ABSTRACT
Electrical impedance tomography (EIT) is a medical imaging modality that typically uses a single ring of external electrodes to image the impedance changes within the body. Impedance refers to the complex value that is used in electrical engineering to describe the electrical properties, resistance and reactance, of an object. In a biological system such as the human body, electrolytes such as Na+, Ka+, and Cl− are abundant. The distribution of these ions in the body results in the different electrical impedances of each tissue and by extension each organ of the body. Physiological processes as fundamental as breathing or circulation of blood change the electrical impedance of the corresponding anatomy, which can be imaged using EIT. The estimation of internal impedance is achieved by the excitation of the electric eld in the body using a small electrical current at high frequency, typically less than 5 mA at 50 kHz through two of the 16 or 32 surface electrodes. Changes in voltages at the same frequency (50 kHz) can be measured via the rest of the surface electrodes. Mathematical modeling can then be used to infer the internal distribution of impedance through these voltage measurements (Figure 12.1).
