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8 ELECTRICAL SOURCE IMAGING
Pages 133-142

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From page 133...
... The classical approach to studying brain electrical activity involves recognizing patterns in a set of waveforms showing voltage as a function of time, acquired from about 20 electrodes placed on the scalp. While frequency analysis methods can indicate probable ATzheimer's disease by the 133
From page 134...
... Non-invasive electrocardiography requires simultaneous recordings of electrical potential from 100 to 250 torso sites in order to map the body surface potential. These body surface potential maps (BSPMs)
From page 135...
... Recently, multielectrode probes have been developed with 65-100 evenly distributed electrodes on their surface that do not occlude the ventricular cavity and thus can be used in the catheterization laboratory for clinical and basic medical science studies. Since the probe electrodes are not in direct contact with the endocardium, the measured potentials can provide only a Tow-resolution reflection of the endocardial potentials, smoothed out by the high conductivity of the intracavitary blood.
From page 136...
... The numerical solution for the inverse problem requires two steps. The first step is the construction of a mathematical expression describing the potential field throughout the torso volume conductor, particularly on the body surface, as a function of epicardial potential data (the forward problem)
From page 137...
... After manipulation and regrouping of terms, the following matrix equation is obtained: VT = AVE, where VE is the vector of cerebral cortex or epicardial potentials, VT is the vector of scalp or torso potentials, and A is the transfer matrix between heart and torso that depends on anatomic and electrical properties of hearttorso geometry. The inverse problem requires an inversion of A to give VE in terms of VT Inversion is performed with a least-squares approach, minimizing the residual vector with respect to VE, :Leading to the solution VE = (A A)
From page 138...
... A VT S.2.3 Temporal Regularization The electrical mapping problem is also characterized by the fact that brain and cardiac activation is a continuous process in time. Incorporation of the continuous time progression into the regularization procedure has been done using Twomey regularization.
From page 139...
... Solid contour lines indicate positive potentials; broken contour lines indicate negative potentials. RE: relative error.
From page 140...
... The following research areas need further development before the potential of EST can be realistically gauged. The inverse computation of cerebral cortex and epicardial potentials requires knowledge of the geometries and con~luctivities of intervening tissues.
From page 141...
... 4. I,aw, S.K., Nunez, P.I,., and Wijesinghe, R.S., High-resolution BEG using spline generated surface laplacians on spherical and ellipsoidal surfaces, [EKE Trans.


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