雙頻高強度共聚焦超音波與超音波顯影劑合用於熱治療之探討

Abstract

雙頻高強度聚焦式超音波相對於單頻高強度聚焦式超音波而言，擁有較低的平均聲能量強度，產生較少的非線性諧波聲場，而在介質中被吸收的比率較少。因此雙頻超音波擁有較高的組織穿透力，但焦斑生成能力卻較弱。除此之外，雙頻聚焦式超音波因為可產生遠較主聲場為低的頻率的差頻聲場，在與超音波顯影劑合用時，在聚焦點上可能製造出較單頻聚焦式超音波與超音波顯影劑合用時更大且更準確的焦斑。本實驗中，我們使用了兩種不同的組織仿體以檢證這樣的假說，這兩種組織仿體分別是以poly-acrylamide為基礎的蛋白仿體以及活體大鼠肝臟。而在本實驗中，我們選用Sonovue作為超音波顯影劑。 在實驗中，我們使用了特製的雙陣元共焦式超音波換能器以產生單頻與雙頻超音波聲場。為了檢測這些換能器的效能，我們運用以雷利-薩瑪菲爾德繞射積分式為基礎的MATLAB程式事先模擬各種聲場強度分佈，並以超音波水診器實際檢測超音波換能器所實際產生的聲場分佈。除此之外，我們也使用了微量天平以檢驗這些超音波換能器的電聲轉換效率。在蛋白仿體實驗中，我們使用了四種顯影劑濃度，分別是0%, 0.001%, 0.01%, 0.1%，超音波強度為50+50瓦，進行了60秒的燒灼。而在活體大鼠肝臟實驗中，我們則實驗了三種超音波顯影劑濃度，分別是4.5l/kg, 45l/kg, 以及450l/kg，超音波強度為30+30瓦，進行了30秒的燒灼。 實驗結果顯示出，在與超音波顯影劑合用時，單頻共焦式高強度聚焦超音波能產生的焦斑大小大於雙頻超音波，但是後者所造成的焦斑位移卻小於前者。可能由於雙頻超音波能在焦點處產生低頻的差頻聲場，大幅強化空蝕作用，藉以在超音波焦點處產生較為準確的焦斑。 關鍵字：拍音，雙頻共焦式高強度超音波，焦斑體積，焦斑位移，超音波顯影劑

Dual-frequency high-intensity-focused-ultrasound (HIFU), due to its lower average acoustic pressure, has better penetration but poorer lesion-producing ability than single-frequency HIFU. However, its ability to produce beating sound wave, of which the frequency is much lower than original ultrasound frequency, may be able to produce a larger thermal lesion when combined with ultrasound contrast agent (UCA) and may be more precisely located at the desired location than those produced by single-frequency HIFU. We studied the hypothesis with poly-acrylamide based egg-white phantom and rat liver in vivo. We used a customized two-element ultrasound transducer to produce single- and dual-frequency ultrasound for experiments. Computer simulation based on Rayleigh integral was used to simulate the acoustic pressure and the transducer was evaluated with hydrophone to check the pressure distribution. Sonovuewith different concentration was applied to phantoms (0%, 0.1%, 0.01%, and 0.001%) and rats (4.5 l/kg, 45 l/kg, and 450 l/kg). The electric power and heating time were 50+50 Watt and 60 sec in phantom experiments and 30+30Watt and 30 sec in rat liver experiments. The experimental results showed that single-frequency HIFU produced larger thermal lesion. Each experiment showed that when UCA applied, the distance of lesion shifted from the focus was shorter in dual-frequency HIFU than in single-frequency HIFU. It may be because the difference acoustic field generated by dual-frequency ultrasound at focal area enhanced the cavitation effect and therefore produced thermal lesion more close to focus. Keywords: beatings, confocal dual-frequency high intensity focused ultrasound, lesion size, lesion shift, ultrasound contrast agent