Raimund Hibst

Spatially resolved absolute diffuse reflectance measurements for noninvasive determination of the optical scattering and absorption coefficients of biological tissue

The absorption and transport scattering coefficients of biological tissues determine the radial dependence of the diffuse reflectance that is due to a point source. A system is described for making remote measurements of spatially resolved absolute diffuse reflectance and hence noninvasive, noncontact estimates of the tissue optical properties. The system incorporated a laser source and a CCD camera. Deflection of the incident beam into the camera allowed characterization of the source for absolute reflectance measurements. It is shown that an often used solution of the diffusion equation cannot be applied for these measurements. Instead, a neural network, trained on the results of Monte Carlo simulations, was used to estimate the absorption and scattering coefficients from the reflectance data. Tests on tissue-simulating phantoms with transport scattering coefficients between 0.5 and 2.0 mm(-1) and absorption coefficients between 0.002 and 0.1 mm(-1) showed the rms errors of this technique to be 2.6% for the transport scattering coefficient and 14% for the absorption coefficients. The optical properties of bovine muscle, adipose, and liver tissue, as well as chicken muscle (breast), were also measured ex vivo at 633 and 751 nm. For muscle tissue it was found that the Monte Carlo simulation did not agree with experimental measurements of reflectance at distances less than 2 mm from the incident beam.

Pulsed erbium:YAG laser ablation in cutaneous surgery

Among the various pulsed midinfrared‐lasers studied in skin surgery the 2.94 μm Erbium: YAG laser has been shown to combine most efficacious ablation with least thermal damage due to its unique absorption characteristics in tissue water.

Detection of Occlusal Caries by Laser Fluorescence: Basic and Clinical Investigations

Summary Objectives: Motivated by the poor sensitivity of visual inspection and radiography in the detection of occlusal caries hidden under a macroscopically sound surface, we explored the implementation of fluorescence as an alternative diagnostic tool. Methods and Results: Fluorescence spectroscopic investigations revealed considerable contrast between sound and carious tooth hard tissue when excited by red (655 nm) light. For this excitation wavelength on inorganic tooth components only negligible fluorescence was observed. Advanced caries, however, show readily detectable fluorescence. The latter, at least in part, originates from porphyrins produced by bacteria. Based on red excited fluorescence a novel caries detector was constructed (DIAGNOdent™, KaVo, Biberach). The system measures fluorescence quantitatively. The minimum amount of protoporphyrin IX that can be detected is approximately 1 pmol. Clinical investigations yielded a sensitivity ≥0.92 in detection of occlusal caries, compared to 0.63 for bitewing radiography, and ≤0.62 for visual inspection. Intra- and interexaminer reproducibility tests resulted in Cohens Kappa value of 0.93 in vivo. Conclusions: Detection of occlusal caries with the DIAGNOdent™ system has a much better sensitivity than conventional methods. Because the readings are correlated to the status of the carious lesion, treatment decisions can be aided by the measurement. Additionally, excellent reproducibility allows monitoring of lesions over time to facilitate preventive based management of dental decay.

Anisotropy of light propagation in biological tissue

We investigated the propagation of light in biological tissues that have aligned cylindrical microstructures (e.g., muscle, skin, bone, tooth). Because of pronounced anisotropic light scattering by cylindrical structures (e.g., myofibrils and collagen fibers) the spatially resolved reflectance exhibits a directional dependence that is different close to and far from the incident source. We applied Monte Carlo simulations, using the phase function of an infinitely long cylinder, to explain quantitatively the experimental results. These observations have consequences for noninvasive determination of the optical properties of tissue as well as for the diagnosis of early tissue alterations.

Light propagation in dentin: influence of microstructure on anisotropy

We investigated the dependence of light propagation in human dentin on its microstructure. The main scatterers in dentin are the tubules, the shape of which can be approximated as long cylinders. We calculated the scattering of electromagnetic waves by an infinitely long cylinder and applied the results in a Monte Carlo code that simulates the light propagation in a dentin slab considering multi-scattering. The theory was compared with goniometric measurements. A pronounced anisotropic scattering pattern was found experimentally and theoretically. In addition, intensity peaks were measured which are shown to be caused by light diffraction by the tubules.

