Experimental setup for bubble cloud monitoring at a tissue-water interface using optical attenuation and acoustic backscatter methods. Light source and CCD camera in dashed circle are set up for high speed imaging in water. However, at a tissue-water interface, the light source was blocked by the tissue and the imaging could not be used with the optical attenuation detection.
An image of the bubble cloud taken by a high speed camera. The bubble cloud was generated by a single pulse of three cycles in gas saturated free water. It was used for the laser beam alignment. The ultrasound pulse propagated from the left to the right side of the image.
Light attenuation signal caused by a bubble cloud recorded as a photodetector output. The bubble cloud was generated by a 6-cycle histotripsy pulse in water. The top left arrow indicates the arrival of the histotripsy pulse at the therapy transducer focus where the laser beam is projected. The inset is an expanded view (compressed in vertical direction and expanded in horizontal direction) of the optical signal tracking the ultrasound pulse wave form.
Light attenuation, variable acoustic backscatter and corresponding bubble cloud imaging in water. (a) Light attenuation. Each vertical line is a light intensity signal corresponding to one ultrasound pulse, with light intensity encoded in gray color. Dark color indicates a decrease in the light intensity. The horizontal axis is pulse number. (b) The light attenuation duration for each pulse vs pulse number. (c) Acoustic backscatter in fast time-slow time display. Each vertical line is an -line acoustic backscatter signal where the signal amplitude is encoded in gray color. (d) The normalized backscatter power moving SD vs pulse number. (e) Example bubble cloud image. Each image was taken after the arrival of a pulse. (f) Bubble cloud cross-sectional area vs pulse number. Horizontal arrows indicate the arrival of the histotripsy pulse.
Wave forms of synchronized light attenuation, acoustic backscatter signals and corresponding bubble cloud images from Fig. 4. The ultrasound pulse propagated from the left to the right side of the image. The short arrows in the top two rows indicate the arrival of the histotripsy pulse at the focus of the therapy transducer. The light attenuation (after the pulse) and variable acoustic backscatter signals were only detected when a bubble cloud was observed using high speed imaging. The insets in the top row are expanded views of the optical signal tracking ultrasound pulse wave form during the pulse.
Optical attenuation signal tracking the ultrasound pulse wave form without the formation of a bubble cloud. The peak amplitude of the signal increased with increasing pressure.
Initiations of the light attenuation and the variable acoustic backscatter at a tissue-water interface. Panels (a)–(d) display the light attenuation and acoustic backscatter signals in the format described in Fig. 4. Initiation of the variable backscatter corresponded well to initiation of the light attenuation.
Extinctions and re-initiations of the light attenuation and the variable acoustic backscatter at a tissue-water interface. Panels (a)–(d) display the light attenuation and acoustic backscatter signals in the format described in Fig. 4.
Wave forms of light attenuation and acoustic backscatter signals in Fig. 8. The short arrows indicate the arrival of the histotrispy pulse at the focus of the therapy transducer. Variable backscatter was observed with light attenuation. Without light attenuation, a high amplitude but stable backscatter was observed.
Bubble cloud rebounds detected as the additional light attenuations (pointed by arrows) after the main light attenuation signal. The durations of the main light attenuation and the second peak are longer with higher pressure. Interestingly, the additional light attenuation peak in dashed circle always occurred at after the ultrasound pulse even as the pulse pressure increased.
A bubble cloud was first generated by a high amplitude histotripsy pulse (, ) which caused the light attenuation. One ms after the onset of the histotripsy pulse, a lower amplitude pulse (, ) was delivered. The lower amplitude pulse, which could not produce a bubble cloud by itself, regenerated the bubble cloud resulting in another light attenuation. A pulse duration of 6 cycles and a gas concentration of 22–24% were employed.
The peak attenuation level of the light attenuation caused by the lower amplitude pulse as a function of the delay time. The peak attenuation level has a linearly decreasing trend on a log scale axis. On a linear scale, the peak attenuation level displayed an exponential-like decay.
Acoustic parameters used in all figures.
Article metrics loading...
Full text loading...