Three-dimensional passive cavitation mapping and correlation with stone damage during laser lithotripsy treatment

Introduction: Kidney stone disease is a condition where solid deposits of minerals and salts form inside the kidneys or urinary tract, affecting about 12% of the world population. In this study, we developed the three-dimensional passive cavitation mapping (3D-PCM) method to monitor the cavitation activities during laser lithotripsy (LL) with a 2D array transducer. The 2D array was calibrated with the hydrophone and characterized in terms of spatial resolution and acoustic field sensitivity. The bubble tracking method of Ultrasound Localization Microscopy technology was involved in the 3D-PCM method to improve the localization accuracy. Using the 3D-PCM approach, we imaged and analyzed laser-induced on-axis and off-axis single cavitation bubble. We monitored LL-induced cavitation bubble collapse on glass and Bego stone during the LL procedure (from pulse number 1 to 100). We also studied the dose dependency of LL and analyzed the correlation between bubble dynamics and stone damage. Consequently, our results have demonstrated the high special-temporal accuracy and reliability of the 3D-PCM approach regarding bubble collapse localization and prediction during LL treatment. We expected to apply this method to clinical kidney stone treatment settings, which can provide high-accuracy bubble distribution information for treatment monitoring and pre-treatment prediction regarding optimized stone damage performance.

Materials and Methods:: A. Laser lithotripsy treatment on quartz glass and Bego stone surface

A commercial Ho:YAG laser lithotripter (Dornier MedTech, Germany) was operated at Ep = 0.8 J and Freq = 10Hz. The pulsed laser is delivered with a 365-um core diameter fiber into a container filled with degassed water.

B. Passive cavitation detection with two-dimensional transducer array.

To monitor the entire three-dimensional cavitation activities on the stone surface during LL treatment, a custom-made 2D array (Imasonic SaS, France) was applied. It consists of 256 piezoelectric elements and has a central frequency of 4 MHz with 60% bandwidth. The data acquisition of the 2D array was performed by a Verasonics scanner with 15 MHz of sampling rate.

C. Sliding-window delay-and-sum reconstruction method and bubble localization

Based on the traditional delay and sum method, the reconstruction window with a fixed window size, slides along the time axis to search for the correct time origin of each individual collapse point. Ultrasound Localization Microscopy technology was used in our collapse points localization. The temporal resolution of 3D-PCM is around 0.256 us and spatial resolution is 433 um, 440 um and 186 um along the lateral, elevational, and axial axis.

E. Ultrasound B-mode imaging and high-speed camera shadowgraph imaging.

We performed B-mode imaging and high-speed camera shadowgraph imaging of LL-induced bubbles on a solid surface. The position of shockwave origin and collapse time estimation acquired from the shadowgraph images were used as validation of 3D-PCM data of the whole bubble collapse process on the solid surface.


Results, Conclusions, and Discussions:: Results:

A. Calibration and characterization of the customized 2D array.

B. 3D-PCM validation with the laser-induced single bubble in free field (on and off-axis). Five different laser energy levels were employed, which produced different bubble sizes. As shown in Figure 3(g), the collapse time captured by the camera and the collapse time captured by the 2D array is consistent. The 2D array has a much higher temporal resolution (around 0.128 μs) than the high-speed camera (up to 5 Mfps).

C. 3D-PCM of LL-induced bubble collapse on Bego stone. The experiment of LL-induced bubble collapse on Bego validated that 3D-PCM has a high accuracy of bubble localization with spatial resolution of 300 μm. In the actual treatment, the camera cannot be used to record the bubble-bursting activity in the human kidney while 3D-PCM could potentially be used to capture the detailed bubble collapse on stone.

D. Relevance analysis between bubble dynamics and stone damage. The results shown in Fig. 5 indicate the cavitation from bubble collapse plays an important role in craters formed on the Bego stone surface.

Conclusion:

In this manuscript, we developed the three-dimensional cavitation mapping method based on a 2D array. The 2D array was calibrated and characterized in terms of spatial resolution and acoustic field sensitivity. The temporal and spatial accuracy of the method was validated on laser-induced single bubbles. We have demonstrated statistically that 3D-PCM captures the bubble collapse precisely no matter whether it is located on-axis or off-axis. The 3D-PCM was then incorporated into a laser lithotripter and the collapsed bubbles were reconstructed and displayed. We expected to apply this method to clinical kidney stone treatment settings and provide the high accurate bubble distribution information for treatment monitoring.

Discussions:

The speed limitation of 3D-PCM is primarily due to the time required to reconstruct and track the acoustic emissions from bubbles generated by cavitation. As for the future work of developing a real-time display of bubble activities during clinical treatment, this is a promising direction. Real-time displays can provide immediate feedback on the effectiveness of the treatment and allow for adjustments to be made if necessary.


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