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Biomedical subjects

David W Hahn

Publications and source records attributed to David W Hahn.

11 recordsLinked to original sources
PubMed2007

Development and numerical solution of a mechanistic model for corneal tissue ablation with the 193 nm argon fluoride excimer laser.

We detail the development and implementation of a global ablation model that incorporates a dynamically changing tissue absorption coefficient. Detailed spectroscopic measurements rule out plasma-shielding effects during the laser-tissue interaction and thereby support a photochemical mechanism. The model predicts ablation rate behavior that agrees well with a variety of experimental ablation rate data and that substantially deviates from a static Beer-Lambert model. The dynamic model predicts an enhancement in the tissue absorption coefficient of about 25%-50% as compared with the initial, static value. In addition, the model predicts an increase in the tissue ablation rate as corneal hydration increases, which may provide additional insight into variations in refractive surgery outcome.

Journal Article↗
PubMed2005

Assessment of soot particle vaporization effects during laser-induced incandescence with time-resolved light scattering.

Although laser-induced incandescence (LII) has been successfully used for soot volume fraction and particle size measurements, uncertainties remain regarding issues of soot vaporization leading to mass loss and morphological changes occurring in soot due to intense heating. Prompt LII detection schemes are often based on the assumption that the associated time scale is shorter than the time scale of soot vaporization or sublimation. The validity of such assumptions is the focus of the current study. Time-resolved light-scattering measurements were made in combination with LII measurements to quantify soot particle vaporization effects resulting from the LII laser pulse. The light-scattering measurements revealed a sharp decrease in total soot particle mass during the time course of the 25 ns full-width LII laser pulse for fluences in the range of 0.5 J/cm2. Light-scattering theory was used to invert the scattering data, revealing approximately 80%-90% reductions in the soot particle volume for LII fluences of 0.47 and 0.61 J/cm2. In addition, the time-resolved scattering measurements show that the time scale of soot vaporization is completely confined to the LII laser pulse itself. Light scattering revealed no soot vaporization only for fluences of approximately 0.1 J/cm2, which is consistent with recent work on low-fluence LII. Possible mechanisms for soot vaporization are discussed, notably for near-threshold fluences.

Journal Article↗
PubMed2004

Measurement of small-signal absorption coefficient and absorption cross section of collagen for 193-nm excimer laser light and the role of collagen in tissue ablation.

A 193-nm ArF excimer laser transmission was measured at subablative fluence through varying strength solutions of dissolved collagen, yielding an absorption cross section of 1.14 x 10(-17) cm2 for the peptide bond, which accounts for 96% of the total collagen attenuation that is based on additional transmission measurements through solutions of isolated constituent amino acids. The measured absorption cross sections, in combination with typical corneal tissue composition, yield a predicted corneal tissue absorption coefficient of 16,000 cm(-1). In addition, dry collagen films were prepared and ablation-rate data were recorded as a function of laser fluence. Ablation rates were modeled by use of a Beer-Lambert blow-off model, incorporating a measured ablation threshold and an absorption coefficient that are based on the measured collagen absorption cross section and the film bond density. The measured ablation rates and those predicted by the model were in very good agreement. The experiments suggest that collagen-based absorption coefficients are consistent with predicted corneal tissue ablation rates and previously observed dynamic changes in tissue properties under ablative conditions.

Animals↗
PubMed2004

Evaluation of XRF and LIBS technologies for on-line sorting of CCA-treated wood waste.

Contamination of wood waste with chromated copper arsenate greatly limits recycling opportunities for the wood waste as a whole. Separation of CCA-treated wood from other wood types is one means by which such contamination can be removed. The purpose of the current study was to evaluate two detector technologies for sorting CCA-treated wood from other wood types. The detector technologies evaluated included X-ray fluorescence spectroscopy (XRF) and laser induced breakdown spectroscopy (LIBS). The XRF detector system utilized in this study was capable of rapidly detecting the presence of CCA in painted wood, wet wood, heartwood, sapwood, and at portions of the wood containing knots. Furthermore, the XRF system was capable of distinguishing between CCA-treated wood and wood treated with alternative wood treatment preservatives, but was limited by the fact that it was not designed for on-line operation so tests were conducted in a batch mode on a conveyor. The analysis time used in this study (3 s) can be decreased significantly for an XRF system designed specifically for on-line operation. The LIBS system developed for this study was found to effectively identify CCA-treated wood for pieces ranging in thickness from 1 to 8 cm. High sorting efficiencies were noted when 10 laser shots were taken on a piece of wood. Furthermore, the LIBS system was found to be effective for identifying wood that has been coated with stains and paints in addition to identifying wood that has been CCA treated. The major drawback with the LIBS system developed in this study was the limited laser pulse energy. With an increase in laser pulse energy it is anticipated that the working focal length of the LIBS system can be increased to enable the monitoring of wood samples of more variable thicknesses. Limitations associated with analysis of very rotted pieces of wood and wet wood can also be overcome by using a higher pulse energy laser. Overall, both technologies show incredible promise for sorting CCA-treated wood from other wood types. The next recommended step would be to run an improved full-scale operation at one facility to document sorting efficiencies and fine-tune the improvements proposed in the current study. Such a study could potentially open-the-door for more widespread sorting of wood waste.

Arsenates↗
PubMed2003

Comparison of nonintensified and intensified CCD detectors for laser-induced breakdown spectroscopy.

