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Recent studies suggest that cavitation effect following laser induced vapor bubble collapse is more dominant than the photothermal effect in stone ablation during laser lithotripsy. Our research aims to introduce an experimental study design that precisely measures each effect's contribution using gypsum phantom stones. To isolate the cavitation-only mechanism after the collapse of the laser-induced vapor bubble, a phantom stone was submerged in a dye solution. The dye solution absorbed all laser light, generating cavitation, with additional experiments confirming the absence of any photothermal effect when the dye was not used. The fiber was positioned both parallel to the stone surface and perpendicular at a 1mm distance, exposing it solely to cavitation. In another set of experiments, a phantom stone was submerged in water and 2μm light from a thulium yttrium aluminum garnet (Tm:YAG) laser was delivered via the same optical fiber positioned (this time) perpendicular to the stone surface. In this case, both optical absorption and cavitation effect from laser-induced vapor bubble collapses were observed but the measured pressure transients showed significantly lower peak pressures compared to the first set. In a final set of experiments, these conditions remained constant, except the fiber was positioned parallel to the stone surface, once again exposing it to only the cavitation from the collapse of the laser induced vapor bubble. Craters created by all methods were imaged using an optical coherence tomography (OCT) system. Measured volumes showed that stone ablation was dominated by photo-thermal, and not by cavitation from the vapor bubble collapse. In fact, in two of the three trials of stone experiments (n=5, each trial) that were subjected to cavitation-only, there was no observable ablation. One trial produced an average volume that was 50% smaller than the average resulting from a single photo-thermal-only case (p = 0.0022 < 0.05). Our results suggest that finetuning of lithotripsy procedures with focus on energy transmission to the stone can provide optimal results.
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