume within which the study can be done. Given its high resistivity, the skull limits the spread of bioelectrical currents due to brain sources and it leaves only few holes for current flow, namely the occipital hole and the openings for the optic nerves. This is a simulation study performed adopting realistic head models extended to different percentages of the whole head volume conductor to determine the possibility of limiting the model volume for bioelectric field analysis. A realistic head model extended to the chin was used as reference model to analyze three reduced model extensions: 80% of the volume of the reference model (including the neck upper part), 70% (including all the skull but not the neck) and 60% (cutting the head at cerebellum level). The lower limit of the reduced model was named "cut-plane". We simulated the head electrical potential generated by various dipole current sources within the brain, either far from or near the cut-plane and either orthogonal or parallel to it. The scalp potential distributions were compared between each reduced model and the reference model by means of relative-difference measure (RDM). The larger differences were found for sources near the cut-plane and for sources orthogonal to it. The differences increased non linearly with model volume reduction, dramatically augmenting as the skull was intercepted by the cut-plane. The same model performed differently according to source position relative to the particular head structure.

Action Potentials↗

PubMed2002

Improving lesion conductivity estimate by means of EEG source localization sensitivity to model parameter.

EEG-based source localization techniques use scalp-potential data to estimate the location of underlying neural activity. EEG source location reconstruction requires the assumption of a source model and the assumption of a conductive head model. Brain lesions can present conductivity values that are dramatically different from those of surrounding normal tissues and have to be included in head models for accurate neural source reconstruction. It is therefore necessary to analyze subjects' anatomic images (using MRI or computed tomography) to identify lesion type and to assign the appropriate conductivity value. Source localization accuracy may be influenced by uncertainties in tissue conductivity assignment during head model construction. The authors present a sensitivity study quantifying the effect of uncertainty in brain lesion conductivity assignment on EEG dipole source localization. They adopted an eccentric-spheres head model in which an eccentric bubble approximated the effects of actual brain lesions. After simulating EEG signal measurement in 64 different pathologic situations, an inverse dipole fitting procedure was carried out, assuming an incorrect lesion conductivity assignment ranging from a half to twice the real value. Incorrect lesion conductivity assignment led to markedly wrong source reconstruction for highly conductive lesions like liquid-filled ones (localization errors as much as 1.7 cm). Conversely, low sensitivity to uncertainties in conductivity assignment was found for lesions with low conductivity like calcified tumors. The authors propose a method based on residual error analysis to improve the lesion conductivity estimate. This procedure can identify lesion tissue conductivity with only a few percent error and guarantees source localization errors less than 5 mm.

Brain↗

PubMed2002

Accuracy of EEG source reconstruction in the presence of brain lesions: modelling errors and surface electrodes' placement.

Source localization techniques based on electroencephalography (EEG) use scalp potential data to infer the location of brain neural activity. A volume conductor model describing the electrical properties of the human head is needed. Lesions have conductivity considerably different from that of normal brain and should be included in the head model because the differences between the actual head and the model can cause source reconstruction errors. We performed a simulation study investigating EEG dipole source reconstruction errors, caused by brain lesions neglecting, using different measurement montages. The scalp was sampled by 64 electrodes (simulating clinical practice) and by 128 electrodes (extended configuration). The human head was represented by an eccentric-spheres model in which a modifiable eccentric bubble approximated various brain lesions. We analyzed 64 pathological situations. Results showed that neglecting brain lesions in source reconstruction procedures could cause large source localization errors which depended on source location and orientation, and varied with EEG montage. The maximum source localization errors (LE) were 2.5 cm and 1.4 cm for the 64 and 128 electrode configurations respectively. The largest errors occurred for sources nearby the lesion. LE was not systematically smaller with 128 rather than 64 electrodes. Maximum intensity errors were similar for 128 and 64 electrodes. The EEG inverse dipole solution was maximally sensitive to the electrode configuration on the scalp when the source was located deep in the brain (e.g. in the brain stem). We concluded that deep source localization needed an extended sampling of the scalp.

Brain Diseases↗

PubMed2002

Lesion type misidentification: EEG potential sampling and source reconstruction errors.

Accurate EEG source reconstruction needs an appropriate volume conductor head model including, in the presence of a morphological brain lesion, a lesion compartment. Lesion electrical properties (conductivity) can not be measured in vivo and need to be retrieved from literature on the base of lesion type identification, performed by means of diagnostic imaging. However, different pathologies can appear similar at bioimages inspection, leading to uncertain diagnosis and in turn to wrong lesion conductivity assumption. Besides many factors, source reconstruction accuracy depends on lesion conductivity value and on electrodes placing (scalp potential sampling). We investigated the relation between electrodes sampling of the EEG and source reconstruction accuracy in case of uncertain lesion type identification. We adopted an eccentric-spheres head model (including the lesion) and a dipole source. We simulated several pathological conditions considering different electrode montages on the scalp: a "clinical like" sampling with 64 electrodes and an extended sampling with 128 electrodes. Source reconstruction was performed assuming wrong lesion type identification (i.e. introducing an error in the model parametrical setting). We found large errors for both source localization (LE) and source intensity estimations. Maximum LE could be reduced (from 30 to 27 mm) by the extended sampling. Conversely, the mean values of LE mostly increased using more electrodes. The benefit of enlarging the sampling was clear for the intensity estimation. In conclusion, only the most inaccurate source reconstruction could be improved changing electrodes placing; the inherent source reconstruction error due to model parameter setting could not be avoided.

Brain Diseases↗