Impedance imaging of lung ventilation: do we need to account for chest expansion?

Electrical impedance tomography uses surface electrical measurements to image changes in the conductivity distribution within a medium. When used to measure lung ventilation, however, measurements depend both on conductivity changes in the thorax and on rib cage movement. Given that current reconstruction techniques assume that only conductivity changes are present, certain errors are introduced. We use a finite element model to calculate the effect of chest expansion on the reconstructed conductivity images. Results indicate that thorax expansion accounts for approximately 20 percent of the reconstructed image amplitude, and its contribution is relatively independent of inspiration depth. We propose that chest expansion can contribute significantly to the conductivity images of lung ventilation, and should be taken into account in interpreting these images