Spectroscopic 16 channel magnetic induction tomograph: The new Graz MIT system

Abstract

Frequency differential magnetic induction tomography allows the reconstruction of images of relative conductivity spectra with the aid of magnetic AC fields and coil arrays. We describe a new 16-channel hardware with an analog bandwidth of 50 kHz - 1.5 MHz which covers a good part of the β-dispersion of many tissues. At each acquisition time 8 of the 16 excitation coils are simultaneously driven by individual power amplifiers with up to 3 Ass at multiple frequencies. These amplifiers are configured as current sources so that every coil termination is high enough so as not to perturb the field of all other coils. For encoding the excitation coils their individual carrier frequencies differ by 200 - 500 Hz. In this way every excitation frequency is split into 8 subcarriers within a bandwidth of 1.6 - 4 kHz so that they can be approximately considered as a single peak in the comparatively flat tissue spectrum. The magnetic fields are received by 16 planar gradiometers and the induced voltages are amplified by low-noise amplifiers before being fed into the ADC. The real and imaginary parts of the received signals are extracted efficiently with FFT. The SNR is reduced by a factor of 5 when compared to the theoretical limit of our previous single-excitation system but due to the much higher acquisition speed drifts and physiological noise are considerably lower. © Springer-Verlag 2007.