Use of overpressure to assess the role of bubbles in focused ultrasound lesion shape in vitro

Overpressure-elevated hydrostatic pressure-was used to assess the role of g as or vapor bubbles in distorting the shape and position of a high-intensit y focused ultrasound (HIFU) lesion in tissue, The shift from a cigar-shaped lesion to a tadpole-shaped lesion can mean that the wrong area is treated. Overpressure minimizes hubbies and bubble activity by dissolving gas bubbl es, restricting bubble oscillation and raising the boiling temperature, The refore, comparison with and without overpressure is a tool to assess the ro le of bubbles. Dissolution rates, bubble dynamics and boiling temperatures were determined as functions of pressure. Experiments were made first in a low-overpressure chamber (0.7 MPa maximum) that permitted imaging by B-mode ultrasound (US), Pieces of excised beef liver (8 cm thick) were treated in the chamber with 3.5 MHz for 1 to 7 s (50% duty cycle). In situ intensitie s (I-SP) were 600 to 3000 W/cm(2). B-mode US imaging detected a hyperechoic region at the HIFU treatment site. The dissipation of this hyperechoic reg ion following HIFU cessation corresponded well with calculated bubble disso lution rates; thus, suggesting that bubbles were present, Lesion shape was then tested in a high-pressure chamber. Intensities were 1300 and 1750 W/cm (2) (+/- 20%) at 1 MHz for 30 s, Hydrostatic pressures were 0.1 or 5.6 MPa, At 1300 W/cm(2), lesions were cigar-shaped, and no difference was observed between lesions formed with or without overpressure, At 1750 W/cm(2), lesi ons formed with no overpressure were tadpole-shaped, but lesions formed wit h high overpressure (5.6 MPa) remained cigar-shaped. Data support the hypot hesis that bubbles contribute to the lesion distortion. (E-mail: bailey@apl .washington.edu) (C) 2001 World Federation for Ultrasound in Medicine & Bio logy.