A multifrequency magnetic induction tomography system using planar gradiometers: data collection and calibration

Abstract

We developed a 14-channel multifrequency magnetic induction tomography system (MF-MIT) for biomedical applications. The excitation field is produced by a single coil and 14 planar gradiometers are used for signal detection. The object under measurement was rotated (16 steps per turn) to obtain a full data set for image reconstruction. We make measurements at frequencies from 50 kHz to 1 MHz using a single frequency excitation signal or a multifrequency signal containing several frequencies in this range. We used two acquisition boards giving a total of eight synchronous channels at a sample rate of 5 MS s(-1) per channel. The real and imaginary parts of DeltaB/B(0) were calculated using coherent demodulation at all injected frequencies. Calibration, averaging and drift cancellation techniques were used before image reconstruction. A plastic tank filled with saline (D = 19 cm) and with conductive and/or paramagnetic perturbations was measured for calibration and test purposes. We used a FEM model and an eddy current solver to evaluate the experimental results and to reconstruct the images. Measured equivalent input noise voltage for each channel was 2 nV Hz(-1/2). Using coherent demodulation, with an integration time of 20 ms, the measured STD for the magnitude was 7 nV(rms) (close to the theoretical value only taking into account the amplifier's thermal noise). For long acquisition times the drift in the signal produced a bigger effect than the input noise (typical STD was 10 nV with a maximum of 35 nV at one channel) but this effect was reduced using a drift cancellation technique based on averaging. We were able to image a 2 S m(-1) agar sphere (D = 4 cm) inside the tank filled with saline of 1 S m(-1).

Preliminary Results on Brain Monitoring of Meningitis Using 16 Channels Magnetic Induction Tomography Measurement System

“…When magnetic induction tomography is used in medical image, due to the low conductivity of biological tissue, a high accuracy phase measurement system is necessary. To increase the phase accuracy, the gradiometer coils are used in the magnetic induction measurement system [11][12][13]. The coaxial gradiometer coils consist of two detection coils and an excitation coil.…”

“…Riedel et al described a multichannel planar-array MIT system with a 2 × 2 planar matrix of axial gradiometers [11,12]. Rosell-Ferrer (2006) proposed a multifrequency 14-channel MIT system using planar gradiometers [13]. But these systems did not provide any clinical experimental results.…”

Numerical modeling of magnetic induction tomography using the impedance method

“…We used the frequencies 100 kHz, 1 MHz, and 10 MHz. The dimensions of the object and the coil are realistic when compared to prototypes of tomographs reported in the literature [14,16]. Solutions to the mesh equations were obtained by the successive-over-relaxation Gauss-Seidel method using the acceleration factor of 1.84.…”

A Large and Quick Induction Field Scanner for Examining the Interior of Extended Objects or Humans

“…The most frequently described MIT systems consist of a circular [7][8][9] or hemispherical [5] arrangement of multiple transmitter and receiver coils placed around the measurement volume. The setup is often surrounded by a metallic shield to protect the delicate signals from incident electromagnetic interference (EMI) from the environment.…”