ultrsonic nonlinear imaging analysis

• Main Authors: Che-Chou Shen, 沈哲州
• Other Authors: Pai-Chi Li
• Format: Others
• Language: zh-TW
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/24394853479497817563

碩士 === 國立臺灣大學 === 電機工程學研究所 === 88 === Performance issues related to tissue harmonic imaging are discussed in this thesis. Tissue harmonic signal is generated due to finite amplitude distortion as sound waves propagate through the body. The intensity of harmonic is low in the near field, thus limiting sensitivity and penetration of tissue harmonic imaging. A multiple transmit focusing approach has been proposed to overcome the problem. In addition to the primary focus, a secondary focus is added to increase harmonic generation. Using both simulations and experiments, it is shown that the near-field harmonic intensity increases by the addition of the secondary focus. However, adding a secondary focus also results in elevated sidelobes. This degrades contrast resolution in the harmonic image. Nevertheless, contrast resolution is still improved over conventional fundamental imaging. Effects of harmonic leakage on image quality were also addressed. Harmonic leakage is often present since the transmitted signal may contain harmonics induced by the imaging system. The leaked harmonic signal has different beam-forming characteristics from those of tissue harmonics. It is shown that harmonic leakage degrades contrast resolution of tissue harmonic imaging. We compared effects of various transmit signals for their leakage using both simulations and experiments. Impact of system nonlinearity was also studied using hydrophone beam pattern measurements.