Single-plane multifrequency electrical impedance instrumentation.

When tissue interacts with electromagnetic radiation it exhibits resistivity and permittivity changes, which decrease with frequency. Above 100 kHz it is expected that dielectric changes in tissue (permittivity) will allow one to distinguish damaged and necrotic tissue. Furthermore, tissue impedance at medium frequencies (100 kHz-1 MHz) have not been well characterized. The aim of this work was to design instrumentation for an impedance tomographic spectrometer to cover the minimum band 10 kHz-1 MHz. In order to produce images sensitive to small changes in resistivity, voltage measurement must be accurate to at least 0.1%. Using commercially available operational amplifiers, PSPICE simulations demonstrated 0.1% accuracy up to 800 kHz, falling off to 0.5% at 1 MHz. Implementation achieved a reasonably flat amplitude (+/- 0.5 dB) and a phase shift of 50 degrees from 10 kHz to 3 MHz and a receive response of 0.13 dB to 5 MHz and phase shift of -40 degrees at 3 MHz. With channel correction this design will provide useful readings up to 3 MHz.

Raw data interchange format for electrical impedance tomography.

A data interchange format is described to allow groups working on electrical impedance tomography (EIT) with disparate algorithms and instruments to compare results. The procedure has been tested by exchanging data by e-mail. The format is defined in the appendix.

Multifrequency electrical impedance tomography.

Multifrequency tomography may be conveniently achieved by sequentially sweeping the probing drive current and measuring the resultant voltages at each frequency. If events change during measurement comparisons between frequencies cannot be made. Mixing several frequency components may decrease acquisition time but increase the complexity of the instrumentation. A third method is described using Fourier transformation that enables simultaneous multifrequency measurements without an increase in instrumentation. A signal is constructed from a number of sinusoidal components of known amplitude and phase. This group of components is transformed into a time series by the inverse Fourier transform and applied to the object via a voltage or current source. Transforming the detected voltage back into Fourier components will provide the frequency response of the object. Data are collected in this way for all projections and tomograms reconstructed for each frequency. This has the advantage that no special detector is required; both in-phase and quadrature components are available and systematic errors may be easily corrected by software. This technique is demonstrated using a resistor phantom with known frequency-dependent perturbations.