高強度聚焦型超音波對血管之熱效應模擬

Abstract

超音波應用在醫學上的研究十分廣泛，從超音波影像、減少痙攣、消炎、加速傷口癒合、神經再生，到利用熱燒灼治療腫瘤等等。但若要了解超音波能量在人體內的分佈，只有當醫師於治療時使用高強度聚焦型超音波，同時透過溫度導引系統來觀察，或是購買相關儀器進行超音波實驗方能得知。本研究透過有限元素數值分析高強度聚焦型超音波在血管組織模型中產生之能量分佈及熱效應，盼能建立出一套可預測超音波能量與溫度在人體內變化情況之模擬。本研究首先以波動方程式為基礎計算出超音波產生之壓力場大小，接著利用超音波能量在介質內會累積的特性，透過生物熱力學理論加以運算得出超音波在模型中所造成的溫度分佈。本研究探討不同血管組織模型、超音波頻率、超音波能量及時間等因素對於超音波在人體組織內焦區及溫度變化之影響與關係。經過數值分析得知當採用線性且靜態之模擬時，惟有超音波頻率及組織特性才會改變壓力及強度焦區的大小；超音波的能量(法線位移)則會影響聚焦區在一定時間內溫度上升的度數；中空及非中空形狀之超音波探頭，此兩者加熱組織的能力僅有 0.2% 之差異；頻率的增加會使得聚焦區域較集中且組織溫度增幅劇烈。而在後續的研究中可利用此模型搭配血液流速及血管形變等理論，實現更高預測性的有限元素分析，以作為未來進行高強度聚焦型超音波治療參數之參考。

Ultrasound can be used in many kinds of medical studies, such as ultrasound imaging, reducing cramps, anti-inflammation, accelerating wound healing, nerve regeneration, ablation therapy for tumor, etc. However, there are only two ways to understand the distribution of ultrasonic energy in the human body: one the MRI thermometry to monitor ultrasound-induced temperature inside the body, the other is the pressure measurement in free field to estimate focal intensity at the target tissue. Analyses of the energy distributions and thermal effects were performed by finite element method, to establish a predictable set of simulation model of ultrasonic energy and the temperature changes inside the human body. Firstly, the pressure field generated from a focused ultrasound transducer was calculated on the basis of the wave equation, and the temperature distribution was obtained in the model by Bio-thermodynamics. Effects of the ultrasonic frequency, ultrasonic energy and sonication time on the focal zone and the tissue temperature were investigated in various blood vessel models. Through results of static simulations showed that: the frequency of ultrasound and the tissue properties affect both the focal pressure and focal size. The result of dynamic simulations demonstrated that, the ultrasonic energy (also known as normal displacement amplitude) affected the temperature within the focal zone. Additionally, the heating effect of the blood vessel by the transducer with a central hole was similar with that by the transducer without the hole. The focal size was decreased and the temperature rise became fast as the ultrasonic frequency was increased. This model will be combined with parameters of blood flow rate and vascular deformation to achieve higher predictive finite element analysis as a reference for high-intensity focused ultrasound treatment.