| Summary: | 博士 === 國立臺灣大學 === 電信工程學研究所 === 92 === Spatial resolution, agent-to-tissue ratio, life time of contrast agents, and attenuation effect are very important factors in evaluating the performance of contrast imaging techniques. Higher center frequency and larger bandwidth of transmitted signals, better signal-to-interference ratio, lower transmitted pressure, and lower center frequency are the direct methods to achieve these performance indexes. We extend the two-frequency analytic solutions of Newhouse et al. to approximate the short-pulse responses of microbubbles in a low-amplitude field. Based on their results, there is an expected component near DC in the spectrum of bubble echoes excited by a short pulse. Here this component is named the low-frequency response, and its theoretical properties are verified experimentally. Our work has determined the constraint conditions under which this approximated solution can be used to analyze the weak short-pulse responses. This thesis also provides the amplitude and spectral properties of these responses. The low-frequency response has a special bandwidth-dependent property and has potential applications in imaging and bubble sizing. We have established the procedure of low-frequency imaging and the imaging resolution, SIR and SNR under attenuation effect are also analyzed. The demonstration of experimental images shows that the SIR in low-frequency imaging is better than that in fundamental imaging under different attenuation conditions.
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