Principle of electrical impedance tomography (EIT) and the functional EIT image (fEIT). Electrical excitation currents are applied between pairs of adjacent surface electrodes (1 to 16); the resulting voltages are measured between the other electrodes (U). In the fEIT image, impedance variation induced by the tidal volume is divided into a 32 × 32 matrix. Each pixel contains the individual tidal impedance variation, creating an image of ventilation distribution. The ventral to dorsal oriented ROIs are marked in gray in the right panel.