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

Abstract

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 receivercoil. 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 x 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 beta-dispersion range of biological tissues is experimentally demonstrated. An improvement of the resolution towards SCR = 10(-7) with an SNR of > or = 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.

Biological tissue characterization by magnetic induction spectroscopy (MIS): requirements and limitations

“…Fig. 2 shows the principle of a single measuring channel for a MIS system, as published previously in [8], [19]. The excitation coil (EXC) is fed by a power amplifier (PA) with a sinusoidal current.…”

“…This is the case for biological objects conductivity S/m up to about 10 MHz. A semi-analytical solution which remains valid also at very low penetration depths has been presented in [8] for a small conducting and paramagnetic sphere located in an arbitrary point between two circular coils. In the following, the key quantity shall be called the "signal/carrier ratio" (SCR) [8].…”

“…A semi-analytical solution which remains valid also at very low penetration depths has been presented in [8] for a small conducting and paramagnetic sphere located in an arbitrary point between two circular coils. In the following, the key quantity shall be called the "signal/carrier ratio" (SCR) [8]. It is a function of the location of the perturbation and of the coil system [7].…”

“…A promising alternative to some electrode-based methods is magnetic induction spectroscopy (MIS) [7], [8]. This new technique is a multifrequency version of the recently developed magnetic induction tomography (MIT) [9], [10].…”

Three-dimensional multiexcitation magnetoacoustic tomography with magnetic induction

“…Magnetic induction tomography ͑MIT͒ is another bioimpedance imaging modality that employs both magnetic transmitting and sensing technologies. 7,8 In MIT, measurements of the secondary magnetic field produced by the induced eddy current in a conductive tissue object are taken by small coils arranged around the object for impedance image reconstruction. However, spatial resolution of current MIT technique is still quite limited.…”