E. Tri-modality cavitation imaging of shock wave-induced bubble clusters

After validating the tri-modality cavitation mapping system on laser-induced single bubbles, we further applied the system to study the lithotripter shock wave induced cavitation clusters. Shock wave pulses were generated by a focused shock wave generator (Piezoson 100, Richard Wolf, Germany), which was mounted at the bottom of the water tank. The shock wave generator transmitted microsecond shock wave pulses with a pulse repetition rate of 1 Hz and a focal distance of 65 mm. The resultant cavitation bubble clusters distributed sparsely in a volumetric space around the focal zone, which was approximately 1 cm laterally by 2 cm axially.

Again, we used the high-speed camera and the linear-array ultrasound probe to capture the shock wave-induced bubble clusters. With a relatively large depth of focus, the high-speed camera can project all the bubbles throughout the whole shock wave focal zone into a single 2D image. However, the ACM and PCM can only detect bubbles located within the ultrasonic transducer's imaging plane, i.e., a thin 2D slice cutting through the shock wave focal zone. In other words, the ultrasonic transducer would only detect a portion of the bubbles captured by the high-speed camera. Therefore, it is challenging to directly correlate cavitation bubbles captured by the high-speed camera and the ultrasonic transducer. To resolve this issue, we implemented a 3D camera system (^{Sankin et al., 2012}). As shown in Fig. 2(a), we constructed two orthogonal white-light illumination beams that first passed through the shock wave focal zone, and then transmitted through, respectively, a red filter and a blue filter. The filtered light beams were combined by a dichroic mirror before captured by a color CCD camera. The red and blue channels of the camera images were recorded separately. The red channel image provided the x-z projection of the bubbles, and the blue channel provided the y-z projection, with the origin of the coordinates located at the upper left corner of the RGB image. A 3D reconstruction of the bubble cluster was extracted from the two projections. We aligned the imaging plane of ultrasonic transducer through the shock wave focus, with an angle of 45° relative to both illumination paths. The lateral direction of the ultrasonic transducer corresponds to the z direction of camera captured images [Fig. 2(b)], and the origin of the ultrasonic transducer is located at the topmost point of the transducer's elements as shown in the dashed rectangle in Fig. 2(a). The images from the camera and the ultrasonic transducer are registered through a pointer with its tip located at the shock wave focus. From the 3D reconstruction of the camera-captured bubbles, we extracted the group of bubbles within the ultrasonic transducer's imaging plane to validate the ACM/PCM images. The time sequence of shock wave transmission, camera recording, and ACM/PCM detection is shown in Fig. 2(c). The camera imaging and shock wave transmission were synchronized. A total of 55 camera frames were captured for each shock wave pulse, with a frame rate of 50 kHz. To acquire the ACM/PCM images, we applied a delay time of∼50 μs after the shock wave transmission, allowing the shock waves to completely pass through the ACM/PCM imaging plane and avoiding the acoustic signal interference. The delay also allowed the bubbles to grow substantially, providing stronger ACM signals.

(Color online) Tri-modality cavitation mapping system of lithotripter shock wave-induced bubble cluster. (a) Schematics of the experimental setup, showing the two orthogonal illumination beams and the 45° ultrasonic transducer. UST: ultrasonic transducer. The relative position of the lithotripter aperture and the ultrasonic transducer is shown in the dashed rectangle. (b) Photograph of experimental setup. (c) Time sequence of shock wave transmission, camera recording, and ACM/PCM detection. SW TX: shock wave transmission; Tx: ultrasound transmission; Rx: ultrasound/cavitation signal receiving.