Magnification--a new look at a long-known optical property of dentin.

Light propagation in human dentin exhibits a strong directional dependence featuring the long-known optical magnification property. We hypothesized that this anisotropic effect is caused by multiple scattering at the dentin tubules, and not by fiberoptic effects, as had been previously assumed. We performed measurements of the transmitted intensity from dentin disks and compared them with Monte Carlo simulations of light propagation in dentin, considering the scattering by the tissue's microstructure. We found that the optical anisotropy of dentin can be fully explained with this model. We concluded that the magnification property of dentin is due to multiple scattering by the dental microstructure.

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.

DIAGNOdent: an optical method for caries detection.

The onset of caries is characterized by demineralization of dental hard tissues. Optimal fluoridation with respective oral hygiene habits and diet may stop the progression of a lesion and even allow for its remineralization. The aim of modern dentistry must be a preventive approach rather than invasive repair of the disease. This is possible only with early detection and respective preventive measures. Some of today's diagnostic tools are not sensitive enough to detect this early onset of destruction. Tools based on fluorescence could have the possibility to overcome this problem. This overview will focus on today's knowledge of one possible tool, the DIAGNOdent.

[Intracanalicular trabeculostomy with the Er:YAG laser]. / Intrakanaläre Trabekulostomie mit dem Er:YAG-Laser.

PURPOSE: To develop a new ab externo technique for glaucoma trabecular surgery using the infrared Er:YAG laser radiation (2.9 micrometer) guided into Schlemm's canal by an optical fiber. MATERIAL AND METHODS: In order to create a drainage canal to the anterior chamber, a quartz fiber (core diameter: 100 micrometer) coupled to an Er:YAG laser was shielded by a metal canula (diameter: 280 micrometer) bent in conformance to the curvature of Schlemm's canal. A 45 degrees -mirror enabled the laser radiation to exit the canula perpendicular to the fiber axis. The complete surgery device was tested on agar medium and enucleated human eyes. RESULTS: Using the unshielded quartz fiber, eight pulses of 8 mJ (frequency: 7 Hz, pulse duration: 150 microseconds) were sufficient to perforate the trabecular meshwork of the human eye. Histology showed a rippled canal with 50 micrometer average diameter and a surrounding necrosis zone of 15-35 micrometer. The complete device could be easily inserted into Schlemm's canal similar to the classic trabeculostomy probe and a conical-shaped canal with a length of 2-3 mm could be created in agar medium within a few seconds. CONCLUSIONS: Initial experience in an experimental set-up show the ab externo creation of a draining canal between Schlemm's canal and the anterior chamber with only minimal irritation of the surrounding tissue. Further refinements of the laser parameters and the biomechanical set-up resulted in a new hand-held device with improved function. The Er:YAG laser intracanalicular trabeculostomy could become a new ab externo technique for minimal invasive therapy of open angle glaucoma.

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

Time-gated in vivo autofluorescence imaging of dental caries.

Laser-induced time-resolved autofluorescence from carious lesions of human teeth was studied by means of ultrashort pulsed laser systems, time-correlated single photon counting and time-gated imaging. Carious regions exhibited a slower fluorescence decay with a main 17 ns fluorescence lifetime than healthy hard dental tissue. The long-lived fluorophore present in carious lesions only emits in the red spectral region. Fluorescence decay time and spectral characteristics are typical of fluorescent metal-free porphyrin monomers. The spatial distribution of the long-lived endogenous porphyrin fluorophore within the tooth material was detected by time-gated nanosecond autofluorescence imaging. In particular, high contrast video images were obtained with an appropriate time delay of 15 ns to 25 ns between excitation and detection due to the suppression of short-lived autofluorescence of healthy tissue. First in vivo applications are reported indicating the potential of time-resolved fluorescence diagnostics for early caries- and dental plaque detection.

Erbium:YAG laser application in caries therapy. Evaluation of patient perception and acceptance.

