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  • 學位論文

以近場影像與電腦斷層掃描提升剪切波彈性影像解析度之研究

Improving Resolution of Shear Wave Elasticity Imaging – Near Field Imaging and Shear Wave Computed Tomography

指導教授 : 李百祺

摘要


高頻超音波系統,相較於目前大部分超音波系統,能提供較高的空間解析度,亦能偵測到較小的剪切波位移,特別適合用於臨床前小動物實驗或三維細胞培養觀測上。在之前的研究中,我們成功地將剪切波彈性影像架設於此系統上。本研究則是進一步修改先前的架構,改為接收近場的剪切波,以提高剪切波彈性影像的解析度及增加其準確性。接著嘗試結合近場剪切波彈性影像及電腦斷層掃描,希望能進一步獲得待測物各個角度的資訊,及更加提升影像的解析度。大部分的剪切波彈性影像都是藉由計算剪切波於組織中的傳播速度以求得彈性,然而剪切波會隨著傳遞的距離衰減甚至變形,因此遠離剪切波產生位置的速度計算結果便會因為訊雜比的降低而較不準確。因此,我們提出近場剪切波彈性影像的方法,希望藉由量測近場的剪切波,減少傳統方法中剪切波可能在傳遞過程中大幅衰減且變形的影響,來增加剪切波彈性影像的準確性及提高解析度。在仿體的實驗結果中,Contrast-to-noise ratio由先前架構的1.45,在近場架構中提升至2.00;硬塊仿體計算出的速度偏差也由先前架構-19.04%降低至近場架構的1.44%,可見近場剪切波彈性影像確實能有效提高硬塊與背景的對比以及數值的正確性。在活體實驗中,亦可以更有效觀察到腫瘤的位置和速度變化。在近場剪切波彈性影像的基礎下,我們提出了剪切波彈性影像電腦斷層攝影。在現有的超音波電腦斷層攝影中,可以重建出整個掃描範圍內聲波的速度分布。我們希望用類似的方式,但將原本計算聲波的速度改為計算剪切波的速度,以現有的剪切波彈性影像系統,重新設計掃描的方式,藉由許多次不同角度、位置的發射及接收資訊,以time-of-flight的方式得到剪切波的到達時間,再搭配既有的電腦斷層攝影重建方法,來算出整個掃描範圍內的剪切波速度分布,也就相當於彈性分布。斷層掃描方法目前重建結果仍有顯著的誤差,其可能的原因在本論文中亦有討論。

並列摘要


High frequency ultrasound imaging, which provides higher spatial resolution and better sensitivity for detecting shear wave displacement, is suitable for pre-clinical small animal and 3-D cell culture system studies. In our previous study, we have implemented shear wave elasticity imaging (SWEI) on our high frequency single element system. However, due to attenuation and diffraction, the signal-to-noise ratio (SNR) and estimation accuracy decrease with distance. The measured speed map tends to be incorrect for large region of interest (ROI). In this research, we hypothesize that by adopting near field SWEI (i.e., push transducer and image transducer are kept close to each other) SWEI resolution and accuracy can be improved. Furthermore, we try to combine the near field SWEI and computed tomography, hoping to have an even higher resolution. Our results show that in phantom experiments, the contrast-to-noise ratio is 1.45 for the previous setup, and 2.00 for the near field setup; the shear wave velocity bias of inclusion is -19.04% for the previous setup, and 1.44% for the near field setup. We were also able to obtain a 2D elasticity map of a mouse tumor with better agreement with B-mode morphology compared to the conventional setup. Finally, large errors exist in the results from shear wave computed tomography. Possible sources of errors are discussed.

參考文獻


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[5] J. Bercoff, M. Tanter, and M. Fink, "Supersonic shear imaging: a new technique for soft tissue elasticity mapping," IEEE Trans UltrasonFerroelectrFreq Control, vol. 51, pp. 396-409, Apr 2004.

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