ELECTRICAL-IMPEDANCE TOMOGRAPHY OF COMPLEX CONDUCTIVITY DISTRIBUTIONSWITH NONCIRCULAR BOUNDARY

Electrical impedance tomography (EIT) uses low-frequency current and v oltage measurements made on the boundary of a body to compute the cond uctivity distribution within the body. Since the permittivity distribu tion inside the body also contributes significantly to the measured vo ltages, the present reconstruction algorithm images complex conductivi ty distributions. A finite element model (FEM) is used to solve the fo rward problem, using a 6017-node mesh for a piecewise-linear potential distribution. The finite element solution using this mesh is compared with the analytical solution for a homogeneous field and a maximum er ror of 0.05% is observed in the voltage distribution. The boundary ele ment method (BEM) is also used to generate the voltage data for inhomo geneous conductivity distributions inside regions with noncircular bou ndaries. An iterative reconstruction algorithm is described for approx imating both the conductivity and permittivity distributions from this data. The results for an off-centered inhomogeneity showed a 35% impr ovement in contrast from that seen with only one iteration, for both t he conductivity and the permittivity values. It is also shown that a s ignificant improvement in images results from accurately modeling a no ncircular boundary. Both static and difference images are distorted by assuming a circular boundary and the amount of distortion increases s ignificantly as the boundary shape becomes more elliptical. For a homo geneous field in an elliptical body with axis ratio of 0.73, an image reconstructed assuming the boundary to be circular has an artifact at the center of the image with an error of 20%. This error increased to 37% when the axis ratio was 0.64. A reconstruction algorithm which use d a mesh with the same axis ratio as the elliptical boundary reduced t he error in the conductivity values to within 0.5% of the actual value s.