Electrical impedance tomography (EIT) is a radiation-free functional imaging modality that allows non-invasive bedside monitoring of both regional lung ventilation and arguably perfusion. Commercially available EIT devices were introduced for clinical application of this technique, and thoracic EIT has been used safely in both adult and pediatric patients

3. EIT Measurements and Image Reconstruction

Electrodes are either single self-adhesive electrodes (e.g., electrocardiogram, ECG) that are placed individually with equal spacing in-between the electrodes or are integrated in electrode belts . Also, self-adhesive stripes are available for a more user-friendly application .Chest wounds, chest tubes, non-conductive bandages or conductive wire sutures may preclude or significantly affect EIT measurements. Commercially available EIT devices usually use 16 electrodes, but EIT systems with 8 or 32 electrodes are also available

Different available color codings of EIT images in comparison to the CT scan. The rainbow-color scheme uses red for the highest relative impedance (e.g., during inspiration), green for a medium relative impedance, and blue for the lowest relative impedance (e.g., during expiration). A newer color scales use instead black for no impedance change), blue for an intermediate impedance change, and white for the strongest impedance change.

4. Functional Imaging and EIT Waveform Analysis

Analysis of EIT data is based on EIT waveforms that are formed in individual image pixels in a series of raw EIT images over time (A region of interest (ROI) can be defined to summarize activity in individual pixels of the image. In each ROI, the waveform displays changes in regional conductivity over time resulting from ventilation (ventilation-related signal, VRS) or cardiac activity (cardiac-related signal, CRS). Additionally, electrically conductive contrast-agents such as hypertonic saline can be used to obtain an EIT waveform (indicator-based signal, IBS) and may be linked to lung perfusion. The CRS may originate from both the cardiac and lung region and may be partly attributed to lung perfusion. Its exact origin and composition are incompletely understood Frequency spectrum analysis is frequently used to discriminate between ventilation- and cardiac-related impedance changes. Non-periodic changes in impedance may be caused by changes in ventilator settings.

Functional EIT (fEIT) images are generated by applying a mathematical operation on a sequence of raw images and the corresponding pixel EIT waveforms. Since the mathematical operation is applied to calculate a physiologically relevant parameter for each pixel, regional physiological characteristics such as regional ventilation (V), respiratory system compliance as well as regional perfusion (Q) can be measured and displayed Data from EIT waveforms and simultaneously registered airway pressure values can be utilized to calculate the lung compliance as well as lung opening and closing for each pixel using changes of pressure and impedance (volume). Comparable EIT measurements during stepwise inflation and deflation of the lungs allow the displaying of pressure-volume curves on a pixel level. Depending on the mathematical operation different types of fEIT images may address different functional characteristics of the cardio-pulmonary system.