Hardware for quasi-single-shot multifrequency magnetic induction tomography (MIT): the Graz Mk2 system

Abstract

Magnetic induction tomography (MIT) has been suggested by several groups for the contact-less mapping of the passive electrical properties of tissues via AC magnetic fields in the frequency range between several tens of kHz and several tens of MHz. Multifrequency MIT as an analog to multifrequency EIT has been tried and first image reconstructions have been demonstrated with phantoms. MIT appears to yield comparable images to EIT but offers the advantage of being non-contacting. In the beta-dispersion range of most tissues the method is challenging because the signals are very small and buried in noise. In order to minimize drifts and systematic errors fast data acquisition is therefore pivotal. This paper presents a method for single-shot MIT which allows us to acquire the data for a multifrequency image with an analog bandwidth of 50 kHz-1.5 MHz which covers a good part of the beta-dispersion of many tissues. The transmit (TX) coils are simultaneously driven by individual power amplifiers with a multisinus pattern with up to 3 A(pp). The amplifiers are configured as current sources so as not to perturb the excitation fields by inappropriately terminated coils. The separation of the different TX channels after reception is achieved by splitting up the carrier frequencies into individual subcarriers with a narrow spacing of at most 300 Hz. In this way every TX coil is identifiable by its own subcarrier but the whole excitation band is contained within a few kHz. The real and imaginary parts of the received signals are extracted efficiently with FFT. The system noise and the sources for low-frequency perturbations are analyzed and characterized.

Forward modelling of magnetic induction tomography: a sensitivity study for detecting haemorrhagic cerebral stroke

“…Holder [36] has, however, shown in an animal model that the conductivity changes can be by as much as 60% at 50 kHz. MIT researchers are now designing hardware for operation down to such frequencies [37]. Even so, an MIT system capable of detecting just haemorrhagic stroke could still be of clinical value for excluding haemorrhage prior to administration of thrombolytic drugs.…”

Reduction of low-frequency noise in magnetic induction tomography systems

“…When demodulating these rotated replicas s' and m' with respect to the LO frame (Re LO , Im LO ) a strong spurious imaginary modulation m s appears although in reality there is none in the physical frame (assumption 2). The imaginary signal s s is the projection of s' to the axis Im LO and does not reflect correctly the magnitude of the true signal s. Hence mechanical vibrations can heavily deteriorate the SNR of the measured imaginary signal [7]. Also other errors which affect mainly the true real part (moving metallic parts, thermal drifts of the RXC [5]) project into Im LO .…”

“…In order to avoid the above spurious projections we must back-rotate the signals by ψ = (θ 1 +θ 2 +ϕ) whereas θ 1 in previous concepts was provided by a separate reference coil which picked up the primary magentic flows from all TXC at once [7]. In the new approach the correction is achieved by rotating the signals until the spurious modulation m S vanishes, i. e. Var(s S ) is minimum.…”

“…The lowest frequency so far tried was found in [6] (11.6 kHz). We have developed a fast parallel multi-frequency imaging system which operates from 50 kHz to 1 MHz [7]. It comprises 16 RXC and 16 TXC and allows frequency differential imaging [8], whereas 8+8 coils can be operated in single shot mode.…”

“…It comprises 16 RXC and 16 TXC and allows frequency differential imaging [8], whereas 8+8 coils can be operated in single shot mode. However, due to the the low sensitivity vibrations and thermal drifts cause significant errors [7]. This paper describes a method for eliminating such errors.…”