We present a novel two-dimensional (2D) MAET scanner, with a rotating object of interest and two fixed pairs of electrodes. Such an acquisition scheme, with our novel reconstruction techniques, recovers the boundaries of the regions of constant conductivity uniformly well, regardless of their orientation. We also present a general image reconstruction algorithm for the 2D MAET in a circular chamber with point-like electrodes immersed into the saline surrounding the object. An alternative linearized reconstruction procedure is developed, suitable for recovering the material interfaces (boundaries) when a non-ideal piezoelectric transducer is used for acoustic excitation. The work of the scanner and the linearized reconstruction algorithm is demonstrated using several phantoms made of high-contrast materials and a biological sample.

A multi-frequency data acquisition system is realized for subsurface conductivity imaging of biological tissues. The measurement procedures of the system at different frequencies are same. The only difference between the single frequency experiments and the multi-frequency experiments is the hardware, i.e. the sensor and the power amplifier used in the single frequency experiments was different than that were used in the multi-frequency experiments. To avoid confusion the measurement system with which the single frequency experiments were performed is named as prototype system and the measurement system with which the multi-frequency experiments were performed is named as multi-frequency system. This system uses magnetic excitation (primary field) to induce eddy currents inside the conductive object and measures the resulting magnetic field due to eddy currents (secondary field). For this purpose, two differential-coil sensors are constructed / one is for the single frequency measurements and the other is for the multi-frequency measurements. Geometrically the coils are same, the only difference between them is the radius of the wires wound on them. The sensor consists of two differentially connected identical receiver coils employed to measure secondary field and in between the receiver coils is placed a transmitter coil, which creates the primary field. The coils are coaxial. In the prototype system the transmitter coil is driven by a sinusoidal current of 300 mA (peak) at 50 kHz. In the multi-frequency system the transmitter coil is driven by a sinusoidal current of 217 mA (peak), 318 mA (peak), 219 mA (peak) and 211 mA (peak) at 30 kHz, 50 kHz, 60 kHz and 90 kHz, respectively. A data acquisition card (DAcC) is designed and constructed on a printed circuit board (PCB) for phase sensitive detection (PSD). The equivalent input noise voltage of the card was found as $146.80 hspace{0.1 cm}nV$. User interface programs (UIP) are prepared to control the scanning experiments via PC (HP VEE based UIP, LabVIEW based UIP) and to analyze the acquired data (MATLAB based UIP). A novel sensitivity test method employing resistive ring phantoms is developed. A relation between the classical saline solution filled vessel (45mm radius, 10 mm depth) phantoms and the resistive ring phantoms is established. The sensitivity of the prototype system to saline solutions filled vessels is 13.2 $mV/(S/m)$ and to resistive rings is 155.02 mV/Mho while the linearity is 3.96$%$ of the full scale for the saline solution filled vessels and 0.12$%$ of the full scale for the resistive rings. Also the sensitivity of the multi-frequency system is determined at each operation frequency by using resistive ring phantoms. The results are in consistence with the theory stating that the measured signals are linearly proportional with the square of the frequency. The signal to noise ration (SNR) of the prototype system is calculated as 35.44 dB. Also the SNR of the multi-frequency system is calculated at each operation frequency. As expected, the SNR of the system increases as the frequency increases. The system performance is also tested with agar phantoms. Spatial resolution of the prototype system is found 9.36 mm in the point spread function (PSF) sense and 14.4 mm in the line spread function (LSF) sense. Spatial resolution of the multi-frequency system is also found at each operation frequency. The results show that the resolving power of the system to distinguish image details increases as the frequency increases, as expected. Conductivity distributions of the objects are reconstructed using Steepest-Descent algorithm. The geometries and the locations of the reconstructed images match with those of the real images. The image of a living tissue, a leech, is acquired for the first time in the literature. Magnetic conductivity spectroscopy of a biological tissue is shown for the first time in electrical conductivity imaging via contactless measurements. The results show the potential of the methodology for clinical applications.

The previous data acquisiton system developed for the electrical impedance imaging via contactless measurements is improved to obtain measurements with a faster scanning speed of 0.15 sec/mm2. This system uses magnetic excitation to induce currents inside the body and measures the magnetic fields of the induced currents with an axial gradiometer. Gradiometer consists of two differentially connected 10000-turn coils with diameter of 30 mm and a transmitter coil of 100-turn coil of diameter 30 mm placed and magnetically coupled between them. Transmitter coil is driven by a sinusoidal current of 200 mA (peak) whose frequency is 14.1 kHz. A Data Acquisition Card (DAcC) is designed and constructed on PCB, thus elliminates the use of the Lock-In Amplifier Instrument (LIAI) in the phase sensitive measurements. User interface programs to control the scanning experiments via PC (MATLAB Scanner 1.0, HP VEE Scanner 1.0) and to analyze the acquired data (Data Observer 1.0) are prepared. System performance tests for the DAcC are made. Error in the phase sensitive measurements is measured to be 0.6% of the test signals. Minimum magnetic field density that can be detected is found to be 7 DT. Output stage performance of the DAcC is improved by using an integrator instead of an amplifier in the output stage. In this manner, maximum linearity error is measured as 6.60*10-4 % of the full scale for the integrator circuit. Thermally generated voltage drift at the sensor output is measured to be 0.5 mV/minute in the ambient temperature. Overall normalized standard deviation at the output of the data acquisition system is observed as to be in the order of 10-4. Mathematical relation between the resistive rings and conductive phantoms is studied. It is derived that maximum resistor value that can be distinguished in the resistive ring experiment which is 461 F, corresponds to the phantom conductivity of 2.7 S/m. Field profiles (i.e., the voltage measurements) for the human left hand is obtained for the first time in literature, employing the LIAI. Agar objects with conductivity value of 1 S/m in a saline solution of 0.2 S/m are scanned and the field profiles are obtained using the DAcC. Image profiles of the scan fit well with the actual locations, geometries, and relative dimensions of the agar objects. A coil winding machine is prepared which enables the operator to design and wind up coils under self-controlled environment and conditions.