Our histopathological examination of rabbit livers directly following high intensity focused ultrasound (HIFU) irradiation revealed tissue degeneration characterized by hemorrhage, cavitation and liquefaction, in addition to thermocoagulation. This suggested a possibility of dispersing tumor cells. We called these histological findings “structural destruction”. This structural destruction may have been due to excessive energy of ultrasound at the focus. We, therefore, have developed the phased array method in which ultrasound intensity is dispersed. We examined, with our histological classification, the tissue degeneration of rabbit liver, which was exposed by phased array HIFU. Average intensities at the focus (range: 30–360 W/cm2) and duration of exposure (range: 1–15 s) were tested. When each combination of average intensity at the focus and duration of exposure was 100 W/cm2/15 s, 120 W/cm2/10 s, 120 W/cm2/15 s and 180 W/cm2/5 s, tissue degeneration seemed moderate without structural destruction. HIFU treatment with phased arrays is expected to become a new technique for the thermal ablation of liver tumors without structural destruction.
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