Determination of Optimum Wavelength for Laser Photofragmentation of Urinary Stones

In order to determine a proper laser wavelength to fragment urinary stones with the minimum amount of energy, the optical absorption characteristics of urinary stones were investigated. The relative absorption spectra of dry stone samples were obtained by diffuse reflectance spectroscopy between 0.8 and 20.0 μm and showed three common major absorption peaks at 3 and 6 μm. To estimate the influence of the water content in vivo, and also to estimate the optical characteristics of the visible region, which has been used for laser photofragmentation so far, we investigated absolute optical absorption utilizing pulsed photothermal radiometry (PPTR) at 515 nm, 1.06 μm, and 5.4 μm. The infrared absorption by inorganic substances in the stone rather than the contained water was found to be dominant. Furthermore, the urinary stones showed strong absorption in the infrared compared with the visible region. The suitable wavelength for stone photofragmentation may exist in the infrared at 3 and 6 μm. At present, the former may be the most appropriate, but with the advancement of long-wave laser delivery systems, a more effective 6-μm band might be used for laser photofragmentation.