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.
| Identifer | oai:union.ndltd.org:METU/oai:etd.lib.metu.edu.tr:http://etd.lib.metu.edu.tr/upload/12607071/index.pdf |
| Date | 01 February 2006 |
| Creators | Ozkan, Koray Ozdal |
| Contributors | Gencer, Nevzat Guneri |
| Publisher | METU |
| Source Sets | Middle East Technical Univ. |
| Language | English |
| Detected Language | English |
| Type | M.S. Thesis |
| Format | text/pdf |
| Rights | To liberate the content for public access |
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