Dusan Sustercic

Human tooth pulp anatomy visualization by 3D magnetic resonance microscopy

Human tooth pulp anatomy visualization by 3D magnetic resonance microscopy Background. Precise assessment of dental pulp anatomy is of an extreme importance for a successful endodontic treatment. As standard radiographs of teeth provide very limited information on dental pulp anatomy, more capable methods are highly appreciated. One of these is 3D magnetic resonance (MR) microscopy of which diagnostic capabilities in terms of a better dental pulp anatomy assessment were evaluated in the study. Materials and methods. Twenty extracted human teeth were scanned on a 2.35 T MRI system for MR microscopy using the 3D spin-echo method that enabled image acquisition with isotropic resolution of 100 μm. The 3D images were then post processed by ImageJ program (NIH) to obtain advanced volume rendered views of dental pulps. Results. MR microscopy at 2.35 T provided accurate data on dental pulp anatomy in vitro. The data were presented as a sequence of thin 2D slices through the pulp in various orientations or as volume rendered 3D images reconstructed form arbitrary view-points. Sequential 2D images enabled only an approximate assessment of the pulp, while volume rendered 3D images were more precise in visualization of pulp anatomy and clearly showed pulp diverticles, number of pulp canals and root canal anastomosis. Conclusions. This in vitro study demonstrated that MR microscopy could provide very accurate 3D visualization of dental pulp anatomy. A possible future application of the method in vivo may be of a great importance for the endodontic treatment.

Influence of water spray on Er:YAG ablation of hard dental tissues

Influence of water-spray cooling on the efficiency of erbium laser ablation of hard dental tissues is investigated experimentally. Consecutive pulses from a dental Er:YAG laser system are applied to dentin and enamel surface in vitro. Depths and diameters of the resulting craters, obtained with and without the use of a pressurized air/water spray, are compared at single-pulse fluences from 10 to 140 J/cm2. Similar measurements are performed also with direct drilling of enamel with a fiber tip. The results show that direct interaction of the cooling water with the laser radiation increases the ablation efficiency for enamel, especially in fiber-tip drilling.

Effects of pulsed CO2 and Er:YAG lasers on enamel and dentin

Enamel and dentin samples were exposed extraorally to pulsed TEA CO2 lasers with pulse durations of 1 microsecond(s) ec and 0.1 microsecond(s) ec. The ablation rate is for energy densities above 5 J/cm2 independent of the CO2 laser energy. For 1 microsecond(s) long CO2 pulses the ablation rate is 3 micrometers /pulse for drilling in enamel, and 8 micrometers /pulse for drilling in dentin. Drilling with 0.1 microsecond(s) CO2 laser results in lower ablation rates of approximately 1 micrometers /pulse in enamel, and 4 micrometers /pulse in dentin. At all experimental energy densities plasma formation is observed, effectively reducing the amount of energy deposition. Compared with these results, experiments with the Er:YAG laser show that 200 microsecond(s) long Er:YAG laser pulses achieve better ablation in the high energy density range because ablation is not diminished as rapidly by the plasma formation. The Er:YAG laser saturated ablation rates are approximately 60 micrometers /pulse for drilling in dentin and 40 micrometers /pulse for drilling in enamel.

Fiber-tip drilling of hard dental tissues with Er:YAG laser

Specifics of fiber-tip ablation of hard dental tissues with mid-infrared laser radiation are investigated in vitro. Sequences of free-generated Er:YAG laser pulses are applied to fresh human dentin and enamel slices at a low repetition rate and the resulting craters inspected and measured by optical microscopy. Influence of the laser pulse fluence (up to 55 J/cm2), number of pulses in the sequence (1 to 20), and the gap between the fiber tip and the tissue surface (0 to 1.0 mm), is determined quantitatively. Additionally, the effect of the optical quality of the fiber tip on the ablation efficiency is assessed qualitatively. The results help us identify the optimum working regime for the dental fiber-tip handpieces in both tissues. Additionally, they provide new clues for understanding the process of mid-infrared ablation of hard biological tissues, especially in presence of the water spray, which interacts directly with the laser radiation.

