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Influence of pulselength on mid-infrared laser ablation of hard dental tissue was studied experimentally by applying free-generated Er:YAG laser pulses with pulselengths from 50 ts to 1 ms and different energies to dentin slices in vitro with no spray cooling. The results show that for pulselengths between 50 and 300 ps, dentin ablation turns up abruptly at a well defined fluence value ofapproximately 4 J/cm2, which is independent of the pulselength. Similarly, volumes of the resulting craters as a function of laser fluence at these pulselengths follow nearly the same curve, which corresponds well to a model of laser beam screening by the ejected debris. However, with 1 ms long laser pulses, the ablation turns up at laser fluences above 4 J/cm2 only gradually, reaching differential ablation efficiency similar as in previous cases at fluence values several times higher than the ablation threshold value. An estimate of this "second threshold" fluence and the pulselengths at which it should in general be observed are derived from a simple model of heat diffusion and ablation front dynamics.

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BackgroundPrecise 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 methodsTwenty 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.ResultsMR 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.ConclusionsThis 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.

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 broadbard microphone was used to detect laser generated acoustic waves in the ambient air. Correlation analysis of tht laser pulse spikes and the response of the optoacoustic probe indicates that each laser spike ablates the har i 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 als) show a significant difference in optoacoustic signals obtained during ablation in caries, enamel and dentin.