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
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.
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).
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