M.Sc Thesis

M.Sc StudentRosen Alexander
SubjectMicro-Bubble Based Pressure Measurements Using Ultrasound: A
Modeling and Signal Processing Study
DepartmentDepartment of Biomedical Engineering
Supervisors PROFESSOR EMERITUS Eitan Kimmel
Full Thesis textFull thesis text - English Version


Micro-bubbles filled with gas and surrounded by a thin elastic shell are used as contrast agents in medical imaging with ultrasound. The idea of using micro-bubbles exposed to ultrasound for blood pressure measurements has been studied at the Technion in the last years. This task requires better understanding of the  acoustic signals induced by the micro-bubbles in ultrasonic field and the conditions for generation of non-linear effects such as appearance of sub-harmonic component (half of the stimulating frequency) in the emitted spectrum.

The response of the micro-bubbles to the driving ultrasound force is defined by their following properties: micro-bubbles resonance frequency which is correlated to the bubble steady state radius, the distribution of the micro-bubble size and resonance frequency, and the gas diffusion process.

The experimental setup designed for this investigation consists of ultrasound signal generation block, composed of two transducers, signal generation equipment, data acquisition and analysis block. Definity™ micro-bubbles with resonance frequency of 2.75 MHz were used during the experiments in water reservoir.  The experiments were conducted using two different driving frequencies of 5.5 and 4.5 MHz. The acoustic pressure varies from 250 kPa to 650 kPa with 50 kPa steps.

The appearance of sub-harmonic component is eminent in the applied acoustic pressures range and under applied frequency of twice micro-bubbles resonance frequency (5.5 MHz). The experiments showed linear correlation between the applied acoustic pressure and first harmonic-to-sub-harmonic amplitude ratio. 

Further experiments, investigating also the quantitative dependence of sub-harmonic amplitude modulation as a result of ambient pressure variations are needed.