PubMed · 8057061
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V C Wright. 1994. Contemporary electrosurgery: physics for physicians.. https://pubmed.ncbi.nlm.nih.gov/8057061/
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In the present paper DNA is treated as a crystal with symmetry corresponding to a double-helix surface charge density, due to the phosphate groups, immersed in a weak electrolyte. The surrounding solvent is treated via the nonlinear Poisson-Boltzmann equation and the boundary conditions of electrostatics are exactly fulfilled on the DNA-solvent interface. Analytical solutions for the electric potentials and fields inside and outside DNA are obtained. The results give the possibility for a map of the surface potential of DNA to be created. They also show that the electric field inside DNA may decay in two different ways if we change the chemical content of the surrounding solvent. According to this we can draw conclusions about the stability of DNA with respect to the internal and changeable parameters of the system such as chemical content of the aqueous solvent. The position of the condensed counterions around DNA in the Manning cloud can be determined.
Magnetoencephalography (MEG) is a non-invasive functional imaging modality based on the measurement of the external magnetic field produced by neural current sources within the brain. The reconstruction of the underlying sources is a severely ill-posed inverse problem typically tackled using either low-dimensional parametric source models, such as an equivalent current dipole (ECD), or high-dimensional minimum-norm imaging techniques. The inability of the ECD to properly represent non-focal sources and the over-smoothed solutions obtained by minimum-norm methods underline the need for an alternative approach. Multipole expansion methods have the advantages of the parametric approach while at the same time adequately describing sources with significant spatial extent and arbitrary activation patterns. In this paper we first present a comparative review of spherical harmonic and Cartesian multipole expansion methods that can be used in MEG. The equations are given for the general case of arbitrary conductors and realistic sensor configurations and also for the special cases of spherically symmetric conductors and radially oriented sensors. We then report the results of computer simulations used to investigate the ability of a first-order multipole model (dipole and quadrupole) to represent spatially extended sources, which are simulated by 2D and 3D clusters of elemental dipoles. The overall field of a cluster is analysed using singular value decomposition and compared to the unit fields of a multipole, centred in the middle of the cluster, using subspace correlation metrics. Our results demonstrate the superior utility of the multipolar source model over ECD models in providing source representations of extended regions of activity.
A homogeneous spherical volume conductor is used as a model system for the purpose of calculating electric fields induced in the human head by externally applied time-varying magnetic fields. We present results for the case where magnetic field gradient coils, used in magnetic resonance imaging (MRI), form the magnetic field, and we use these data to put limits on the rates of gradient change with time needed to produce nerve stimulation. The electric field is calculated analytically for the case of ideal longitudinal and transverse linear field gradients. We also show results from computer calculations yielding the electric field maps in a sphere when the field gradients are generated by a real MRI gradient coil set. In addition, the effect of shifting the sphere within each gradient coil volume is investigated. Numerical analysis shows similar results when applied to a model human head.