|M.Sc Student||Rukinglaz Yury|
|Subject||Cavitation in Ultrasound Field: Generation and Monitoring|
|Department||Department of Biomedical Engineering||Supervisor||Professor Eitan Kimmel|
|Full Thesis text|
Ultrasonically induced cavitation is referred to as the generation of vapor-filled free bubbles by ultrasound. The free bubbles might be generated by a focused beam in a chosen location anywhere in the body for various targeted medical applications such as facilitated ultrasonic heating or enhanced drug delivery. In order to generate bubbles there is a defined threshold acoustic pressure which needs to be overcome - a cavitation threshold.
The objective of the work was to study the conditions required for generation of bubbles and to identify the special spectral features that appear only when bubbles exist in the system. In order to achieve this goal, two setups- for generation and detection of free bubbles in both flow and no-flow conditions- were designed and built. Bubbles were generated using focused ultrasound, frequency of 1.5 MHz (f), pulse width of 5 ms, and repetition interval of 15 ms. The RMS value of the acoustic pressure amplitude was changed from 13.1 to 345.8 kPa. All tests duration was 100 sec. Detection of backscattering was made by a needle hydrophone. We studied the effect of cavitation both in flow and no-flow conditions. Tap water, saline, blood and gel were tested.
Bubbles are characterized by enhanced nonlinear backscattering that result by exclusive appearance of sub-harmonic (f/2) and ultra harmonics (3f/2, 5f/2…) and multiplied harmonics (2f, 3f…) that can be also generated by the experimental system.
Pressure threshold which is required to initiate bubbles generation (cavitation threshold) for saline (247 kPa) is higher than for water (101.3kPa), and for blood it is even higher (321.1 kPa, dilution of 50%) than saline. The acoustic threshold for whole blood couldn't be found nor for ultrasound gel.
In order to decrease threshold a new technique (“provoked cavitation method”) was developed. The use of this method has been successful in leveling down the cavitation threshold for a saline, e.g., to 200 kPa.
In flow conditions the threshold was reduced with increasing of flow rate, probably due to more cavitation seeds that were entered into the focal zone of an ultrasonic beam. As for no- flow conditions it was 106.2 kPa and it decreased to 71.6 kPa when flow rate was 142.9 ml/min.
In all measurement we used the high frequency analyze of a backscattering signal. Experimental results confirmed the existence of unique characteristics for the harmonic responses as predicted by theory and demonstrated by simulation results.