Influence of the phase function on determination of the optical properties of biological tissue by spatially resolved reflectance

Spatially resolved reflectance measurements are widely used for determination of the optical properties of biological media. However, the influence of the phase function on these measurements has not been quantified. We show that errors in the derived reduced scattering and absorption coefficients are as great as 100% for both absolute and relative spatially resolved reflectance measurements if a standard solution of the diffusion equation is used in the analysis. In addition, we investigated nonlinear regressions, using Monte Carlo simulations and an additional fitting parameter that characterizes the phase function, and found that the errors in the obtained optical coefficients were </=20% .

Why do veins appear blue? A new look at an old question

We investigate why vessels that contain blood, which has a red or a dark red color, may look bluish in human tissue. A CCD camera was used to make images of diffusely reflected light at different wavelengths. Measurements of reflectance that are due to model blood vessels in scattering media and of human skin containing a prominent vein are presented. Monte Carlo simulations were used to calculate the spatially resolved diffuse reflectance for both situations. We show that the color of blood vessels is determined by the following factors: (i) the scattering and absorption characteristics of skin at different wavelengths, (ii) the oxygenation state of blood, which affects its absorption properties, (iii) the diameter and the depth of the vessels, and (iv) the visual perception process.

Infrared Absorption Bands of Enamel and Dentin Tissues from Human and Bovine Teeth

Abstract The chemical components present in the hard tissue such as water, phosphate, carbonate and organic material strongly absorb infrared radiation. The research in dentistry area has a fundamental interest in identifying the chemical origin of all the bands present in the infrared region to study the chemical and thermal action on the hard dental tissues. To improve this technique it is important to identify chemical alterations in the tissues and characterize their properties in a more natural form. Transmission and reflection spectra in the infrared range from 2.5 µm to 25 µm of the enamel and dentin tissues from human and bovine teeth were acquired. Sliced samples were used for the transmission spectra, polished blocks and natural surfaces for the reflection spectra. Tissue spectra were registered between 4000 and 400 cm−1 in a FTIR Spectrometer. No differences were found between the mineral matrix of human and bovine tissues, only differences in intensity among bands were observed. Regarding the organic matrix, the bands observed in the dentin were not observed in the enamel tissue and, as the same, the bands observed in the enamel tissue were not observed in the dentin tissue with a significant intensity. Using infrared spectroscopy of sliced samples or blocks forms it is possible to characterize the chemical and optical alterations produced in the dental hard tissues by either thermal or chemical treatment, or an association of the two treatments, Therefore this technique can be used in dental research that study the effect of laser irradiation on hard tissues and the acid attack produced by intraoral bacteria.

Effects of laser parameters on pulsed Er-YAG laser skin ablation

Previous studies demonstrated that pulsed 2.94μm Er-YAG laser radiation allows a precise etching of organic tissue with only minimal thermal damage. This makes the Er-YAG laser a promising tool for the careful removal of superficial skin lesions. In order to provide optimized laser parameters for potential clinical use and to enhance our understanding of the mid-infrared ablation process, we measured the ablation rate, temperature profile and damage zones for various pulse numbers, radiant energies and pulse repetition rates. Ablation is very efficient (about 6μm J−1 cm2 for high radiant exposure) and the crater depth is exactly (1Hz) or nearly (2 Hz) linearly related to the radiant exposure. In contrast, no significant effects of the laser parameters on the thermal damage of the epidermis and the crater bottom were observed. In conclusion, for a future clinical use high radiant energies should be applicable without the disadvantage of enhanced damage.

Effects of Er:YAG laser on enamel bonding of composite materials

Cylinders of microfil light-cured composite resin were bonded to the labial enamel surface of bovine incisor teeth which had previously been subjected to different laser treatments. One part of the tooth surfaces were laser treated in a defocused way only, another part with a different overlaying pattern of focused laser pulses. Specimens were thermocycled and the adhesion of the composites were determined by tensile strength tests. The best results were obtained by laser conditioning of the enamel surface in a defocused way with an overlaying fine pattern of focused single shots. The tensile strength reached 92.5% of the acid etched bonding.