HIFU-Induced Hyperecho in Ultrasound Images, Cavitation Activity and Thermal Behavior

Abstract

High Intensity Focused Ultrasound (HIFU) treatment of soft tissues has been shown to result in a hyperechoic region in B‐mode ultrasound (US) images. This is believed to result from bubble activity at the HIFU focus. Here we report our in vivo results of detecting inertial and stable cavitation in correlation with the appearance of a hyperechoic region, along with in vitro confirmation of these results that included measurement of the temperature at the HIFU focus. The ultrasound system consisted of a HIFU transducer (3.3 MHz), a broadband A‐mode transducer for active and passive cavitation detection (ACD and PCD), and an US‐imaging probe that were all co‐focal and synchronized. HIFU, at in situ intensities of 220– 1,710 W/cm2, was applied for 10 s to pig muscles in vivo or polyacrylamide in vitro at a focal depth of 2 cm. A thermocouple placed at the HIFU focus was added to the above system during the in vitro portion of this study. ACD and PCD results showed a strong correlation between the onset of cavitation and the appearance of a hyperechoic region. In vivo PCD results showed that inertial cavitation typically occurred prior (within 0.5 s) to the appearance of a hyperechoic region. In vitro PCD results show that inertial cavitation occurred at or within 1–2 pulses prior to the appearance of a hyperechoic region and typically preceded rapid heating up to 110 °C at the HIFU focus within 1–2 pulses. The observed cavitation activity suggests that bubbles are present during the formation of a hyperechoic region at the HIFU focus and that boiling occurs rapidly after the onset of cavitation. Further investigation is needed to determine if the hyperechoic region in the US image originates from bubbles formed during cavitation alone or during cavitation‐induced boiling.