Steffan Puwal

NUMERICAL SIMULATIONS OF SYNCHRONIZED PACING

Pak et al.1 demonstrated an experimental technique for termination of fibrillation in the heart. Their method used feedback pacing, and resulted in an eight-fold increase in the success rate compared to conventional overdrive pacing. Our goal is to study this technique numerically. Computer simulations were performed using the Fenton-Karma model of membrane excitability, with a correction introduced to allow more realistic modeling of external stimulation. We found that both overdrive pacing and independent synchronized pacing resulted in significantly improved success compared to spontaneous termination of fibrillation. We conclude that synchronized pacing may provide a low-energy alternative to traditional defibrillation.

Fourier analysis in magnetic induction tomography: mapping of anisotropic, inhomogeneous resistivity

Magnetic Induction Tomography is an electromagnetic-based technique for mapping the passive electromagnetic properties of conductors and has the potential for applications in biomedical imaging. In a previous analysis we approached the inverse problem of determining isotropic resistivity with a Fourier-based analysis. Here, we extend that analysis to anisotropic media. The proposed Fourier-based solution method, when properly filtered, robustly handles noise to accurately map the inhomogeneous terms of the resistivity tensor. We observe a random variation in the measure of accuracy (mean deviation) that is resolved with independent spatial frequencies in the x- and y-directions in the applied field. Further, the formation of improper images we noted in our previous analysis is addressed through this use of independent spatial frequencies and through the use of additional applied fields. We conclude with a discussion of computation time for the large system of linear equations this method requires and propose methods for limiting memory usage.

Heterogeneous anisotropic magnetic susceptibility of the myelin-water layers causes local magnetic field perturbations in axons†

One goal of MRI is to determine the myelin water fraction in neural tissue. One approach is to measure the reduction in T2* arising from microscopic perturbations in the magnetic field caused by heterogeneities in the magnetic susceptibility of myelin. In this paper, analytic expressions for the induced magnetic field distribution are derived within and around an axon, assuming that the myelin susceptibility is anisotropic. Previous models considered the susceptibility to be piecewise continuous, whereas this model considers a sinusoidally varying susceptibility. Many conclusions are common in both models. When the magnetic field is applied perpendicular to the axon, the magnetic field in the intraaxonal space is uniformly perturbed, the magnetic field in the myelin sheath oscillates between the lipid and water layers, and the magnetic field in the extracellular space just outside the myelin sheath is heterogeneous. These field heterogeneities cause the spins to dephase, shortening T2*. When the magnetic field is applied along the axon, the field is homogeneous within water‐filled regions, including between lipid layers. Therefore the spins do not dephase and the magnetic susceptibility has no effect on T2*. Generally, the response of an axon is given as the superposition of these two contributions. The sinusoidal model uses a different set of approximations compared with the piecewise model, so their common predictions indicate that the models are not too sensitive to the details of the myelin‐water distribution. Other predictions, such as the sensitivity to water diffusion between myelin and water layers, may highlight differences between the two approaches. Copyright