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Background and Objective: The purpose of this study was to develop a feedback control system for the pulsed holmium: YAG medical laser that enhances tissue selectivity and safety by discriminating between soft and hard biological tissue such as urinary and biliary calculi and bone. Study Design/Materials and Methods: The ability to discriminate is achieved by monitoring prompt laser-induced visible/ NIR photoemissions via retrograde transmission over the laser delivery fiber in conjunction with a developed detection algorithm. Results: Experimental data are presented for a system that employs this discrimination scheme with an electro-optic shutter for rapid intrapulse feedback control of holmium laser-based lithotripsy procedures. The results demonstrate the feasibility of a lithotriptor that can deliver 1 J per pulse to calculi yet limit errant discharges to surrounding urinary tract tissue to Շ 0.1 J. Conclusion: Based on animal tissue safety data, the laser margin of safety is improved by an order of magnitude. Lasers Surg. Med. 21:20-28, 1997 © 1997 Wiley-Liss, Inc.

A compact, portable spectrofluorimeter which measures broad-band fluorophores with high sensitivity is described. This instrument is specifically designed for acquisition of human tissue autofluorescence spectra in vivo. Pulsed excitation laser light at 337 nm and in the 370–700 nm wavelength range is provided, and the resulting fluorescence can be measured with 10-nm resolution. The system produces single-pulse spectra in human artery wall tissues with a signal-to-noise ratio of 100:1. An optically shielded fiber-optic probe for light delivery and collection permits calibrated intensity information to be obtained.