Electrical capacitance tomography

Electrical capacitance tomography (ECT) is a method for determination of the dielectric permittivity distribution in the interior of an object from external capacitance measurements. It is a close relative of electrical impedance tomography and is proposed as a method for industrial process monitoring, although it has yet to see widespread use. Potential applications include the measurement of flow of fluids in pipes and measurement of the concentration of one fluid in another, or the distribution of a solid in a fluid. 
Although capacitance sensing methods were in widespread use the idea of using capacitance measurement to form images is attributed to Maurice Beck and co-workers at UMIST in the 1980s.
Although usually called tomography, the technique differs from conventional tomographic methods, in which high resolution images are formed of slices of a material. The measurement electrodes, which are metallic plates, must be sufficiently large to give a measureable change in capacitance. This means that very few electrodes are used and eight or twelve electrodes is common. An N-electrode system can only provide N(N−1)/2 independent measurements. This means that the technique is limited to producing very low resolution images of approximate slices. However, ECT is fast, and relatively inexpensive.