Abstract:

Electrical impedance tomography is an accepted and validated tool to analyze and support mechanical ventilation at the bedside. In the future it could furthermore clinically provide information of the pulmonary perfusion and other blood volume changes within the thorax by exploiting a cardiosynchronous EIT component. In the presented study, the spatial forward sensitivity against different background lung tissue distributions was analyzed. Spheres with a 10% change of the background conductivity were introduced in the lungs and in the heart. The cranio-caudal distribution of sensitivity had a bell shape and was similar between all simulated scenarios, varying only in magnitude. If the background tissue conductivity within the lungs was chosen to be the one of deflated tissue, the overall sensitivity was 46% smaller compared to the overall sensitivity against inflated lung tissue conductivity. Within the heart region, the sensitivity was increased for fully deflated lung tissue conductivity (23% relative to the sensitivity in the lungs) compared to a homogeneous distribution of inflated lung tissue conductivity (10% relative to the sensitivity in the lungs).

Abstract:

Electrical Impedance Tomography (EIT) is a radiation-free functional imaging modality used in biomedical applications, primarily, to obtain real-time images of the lungs. Since its invention, there has been an increasing scientific and clinical interest in this technology driven by the clinical need for monitoring and assessing the regional lung function at the bedside. One of the existing drawbacks of this measurement technique is, that it generates two-dimensional images from data which is influenced by three-dimensional parameters. One such parameter is pulmonary perfusion. This thesis aims to analyze the sensitivity of the measurements to a change in the conductivity due to perfusion at certain locations within the body. The aim is to estimate the contribution of varying conductivity on the readings and consequentially, generate a three-dimensional sensitivity profile which could help in better interpretation and understanding of the EIT. This profile can be used as a guideline by technicians, while developing new reconstruction algorithms, to correctly estimate the context of the measured signals. In order to achieve this, forward simulations were carried out on an FEM model of the human thorax. The model was generated by segmentation of MRI images. It was infused with spheres with higher conductivities at distinct locations in the lungs to simulate pulmonary perfusion. The results generated by the simulated measurements were calculated and compared to quantify the change in the output with respect to change in the position of the sphere relative to the belt. The main results that were sought out were: the sensitivity of the EIT measurements to perfusion and how the reconstructed images projected the effects of pulmonary perfusion in 3D, on the 2D reconstruction plane. The results obtained were in accordance with the hypotheses. The sensitivity analysis showed a varying profile which showed that the sensitivity reduced as the perfusion occurred higher up in the lung. The expected effects were also visible in the reconstructed images, which showed the position error as well as amplitude response to the perfusion changes.