Electrical impedance tomography (Part I)

Electrical impedance tomography (EIT) is a method of imaging the spatial distribution of conductivity inside the object (in particular human organs), by measuring the voltage on its surface. This voltage is excited by small current that flows through the object in various combinations. EIT is based on fact that different areas inside the object under investigation have different conductivity, thereby current flow changes its direction. That effect leads to boundary potential changes, that can be detected.

Since the 90s of XX century EIT has become a rapidly growing area of research, which involves dozens of research groups in the world. At present, the main areas of EIT are [1]:
- geophysics;
- monitoring and control of industrial processes;
- nondestructive testing;
- medical diagnosis.

In medicine EIT is used for:
- impedance analysis;
- impedance cardiography;
- impedance tomography;
- impedance spectroscopy;
- impedance mammography.
Again EIT of human body is based on fact that different tissues have different impedance, and it is possible to differentiate their image and to detect some physiological changes.

Despite of the poor spatial resolution, EIT has some qualities that are absent in modern high-resolution devices. EIT devices are safe (in comparison with devices that use x-rays), portable, inexpensive, EIT easy to configure and use (as compared with CT and MRI scanners). EIT along with reconstruction of static images allow to get dynamic images. In many cases, the EIT can provide the optimal solution for the criterion of price/performance.

The impedance tomographic image reconstruction is an ill-posed inverse problem by its physical nature [2]. The complexity of EIT compared to, for example, X-ray tomography has the property of nonlocality [1], consisting on the fact that: - that electric current does not flow in the body in a straight line - a local change in electrical conductivity anywhere inside the object has an effect on the potentials at all measuring electrodes. To obtain a stable solution of incorrect inverse problem of EIT some priori information may have used.

Sources:
1. Polydorides, N. Image Reconstruction Algorithms for Soft-Field Tomography: Ph.D. thesis / Polydorides — Manchester, United Kingdom: N. UMIST, 2002. —250 p.
2. Vauhkonen M. Electrical impedance tomography and prior information: Ph.D. thesis / Vauhkonen M. — Kuopio University Publications, 1997. —110 p.