Magnetic induction tomography: Hardware for multi-frequency measurements in biological tissues

Magnetic induction tomography (MIT) is a contactless method for mapping the electrical conductivity of tissue. MIT is based on the perturbation of an alternating magnetic field by a conducting object. The perturbation is detected by a voltage change in a receiver coil. At physiologically interesting frequencies (10 kHz-10 MHz) and conductivities (<2 S m^-1) the lower limit for the relative voltage change (signal/carrier ratio = SCR) to be resolved is 10^-7-10^-10. A new MIT hardware has been developed consisting of a coil system with planar gradiometers and a high-resolution phase detector (PD). The gradiometer together with the PD resolves an SCR of 2.5 × 10^-5 (SNR = 20 dB at 150 kHz, acquisition speed: 100 ms). The system operates between 20 and 370 kHz with the possibility of extending the range up to 1 MHz. The feasibility of measuring conductivity spectra in the β-dispersion range of biological tissues is experimentally demonstrated. An improvement of the resolution towards SCR = 10^-7 with an SNR of ≥20 dB at frequencies > 100 kHz is possible. On-line spectroscopy of tissue conductivity with low spatial resolution appears feasible, thus enabling applications such as non-invasive monitoring of brain oedema.