Microbubbles, are produced inside the human body by several mechanisms which may cause many serious health problems or even death. The ability of a gas bubble to scatter ultrasound waves has led to the testing of various ultrasonic devices to monitor microbubble formation in the blood stream. These have included pulsed echo, acoustic-optical imaging, Doppler technique and the through-transmission technique. In this work, a pulse through transmission method for measuring ultrasound velocity in bubbly gel phantoms was used. The bubble size has been assessed by measurement of the pulse-wave velocity. A specially designed pressure chamber ensured the measured velocity was directly related to the volume of microbubble by Boyle's law. This velocity is an average indicator of the bubble size between transmitting and receiving transducers. The average bubble radius was 0.1 mm. For best results the measurement were carried out around 1-3 MHz frequency range. It is shown that a large change in velocity of ultrasound occurs as a result of small changes in the volume of microbubble. When the fraction by volume of gas in gel is changed from 0.6 percent to 0.8 percent the velocity changed from 1500 to 500 m/s respectively. This large change in velocity should provide a good base for detecting and more importantly, sizing microbubbles in the blood stream. The measured results were compared with theoretical prediction with good agreement between theory and experiment. This technique can be used as a simple real time method to monitor and measure the volume of the microbubles in the blood stream. This technique has many application in medical field such as; open heart surgery, blood dialysis and deep sea diving (decompression sickness).
The characterization and calibration of piezoelectric transducer beam profiles form an essential pre-requisite for ultrasonic applications in non-destructive evaluation (NDE) and medical imaging. Although considerable information is available concerning transducer temporal behavior and spatial field characteristics, the influence of the transmitting electronics, on transmitted pulse and transducer beamforming has been somewhat neglected. This work involves a study of electrical factors which control the beam profiles of wide band piezoelectric transducers usually used in medical and NDE application. The influence of the transmitting electronics, such as; turn-on time and matching circuitry, was examined with respect to pulse shape and frequency spectrum and, more importantly, transducer beam profiles. These factors and method of detection are shown to exert a considerable influence on measured beam profiles.
In many ultrasonic applications, such as ultrasonic imaging and signal processing, it is important that the transmitted pulse shape and ultrasonic beam profile remain consistent throughout the entire radiated field. The principles of diffraction strongly influence the ultrasonic beam profile and pulse shape and causes a considerable fluctuation in the near field region and a significant beam spreading in the far field region. In this work, different voltage distribution such as, logarithmic,linear and Gaussian across the transducer surface was investigated. The effect of a new electrode configuration based on a resistive taper on transducer performance was also discussed. Several transducers have been built to illustrate this design approach with good agreement between theory and experiment.