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.
| Identifer | oai:union.ndltd.org:METU/oai:etd.lib.metu.edu.tr:http://etd.lib.metu.edu.tr/upload/12606521/index.pdf |
| Date | 01 April 2005 |
| Creators | Colak, Evrim I. |
| 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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