雙頻高強度聚焦超音波熱治療之探討

Abstract

有研究顯示，兩頻率相近之雙頻超音波換能器可能增大高能超音波燒灼的區域。在本研究中，使用了雷利積分式來模擬雙頻超音波線性聲場，在雙頻聲場中，兩頻率相近會產生隨時間改變的拍音效應，為求分析此種時變聲場，我們必須建立在時域上的分析方法。拍音會隨時間變化，聲場強度亦隨之以拍音的頻率變化，亦為一週期函數，故我們可以求得在一週期內聲場強度之平均值，來模擬溫度場與熱劑量的分佈。我們採用了0.5MHz切半之半球形超音波換能器，以雙頻驅動來做進一步的探討（直徑/曲率半徑：10cm/10cm）。 在模擬的部分裡，雙頻兩元素同軸環形之超音波換能器的聲場分佈會有朝軸向延伸的情形發生，對於HIFU治療時要盡量避免燒灼到正常組織來說，是較不利的，另一種切半球形之超音波換能器之聲場強度則有朝橫向延伸的情形，形成一截面為橢圓形之焦斑，在線性的探討上，模擬的結果顯示雙頻超音波換能器所能產生的焦斑體積與單頻所產生的體積並無明顯差異。 另外在此研究中進行了初步的實驗探討，結果證實雙頻超音波換能器會形成截面為橢圓形之焦斑，然而在體積的定量上有其難度故不予計算，所使用的電功率為25W + 25W。

The dual-frequency high-intensity focused ultrasound was shown to have the potential ability to increase the size of HIFU lesions. In this study, we proposed a Rayleigh integral method to predict the linear pressure field of the dual-frequency driven transducers that dual frequencies were slightly different. Because two slightly different frequencies generate time-varying beats, time-domain analysis should be employed. With time-varying beats, acoustic intensity also changes with time at the beat frequency. That time-varying intensity is a periodic function with a short period（1 / beat frequency）, and we can apply time-average intensity to study temperature and thermal lesions. The dual-frequency ultrasound was generated by a 0.5MHz（diameter and radius of curvature：10 cm）confocal transducer, which was cut from a bowl-shape piezo-electrical element into two half-hemispherical elements. In the simulation part, the intensity of dual-element annular confocal ultrasound transducer has some extension in the axial direction. This feature is a shortcoming in HIFU therapy. The other one, dual-element spherical ultrasound transducer driven by dual-frequency, has some extension in the transverse direction. An ellipse shape lesion was formed at the focal plane and it is more suitable for HIFU therapy. So we chose the latter one to do some experiments. The volume of the lesion didn’t show a significant difference in this linear study. Some preliminary experiments demonstrated the formation of the ellipse shape lesion by the dual-frequency sonication. However, there is no precisely quantitive method of the lesion in the porcine muscle, the lesion volume was not calculated. The power of both sonication（dual & single）is 25 walt in each element and 50 walt in total.