| 題目：醫用超音波非線性影像技術: 以三倍頻發射相位法為例 (Medical ultrasound nonlinear imaging: The 3f0 phasing technique as an example) 主講人：沈哲州 教授(國立臺灣科技大學 電機工程系) 時間：103年6月11日(星期三13:30-15:30) 地點：三峽校區商學院商1F01教室 大綱： Ultrasound nonlinear imaging provides better image contrast over conventional linear mode and has become an essential tool in clinical diagnose. However, since the nonlinear signal is generated either during finite-amplitude acoustic propagation or from the nonlinear oscillation of microbubble contrast agents, it is relatively weak compared to its linear counterpart. The method of third harmonic (3f0) transmit phasing can be utilized by phasing an additional 3f0 transmit signal to manipulate the magnitude of nonlinear signal. Specifically, generation of tissue harmonic signals during acoustic propagation is based on the combined effect of two different spectral interactions of the transmit signal. One produces harmonic whose frequency is the sum of transmit frequencies. The other results in harmonic at difference-frequency of the transmit signals. The relative phasing between the frequency-sum component and the frequency-difference component markedly changes the amplitude of the second harmonic signal. For harmonic enhancement, approximate 6 dB increase of second harmonic amplitude can be achieved while the lateral harmonic beam pattern is also improved. To further increase the SNR without excessive transmit pressure, coded excitation can be incorporated in 3f0 transmit phasing to boost the tissue harmonic generation. The phase-encoded Golay excitation is suitable in 3f0 transmit phasing due to its superior transmit bandwidth efficiency. The resultant frequency-sum and frequency-difference components of tissue harmonic signal can be simultaneously Golay-encoded for SNR improvement. For harmonic suppression, the second harmonic signal could also be significantly reduced by about 11 dB when the frequency-difference component is out of phase with the frequency-sum component. Generally, with the selection of 3f0 transmit phase that leads to maximal tissue suppression, the CTR can be optimally improved in contrast harmonic imaging. |
