Peter Hering

The ablation threshold of Er:YAG and Er:YSGG laser radiation in dental enamel.

Abstract.The scientific investigation of fundamental problems plays a decisive role in understanding the mode of action and the consequences of the use of lasers on biological material. One of these fundamental aspects is the investigation of the ablation threshold of various laser wavelengths in dental enamel. Knowledge of the relationships and influencing factors in the laser ablation of hard tooth tissue constitutes the basis for use in patients and the introduction of new indications. The present paper examines the ablation threshold of an Er:YAG laser (λ=2.94 µm) and an Er:YSGG laser (λ=2.79 µm) in human dental enamel. To this end, 130 enamel samples were taken from wisdom teeth and treated with increasing energy densities of 2–40 J/cm2. The sample material was mounted and irradiated on an automated linear micropositioner. Treatment was performed with a pulse duration of τP(FWHM)≈150 µs and a pulse repetition rate of 5 Hz for both wavelengths. The repetition rate of the laser and the feed rate of the micropositioner resulted in overlapping of the single pulses.The surface changes were assessed by means of reflected light and scanning electron microscopy. On the basis of the results, it was possible to identify an energy density range as the ablation threshold for both the Er:YAG and the Er:YSGG laser. With the Er:YAG laser, the transition was found in an energy density range of 9–11 J/cm2. The range for the Er:YSGG laser was slightly higher at 10–14 J/cm2.

Fast exponential fitting algorithm for real-time instrumental use

We report on a very fast fitting algorithm for single exponential functions which is based on the method of successive integration. The algorithm corrects the systematic error of trapezoidal integration. The new algorithm needs only 150 μs for a dataset of 1536 points and is around 700 times faster than the nonlinear Levenberg–Marquardt fit provided by LABVIEW. This makes it suitable for real-time instrumental use. Beside the better time resolution, the acceleration allows more averaging, which leads to higher precision. In our experiment instrumental sensitivity was improved by a factor of 3.7.

Parts per trillion sensitivity for ethane in air with an optical parametric oscillator cavity leak-out spectrometer

Spectroscopic detection of ethane in the 3-microm wavelength region was performed by means of a cw optical parametric oscillator and cavity leak-out. We achieved a minimum detectable absorption coefficient of 1.6 x 10(-10) cm 1/square root of Hz, corresponding to an ethane detection limit of 6 parts per trillion/square root of Hz. For 3-min integration time the detection limit was 0.5 parts per trillion. The levels are to our knowledge the best demonstrated so far. These frequency-tuning capabilities facilitated multigas analysis with simultaneous monitoring of ethane, methane, and water vapor in human breath.

Improvements in Shape-From-Focus for Holographic Reconstructions With Regard to Focus Operators, Neighborhood-Size, and Height Value Interpolation

This paper presents a shape-from-focus method, which is improved with regard to the mathematical operator used for contrast measurement, the selection of the neighborhood size, surface refinement through interpolation, and surface postprocessing. Three-dimensional models of living human faces are presented with such a high resolution that single hairs are visible.

Osteotomy with 80-μs CO2 laser pulses: histological results

Haemostatic and aseptic effects and intricate cut geometry are beneficial aspects of non-contact laser osteotomy. Collateral thermal damage, however, has severely limited the use of conventional lasers. The purpose of this study was to test the side effects on bone after cutting it with short CO2 laser pulses and simultaneous application of a fine air–water spray. The 10.6 μm CO2 laser emitted 80-μs pulses of 46 mJ energy, f=100 Hz, focused to a spot diameter of 130 ìm. Scan rate amounted to 40 mm/s. To approximate live conditions 10 samples of cortical bone and 10 rib segments were prepared immediately after sacrificing of pigs. A reference cut with a bandsaw and three laser cuts with an increasing number of beam passes (4, 16, 64) were performed on each sample. Half of the samples were decalcified in EDTA. The others were embedded in plastic to cut non-decalcified sections. The laser incisions were not accompanied by carbonisation. The incisions with slightly convergent walls were 150 ìm wide. The depths of the cavities increased with the number of the beam passes from approximately 0.5 mm (4 passes) to 3 mm (64 passes). At the border of the incisions two narrow zones of damage were noted: an amorphous intensively stained zone of 1–3 μm width and a wider, also sharply demarcated but faintly stained zone of 7–10 μm. A broader zone of about 50 μm was characterised by empty lacunae and osteocyte damage. These effects were not predictable; intact osteocytes were also observed near to the cut surface. Polarised light microscopy showed no alterations in the inorganic structure of the bone at the cut borders. The histological results indicated only minimal damage to bone ablated at the specified parameters. The described laser procedure might have advantages over mechanical instruments.

Bone Tissue Ablation with sub-µs Pulses of a Q-switch CO2 Laser: Histological Examination of Thermal Side Effects

Abstract.The goal of this study is an in vitro evaluation of thermal side-effects by the application of short sub-µs CO2 laser pulses in combination with an air–water spray on different types of bone tissue. A mechanically Q-switched CO2 laser delivered 300 ns pulses at 9.6 µm wavelength, which were focused down to a spot size of 440 µm on the tissue (a corresponding energy density of 9 J/cm2). Bone samples (blocks from pig femur, rib, or cartilage) were moved through the beam repeatedly until 1–5 mm deep cuts were produced. An air driven water spray was applied to prevent the tissue dehydration. Subsequent visual and histological examinations revealed no carbonisation, melting traces or fissuring of the tissue. An extremely narrow, 2–6 µm thick thermally altered layer was observed at the cut border in compacta and cartilage. No accumulation of the thermal damage occurred with increasing cut depth. Laser incisions in trabecular tissue were accompanied with a 100–200 µm thick zone of thermal necrosis in bone marrow. The difference from compacta and cartilage can be explained considering the particular character of the spreading of the ablation products in the trabecular meshwork. Minor thermal side effects make the Q-switched and probably other short pulsed CO2 laser systems interesting for hard tissue surgery.

