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G H Pettit

Publications and source records attributed to G H Pettit.

15 recordsLinked to original sources
PubMed1997

In vitro measurements of cytotoxic effects of 193 nm and 213 nm laser pulses at subablative fluences.

BACKGROUND AND OBJECTIVE: The frequency-quintupled q-switched Nd:YAG laser is being studied as an alternative to the ArF excimer laser for photorefractive procedures. The present report describes two experiments comparing biologic effects of these laser devices. STUDY DESIGN/MATERIALS AND METHODS: Bovine corneas were irradiated with subablative laser pulses in liquid nitrogen and analyzed by electron paramagnetic resonance spectroscopy to assess free radical production. Aqueous bacterial suspensions were irradiated with low-intensity laser pulses and assayed for cell survival. RESULTS: Electron paramagnetic resonance spectra were very similar in both amplitude and shape for exposure at the two wavelengths. Bacterial survival was markedly less for 213 nm irradiation than 193 nm exposure and displayed a different dependence on cumulative exposure. CONCLUSION: Free radical production by 213 nm laser exposure is quite comparable to that seen previously for 193 nm irradiation. However, cell lethality appears to be significantly greater at the longer ultraviolet wavelength. This difference may contribute to complications observed after corneal photoablation with the 213 nm device.

Animals↗
PubMed1996

Electron paramagnetic resonance spectroscopy of free radicals in corneal tissue following excimer laser irradiation.

BACKGROUND AND OBJECTIVES: Free radicals, detected previously in corneal tissue following 193 nm laser irradiation, may be important agents in the laser/tissue interaction. Electron paramagnetic resonance spectroscopy (EPR) has been used to examine such radical formation in detail. STUDY DESIGN/MATERIALS AND METHODS: Bovine corneal strips were frozen in liquid nitrogen, irradiated with excimer laser pulses, and assayed by EPR. Exposure conditions were varied to study radical formation dependence on laser intensity and repetition. Results were measured against a quantifiable standard to calculate radical quantum yield. RESULTS: Either weak or intense laser fluences produced comparable tissue EPR signals. Radicals accumulated in frozen tissue for at least 10 initial ablation pulses. Radical quantum yield in cornea was 0.15%. CONCLUSION: Corneal radical formation is largely a photochemical process driven by the 193 nm laser radiation. Reactive radical species are produced in substantial numbers and likely have a significant clinical role.

Animals↗
PubMed1995

Dynamics of ablation plume particles generated during excimer laser corneal ablation.

BACKGROUND AND OBJECTIVE: Although the empirical characteristics of ArF excimer laser corneal ablation have been well documented, the exact ablation mechanisms are not well understood. The present paper reports a quantitative analysis of corneal ablation plumes using in situ time resolved laser light scattering and Raman spectroscopy. STUDY DESIGN/MATERIALS AND METHODS: Bovine corneas were used as the ArF excimer laser ablation targets. Light scattering data were recorded from the ablation plume as a function of height above the tissue surface and as function of delay time with respect to the ablative ArF laser pulse. RESULTS: Raman spectra of the ablation plume allow identification of the particles as water. Mean plume particle diameters are found to decrease with height, while the particle volume fractions are relatively constant. The total volume of plume particles correlates well with the total volume of water in the ablated corneal tissue. CONCLUSION: The finding of a non-evolving plume composed of water spherules, combined with the excellent agreement between total volume of water in the plume and the content of water in the ablated corneal tissue, support the concept of photodecomposition or "cold ablation" for corneal tissue during ArF excimer laser ablation.

Aluminum Silicates↗
PubMed1994

Enhanced ArF laser absorption in a collagen target under ablative conditions.

The time-resolved transmission of collagen targets during ArF excimer laser ablation has been measured. The transmitted excimer pulse measurements demonstrate enhanced media attenuation that is a nonlinear function of incident laser fluence. Forward scattering of the transmitted pulse has been assessed to be a negligible contributor to the observed phenomena. Results of pump/probe interrogation of the ablation site indicate that the onset of enhanced attenuation occurs on the time course of the ablating laser pulse and persists for times on the order of hundreds of microseconds.

