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Monitoring lung air and fluid volume is a promising EIT application. Respiratory activity causes both chest expansion and position changes of tissues/organs in the thorax, which contributes measurement error. In EIT application, the electrodes are placed around chest in a two-dimensional ring, and image is reconstructed on the assumption that the object is two-dimensional and the images are due to resistivity change. EIT reconstruction algorithms generally do not consider organ and tissue positional change. Finite difference models (FDM) were developed based on MR images with different air volumes and used to study the effect of tissues/organs' motion. Results show that the tissues/organs' motion influence the region around the heart, which introduce the artifacts in the middle anterior region. The tissues/organs' motion also increases the resistivity change in the lung regions, which cannot be neglected. We conclude that tissues/organs' motion can cause serious error in the interpretation of EFT ventilation images.