Purpose: To determine the threshold radiant exposures (J/cm2) needed for ablation or fragmentation as a function of infrared wavelengths on various urinary calculi and to determine if there is a relation between these thresholds and lithotripsy efficiencies with respect to optical absorption coefficients. Materials and Methods: Human calculi composed of uric acid, calcium oxalate monohydrate (COM), cystine, or magnesium ammonium phosphate hexahydrate (MAPH) were used. The calculi were irradiated in air with the free electron laser (FEL) at six wavelengths: 2.12, 2.5, 2.94, 3.13, 5, and 6.45 μm. Results: Threshold radiant exposures increased as optical absorption decreased. At the near-infrared wavelengths with low optical absorption, the thresholds were >1.5 J/cm2. The thresholds decreased below 0.5 J/cm2 for regions of high absorption for all the calculus types. Thresholds within the high-absorption regions were statistically different from those in the low-absorption regions, with P values much less than 0.05. Conclusions: Optical absorption coefficients or threshold radiant exposures can be used to predict lithotripsy efficiencies. For low ablation thresholds, smaller radiant exposures were required to achieve breakdown temperatures or to exceed the dynamic tensile strength of the material. Therefore, more energy is available for fragmentation, resulting in higher lithotripsy efficiencies.
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