The use of transparent aqueous gels for observing and recording cavitation activity produced by high intensity focused ultrasound

Low gelling temperature agarose gels are exceptionally clear and owe their rigidity to the formation of a homogenous lattice of polymer molecules. We have observed cavitation bubbles moving though these gels leaving visible tunnel-like tracks whose walls can be visualized at high magnification, especially with phase contrast illumination. These tracks provide a record of the limits of each individual bubble´s dynamic behavior and demonstrate a wide range of bubble activities including coalescence, fragmentation and bubble-bubble interactions. A HIFU transducer with 3.75 cm radius of curvature was operated at 0.78 MHz using tone burst signals from a gated oscillator and power amplifier. Agarose gels (1% to 8% w/v in saline)were cast as 1×1×3 cm blocks or 2.5 cm diameter discs, 2-3 mm thick. Bubble motion within the gel was achieved either by initiating cavitation in the water bath directed toward the uncovered surface of the agar, or by embedding a gas microbubble contrast agent (Optison®) within the agar and interposing a thin plastic film between the gel and the water bath. Various water bath configurations were used to expose the gels to ultrasound under free-field and reflecting conditions while simultaneously viewing the focal volume. The tunnels formed following nucleation in the water bath were liquid-filled, and when the gel was dried slightly, these tunnels filled with air. However, tunnels formed entirely within the gel from embedded nuclei could not be drained. Analysis of high speed video recordings made during exposure assisted in the interpretation of the tunnel formation. The diameter of the tunnels tended to increase with increasing pressure amplitude, and the length increased with both pressure amplitude and exposure duration. For example, tunnels formed during 2 MPa exposure for 200 ms were 50 - 100 microns in diameter and up to several 100 microns long.