The investigation of contrast-agent enhanced ultrasound thermal effect

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 experimental groups (adding UCA or increasing power) to the volume of controlled the sample, 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 SOW 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 100 W 30% reduction of the power level was achieved. However, the shift of the most heating position also increased with the concentration of UCA. investigated. For example, at UCA concentration of 0.015%, the shift could be 2.16 cm from the geometric focus. For concentration higher than 0.1%, the lesion was basically formed at the surface of the phantom. The mechanism of lesion formation and transformation was also investigated. For samples without UCA insonified by high intensity ultrasound, the tadpole-shaped lesion was formed due to boiling effect. The larger lesion formed in samples with UCA was because of the generation of inertial cavitation and bubbles which scattered sound waves and evenly distributed energy absorption in a larger volume. In conclusion, UCA could increase the size of lesion by enhancing scattering. However, the most heating center also moved toward the transducer when UCA concentration increased. A lower concentration (0.001%) of UCA and lower power (50 W) were enough to produce a lesion 39 times larger, and produced a minimal lesion movement (0.65 cm).