Electrical impedance tomography based on relaxation measurements in time domain

电阻抗断层扫描（EIT）是一种实时功能成像技术，不需要电离辐射或强磁场，适用于床边，也可能适用于移动的应用。市场上的大多数系统使用单频测量，并且能够很好地区分具有非常不同阻抗的材料（空气、组织）。目前的研究集中在多频系统，这也允许区分不同的组织。目前的多频测量方法只能测量少数几个频率（多正弦）或导致低帧率（步进正弦）。作为一个优雅的替代方案，我们认为时域测量是一个有前途的方法来处理这些限制。为了实现现有的多频EIT方法的基础上在时域中使用弛豫光谱测量提供了几个优点。当施加宽带激励信号时，可以同时测量任何数量的时间点和频率。对于多频率EIT，这意味着可以进一步增加扫描速率以更好地可视化快速生理过程。也可以在给定的扫描速率下增加测量频率的数量，以便更好地区分组织。结合非等距采样，可以大大减少弛豫测量中的数据量。在高EIT扫描速率、多个采样点和大量电极的情况下，要传输和处理的数据量越来越成为瓶颈。在这里，非等距扫描可以设定新的标准。弛豫光谱可以用最少的硬件来实现，这也意味着设备的形状因子更小。这允许将测量仪器直接集成到有源电极中，并且仅将数字信号传输到控制单元。一方面，这将消除作为干扰因素的长测量引线，另一方面，测量技术的短连接将允许测量更高的频率范围，这也导致更准确的组织表征。然而，为了使这些潜力可用于EIT，重要的基础知识需要进行调查。本研究的目的是为时域EIT测量建立理论和实践基础，从而通过时域测量的优势大大扩展EIT的应用范围。

Electrical impedance tomography (EIT) is a real-time functional imaging technique that does not require ionizing radiation or strong magnetic fields and is suitable for bedside and potentially for mobile applications as well. Most systems on the market use single frequency measurements and are able to discriminate well between material with very different impedance (air, tissue). Current research focuses on multi-frequency systems, which also allow distinguishing between different tissues. Present approaches to multi-frequency measurement can only measure a few frequencies (multi-sine) or lead to a low frame rate (step-sine). As an elegant alternative, we consider time domain measurements to be a promising approach to deal with these limitations. To realize the existing multi-frequency EIT methods based on measurements in time domain using relaxation spectroscopy offers several advantages. When applying a broadband excitation signal, any number of time points respectively frequencies can be measured simultaneously. For multi-frequency EIT, this means that the scan rate can be further increased to better visualize fast physiological processes. It is also possible to increase the number of measured frequencies at the given scan rate in order to better differentiate the tissues. In combination with non-equidistant sampling, the amount of data can be reduced substantially in relaxation measurements. At high EIT scan rates, multiple sampling points, and a large number of electrodes, the amount of data to be transmitted and processed becomes more and more a bottleneck. Here, non-equidistant scanning can set new standards. Relaxation spectroscopy can be realized with a minimum of hardware, which also means a smaller form factor of the device. This allows to integrate the measurement instrumentation directly into active electrodes and to transmit only digital signals to a control unit. On the one hand, this would eliminate long measurement leads as an interference factor, and on the other hand, the short connection of the measurement technology would allow higher frequency ranges to be measured, which also results in more accurate tissue characterization. However, in order to make these potentials available for EIT, important basics need to be investigated. The aim of this research project is to establish a theoretical and as well practical basis for EIT measurements in time domain and thus to substantially expand the range of applications of EIT through the advantages of time domain measurement.