|M.Sc Student||Hoogi Assaf|
|Subject||Pressure Dependent Frequency Response of Microbubbles in|
|Department||Department of Biomedical Engineering||Supervisors||Professor Eitan Kimmel|
|Professor Emeritus Dan Adam|
|Full Thesis text|
Encapsulated microbubbles are used as ultrasound contrast agents (UCA) and may have potential applications in diagnosis and therapy. Potential utilization of microbubbles for blood pressure measurement is the subject of our study. This application is based on the non-linear frequency response of UCA to acoustic drive and its sensitivity to ambient pressure. Properly driven, the microbubbles generate a strong subharmonic component, a response which is exclusive for bubbles and differs from surrounding tissues e.g. that transmit the fundamental frequency and generate a second harmonic component.
Attenuation of acoustic signals through a bubble solution (DefinityTM) was tested. Ultrasound pulses of 15 cycles in length were applied in discrete frequencies within the range of 2MHz and 5MHz at low acoustic pressure. The transmitted signal was measured by a hydrophone. Resonance frequency of 2.7MHz was determined - in accord with the size distribution of the microbubbles. Backscattering from a tube filled with DefinityTM solution was measured for acoustic pressures of 100 to 630 kPa and threshold conditions for appearance of the subharmonic signal were found. Subharmonic to fundamental harmonic ratio was calculated. At frequency of about 5.5MHz, twice the resonance, a maximal value for this ratio was observed. When cyclic ambient pressure was applied, it seems that it may be better to transmit ultrasonic signal at frequency which is lower than twice the resonance in order to get higher sensitivity to ambient pressure.