Magnetic induction tomography (MIT) allows the reconstruction of conductivity distributions for a wide variety of industrial and medical applications. The advantage of this technique is the contactless and non-invasive way of collecting information on the tissue. An MIT system consists of excitation coils that produce a primary magnetic field that causes eddy currents in a conductive object. The eddy current produces a secondary magnetic field that can be detected by an array of receiving coils. The article presents the setup of a 16 channel MIT system featuring parallel readout of the 16 receiver coil array. To achieve the parallel readout the high-frequency signals used for the measurements are converted to a lower frequency by heterodyne downconversion and then sampled by high quality audio sampling equipment. The sampled low frequency signals are processed by digital signal processing algorithms in a standard computer. This allows the replacement of the commonly used lock-in amplifier and enables the processing of all 16 receiving channels in parallel.
Magnetic Induction Tomography (MIT) and Magnetic Induction Spectroscopy (MIS) enable a contactless method of imaging and mapping of all three passive electromagnetic properties (conductivity, permittivity and permeability) for a wide variety of industrial and medical applications. MIT applies a magnetic field from an excitation coil to induce eddy currents in the material. The secondary magnetic field generated by the eddy currents can be detected by receiver coils. The article presents the setup of a 16 channel MIT system featuring parallel readout of the 16 receiver coil array. To achieve the parallel readout the high-frequency signals used for the measurements are converted to a lower frequency by heterodyne downconversion and then sampled by high quality audio sampling equipment. The sampled low frequency signals are processed by digital signal processing algorithms in a standard computer. This allows the replacement of the commonly used lock-in amplifier and enables the processing of all 16 receiving channels in parallel.
II. Description of the system
The multi-coil system consists of excitation and receiver coils. A sinusoidal current flows through the excitation coil and generates a primary alternating magnetic field. Typical frequencies of the sinusoidal current are from 1 MHz to 10 MHz. In a conductive medium, the alternating field induces eddy currents, which are proportional to the conductivity of the medium. The eddy currents generate a secondary alternating electromagnetic field that is also proportional to the conductivity. If the skin depth is large compared to the thickness of the sample, which is generally true for biological tissues, the secondary field is nearly 90º phase shifted to the primary field and can be detected by the receiver coil and a suitable signal processing equipment.
B. Mechanical setup
The MI-Tomograph consists of a cylindrical shaped tank made of aluminum with excitation and receiver coils mounted on the inner wall of the tank. The inner diameter of the tank is 35 cm. In the current setup 16 excitation coils are arranged circularly, shaping the outer ring of coils. The inner circle of coils is formed by 16 receiver coils (see Fig. 1).
Fig.1 Setup of the MI-Tomograph: Top-view into the tomograph
The electronic modules for the excitation and receiving coils are mounted on the outer wall of the tank. Each of the electronic modules is placed in a separate metallic case for shielding purposes (see Fig. 2). A removable coping provides a secondary electrical shielding between the receiver and the excitation modules and reduces perturbations from the outside.The coils are made of PCB material (FR4, 1 mm thick). In the current setup coils with two windings of 50 mm in diameter are used, one winding on the top layer and the other on the bottom layer of the PCB.
Fig.2 MI-Tomograph with electronic modules and A/D-converters
C. Electrical Setup
Figure 3 shows a block diagram of the measurement setup. Two synchronized generators (Agilent 33220A) are used for signal generation. Signal generator 1 provides the reference signal for the excitation circuits. The signal is split up by a power splitter (MiniCircuits ZFSC-16-12) and then distributed to the excitation modules. The excitation modules include a switch to turn the excitation signal on or off. The switch is controlled by an USB converter interface box (National Instruments DAQ 6501). The excitation modules amplify the reference signal to a level of about VRMS = 1.7 V and IRMS = 50 mA at the excitation coil. The gain of the excitation module is fixed and cannot be controlled by the measurement software.
Fig.3 Block diagram of the measurement setup (RF=radio frequency, IF=intermediate frequency, LO=local oscillator signal)
The second signal generator provides the sinusoidal signal for the downconversion (LO) of the measurement signal in the receiver module. It is split up by a power splitter and then distributed to the receiver modules, too. These modules include fixed gain amplifiers for the measured signal received by the receiver coil. The receiver modules implement a downconversion of the received radio frequency (RF) signal to a lower intermediate frequency (IF). The current setup uses an IF of 10 kHz. The IF outputs of the receiver modules are connected to an A/D converter device.In the current setup an audio sampling equipment (MOTU HD 192, Manufacturer: Mark of the unicorn) is used. The MOTU HD 192 device allows the parallel sampling of 12 channels. All channels are synchronized internally. Beneath the possible parallel sampling of the channels it features low noise (0.0005% THD+N) and 24-bit resolution of the sampled values at a sampling rate of 192 kHz.The sampled data are transferred to a special PCI-interface card (MOTU PCI-424, Manufacturer: Mark of the unicorn) by an Audiowire cable. The Audiowire bus is a proprietary standard based on the Firewire protocol. The PCI card has four Audiowire ports and enables the connection of up to four MOTU HD 192 devices at the same time, which results in the possibility to sample 48 synchronized channels in parallel.
Fig.4 Block diagrams of receiver (upper diagram) and excitation modules (lower diagram)
The entire measurement setup is controlled by a standard PC. The control software is written in National Instruments Labview, a commonly used graphical programming language for measurement automation. The software controls the signal generators and the excitation modules. Further on it processes the sampled data coming from the MOTU HD 192 devices. By using pre-calculated matrices the Labview software is capable of creating images of the conductivity distribution based on the measured data set directly.Figure 4 shows block diagrams of the receiver and excitation modules. The receiver consists of a low noise amplifier for the RF measurement signal coming from the receiver coil. The LO signal coming from the second signal generator is amplified too. Both signals are multiplied to an intermediate frequency by an analogue frequency mixer. The intermediate frequency is then amplified and low pass filtered to suppress the high frequency content. The excitation module amplifies the reference signal coming from the signal generator. It consists of a pre-amplifier, a power amplifier and a high-speed buffer amplifier. An electromagnetical relay connects the output of the buffer amplifier to the excitation coil. It enables the control software to switch the amplified signal on or off.
Figure 5 shows the result of an experiment with four plastic bottles, each filled with 250 ml saline solution of different conductivities (upper bottle 0.5 Sm-1, left one 0.75 Sm-1, right one 1 Sm-1 and lower one 1.25 Sm-1). The upper picture shows the arrangement of the plastic bottles in the tank. The lower picture shows the reconstructed conductivity distribution. Due to some arbitrary values in the reconstruction algorithms the picture does not give quantitative information about the measured conductivity, but a qualitative one. However, the rising conductivity from the upper to the lower bottle is clearly visible by the increasing intensity of the color.
Fig.5 Measurement setup (left) and reconstructed image (right) of 4 plastic bottles filled with saline solution (conductivities 0.5, 0.75, 1 and 1.25 Sm-1)