Mid-infrared cavity leak-out spectroscopy for ultrasensitive detection of carbonyl sulfide

We present a ringdown absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near lambda = 5 microm. Using a linear ringdown cavity (length, 0.5 m) with R > = 99.99% mirrors, we observed a noise-equivalent absorption coefficient of 7 x 10(-11) cm(-1) Hz(-1/2). This is 2 orders of magnitude improved compared with previous values. With this setup we studied the spectroscopic detection of carbonyl sulfide (here abbreviated OCS) traces in ambient air and in exhaled breath. We achieved a detection limit of 7 parts in 10(12) (parts per trillion) OCS in ambient air, which is unprecedented and shows great promise for environmental and biomedical applications.

The presence of nitrite during UVA irradiation protects from apoptosis

Nitrite occurs ubiquitously in biological fluids such as blood and sweat, representing an oxidation product of nitric oxide. Nitrite has been associated with a variety of adverse effects such as mutagenicity, carcinogenesis, and toxicity. In contrast, here we demonstrate that the presence of nitrite, but not nitrate, during irradiation of endothelial cells in culture exerts a potent and concentration‐dependent protection against UVA‐induced apoptotic cell death. Protection is half‐maximal at a concentration of 3 mM, and complete rescue is observed at 10 mM. Nitrite‐ mediated protection is mediated via inhibition of lipid peroxidation in a similar manner as seen with butylated hydroxytoluene, a known inhibitor of lipid peroxidation. Interestingly, nitrite‐ mediated protection is completely abolished by coincubation with the NO scavenger cPTIO. Using electron paramagnetic resonance (EPR) spectroscopy or Faraday modulation spectroscopy, we directly prove UVA‐induced NO formation in solutions containing nitrite. In conclusion, evidence is presented that nitrite represents a protective agent against UVA‐induced apoptosis due to photodecomposition of nitrite and subsequent formation of NO.

Computer-guided CO2-laser osteotomy of the sheep tibia: technical prerequisites and first results.

OBJECTIVE The purpose of this study was to examine for the first time the feasibility of performing complete osteotomy of sheep tibia using a computer-guided CO2-laser osteotome, and to examine bone healing under functional loading. BACKGROUND DATA Bone cutting without aggravating thermal side effects has been demonstrated with scanning CO2-laser osteotomy. Further research is necessary to develop a clinically usable laser osteotome, which may allow new types of bone surgical procedures. MATERIALS AND METHODS The scanning parameters for performing tibial osteotomies were determined in preliminary ex vivo trials. Osteotomies were performed in the mid-diaphysis of sheep tibia using either the prototype laser osteotome (osteoLAS, study group; n = 12), or an oscillating saw (control group; n = 12). Both groups were divided into two subgroups each (n = 6), and the two groups were sacrificed after 4 and 12 wk. Radiographs were taken postoperatively and after 4, 8, and 12 wk to compare the course of bone healing. RESULTS Laser osteotomies of sheep tibia up to a depth of 20 mm were possible without visible thermal damage to the bone. A sequential PC-controlled cut geometry with artificial widening of the osteotomy gap was required for a complete osteotomy. Both clinically and radiologically, the laser and control groups showed undisturbed primary gap healing. Bone healing was similar and undelayed after both laser osteotomy and osteotomy done by mechanical saw. CONCLUSIONS Osteotomy of multi-layered bones with a scanning CO2-laser demonstrates clinical and radiological healing patterns comparable to those seen with osteotomy done by standard mechanical instruments. It is, however, a technically demanding procedure, and complete laser osteotomies of long bones are only reasonable in bones with a diameter <20 mm, which will likely restrict the use of this technique to bones 7-10 mm thick. Through the use of computer guidance, extremely precise osteotomies and sophisticated cut geometries are possible using this technique. For practical applications, precise control of the depth of laser cutting and easier manipulation of the osteotome are required.

Infrared laser spectroscopy for online recording of exhaled carbon monoxide—a progress report

Despite the growing number of reports on breath CO measurements, the development of rapid and sensitive analysis techniques for measurements of this breath constituent still remains a challenge. We demonstrate the application of infrared laser spectroscopy for exhaled CO analysis. The breath samples are analyzed in real-time during single exhalations by means of cavity ring-down spectroscopy. This is an ultra-sensitive laser-based method for the analysis of trace gases with precision on the ppb level (parts per billion). The noise-equivalent CO level of this method is 7 ppb Hz(-1/2); the time resolution is around 1 s. The expirograms were recorded with exhalation flow rates varying from 4 l min(-1) up to 50 l min(-1). Alveolar phase (phase III) of expiration shows a remarkable flow-rate dependence. Also, expirograms were recorded after a breath holding time between 0 s and 60 s. The normalized slope of the alveolar plateau (S(n)) was determined, which is between 0.004 l(-1) and 0.15 l(-1).