Application of electrical impedance tomography to the determination of lung volume.

Trans-thoracical electrical impedance measurements on a human subject have been performed using the Sheffield equipment. These measured transfer impedances are compared with computed ones, based on an inhomogeneous, torso-shaped volume conductor, for different values of the overall electrical conductivities sigma l and sigma r of left and right lung. Each measured set of impedances is compared with all calculated sets. The remaining (minimum) residual difference epsilon is used as a measure of the goodness of fit, and the values of sigma l and sigma r for which epsilon is minimal are taken as representing the overall conductivities of the lungs. For data measured at various stages within the respiratory cycle, sigma l and sigma r have been determined. The results are found to relate well to physiology: for a measurement taken at maximum expiration sigma l and sigma r indicate almost homogeneity, whereas for end inspiratory level sigma l and sigma r tend towards values of 0.2, which is a realistic value for the relative conductivity of lung tissue.