諧波合頻成像

Abstract

組織諧波成像比起傳統的基頻成像，已證實能改善超音波影像品質，在對比度與空間解析度得到明顯提升，在臨床上乳房與甲狀腺的諧波成像已在使用，而諧波訊號的產生機制，在於超音波在組織傳遞過程中，波形會產生有限幅變形，然而組織諧波強度再強也不會超過基頻強度，諧波強度是遠小於基頻強度的，所以諧波成像最大問題在於信雜比不足導致穿透深度受限制，若能改善諧波信雜比強度的話，將能增加深層組織的可視性。 傳統上諧波成像的發射訊號為簡單脈衝波，而脈衝波要得到高解析度影像，其發射訊號必須為大頻寬的短脈衝波，但這樣會造成發射訊號強度下降，而編碼技術能提高發射訊號強度，增強諧波訊號的信雜比，而編碼技術有著空間上旁辦假像問題，旁辦假像產生原因在於頻譜上諧波成份與基頻成份重疊，所以需透過其他方法抑制基頻，目前壓抑基頻產生最有效方式為脈衝反相技術，可使旁辦假像問題壓至最低，但編碼技術使用在臨床上仍有問題，問題在於組織衰減影響，所以再發射與接收時需補償，但編碼技術在補償做法較困難。 而本研究提出的諧波合頻技術同樣能提高諧波的信雜比，重要的是在發射與接收的補償做法容易，諧波合頻技術做法為發射多個不同頻率的窄頻寬長脈衝 訊號，利用帶通濾波器將回波中的諧波成份濾出，用諧波訊號相加做合頻，實際上在組織內這些不同頻率諧波訊號一樣會受到衰減的影響，衰減的原因主為探頭與組織影響，而發射窄頻寬的好處為可視為單一常係數做補償，而論文中會探討諧波合頻原理，與諧波合頻問題，並進一步的改善。

Tissue harmonic imaging has demonstrated that it can provide superior image quality compared to conventional fundamental imaging, contrast and spatial resolution can be substantially increased. Harmonic imaging has been used for detecting thyroid and breast. Harmonic generation mechanism of the acoustic wave propagates in tissues of the waveform distortion. However tissue harmonic intensity remains lower than that of the fundamental intensity, harmonic intensity is smaller than the fundamental intensity, so Tissue harmonic imaging suffers from the signal-to noise ratio(SNR) degradation, resulting in limited penetration depth . If there is a way can improve the harmonic signal SNR, Enhance the SNR could further improve the visualization of tissue at depth. Traditionally, the transmitted signal as a pulse in harmonic imaging, the pulse to obtain high-resolution images, the transmitted signal must be wideband pulse, but this will result in degradation of the transmitted energy. Coded excitation can improve the transmitted energy, it can enhance to harmonic signal SNR, but coded excitation could suffer from the increased sidelobe artifact, caused by the spectral overlap between the fundamental and harmonic components. So need to suppress the fundamental frequency through other methods. According to research, pulse inversion shows the best result to suppress the fundamental frequency, effective rejection of the sidelobe artifact. Coding excitation still has problems on the clinical. The problem is attenuation in tissue. So to be compensated on transmit and receive, however coding excitation hard to compensate. In my research, synthetic spectrum also can improve the harmonic signal SNR. It’s important to compensate easily. In Synthetic Spectrum, more pulses are transmitted with different frequencies. Use a Band-pass filter to get harmonic components. Sum of harmonic signals is synthetic spectrum. In fact different frequency harmonic signals will attenuate in the tissue. The main reason for attenuation is Transducer and tissue. Narrow bandwidth of transmission signal can be regarded as constant coefficient compensation. This paper will explore the basic principles of synthetic spectrum for further to improve.