טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBurla Elina
SubjectThe Study of Noninvasive Pressure Estimation Using
Ultrasound Contrast Agents
DepartmentDepartment of Biomedical Engineering
Supervisor Professor Emeritus Dan Adam


Abstract

Noninvasive pressure estimation within the heart cavities, the coronary arteries or within other internal organs is essential for providing information on blood perfusion and valiability of the organ. Some ultrasound contrast agents (UCA), which constitute gas microbubbles with or without a shell, may be suited for pressure estimation because pressure changes affect the reflectivity of the microbubbles. Based on results of earlier studies, we hypothesized that UCA, even those encapsulated in shells, demonstrate sensitivity to the ambient pressure, by changes in their different components of their frequency response. To validate the hypothesis computer simulations and ‘in-vitro’ experiments were conducted.

Computer simulations, based on a modified Rayleigh-Plesset model of an oscillating microbubble, provided us a valuable tool for comparing the efficiency of 4 types of the UCA as a pressure gauge, for evaluating the optimal acoustic setup and assessing the parameters of the backscattered signals, which should be most sensitive to the changes of the ambient pressure.

‘In-vitro’ experiments were carried out with UCA Optison for 7 acoustic setups. Two problems were tackled: estimation of non-varying hydrostatic pressure in the range of physiological pressures P=Patm : Patm+150mmHg, and estimation of cyclic pressure changes with an amplitude of 75mmHg and f=0.008 & 0.05 Hz.

For estimation of non-varying pressures, the ratio of the 1st to the sub-harmonic amplitiudes of the backscattered echoes from Optison is proposed as a possible indicator of the ambient pressure. A sensitivity of <5kPa (37mmHg) was achieved in ‘in-vitro’ experiments. Consistency of the experimental results with the numerical simulation results has been proven for the designed acoustical setup. It was shown that the estimation of varying pressures is a more challenging task. The 1st/2nd harmonic ratio demonstrates the best correlation with cyclic pressure changes, but only for a limited number of cycles. Very promissing results were achieved when studying the attenuation parameter as a pressure gauge,  for a specific acoustic setup and with very low Mechanical Index (MI=0.05). Reversibility of the pressure effects on UCA behavior was demonstrated and excellent positive correlation between the attenuation prameter and the hydrostatic pressure was exhibited.