Free electron laser lithotripsy: Threshold radiant exposures

Purpose: To determine the threshold radiant exposures (J/cm(2)) 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 thres holds and lithotripsy efficiencies with respect to optical absorption coeff icients. Materials and Methods: Human calculi composed of uric acid, calcium oxalate monohydrate (COM), cystine, or magnesium ammonium phosphate hexahydrate (M APH) were used. The calculi were irradiated in air with the free electron l aser (FEL) at six wavelengths: 2.12, 2.5, 2.94, 3.13, 5, and 6.45 mu m Results: Threshold radiant exposures increased as optical absorption decrea sed. At the near-infrared wavelengths with low optical absorption, the thre sholds were >1.5 J/cm(2). The thresholds decreased below 0.5 J/cm(2) for re gions 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 threshol ds, smaller radiant exposures were required to achieve breakdown temperatur es or to exceed the dynamic tensile strength of the material, Therefore, mo re energy is available for fragmentation, resulting in higher lithotripsy e fficiencies.