Residual capacitive coupling and the measurement of permittivity in magnetic induction tomography.

In an ideal magnetic induction tomography (MIT) system, the coupling between the coils and the sample is entirely by the magnetic field. In a practical system, unwanted electric-field (capacitive) coupling can also exist and cause large errors in the MIT measurements unless the hardware is designed carefully. A series of tests was carried out to assess the magnitude of capacitive coupling present in a 10 MHz MIT system designed for biomedical use and other applications involving low-conductivity samples (<or=10 S m(-1)). The tests indicated that, even with the individual coils left unscreened, the signal contamination from capacitive coupling was very small compared with the true MIT signal. Because the contamination was small, it was demonstrated possible to derive the permittivity of the sample from the real part of the MIT signal. This was shown to work well when the conductivity of the sample was less than about 0.5 S m(-1), but for higher conductivities, when the skin depth became comparable with the width of the sample, the commonly used theoretical expression for the MIT signal began to break down. This implies that the measurement of permittivity (and permeability) in real biological tissues (which have conductivities of up to 2 S m(-1)) will require a more detailed derivation taking into account both the real and imaginary parts of the signals.