OBJECTIVES: In previous studies it has been demonstrated that the Er:YAG laser can be used to prepare cavities efficiently and without thermal damage to the adjacent dental hard and soft tissues. To investigate the patients' response to Er:YAG laser preparation of teeth, a prospective clinical study was performed in five dental hospitals. METHODS: To evaluate patients' perception and response to cavity preparation a direct comparison was made between conventional mechanical preparation and Er:YAG laser preparation of caries in dental hard tissues. Half of the preparations were completed by the laser alone with standardized parameters, with the other half being mechanically prepared. The sequential order of treatment was randomized, and clinical parameters such as depth and location of the cavities were carefully balanced. A three-score evaluation scheme of patient responses was used: comfortable, uncomfortable, very uncomfortable. In addition the patients were asked to decide which was the more uncomfortable form of treatment and the preferred treatment for future caries therapy. RESULTS: The study included 103 patients with 206 preparations distributed amongst 194 teeth. All teeth gave vital responses (ice test) before and after both types of treatment. The laser treatment was found to be more comfortable than the mechanical treatment, with high statistical significance. During treatment, the need for local anaesthesia was 11% for mechanical preparation compared to 6% during laser application. It was found that 80% of the patients rated the conventional preparation as more uncomfortable than the laser treatment and 82% of the patients indicated that they would prefer the Er:YAG laser preparation for further caries treatment. CONCLUSIONS: The application of the Er:YAG laser system is a more comfortable alternative or adjunctive method to conventional mechanical cavity preparation.

[Intracanalicular trabeculostomy--a new approach to glaucoma surgery]. / Intrakanaläre Trabekulostomie--Ein neuer Ansatz in der Glaukomchirurgie.

BACKGROUND: A new "ab externo" technique for glaucoma trabecular surgery creating multiple perforations of the trabeculum to improve outflow facility was tested. MATERIAL AND METHOD: In order to perform a draining canal to the anterior chamber, a quartz fiber (phi: 300 microns) with a side-firing tip, coupled to an Er:YAG laser (2.94 microns), was inserted into Schlemm's canal of an enucleated human eye. RESULTS: Eight pulses of 8 mJ each were sufficient to perforate the trabecular meshwork. Histologic analysis showed a rippled canal with 50 microns average diameter and a surrounding necrosis zone of 15-35 microns. CONCLUSIONS: The Er:YAG laser trabeculostomy, in conjunction with special fibers, shows promise as a new "ab externo" technique for minimally invasive therapy of open angle glaucoma.

Laser-induced autofluorescence spectroscopy of dental caries.

We studied spectral autofluorescence characteristics of dental caries. A wide range of carious lesions revealed characteristic emission of endogenous fluorophores with strong fluorescence bands in the red spectral region when excited with 407 nm line radiation of a krypton ion laser. Healthy hard dental tissue exhibited no emission bands in the red. The fluorescence spectra, fluorescence excitation spectra as well as the reflectance spectra of carious lesions were found to be typical for fluorescent porphyrins, mainly protoporphyrin IX. A possible source of these porphyrins within carious tissues is bacterial biosynthesis. Non-invasive sensitive in vivo caries detection by means of appropriate excitation sources and porphyrin fluorescence detectors should be possible.

[Modified radiation administration in Er:YAG laser ab externo sclerostomy]. / Modifizierte Strahlungsapplikation bei der Er:YAG-Laser- ab-externo-Sklerostomie.

BACKGROUND: Ab externo sclerostomy enables the restrained aqueous humour drain via a microgram wide corneoscleral canal into the subconjunctival space being an alternative surgical method in glaucoma therapy. The tissue ablative infrared laser radiation (2.94 microgram) was applied until now via a quartz fiber manually pushed forward in permanent tissue contact. Our aim was the development of a new applicator tip without manual feed in order to achieve a reduction in times for construction and maintenance of the former tip as well as in the risk of an infection and mechanical damage of intraocular structures. MATERIAL AND METHOD: In pig eye experiments, the laser parameters were varied in histological and scanning electron microscopic evaluation (pulse energy: 10-60 mJ, repetition rate: 5-17.5 Hz, combinations of constant power: 200 mW). Their influence on the canal shape and the morphology of the internal ostium has been proven in special tests. RESULTS: The new development of the applicator tip (core diameter: 550 microgram) enables a conical profile of the beam (diameter 500-150 microgram) within a distance of 2-3 mm, now without pushing the fiber forward ("fixed fiber"). Smooth canal surfaces, a small homogeneous necrosis zone (25 microgram) and only few tissue ruptures have been seen histologically and on SEM. The correlations of ablation rates/-speed as well as canal characteristics are more significant to energy densities than to repetition rates. First clinical application was technically successful. CONCLUSIONS: The new applicator tip optimizes the method of Er: YAG-laser ab externo sclerostomy from the technical and surgical point of view.

