Gel phantom study with HIFU: Influence of metallic stent containing either air or fluid

Abstract

Purpose: We aimed to investigate whether a cylindrical structure containing either air or fluid and with or without a metallic stent affects the volume and density of cavitation produced by high-intensity focused ultrasound (HIFU) via a gel phantom study. Materials and Methods: Sixteen phantoms with a cylindrical hole were divided into 4 groups of 4 phantoms with air in the holes (Group 1), 4 phantoms with fluid in the holes (Group 2), 4 phantoms with air-containing metallic stents (Group 3), and 4 phantoms with fluid-containing metallic stents (Group 4). The VIFU-2000 small animal HIFU unit (ALPINION Medical Systems, Seoul, Korea) was used with acoustic power (100W), exposure time (36 sec.), duty cycle (50%) and pulse repetition frequency (40 Hz). The focus of the HIFU beam was placed at the posterior wall of the hole in the phantom. The size of cavitation on x-, y-, and z-axis was measured, and the volume of cavitation and coagulation was calculated using the formula for the volume of an elliptical cone. The density of cavitation was measured in the tissue phantom anterior to the hole with 1cm x1cm square region of interest. For statistical analysis, Kruskal-Wallis test and Mann-Whitney U test were used. Results: The volume of anterior cavitations of Groups 1 and 3 were significantly larger than those of Groups 2 and 4 (P < .05). The volume of posterior cavitations of Groups 1, 2, 3 and 4 were not significantly different each other (P > .05). The size on all axes and volumes of anterior cavitations were significantly larger than those of posterior cavitations only in Groups 1 and 3 (all P < .05). Regarding the density, anterior cavitations of Groups 1 and 3 were significantly denser than that of Groups 2 and 4. (P < .05). Conclusion: Phantoms with air-containing holes developed larger and denser cavitations anterior to the focus without unnecessary coagulation posterior to the focus, regardless of the presence of stents. The result of this study might be applied to maximize cavitation to enhance drug delivery into tumors before air-containing duct or stent.