Pulsed ultraviolet laser ablation: Theoretical considerations and applications in medicine
Pettit, George H.
Sauerbrey, Roland A.
Doctor of Philosophy
Pulsed ultraviolet lasers can be used to induce clean etching in organic and some inorganic materials. This effect is called photoablation or ablative photodecomposition. Although the precise cause remains unknown, this phenomenon is becoming of vital importance in micromachining, materials processing, and medicine. To better understand the photoablation process, a theoretical description of the process has been developed. This description is based on a treatment of the radiation transport of intense ultraviolet light pulses through absorbing organic material. The theory predicts deviations from Beer's Absorption Law at high intensities which are in fact observed. These discrepancies are due to three main effects: saturation of the finite number of chromophores in the material, multiphoton absorption, and attenuation of laser light by ablation products. Using the analysis to model the ablation process it is possible to describe observed ablation behaviors for a variety of synthetic and biological substrates. Excimer laser photoablation has also been studied experimentally as a means of removing occlusive arterial thrombi. Thrombi induced in canine coronary arteries were removed with XeF excimer laser light (351 nm) delivered via flexible optical fiber. The results of this study indicated that it was possible to remove significant thrombi (27 mg mass) within 3 minutes, without causing injury to the adjacent arterial wall. In addition, the ablation products consisted of a minute volume of gas and small particulate debris ($>$100 $\mu$m), which would not be a cause of concern in a clinical procedure. Excimer laser induced autofluorescence signatures have been analyzed as possible diagnostic aids in laser angioplasty procedures. Spectra acquired from cadaver artery samples using low intensity laser light revealed distinct differences in fluorescence response between healthy and atherosclerotic arterial wall. Fluorescence spectra acquired during tissue ablation with intense laser pulses also exhibited variation between healthy and diseased sites. Spectral examination of tissue sites after ablative tissue removal showed changes in fluorescence response which coincided with penetration of diseased lesions. These findings indicated that it should be possible to use UV laser fluorescence spectroscopy to target atherosclerotic lesions and to monitor in real time the ablative removal of these vessel obstructions.