The Investigation of Contrast Agent Enhanced Thermal Effect of High Intensity Focused Ultrasound

• Main Authors: Yao-Sheng Tung, 童耀生
• Other Authors: 陳文翔
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/07463255313172873477

碩士 === 國立臺灣大學 === 醫學工程學研究所 === 93 === In recent years, high intensity focused ultrasound (HIFU) was shown to have promising effect on ablating both malignant and benign tumors. Despite its unique advantages such as minimal invasiveness and radiation free, the size of the ablation lesion is small and thus the duration for complete tumor ablation is usually too long. In this study, the effect of using ultrasound contrast agent (UCA) to enhance the ultrasound thermal effect, and thus enlarge the lesion size, was studied. Different concentration of UCA ranging from 0% (control), 0.001% to 0.1% (v/v) was mixed evenly with polyacrylamide gel phantoms containing egg white as a temperature indicator. A 1.85-MHz HIFU transducer was used to form thermal lesions inside the 2 cm*2 cm*4 cm phantoms. For the electric power of 50W and 70W, ''cigar'' and ''tadpole’ shaped lesions were formed inside the control phantoms, respectively. The volume enlargement ratio (VER), defined as the ratio of volume formed in phantoms with UCA to the volume without UCA at the same power level, was used to evaluate the effect of UCA on thermal ablation. When the concentrations of UCA were 0.001%, 0.005%, 0.01% and 0.015%, the VER were 10.58, 39.11, 55.08, 64.65 at 50W, and 2.27, 5.29, 12.09, 10.61 at 70W, respectively. The administration of UCA significantly increased the lesion size up to 65 times! UCA also reduced the necessary power to form a lesion of a certain size. For example, 0.001% of UCA at 50W produced a lesion close to controls at 70W in size. Same concentration of UCA at 70W formed lesions equivalent in size for phantoms without UCA at 100W. 30% reduction of the power level was achieved. Complications of overheating could certainly be reduced when the output power decreased. The forward shift of a lesion, defined as the distance of the most heating position to the HIFU focus, was also investigated. When the UCA concentration was 0%, 0.001%, 0.005%, 0.01% and 0.015%, the lesion shifts at 50W and 70W were 0.1, 0.65, 0.99, 1.32, 1.51 cm and 0.65, 0.80, 1.38, 2.01, 2.16 cm, respectively. If the concentration of the UCA was greater than 0.1%, the lesion was basically formed at the surface of the phantom since most of the incident ultrasound waves were reflected. In order to understand the mechanism of lesion shape transformation at higher intensity (electric power > 70W) and/or introducing UCA, the lesion growing process & the internal temperature change with or without UCA under 70W 30s were continuously monitored. At higher intensity without UCA, the tadpole-shaped lesion was due to boiling effect. The lesion enhanced by UCA was because inertial cavitation, which produced more bubble inside the phantom, and made ultrasound be scattered, finally resulted in larger lesion and wider increase of temperature. In conclusion, UCA could increase the size of lesion by enhancing scattering, and the lesion size increased with the increase of the UCA concentration. However, lesions moved toward the transducer when concentration increased. It would be unable to produce the lesion inside the phantom if the concentration of UCA was too high. Overall, low concentration (0.001%) of concentration agent and low power (50 W) were enough to produce a lesion 39 times larger, and produced minimal lesion movement (0.65 cm).