Thermo-controlled device for inducing deep coagulation in the liver with the Nd:YAG laser.

BACKGROUND AND OBJECTIVE: To increase the effectiveness of laser-induced interstitial thermotherapy (LITT), a new thermo-controlled application system for minimal invasive intervention was designed. Our system consists of a laser applicator of 2.5 mm in diameter, insertion equipment, and a Nd:YAG-laser source. STUDY DESIGN/MATERIALS AND METHODS: A cylindrical light emitting fiber (1-6 cm in length) was placed in the center of the applicator. The surrounding tissue was irradiated through a Duran window at the distal end of the applicator. The power of the laser source was controlled dynamically by thermosensors in a water-cooling system of the laser applicator. The temperature at the surface of the Duran window was kept constant at approximately 60 degrees C, without charring the surrounding tissue. RESULTS/CONCLUSION: We obtained homogeneous coagulation zones. In in vitro experiments with pig livers, we reached ellipsoid coagulation volumes of 3 and 5 cm in diameter within 10 minutes, corresponding to a volume of approximately 25 cm3.

Effects of Er:YAG laser in caries treatment: a clinical pilot study.

BACKGROUND AND OBJECTIVE: Various lasers have been suggested for cavity preparation. Pain reduction is expected as a potential benefit of laser usage. Among the systems studied in vitro, the Er:YAG laser offers a distinct combination of high ablation efficiency and low thermal side effects. Animal studies demonstrated that pulp damage can be avoided when appropriate laser parameters are used. STUDY DESIGN/MATERIAL AND METHODS: The present prospective clinical study was intended to test the practical applicability of the system and to investigate the patient's response. The study population consisted of 67 teeth on 33 subjects. Patient's reaction to pain was documented in four categories: no pain, little pain (like a brief pressure to the tooth), medium pain (like needle sticks), and strong pain (like a thermal shock test). RESULTS: For superficial cavities 200 pulses (SD: 114) were needed. At a pulse repetition rate of 2 Hz, the preparation time was 1.6 min (SD: 2.1 min). Deeper cavities required 391 pulses (SD: 251), or approximately 3.3 min (SD: 2.1 min). No loss in pulp vitality was observed. In 36% of the laser-treated teeth, no pain was felt, and in 57%, little pain was reported. With one exception, no local anesthesia was needed. CONCLUSION: On the basis of this study, we suggest that the Er:YAG: laser can provide caries removal and cavity preparation in an adequate preparation time with minimal patient discomfort.

Optimal parameters for laser treatment of leg telangiectasia.

BACKGROUND AND OBJECTIVE: The optimal parameters for treatment of leg telangiectasia with a pulsed laser are investigated. STUDY DESIGN/MATERIALS AND METHODS: The Monte Carlo method is used to calculate the light penetration and absorption in the epidermis, dermis, and the ectatic blood vessel. Calculations are made for different diameters and depths of the vessel in the dermis. In addition, the oxygen saturation of the blood vessel, the laser beam diameter, and the laser irradiation profile is varied. RESULTS: It is found that for vessels with diameters between 0.3 mm and 0.5 mm wavelengths about 600 nm are optimal to achieve selective photothermolysis for the considered damage model. It is also shown that an elliptical laser beam area has advantages compared to a circular beam area for treatment of leg telangiectasia. CONCLUSIONS: We recommend the treatment of leg telangiectasia with dye laser wavelengths longer than the normally used 577 nm and 585 nm.

