Clinical assessment of near-infrared spectroscopy for noninvasive diabetes screening.

BACKGROUND: Current diabetes screening techniques comprise the fasting plasma glucose (FPG) and oral glucose tolerance tests. Both tests demand patient compliance, and neither test has ideal performance. Near-infrared (NIR) spectroscopy is a noninvasive means of interrogating characteristics of a sample and is evaluated as a novel screening method for type 2 diabetes. METHODS: One hundred fifty-four patients with and without type 2 diabetes were recruited. Their forearm skin was measured with the NIR spectroscopic system, and a capillary blood glucose measurement was also taken. Sixty-six patients returned for a second visit at a later date. A multivariate model, generated from a separate training study, was employed to produce a quantitative risk marker of disease for each NIR spectrum. Sensitivity and specificity (the probabilities that the NIR method will correctly identify a subject as having diabetes or as not having diabetes, respectively) were calculated. As the NIR method produces a continuous rather than categorical classification, various thresholds were evaluated to give several sensitivity and specificity pairs. Test reproducibility was also determined. RESULTS: At a false-positive rate of 70%, the NIR test had a sensitivity of 77.7%, which is comparable to the 77.3% sensitivity for the FPG test as reported for the Third National Health and Nutrition Examination Survey (NHANES III) study. The reproducibility of the NIR test was also similar to the FPG test (inter-day agreement rates of 84.2% and 79.2%, respectively). CONCLUSIONS: A noninvasive NIR spectroscopic measurement of the volar forearm was shown to have comparable performance characteristics with the FPG test. The source of the spectroscopic signal is still uncertain and is the subject of ongoing research.

Preliminary in vitro study of the histological effects of low fluence 193-nm excimer laser irradiation of corneal tissue.

PURPOSE: To determine if moderate numbers of low fluence, 193-nm excimer laser pulses modify or damage the corneal stroma. METHODS: The corneal epithelium of fresh bovine eyes was scraped off and the exposed stroma was irradiated with 200 low fluence laser pulses from an argon fluoride excimer laser. This process was performed on five eyes each at two laser fluences, 10 mJ/cm2 and 30 mJ/cm2. The ten irradiated and three control (unirradiated) corneas were sectioned and studied by electron microscopy. The maximum and minimum thickness of the anterior layer of randomly oriented collagen fibers was measured using electron microscopy. RESULTS: The mean maximum thickness of the anterior randomly oriented layer of collagen was 1.23 +/- 0.45 microm in the control corneas, 0.67 +/- 0.32 microm in the corneas irradiated at 10 mJ/cm2, and 0.10 +/- 0.12 microm in the corneas irradiated at 30 mJ/cm2. The mean thickness of corneal stroma removed was 0.7 microm at a fluence of 10 mJ/cm2 and 1.1 microm at a fluence of 30 mJ/cm2. A thin, electron-dense pseudomembrane was present at both fluences. CONCLUSION: We report removal of bovine corneal stroma at 10 mJ/cm2--below the previously reported ablation threshold of 20 mJ/cm2.

Optical radiation safety considerations for ocular glucose monitoring.

The potential for noninvasive detection of blood glucose is an area of intense academic and commercial research and a subject of keen interest in the diabetic and healthcare communities. A number of techniques are under investigation that attempt to infer blood glucose levels from measurements of optical signals. Frequently, these techniques are based on laser sources that may, under certain circumstances, be capable of inducing ocular injury. This article provides an overview of ocular damage mechanisms and the international standards for laser exposure limits that have been developed. The application of relevant standards to specific implementations of lasers in optical glucose sensing is presented. In addition, the concept of risk versus benefit for consideration of new medical devices is also discussed.

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.

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.

Corneal-tissue absorption coefficients for 193- and 213-nm ultraviolet radiation.

