A multifrequency serial EIT system

Code-division-multiplexed electrical impedance tomography spectroscopy

Electrical impedance tomography uses multiple impedance measurements to image the internal conductivity of an object, such as the human body. Code-division multiplexing is proposed as a new method that can provide simultaneous impedance measurements of the multiple channels. Code division provides clear advantages of a wide frequency range at reduced cost and reduced complexity of sources. A potential drawback is the lack of perfectly orthogonal code sets. This caused an increase of 0.62% in root-mean-square spectral error when two codes were used to record two impedance channels simultaneously on a low-pass filter network. The method described provides images and spectra which are equivalent to the conventional time-multiplexed method, with increases in frequency resolution and measurement speed which may be of benefit in some applications of electrical impedance tomography spectroscopy.

A fundamental study on parameter estimation of layerd local tissue impedance for EIT

Electrical Impedance Tomography (hereinafter referred to as EIT) is 2-D or 3-D image of electrical impedance distribution in a living tissue. Unlike usual imaging methods, i.e., Xray-CT, MRI and US Imagings, EIT is used for imaging the information of tissue structure and functions. This paper provides a new estimation method as a fundamental study to realize a EIT for local biological tissue. Up until now we has proposed a new configuration of the electrodes, called divided electrode, for a high-speed measurement of bio-impedance in a cross section of a local tissue. The cross section of the tissue was represented by space distributed equivalent circuits of tissue structure known, and their parameters were estimated by inverse algorithm. In this paper, we try to estimate the parameter value of a layered structural model, the thickness of the layer, and the boundary without using US-imaging by using the divided electrode. Its capability is examined by computer simulations, where a distributed equivalent circuit is used as a model of the tissue. Estimation of impedance parameter is carried out by use of the Gauss-Newton method. Usefulness of the proposed method is confirmed by computer simulations using a typical layered tissue model.

A review of errors in multi-frequency EIT instrumentation

Multi-frequency electrical impedance tomography (MFEIT) was proposed over 10 years ago as a potential spectroscopic impedance imaging method. At least seven systems have been developed for imaging the lung, heart, breast and brain, yet none has yet achieved clinical acceptance. While the absolute impedance varies considerably between different tissues, the changes in the spectrum due to physiological changes are expected to be quite small, especially when measured through a volume. This places substantial requirements on the MFEIT instrumentation to maintain a flat system frequency response over a broad frequency range (dc-MHz). In this work, the EIT measurement problem is described from a multi-frequency perspective. Solutions to the common problems are considered from recent MFEIT systems, and the debate over four-terminal or two-terminal (multiple source) architecture is revisited. An analysis of the sources of MFEIT errors identifies the major sources of error as stray capacitance and common-mode voltages which lead to a load dependence in the frequency response of MFEIT systems. A system that employs active electrodes appears to be the most able to cope with these errors (Li et al 1996). A distributed system with digitization at the electrode is suggested as a next step in MFEIT system development.