[Ab externo sclerostomy with the Er:YAG laser: report of outcome after 2 years]. / Ab-externo-Sklerostomie mit dem Er:YAG-Laser: Ergebnisbericht nach zwei Jahren.

OBJECT: Results over a period of 24 months of performing the Er:YAG laser ab externo sclerostomy. MATERIAL AND METHODS: The procedure was performed 81 times (chronic glaucoma with open angle (n = 55) and narrow angle (n = 15), 6 secondary glaucoma, 4 pigment glaucoma, 1 normal pressure glaucoma) in 74 eyes of 64 patients at the age of 70 +/- 10 years without any peribulbar anesthesia. The pulse energy was 11 mJ with a repetition rate of 7 Hz. Antimetabolites were not applicated. Success was defined as an IOP of less than 22 mm Hg or a decrease of at least 30%, with or without anti-glaucoma medication. RESULTS: The mean IOP of all cases decreased from preoperatively 28 +/- 8.1 mm Hg (Success group: 26.7 +/- 7.3 mm Hg) to 15 +/- 8.4 mm Hg (Success group: 14.6 +/- 7.8 mm Hg) on the first postoperative day. The success rate was about 50% within the period of two weeks to six months and decreased to about 15% in the 24th month. Complications were iris incarceration/synechia (n = 33), hypotony (n = 36) with consequent choroidal detachment (n = 8), hyphema (n = 19) and more pronounced inflammation symptoms (n = 10). In most cases, the early postoperative outflow failure is caused by iris incarceration into the internal ostium. The scarring tendency of the episcleral tissue leads to closure of the external ostium after months. Resurgery was necessary in 26 cases. CONCLUSIONS: Further efforts should be directed to maintain the intact corneoscleral morphology of the canal, created by an optimized method of the surgical technique. The application of antimetabolites in correct dosage or the implantation of modified silicon draining systems would be helpful.

Pulsed Erbium:YAG laser ablation in cutaneous surgery.

BACKGROUND AND OBJECTIVE: Among the various pulsed midinfrared-lasers studied in skin surgery the 2.94 microns Erbium:YAG laser has been shown to combine most efficacious ablation with least thermal damage due to its unique absorption characteristics in tissue water. A newly developed high-power Erbium:YAG laboratory laser providing output energies (up to 1.5 J/pulse) and repetition rates (up to 15 Hz) appropriate for clinical use enabled us to investigate its potential indications in dermatological surgery. STUDY DESIGN/MATERIALS AND METHODS: Erbium:YAG laser ablation was performed in vitro on pig skin and in vivo on a total of 30 patients presenting with different skin disorders. RESULTS: In vitro ablation efficiency linearly increased with radiant exposure and was inversely correlated with pulse frequency. Ablation rate at 10 Jcm-2 (used clinically) measured from approximately 10 microns (at 10 Hz) to 40 microns (at 1 Hz). Also for high repetition rates thermal necrosis did not exceed 50 microns, corresponding clinically to capillary bleeding after exposure of the dermis. Superficial lesions, such as epidermal nevi, were easily ablated and re-epithelization was unimpaired owing to the absence of tissue necrosis. In tattoos, exposed pigment particles were precisely removed. However, in deeper lesions the casual onset of bleeding impeded the procedure and scar formation was observed after reepithelization. CONCLUSION: Pulsed 2.94 microns Erbium:YAG laser surgery allows an extremely precise etching of delicate superficial skin lesions and also should have a potential for skin resurfacing.

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.

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.

A new optimal wavelength for treatment of port wine stains?

In this study we investigate light penetration into skin in order to define the optimal wavelength for the treatment of port wine stains. A two-layer model (epidermis and dermis) is applied with a cylindric tube representing the ectatic blood vessel. Light propagation is calculated by the Monte Carlo method. Values for the optical properties of the skin were not only taken from the literature, but also derived from measurements of the spatially resolved reflectance on the human forearm applying a multilayer model. Using the new values, the maximal depth of selective vascular injury better fits experimental and clinical observations, compared to the values in literature. In addition, the optimal wavelength for treatment of port wine stains is shifted to longer wavelengths.