טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentSapunar Michal
SubjectStudy of the Controlling Parameters of Hydrostatic Pressure
Estimation Based on Nonlinear Properties of
Contrast Agent
DepartmentDepartment of Biomedical Engineering
Supervisor Professor Emeritus Dan Adam


Abstract

Medical ultrasound is nowadays a well established technique for clinical diagnosis and will continue to play an important role in the foreseeable future. A major advance in ultrasound imaging was achieved with the development of contrast agents. Ultrasound contrast agents are microbubbles filled with air or large molecules gases, with or without a shell, that are injected into the circulatory system to enhance the echogenicity of the blood.

The measurement of pressure in the different chambers of the heart is of great clinical importance, as it provides vital information concerning the well being of the patient. The lack of proper noninvasive techniques has led to studies of various alternative indirect methods for pressure measurements, some related to ultrasound measurements, and specifically to ultrasound interaction with gas bubbles.

Due to the high compressibility of the gas, the size of the gas bubble changes as a function of the local hydrostatic pressure. This change in size affects the acoustic characteristics of the gas bubble, such as its resonance frequency, backscatter and attenuation signals. Therefore, the ambient pressure may be derived from the measurements of these acoustic characteristics by injecting fluid containing gas bubbles into the organ in which the blood pressure is to be measured.

The purpose of this research was to investigate the effects of cyclic hydrostatic pressures on the harmonic performance of Optison® contrast agent. Optison® consists of human albumin microspheres filled with inert fluorocarbon gas octafluoropropane (C3F8).

Two modes of cyclic pressures were examined: one mimicking the left ventricular pressure changes, the second mimicking the aortic pressure changes. We assumed that the cyclic hydrostatic pressure has a reproducible effect on the harmonics behavior of the backscattered signals.

The results for the first mode of pressure changes show a good correlation between the subharmonic amplitude and the cyclic pressure variations. A linear correlation was obtained between the subharmonic amplitude changes and the pressure changes. For the second mode of pressure variations, the correlation obtained was much lower, yet still significant.

The sensitivity of the subharmonic amplitudes to the cyclic pressure variations suggest that it is feasible to estimate pressure changes (but not absolute pressure) from the changes of their amplitude. Still, at small pressure changes, only a small correlation was measured, which implies that in order to obtain proper sensitivity, improvements must be made in the acoustic setup and in the data processing procedures.