Optoacoustic studies of Er:YAG laser ablation in hard dental tissue

Optoacoustic measurements were carried out in order to obtain better understanding of the ablation mechanisms during the illumination of hard dental tissue by Er:YAG laser radiation. A broadband microphone was used to detect laser generated acoustic waves in the ambient air. Correlation analysis of the laser pulse spikes and the response of the optoacoustic probe indicates that each laser spike ablates the hard dental tissue independently of other spikes. This is in agreement with the model of ablation by means of micro explosions. The optoacoustic signal is observed to be approximately linearly related to the ablation efficiency, and is thus demonstrated to be a good measure of the ablation efficiency. The experiments also show a significant difference in optoacoustic signals obtained during ablation in caries, enamel, and dentin.

Debris screening and heat diffusion in Er:YAG drilling of hard dental tissues

A systematical investigation of Er:YAG laser drilling of both human dentin and enamel with pulselengths between 50 microsecond(s) and 1.2 ms is presented. At the shorter pulselengths, the influence of heat diffusion is negligible. Consequently, ablation of tissue starts abruptly at a well-defined fluence value, which is independent of the pulselength. Depths and volumes of the resulting craters show a quasi-logarithmical dependence on the applied laser fluence. An improved analytical model of the laser beam screening by the ejected debris is developed, which fits well to the experimental data. With longer laser pulses, ablation turns up only gradually, with differential ablation efficiency slowly increasing with applied laser fluence. Such double-threshold behavior is explained by a simple model of dynamical interplay between the ablation front and heat-diffusion wave. The model enables us to derive expressions for pulselength and laser fluence ranges in which such behavior should be expected for any combination of tissue and laser properties. Also, it predicts qualitatively the amount of thermal side effects in a general laser ablation process.

Interaction thresholds in Er:YAG laser ablation of organic tissue

Because of their unique properties with regard to the absorption in organic tissue, pulsed Er:YAG lasers are of interest for various applications in medicine, such as dentistry, dermatology, and cosmetic surgery. The relatively low thermal side effects, and surgical precision of erbium medical lasers have been attributed to the micro-explosive nature of their interaction with organic tissue. In this paper, we report on preliminary results of our study of the thresholds for tissue ablation, using an opto-acoustic technique. Two laser energy thresholds for the interaction are observed. The lower energy threshold is attributed to surface water vaporization, and the higher energy threshold to explosive ablation of thin tissue layers.

Optoacoustic effects during Er:YAG laser ablation in hard dental tissue

Optoacoustic method is a very useful tool for studying laser induced processes in hard dental tissues. In principle, the method can also be used for on-line monitoring of laser drilling. Our study, however, shows that at high laser energies the optoacoustic energy is not proportional to the volume of the ablated hard dental tissue. In addition, the optoacoustic signal depends critically on the presence of water on the tooth surface. These observations must be taken into account when attempting to use the optoacoustic method for on-line monitoring of the laser drilling process.

Saturation effects in ablation of hard dental tissues by Er:YAG

Differing reports exist on the dependence of ablation rate in hard dental tissue on Er:YAG laser energy. In order to clarify this point we have carried out a systematic study of Er:YAG laser ablation rates in enamel and dentin extraorally with and without water spray cooling. Our findings confirm the existence of saturation of ablation rates at high laser energy densities which has been already previously attributed to plasma shielding. The drilling efficiency and the saturation effect have been found not to depend significantly on the presence of water spray. It is worth pointing out that a standard water spray, as used for mechanical drilling very efficiently eliminates any charcoaling, without reducing laser drilling efficiency. Diameters of ablated holes have been found to increase with laser energy also above the saturation of drilling efficiency. Measurements of the dependence of the ablation efficiency on pulse duration indicate that there is an optimal pulse duration for Er:YAG laser drilling of approximately 200 microsecond(s) .

Influence of Er:YAG laser power on the ablation efficiency and thermal damage in hard dental tissue

In vitro study on extracted human teeth of the influence of Er:YAG laser power on the ablation efficiency and thermal damage has been carried out. Dependence of the diameter and depth of ablation holes in dentin and enamel was measured as a function of laser pulse energy and repetition rate. The drilling speed per pulse was found to be independent of the repetition rate in the range of 1 - 20 Hz. In the studied laser power range, and under water spray cooling, no visible damage was observed in enamel. In dentin, carbonization was observed only at very high laser powers. Experimental results show that at high laser powers thermal damage occurs more readily at higher energies per pulse than at higher repetition rates. The experiment therefore seems to indicate that dental lasers which allow higher repetition rates are better suited for laser drilling of hard dental tissue.