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標題: | 以脈衝反相為基礎之超音波非線性影像 Pulse Inversion Based Ultrasonic Nonlinear Imaging |
作者: | Che-Chou Shen 沈哲州 |
指導教授: | 李百祺 |
關鍵字: | 非線性影像,組織諧波,移動雜訊,脈衝反相,超音波,諧波溢漏,對比劑, ultrasound nonlinear imaging,tissue harmonic,contrast agents,spectral leakage,motion artifacts,pulse-inversion, |
出版年 : | 2005 |
學位: | 博士 |
摘要: | 本論文主要針對脈衝反相技術(pulse inversion)在醫用超音波非線性影像(nonlinear imaging)上的應用與限制進行探討,脈衝反相技術的原理是發射一組相位相反的正負脈衝並將所收到的回音相加來擷取信號中的非線性成分,其優點是能夠大幅提升超音波非線性影像中的對比度。
本論文分成三大主題,第一部份是脈衝反相技術對組織諧波影像(tissue harmonic imaging)中諧波溢漏現象(spectral leakage)的影響。由於組織諧波影像一般是以濾波方式來擷取非線性信號,因此當超音波系統在諧波影像的接收頻帶因發射波型或系統非線性而有溢漏諧波存在時,會對組織諧波信號產生干擾而影響組織諧波影像的對比。我們的研究是第一個對溢漏諧波現象做了完整探討的文獻,包含發射信號的封包與頻寬對影像品質的影響。此外組織不均勻(tissue inhomogeneities)與影像系統本身的非線性度也是研究的重點。結果發現諧波溢漏現象對組織諧波影像品質有很大的影響,此時若使用脈衝反相技術不但可以提升影像對比度,同時也能保有足夠信號頻寬以及降低發射信號的設計複雜度。 但脈衝反相技術的限制在於影像物體可能在正負兩次發射中位移而產生移動雜訊干擾(motion artifacts),此時所有線性信號會因相消不完全而大幅增加。在本論文第二部份中我們將移動雜訊的影響分成信號強度以及影像品質兩個方面來探討,結果顯示在移動雜訊嚴重時會造成原本應該消除的基頻信號大幅上升,故脈衝反相技術仍須適當的濾波才能進行組織諧波成像,此外無法相消的溢漏諧波也會造成影像對比度的下降。 在本論文的第三部份我們則率先提出了一個以脈衝反相為基礎的創新基頻影像技術(pulse inversion fundamental imaging)來增強超音波對比劑影像(contrast imaging)上對血液灌流區的偵測能力,結果發現脈衝反相基頻影像相較於傳統之基頻影像或二次諧波影像均能大幅加血液灌流區和周遭組織的對比度(contrast-to-tissue ratio),此外不同特性的發射波形所誘發的震盪差異也可被運用於進一步增強此影像方法在對比劑偵測上的表現,但由於移動雜訊會大幅增加組織基頻信號的強度而降低血液和組織間的對比度,故適當修正移動雜訊對脈衝反相基頻影像的對比劑偵測能力十分重要。此方法不僅在臨床應用上極具價值,亦可延伸至超音波分子影像之領域。 本論文在附錄中也介紹了以van Cittert-Zernike理論為基礎的組織諧波影像的空間相關度(spatial covariance)分析,結果顯示諧波空間相關度受限於訊雜比而偏低,因此組織諧波影像應該結合線性信號來進行相位誤差修正(phase aberration correction)才能進一步提升影像品質。 The purpose of this dissertation is to investigate various issues in pulse inversion (PI) based ultrasonic nonlinear imaging. In PI technique, two phase-inverted transmissions are required for each beam. The linear signal is cancelled by summing both echoes together and the nonlinear signal remains in the sum. Image contrast can be significantly improved by the PI technique in ultrasonic nonlinear imaging. The first topic in the dissertation is spectral leakage in nonlinear tissue imaging. Conventional nonlinear imaging suffers from contrast degradation due to spectral leakage when non-negligible harmonic components are present prior to propagation. The leakage signal cannot be filtered out from the received signal due to spectral overlap with the nonlinear signal of interest. Our study is the first complete report in the literature to investigate issues related to spectral leakage in nonlinear tissue imaging. Effects of transmit signal, including envelope and bandwidth, are investigated. Nonlinearity of imaging system and tissue inhomogeneities are also discussed. It is found that the image contrast significantly decreases when spectral leakage is present. The PI technique can restore the image contrast by canceling the leakage signal in the sum, and the trade-off between axial resolution and image contrast is also avoided. Despite of its advantages, the PI technique suffers from frame-rate reduction and potential motion artifacts. Motion artifacts may be critical when there is relative movement between the probe and the imaged tissue during the two firings. In this case, the linear signal increases because of incomplete cancellation in the sum. Effects of motion artifacts on signal intensity and image quality in nonlinear tissue imaging are also studied in the dissertation. It is shown that filtering is still necessary in PI imaging to suppress the uncancelled fundamental signal. Residue spectral leakage also degrades image contrast in the presence of motion artifacts. In the third part of the dissertation, we propose a new PI-based imaging method for contrast nonlinear imaging by using signals in the fundamental band. This method is also known as the PI fundamental imaging. Compared to that obtained using either conventional fundamental imaging or second-harmonic imaging, the contrast-to-tissue ratio (CTR) is significantly improved in PI fundamental imaging. However, motion compensation is essential because the CTR decreases rapidly due to uncancelled tissue fundamental signal when the tissue moves. In addition to its clinical applications, PI fundamental imaging can also be extended to molecular imaging when microbubbles are used as molecular probes. In the appendix, additionally spatial covariance analysis based on the van Cittert-Zernike theorem is also provided for tissue harmonic imaging. It is suggested that the linear signal at lower frequency should be used for effective correction of phase aberration in tissue harmonic imaging. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/39365 |
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