The small-signal absorption coefficients of 193- and 213-nm nanosecond laser pulses in bovine corneal tissue have been studied. The absolute reflectance of a planar quartz-cornea interface was measured at various angles of incidence for low-intensity laser irradiation (i.e., pulse fluences 3 orders of magnitude below the ablation threshold). The reflectance-versus-angle data were analyzed by the use of Fresnel theory to estimate the effective complex index of refraction of the tissue. This analysis indicated corneal absorption coefficients of 39,900 ± 9800 cm(-1) at 193 nm and 21,400 ± 4900 cm(-1) at 213 nm.

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.

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.

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.

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.

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.

Dynamic optical properties of collagen-based tissue during ArF excimer laser ablation.

Various attenuation mechanisms affecting the absorption of ArF excimer laser light in collagenous tissues have been studied. Temporal distortion of the laser pulse reflected from the cornea has been observed over a range of incident pulse fluences including the ablation threshold. Reflected pulse shortening begins near the ablation threshold and advances with increasing fluence. The measurement of laser light scattered 30 degrees off specular axis from collagen gel targets indicates that the reflected-pulse distortion is partially a result of scattering. Collagen film transmission studies show an increase in 193-nm light transmission in ablation conditions. These nonlinear attenuation mechanisms may impact significantly on the photoablation process.

Tangential corneal surface ablation with 193- and 308-nm excimer and 2936-nm erbium-YAG laser irradiation.

An argon-fluoride excimer laser operating at 193 nm was used to perform tangential, wide-area surface photoablation of the corneas of bovine eyes. Comparisons were made with similar tangential ablations performed with the use of 308-nm xenon chloride excimer laser and qi/gl 2936-nm erbium-YAG laser irradiation to evaluate potential photochemical and thermal effects. Light and transmission electron microscopy of the corneal tissue revealed minimal tissue architectural changes for ablation at 193 nm. The zone of ultrastructural change at the corneal surface with the 193-nm excimer laser was 120 nm thick. The clinical appearance of the corneal surfaces produced by the tangential ablation at 193 nm was the most satisfactory of those with the three wavelengths tested, being macroscopically smooth and glossy.

Excimer-laser-induced fluorescence of rabbit cornea: radiometric measurement through the cornea.

The laser-induced fluorescence spectrum of rabbit cornea irradiated at ablative intensities was measured. This system directly measured the radiant exposure of fluorescence transmitted through the cornea when the anterior surface of the cornea was irradiated by an ArF excimer laser. Evidence of changing spectral characteristics as a function of total laser dose suggests photochemical changes in the cornea may be occurring. Results are compared with previous data of laser-induced fluorescence in other models and detection schemes.

In vitro production of viable bacteriophage in carbon dioxide and argon laser plumes.

Both CO2 and argon laser ablation of an agar substrate containing high titres of bacteriophage phi X174 create plumes which disperse viable phage particles. Irradiances at the beam impact site ranged from 73 to 215 W/cm2 for the CO2 laser and from 40 to 227 W/cm2 for the argon laser. To increase the absorption of argon laser radiation, oxidized hemoglobin was added to the target material. Plume-borne viable phage were observed to be associated with particles large enough to settle out from the plume within 100 mm of the beam impact site. The ratio of the number of dispersed viable phage to the number of viable phage potentially dispersible by a single, 1-second laser exposure was on the order of 10(-6) to 10(-5).

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.

Infrared optical fibers for surgical applications.

The basic characteristics of fibers that are appropriate for surgical use in the infrared are reviewed. New fiber materials, such as sapphire, fluorozirconate and chalcogenide glasses, and polycrystalline fibers are discussed as well as their applicability for surgical procedures.

In vitro production of viable bacteriophage in a laser plume.

Bacteriophage phi X 174, present at high concentrations in an agar overlay in vitro system, were used as a target for pulsed 2.94 microns (Er:YAG) laser ablation. In this preliminary experiment, the potential for transport of viable viruses in the photoablation plume was investigated for laser energy densities of approximately 3.5 J/cm2 per pulse in exposures of 75-600 laser pulses. Transport of relatively few numbers of viable viruses from the ablation site to the plate assay detector, a distance of approximately 2 cm, was observed.