| Summary: | 碩士 === 國立臺灣大學 === 電信工程學研究所 === 102 === The main goal of this study is that through a complete theory, simulate with adaptive control of transmission signal frequency which can approach to best CTR(Contrast-to-Tissue Ratio).In the progressing of optimal frequency, the microbubble size distribution will be one key factor affecting the optimal frequency; most of previous study using one- bubble simulation, which is not practical. By adding microbubble distribution model into consideration, we can acquire the optimal frequency more precisely.
Using fixed shell properties microbubbles analysis optimal frequency range from linear oscillation theory, illustrate the probe, tissue, and bubbles stochastic effects on the received PSD, and use the nonlinear oscillation simulation, with different emission signal frequency to find the optimal frequency.
Transducer’s center frequency and bandwidth should be choose to matched with tissue attenuation along with bubble size distribution, otherwise the optimal frequency will be meaningless because of high power loss. Considering bubble flow velocity, solution need to be derived from average reference traces, and the numbers depends on the bubble changing rate. Using the signal frequency sweep estimate the optimal frequency range, still need to radius of bubble size distribution of information, and the result will be more close to the ideal value if emission signal using the narrowband signal.
In the situation of low bubble replace rate (lower than 20%), the maximum power and optimal frequency can be obtained with gradient decent algorithm, and the result backscatter power is 7dB higher than obtained power in transducer central frequency.
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