The complex impedance of each kind of tissue depends on the frequency in a characteristic manner. Using appropriate measuring frequencies, EIT can provide a differentiating insight into the interior of a body. Therefore, a knowledge of the tissue impedance spectra of various organs is essential for choosing the appropriate frequencies. The impedance data of various tissues in different states (normal, altered by ischaemia or cancerous) show that the characterizing differences occur at frequencies below 500 kHz and down to a few kilohertz. Moreover, the spectra show that the imaginary component of impedance essentially contributes to the characterization of the kind and state of a tissue, even though the dissipative and reactive components are connected by the Kramers-Kronig relations. The course of a dispersion and the position in the frequency range, determined by the distribution of the time constants in the tissue, are clearly presented by the imaginary component. Tomographic imaging combined with spectroscopy for tissue characterization requires a frequency range of at least 10-800 kHz. The upper frequency limit depends on the fluid content of the tissue under investigation.