Absorption↗
PubMed1993

XeCl laser ablation of atherosclerotic aorta: luminescence spectroscopy of ablation products.

XeCl laser ablation of atherosclerotic aorta tissue was investigated. Luminescence spectra of ablation products were measured for soft and hard arterial tissues. A pronounced difference observed between plume luminescence for various plaques and normal vessel wall correlates with the chemical composition of the tissue. The mechanism of plume luminescence appeared to be thermochemical excitation ablation products (particles, atoms, molecules, etc.) in the air. The process of soft tissue ablation was delayed with respect to the exciting laser pulse at relatively low laser fluences close to the ablation threshold. The kinetics of the ablation process as a function of laser-pulse energy fluence is reported. The data indicate that tissue ejection mechanism, which involves vapor bubbles formation, expansion and explosion, is suitable for the description of the XeCl excimer laser ablation of soft tissues.

Angioplasty, Laser↗
PubMed1993

Transmission of corneal collagen during ArF excimer laser ablation.

The time-resolved transmission of collagen films and 10-microns sections of bovine cornea during ArF laser ablation has been investigated. The film studies were performed on thin layers of extracted bovine corneal collagen, the principal chromophore in 193 nm photoablation. Transmission measurements were made on both dry and water-saturated films to assess the sensitivity of the ablation process to hydration. Distinct transient optical changes were observed in both fully desiccated and rehydrated films. Dehydrated films exhibit rapid reduction in film absorption over the time-course of the ablating laser pulse, presumably due to chromophore bleaching or annihilation. In contrast, rehydrated films demonstrate a short-lived enhancement of the attenuation. In either case, a single ablative laser pulse increased the long-term transmission of the film, although this increase was a factor of five greater for dehydrated films than for rehydrated samples. Results obtained from corneal tissue sections were essentially identical to those derived from hydrated collagen films.

Absorption↗
PubMed1993

Atherosclerotic tissue analysis by time-resolved XeCl excimer laser reflectometry.

The temporal modification of XeCl laser pulses reflected from human aorta tissue immersed in saline has been studied. Dynamic tissue reflectivity of both normal and atherosclerotic tissues has been examined for various incident pulse fluences between 0.7 and 6.5 J/cm2. Changes in reflected pulse duration are observed for fluences at or above 2.6 J/cm2 with normal tissue targets and 3.0 J/cm2 with calcified plaque. Such reflected pulse analysis may prove useful in identifying tissue targets for ablation during laser angioplasty.

Adult↗
PubMed1993

Pump/probe transmission measurements of corneal tissue during excimer laser ablation.

Transient changes in the transmission of a 355 nm probe pulse through corneal tissue during 193 nm ArF laser ablation have been examined. A significant decrease in collimated transmission of the probe beam was observed for time delays between 10 ns and 1 ms after the 193 nm laser pulse. At 10 ns delay the collimated probe transmission was 70% of the preablation level. Minimum collimated transmission (40%) was observed at 30 microseconds delay. Transmitted probe examination by both diode array and integrating sphere measurements indicate the observed attenuation is due to scattering of the incident probe beam and not due to absorption. The significant scattering at nanosecond delay times suggests onset of the ablation process during the ArF pulse. Scattering therefore may affect the deposition of the 193 nm radiation in the ablation target.

Absorption↗
PubMed1993

XeCl laser-induced fluorescence of atherosclerotic arteries. Spectral similarities between lipid-rich lesions and peroxidized lipoproteins.

Autofluorescence spectroscopy of arterial surfaces provides information about the distribution and composition of atherosclerotic plaques. The aim of the study was to determine whether accumulation of peroxidized lipoproteins in arterial walls, a process postulated to play a role in initiating atherosclerotic changes, can be demonstrated by fluorescence spectroscopy. XeCl excimer laser (308 nm)-induced fluorescence of human aortas containing early lipid-rich noncollagenous lesions exhibited marked red shifts and broadening of the fluorescence spectra compared with spectra from nonatherosclerotic aortas. Similar profiles were observed in spectra obtained from oxidatively modified low density lipoprotein but not native low density lipoprotein. In hypercholesterolemic rabbits with early foam cell lesions, spectral shifts resembled those of oxidized beta-very low density lipoprotein, the major lipoprotein accumulating in arteries of rabbits fed cholesterol. XeCl laser-fluorescence spectroscopy of arterial surfaces may be useful for the identification of arteries accumulating modified lipoproteins (oxidized low density lipoprotein), a chemical change indicative of atherosclerosis in its early and probably reversible stages.