The performance and sensitivity of an intensified CCD array system and a nonintensified CCD array detector system are compared for laser-induced breakdown spectroscopy (LIBS). LIBS measurements were recorded in a calcium-based aerosol-seeded gas stream at ambient pressure. The signal-to-noise ratio based on the 393.37-nm calcium emission line was calculated as a function of detector delay with respect to the plasma-initiating laser pulse. Both ensemble-averaging and single-shot spectral analyses were performed. For all conditions, the intensified CCD system provided an enhanced signal-to-noise ratio compared with the nonintensified CCD system.

Journal Article↗
PubMed2003

Conditional data processing for single-shot spectral analysis by use of laser-induced breakdown spectroscopy.

Schemes of conditional data processing are evaluated based on either the peak-to-base ratio or the signal-to-noise ratio as a metric for analyte detection in single-shot laser-induced breakdown spectra. The analyte signal investigated is the 288.1-nm Si I emission line provided by an aerosol stream of monodisperse 2.5-microm-sized silica microspheres. Both the Si emission line and a spectral region corresponding to continuum emission are used to evaluate the statistical distribution of spectral noise. The probability of false hits is determined by evaluating various conditional processing thresholds. As the detection threshold increases, the rate of detected silica particle hits decreases along with the expected fraction of false-particle hits (i.e., spectral noise). For all threshold values the signal-to-noise ratio is found to provide a more robust metric for single-shot analyte detection compared with the peak-to-base ratio.

Journal Article↗
PubMed2003

Geiger photodiode array for compact, lightweight laser-induced breakdown spectroscopy instrumentation.

The development of a unique spectrometer based on an array of Geiger photodiodes has been shown to enhance the performance of laser-induced breakdown spectroscopy (LIBS) instrumentation. These compact, silicon-based detectors eliminate the need for postamplification electronics, allow for the detection of single photons at room temperature, and do not require complex gating-timing circuitry. The detectors have dark-count rates of <500 Hz at room temperature and a good response from the UV to the near IR. Their high sensitivity makes them candidates for standoff analysis as part of a LIBS spectrum analyzer.

Journal Article↗
PubMed2003

Assessment of transient changes in corneal hydration using confocal Raman spectroscopy.

PURPOSE: The degree of corneal hydration has been linked to excimer laser corneal ablation rates. Enhanced precision with excimer laser refractive surgery may result from a better understanding of the transient changes in corneal hydration. To better understand the dynamic nature of corneal hydration, bovine corneas were investigated under different surface treatments. METHODS: Confocal micro-Raman spectroscopy was used to quantify corneal hydration. Water and acetone solutions were used to establish a quantitative response of the relative OH/CH Raman bands, which are consistent with the water and collagen protein bands in cornea, respectively. Intact bovine corneas were manually debrided (designated MD group) or lamellar flaps were created to expose stromal tissue (designated lamellar keratectomy or LK group). Raman spectra were recorded every 30 seconds for 6 minutes while the prepared cornea surfaces were exposed to quiescent air or to a forced nitrogen gas flow across the surface. RESULTS: The OH and CH Raman bands yielded a linear response while the percentage of acetone was varied from 0% to 100%. For the bovine cornea under forced flow drying, the OH/CH Raman band ratio was found to decrease by 41% from the initial value for both the MD and LK treatment groups. These decreases were significantly more (p = 0.0051 and 0.054, respectively) than the 26% decrease in the OH/CH band ratio measured for the control corneas. In quiescent air, the control and MD groups exhibited a 7% and 6% decrease in the OH/CH ratio, respectively, while the LK treatment group revealed a 19% decrease in the OH/CH ratio. CONCLUSIONS: The bovine eye experiments demonstrate that significant changes in corneal hydration are realized under different drying conditions and treatment methodologies. This study elucidates the nature of transient changes in corneal hydration in a bovine model and suggests the need for further study of the role of such variations in surgical outcome for excimer laser corneal refractive procedures.

Acetone↗
PubMed2002

Assessment of the upper particle size limit for quantitative analysis of aerosols using laser-induced breakdown spectroscopy.

The laser-induced plasma vaporization of individual silica microspheres in an aerosolized air stream was investigated. The upper size limit for complete particle vaporization corresponds to a silica particle diameter of 2.1 microm for a laser pulse energy of 320 mJ, as determined by the deviation from a linear mass response of the silicon atomic emission signal. Comparison of the measured silica particle sampling rates and those predicted based on Poisson sampling statistics and the overall laser-induced plasma volume suggests that the primary mechanism of particle vaporization is related to direct plasma-particle interactions as opposed to a laser beam-particle interaction. Finally, temporal and spatial plasma evolution is discussed in concert with factors that may influence the vaporization dynamics of individual aerosol particles, such as thermophoretic forces and vapor expulsion.

Aerosols↗
PubMedDate not supplied

Determination of excimer laser ablation rates of corneal tissue using wax impressions of ablation craters and white-light interferometry.

BACKGROUND AND OBJECTIVE: Efforts to better understand the processes of corneal ablation are aided by a means to precisely measure corneal tissue ablation rates. MATERIALS AND METHODS: A white-light interferometer was used to measure the depth of argon fluoride laser ablation craters and wax impressions of the ablation craters. The technique was validated using polymethylmethacrylate (PMMA), and was then used to measure the ablation rate of bovine corneal tissue. RESULTS: The ablation rate was measured as 0.47 microm/pulse from direct measurement of the PMMA ablation craters and as 0.48 microm/pulse from measurement of the corresponding wax impressions. The ablation rate of bovine corneal tissue was found to be 0.90 microm/pulse. CONCLUSIONS: Wax impressions provide a valid representation of ablation craters. The ablation depth of corneal tissue and PMMA was found to increase linearly with increasing number of laser pulses, and the slope of the regression lines provided a measure of the laser ablation rate.

Animals↗