ABSTRACT
The specific conductance (conductivity) of human tissues varies from 15.4 mS/cm for cerebrospinal fluid to 0.06 mS/cm for bone. The difference in the value of conductivity is large between different tissues (Table 17.1). Cross-sectional images of the distribution of conductivity, or alternatively specific resistance (resistivity), should show good contrast. The aim of electrical impedance tomography (EIT) is to produce such images. It has been shown [Kohn and Vogelius, 1984a,b; Sylvester and Uhlmann, 1986] that for reasonable isotropic distributions of conductivity it is possible in principle to reconstruct conductivity images from electrical measurements made on the surface of an object. Electrical impedance tomography (EIT) is the technique of producing these images. In fact, human tissue is not simply conductive. There is evidence that many tissues also demonstrate a capacitive component of current flow and therefore, it is appropriate to speak of the specific admittance (admittivity) or specific impedance (impedivity) of tissue rather than the conductivity; hence the use of the world impedance in electrical impedance tomography.
Tissue consists of cells with conducting contents surrounded by insulating membranes embedded in a conducting medium. Inside and outside the cell wall is conducting fluid. At low frequencies of applied current, the current cannot pass through the membranes, and conduction is through the extracellular space. At high frequencies, current can flow through the membranes, which act as capacitors. A simple