Angioscopes↗
PubMed1992

XeCl laser ablation of atherosclerotic aorta: optical properties and energy pathways.

The energetics of 308-nm excimer laser irradiation of human aorta were studied. The heat generation that occurred during laser irradiation of atherosclerotic aorta equaled the absorbed laser energy minus the fraction of energy for escaping fluorescence (0.8-1.6%) and photochemical decomposition (2%). The absorbed laser energy is equal to the total delivered light energy minus the energy lost as specular reflectance (2.4%, air/tissue) and diffuse reflectance (11.5-15.5%). Overall, about 79-83.5% of the delivered light energy was converted to heat. We conclude that the mechanism of XeCl laser ablation of soft tissue involves thermal overheating of the irradiated volume with subsequent explosive vaporization. The optical properties of normal wall of human aorta and fibrous plaque, both native and denatured were determined. The light scattering was significant and sufficient to cause a subsurface fluence (J/cm2) in native aorta that equaled 1.8 times the broad-beam radiant exposure, phi o (2.7 phi o for denatured aorta). An optical fiber must have a diameter of at least 800 microns to achieve a maximum light penetration (approximately 200 microns for phi o/e) in the aorta along the central axis of the beam.

Aorta↗
PubMed1991

Excimer laser corneal ablation: absence of a significant "incubation" effect.

Pulse-to-pulse consistency of excimer laser etching of cornea has been examined via two noncontact techniques: photoacoustic probe beam deflection, and time-resolved excimer pulse reflectometry. These methods clearly document the incubation phenomenon accompanying excimer laser ablation of polymethyl-methacrylate and the absence of the effect during polyimide ablation. In comparison, results for corneal ablation indicate consistent tissue etching over a train of pulses. Consequently, incubation appears to have negligible impact on corneal ablation.

Acoustics↗
PubMedDate not supplied

The Alcon/Summit/Autonomous perspective on fixed vs. variable spot ablation.

The LADARVision system utilizes a small fixed diameter excimer laser beam coupled with a high fidelity eye tracking system to perform wavefront-guided refractive surgery. The fixed small beam provides a consistent ablation per pulse. By delivering many identical pulses in a predetermined pattern, which includes an optimized custom blend zone, the laser is able to ablate complex (higher order) corneal shapes accurately. The closed loop eye tracking system ensures that accurate ablation is delivered to the right place on the eye.

Cornea↗
PubMedDate not supplied

Noninvasive monitoring of excimer laser ablation by time-resolved reflectometry.

BACKGROUND: Current excimer laser photorefractive procedures use empiric etch rates to determine specific changes in corneal shape. A real-time analytic method for monitoring the tissue ablation process may be useful in tailoring energy delivery to a specific patient and in detecting detrimental phenomena such as corneal desiccation. METHODS: We monitored excimer laser ablation by studying the amplitude and temporal characteristics of ArF laser pulses reflected from the ablation site. Two target materials were used: polymethylmethacrylate (PMMA, a synthetic polymer that undergoes an incubation phase where no ablation occurs for an initial finite number of laser pulses), and bovine cornea. Observed reflectivity changes during irradiation of PMMA were compared to profilometric ablation depth measurements. Corneal ablation was performed both with and without nitrogen gas flow at the ablation site to study the effect of tissue desiccation. RESULTS: For ablation of PMMA at 160 mJ/cm2, the incubation phase included the initial eight laser pulses. For corneal tissue ablation at a fluence of 125 mJ/cm2, flowing nitrogen gas caused significant shortening and amplitude reduction in the reflected laser signals. CONCLUSIONS: Noninvasive time-resolved reflectometry provided real-time information about target ablation. This technique may have diagnostic utility during laser corneal surgical procedures.

Animals↗