Efforts to ablate soft tissue with conventional lasers have been limited by collateral damage and by concern over potential photochemical effects. Motivated by the thermal-confinement model, past infrared investigations targeted the OH-stretch mode of water with fast pulses from lasers emitting near 3,000 nm (refs 1, 7-9). What does a free-electron laser offer for the investigation of tissue ablation? Operating at non-photochemical single-photon energies, these infrared sources can produce trains of picosecond pulses tunable to the vibrational modes of proteins, lipids and/or water. We report here that targeting free-electron laser radiation to the amide II band of proteins leads to tissue ablation characterized by minimal collateral damage while maintaining a substantial ablation rate. To account for these observations we propose a novel ablation mechanism based on compromising tissue through resonant denaturation of structural proteins.
We have investigated the fluorescence profiles of four common pathogens: S. pneumoniae, S. aureus, M. catarrhalis, and H. influenzae. The steady-state auto fluorescence spectra of bacteria are measured as a function of the incident light from 200 to 700 nm. The spectra for each bacterium are combined into a fluorescence profile or fluorescence finger print. Each bacterium produces a unique in vitro fluorescence profile when measured in a saline suspension. The profiles are reproducible. Suspensions of a bacterial strain, where the identification is not known, can be correctly matched to a small library of previously measured fluorescence profiles using a linear least-squares fitting algorithm. In addition, we have measured the fluorescence and absorption spectrum of the tympanic membrane removed from a chinchilla. The optical properties of the tympanic membrane and the least-squares identification process form precept for a non-invasive, fluorescence based bacterial diagnosis technique to be used in otitis media.
We have carried out simultaneous, time-dependent measurements of the free-electron-laser ͑FEL͒-induced stress transients and ablation plume in gelatin, which serves as a model system for collagenous tissues. The Mark-III FEL is tunable in the mid-IR ͑2-10 m͒ and produces macropulses of microsecond duration comprised of picosecond micropulses separated by 350 ps. The macropulse duration was shortened with a broadband, IR Pockels cell, producing pulse durations as short as 60 ns and energies in the range of 0.1-1 mJ. The IR beam was focused to a diameter of 112-210 m, depending on the wavelength, and measurements were made at 3.0, 3.36, and 6.45 m. For fluences below the ablation threshold, stress transients were measured and accounted for with a standard thermoelastic mechanism. Of particular interest were the measurements with fluences above the ablation threshold, where two classes of dynamics were observed. A cw HeNe beam monitors the plume: at 3.0 m a single maximum of the ''shadow'' is observed, while at 3.36 m and 6.45 m a second maximum also was resolved at later times. In addition, at 3.36 m and 6.45 m the duration of the momentum recoil is about twice as long as that observed for comparable exposure parameters at 3.0 m.
Currently, the identification of the bacteria responsible for acute otitis media requires a painful invasive procedure: tympanocentesis. To develop a rapid and noninvasive technique for bacterial diagnosis, the fluorescence profiles of four common pathogens and the optical characteristics of the tympanic membrane have been investigated. Each bacterium produces a unique in vitro fluorescence profile when measured in a saline suspension. Also, spectrally resolved transmission measurements from the chinchilla tympanic membrane demonstrate an optical window that will transmit sufficient light for in vivo measurement of the fluorescence profiles. Thus, we have established the precept for a fluorescence-based bacterial diagnosis technique to be used in otitis media. This paper presents the theory, optical data, and a discussion of the device engineering involved in the technique.
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