| Summary: | 碩士 === 國立臺灣大學 === 電機工程學研究所 === 90 === Ultrasound imaging is a well-established imaging modality that provides diagnostic information in the form of cross-sectional images of soft tissue. However, for many clinical applications involving subtle tissue structure, the resolution of the conventional ultrasound imaging system that operates between 2 and 10 MHz is inadequate. Recent developments have taken advantage of the fact that resolution increases linearly with frequency. In this paper, a high frequency ultrasonic imaging system with high resolution and multi-format imaging has been developed for noninvasive imaging of small scale superficial structures such as the skin, the anterior chamber of the eye, and mouse embryos for studies in developmental biology.
The major design problem concerning medical high frequency ultrasonic imaging systems is caused by the strong attenuation of the tissue, which limits the maximum depth of penetration and the achievable signal to noise ratio (SNR). In this paper, the coded excitation and pulse compression techniques, which can increase the average transmitted power, are utilized to increase both the depth of penetration and SNR. In addition, since array transducers with dynamic focusing are not available for high frequency system, the image quality is significantly deteriorated in the out-of-focused region. Hence, a filter-based synthetic aperture focusing technique is employed here to improve the degraded beam quality. Such a filter is designed in the LMSE sense and is also known as the optimal filter. Moreover, depth scan technique is also applied to increase the depth of field for this high frequency system.
The experimental results demonstrate that the 50 MHz ultrasonic imaging system developed in this paper has resolution on the order of 100 mm and a 55-dB SNR. Implementation issues of optimal filter to increase the depth of field for a single crystal transducer system are also discussed. Based on the high frequency ultrasonic imaging system constructed in this paper, advanced high frequency ultrasound research on high frequency tissue harmonics, perfusion blood estimation and contrast agent characteristics could be performed. In addition, combined with other medical imaging systems, this system might be able to provide much more valuable information on clinical diagnostics and biomedical research in the future.
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