| Summary: | 碩士 === 國立臺灣大學 === 生醫電子與資訊學研究所 === 104 === Using software for beam-forming in ultrasound systems provides high flexibility, and the large amount of computations required in a software-based system can be performed in real time on a personal computer. However, there is a bottleneck in the very large data transfer rate required from the ultrasound front-end to the personal computer host for real-time operation, which cannot be achieved without appropriate compression. Previous studies have examined JPEG compression of ultrasound RF channel data, but the schemes do not exploit temporal redundancy between adjacent frames. This study utilized MPEG technology to process the ultrasound RF data in order to increase the compression efficiency. Our results indicate that MPEG compression generally provides a better compression ratio than does JPEG compression. As an example, the compression ratio of MPEG compression in an 8-bit channel A/D data under the 5 μm interframe displacement is smaller than 0.13 and thus allowing the real-time data transfer requirements to be met. Moreover, the compression efficiency for motions in different directions is shown to be highly dependent on the frame-to-frame correlation. For Doppler imaging, on the other hand, phase distortion resulting from data compression may not be ignored. In order to minimize the distortion, the QP (quantization parameter) rate control was used. Under the QP rate control method, the RF channel data can be compressed without inter-frame interpolation and the quantization parameter can be chosen to meet the requirements for Doppler processing. With MPEG lossless compression, a compression ratio smaller than 0.12 was achieved on simulation data. The compression efficiency increased with the noise level. When the noise RMS value was larger than 2 bit over 8 bit data, the compression ratio was higher than 0.65. With proper rate control, the compression ratio of noisy data became smaller than 0.3, and can be even smaller if higher quantization was made. Nonetheless, the encoding/decoding speed of current state-of-the-art electronics technologies is still insufficient to implement real-time ultrasound array imaging.
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