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.