Monitoring cavitation dynamics evolution in tissue mimicking hydrogels for repeated exposures via acoustic cavitation emissions.

A 700 kHz histotripsy array is used to generate repeated cavitation events in agarose, gelatin, and polyacrylamide hydrogels. High-speed optical imaging, a broadband hydrophone, and the narrow-band receive elements of the histotripsy array are used to capture bubble dynamics and acoustic cavitation emissions. Bubble radii, lifespan, shockwave amplitudes are noted to be measured in close agreement between the different observation methods. These features also decrease with increasing hydrogel stiffness for all of the tested materials. However, the evolutions of these properties during the repeated irradiations vary significantly across the different material subjects. Bubble maximum radius initially increases, then plateaus, and finally decreases in agarose, but remains constant across exposures in gelatin and polyacrylamide. The bubble lifespan increases monotonically in agarose and gelatin but decreases in polyacrylamide. Collapse shockwave amplitudes were measured to have different-shaped evolutions between all three of the tested materials. Bubble maximum radii, lifespans, and collapse shockwave amplitudes were observed to express evolutions that are dependent on the structure and stiffness of the nucleation medium.

A cost-effective, multi-flash, "ghost" imaging technique for high temporal and spatial resolution imaging of cavitation using "still-frame" cameras.

This paper describes a method for acquiring high temporal and spatial resolution images of cavitation events using a multiple-flash-per-camera-exposure imaging technique. A primary challenge associated with imaging cavitation is that the velocity of the bubble wall reaches its maximum (∼1.5×103 m/s) as the bubble size approaches its minimum (≲1 µm). In order to adequately resolve dynamics on these scales, specialized-often prohibitively expensive-cameras with ultra-high frame-rates and resolutions are generally required. This paper describes low-cost, high-speed light emitting diode (LED) flash sources with minimum pulse widths of 20 ns that can be pulsed at rates of up to 17 MHz. The flashes are used to illuminate images of bubbles captured using high-resolution "still-frame" cameras wherein multiple flashes are issued from the LED(s) at known time intervals within a single camera exposure, resulting in overlapping snapshots of the same bubble at multiple unique time-points in a single image. The overlapping snapshots can be uniquely associated with the known time-points of the flashes based on their relative levels brightness. This paper demonstrate effective frame-rates up to 4 Mfps using this technique and the acquisition of snapshots at up to 13 unique time-points per exposure. Hardware descriptions of the flash sources and the programmable device used to control them are provided.