Imaging of local lung ventilation under different gravitational conditions with electrical impedance tomography

The regional lung ventilation depends on the amount and direction of applied gravitational force. This article presents a technique suitable for microgravity condition, providing online information about the local dynamic behavior of the lungs. Using state-differential images measured by electrical impedance tomography (EIT), for every region of interest (ROI) of the lungs a median signal was calculated. For the detection of regional differences in the lung ventilation, local impedance signals were compared with a reference signal, which was the median value over all pixels inside the lungs.We compared both the difference in magnitude and the phase shifts between the ROIs. The phase information was calculated using the Hilbert transform. The technique was tested on five spontaneously breathing subjects.The phase difference proved to be a very sensitive indicator for changes in the regional ventilation during postural changes and therefore changes of the direction of gravitational forces.