Free electron laser ablation of urinary calculi: An experimental study

Infrared laser ablation of urinary calculi was investigated as a function of wavelength to determine the relation of ablation threshold fluences, ablation depths, and optical absorption. A simple photothermal ablation model was employed to examine this relationship. Human urinary calculi composed of > 95% uric acid, > 95% cystine, > 95% calcium oxalate monohydrate (COM), and > 90% magnesium ammonium phosphate hexahydrate (MAPH) were used. Various wavelengths between 2.1 and 6.5 μm were selected to perform threshold fluence and ablation depth measurements. The laser source for this study was the tunable pulsed infrared free electron laser (FEL) at Vanderbilt University. Experimental results indicated a correlation of threshold fluence and ablation depth to the optical absorption properties of the calculi. When calculus optical absorption increased, the threshold fluences decreased. Although the ablation depths increased with calculus optical absorption, results indicated that in certain calculi the ablation depth was affected by optical attenuation through the ablation plume. These observations were in agreement with the photothermal ablation model, but fractures in striated calculi at higher optical absorptions, indicated the contribution of a photomechanical mechanism. Threshold fluences and ablation depths are a function of the wavelength dependent absorption properties of the calculi. These observations suggest that the 3-μm and 6-μm absorption bands are optimal for ablation or fragmentation of urinary calculus.