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Ultrasound Intensity Exposimetry in Physiotherapy Devices using Micro-Bubbles

Rezayat, Ehsan | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41821 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Zahedi, Edmond
  7. Abstract:
  8. Routine ultrasound calibration methods have some restrictions such as: low bandwidth, low frequency resolution, limited spatial resolution, the high cost of test and risk of damage to the transducer when exposed to high powers. In this thesis, a new technique based on microbubble resonance is proposed. Non-linear resonating microbubbles produce harmonics;the first harmonic may be masked by reflection from larger microbubbles as well as unwanted reflections which cause ambiguity in the determination of the amplitude of the excited pressure. Therefore, the second harmonic is used, with a single characteristic peak. In order to model the physical phenomenon, the second harmonic generated in the nonlinear regime for a continuous ultrasound wave with a frequency of 1 MHz in water is simulated.The parameters are: 10 kPa pressure (Therapeutic Ultrasound) and frequency of 1 MHz for non-coated gas microbubbles (radius of 3.15 microns) and lipid-coated microbubbles (3.84 microns). Sensitivity analysis was also done, using the tolerance of coefficients of available transducer. Model results show that the sensitivity of the exciting transducer at a frequency of 1 MHz is 12.11 kPa/V, whereas the sensitivity of the receiver transducer at the first and second harmonics is 47.17 mV/MPa and 5.51 mV/MPa, respectively. In the setup,background noise was reduced from 19.15 mV (RMS) to 1.1 mV (RMS) using a metallic shield. Free-gas microbubbles were produced with electrolysis using and a fine Remanium wire (200 8m diameter) in water. The passage of these microbubbles increased the RMS value of the first and second harmonics by 11.28 mV and 0.162 mV when using an excitation voltage of 8 V, respectively. Lipid-coated microbubble consisted of 90% hosphatidyl Choline (coating) and perfluorobutane (gas content). This liquid was diluted 100 times, and the diameter distribution was obtained using optical microscope imaging and image processing techniques (R0min = 1 μm and R0max = 7 μm). At an exciting voltage of 8 V and due to the passage of passing coated microbubbles, the RMS value of the first and second harmonic decreased by 9.5 mV and 0.023 mV, respectively. The main inaccuracies are thought to come from lack of laboratory equipment and the effects of radio interference on the signals. In addition, there is a need for more accurate modeling of microbubble oscillation in non-linear regime
  9. Keywords:
  10. Microbubble ; Calibration ; Ultrasonic Waves

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