Anna Matvienko

In vitro detection and quantification of enamel and root caries using infrared photothermal radiometry and modulated luminescence

Artificially created demineralized and remineralized carious lesions on the root and enamel of human teeth were examined by photothermal radiometry (PTR) and modulated luminescence (LUM). Fourteen extracted human teeth were used and a lesion was created on a 1 mmx4 mm rectangular window, spanning root to enamel, using a lactic acid-based acidified gel to demineralize the tooth surface. The lesion was then exposed to a remineralization solution. Each sample was examined with PTR/LUM on the root and enamel before and after treatment at times from 1 to 10 (5 on root) days of demineralization and 2 to 10 days of remineralization. Ten-day (5 on root) demineralized samples were remineralized. After completing all the experiments, transverse microradiography (TMR) analysis was performed to compare and correlate the PTR/LUM signals to the depth of lesions and mineral losses. The PTR and LUM amplitudes and phases showed gradual and consistent changes with treatment time. In this study, TMR showed good correlation coefficients with PTR and LUM. It was also found that the length of the treatment time did not correlate very well to any technique, PTR/LUM or TMR, which implies a significant degree of inhomogeneity of the demireralization and remineralization rates in each and every tooth.

Detection of interproximal demineralized lesions on human teeth in vitro using frequency-domain infrared photothermal radiometry and modulated luminescence

Frequency-domain photothermal radiometry (FD-PTR or PTR) is used to detect mechanical holes and demineralized enamel in the interproximal contact area of extracted human teeth. Thirty-four teeth are used in a series of experiments. Preliminary tests to detect mechanical holes created by dental burs and 37% phosphoric acid etching for 20 s on the interproximal contact points show distinct differences in the signal. Interproximal contact areas are demineralized by using a partially saturated acidic buffer system. Each sample pair is examined with PTR before and after micromachining or treating at sequential treatment periods spanning 6 h to 30 days. Dental bitewing radiographs showed no sign of demineralized lesion even for samples treated for 30 days. Microcomputer tomography (micro-CT), transverse microradiography (TMR), and scanning electron microscopy (SEM) analyses are performed. Although micro-CT and TMR measured mineral losses and lesion depths, only SEM surface images showed visible signs of treatment because of the minimal extent of the demineralization. However, the PTR amplitude increased by more than 300% after 80 h of treatment. Therefore, PTR is shown to have sufficient contrast for the detection of very early interproximal demineralized lesions. The technique further exhibits excellent signal reproducibility and consistent signal changes in the presence of interproximal demineralized lesions, attributes that could lead to PTR as a reliable probe to detect early interproximal demineralization lesions. Modulated luminescence is also measured simultaneously, but it shows a lower ability than PTR to detect these interproximal demineralized lesions.

Theoretical analysis of coupled diffuse-photon-density and thermal-wave field depth profiles photothermally generated in layered turbid dental structures

During the past 3 decades, we have become witnesses to an ever accelerating growth of laser applications, for both clinical treatment and noninvasive diagnostics, in medicine and biology. This is why the processes governing laser-tissue interactions are so thoroughly investigated nowadays. These processes include two main components: optical, i.e., light propagation, and thermal, i.e., energy distribution following optical-to-thermal energy conversion. In order to understand and describe these processes, it is crucial to have accurate information on optical and thermal properties of biological tissues. Moreover, high-resolution noninvasive measurements of optical and thermal properties of tissues can be used as diagnostics of early stages of pathological changes. Numerous studies have been focused on the in vivo evaluation of optical properties of biological tissues. In many cases, these results are based on the radiative transport theory with various modifications particularly, the diffusion approximation 1 depending on the applied measurement technique. The main restriction to applications of the diffusion theory is that scattering effects must be significant, which is, however, usually the case with tissues. Particularly, the requirement that the mean free path for photon scattering should be much larger than the wavelength of light and much smaller than the thickness of the medium allows the description of multiply scattered light intensity by means of a diffusion equation. 2 Additional constraints are related to the size of the scattering particles, which should be small compared to the optical wavelength. 2

Polypyrrole nanoparticles as a thermal transducer of NIR radiation in hot-melt adhesives

Polypyrrole (PPy), like other conducting polymers, has a broad absorption band in the near infrared (NIR) with no evidence of fluorescence emission. We describe the preparation of PPy–EVA blends as potential hot-melt adhesives that can be activated by irradiation with NIR light. The PPy content needed to act as a thermal transducer of NIR radiation is much lower than that needed for conductivity. Blends were prepared in two ways: by blending sterically-stabilized 50 nm diameter PPy particles in water with a dispersion of 800 nm diameter ethylene–vinyl acetate copolymer (EVA) particles, and by synthesizing PPy-coated EVA core-shell particles by precipitation polymerization in water. The PPy nanoparticles and the PPy-coated EVA core-shell particles could be purified by sedimentation followed by redispersion in water to remove Fe salts. Films prepared from these particles, containing 0.1–0.5 wt% PPy, showed a strong NIR absorbance in the range of our spectrometer (700–1100 nm) with a weaker absorbance in the visible region. Photothermal radiometry (PTR) measurements indicate that these blends show good promise as potential NIR-activated adhesives, which are essentially transparent to the eye.

Quantitative one-dimensional thermal-wave cavity measurements of fluid thermophysical properties through equivalence studies with three-dimensional geometries

The thermal-wave field in a photopyroelectric thermal-wave cavity was calculated with two theoretical approaches: a computationally straightforward, conventional, one-dimensional approach and a three-dimensional experimentally more realistic approach. The calculations show that the dimensionality of the thermal-wave field in the cavity depends on the lateral heat transfer boundary conditions and the relation between the beam size of the laser impinging on the thermal-wave generating metallic film and the diameter of the film itself. The theoretical calculations and the experimental data on the photopyroelectric signal in the cavity were compared. The study resulted in identifying ranges of heat transfer rates, beam sizes, and cavity radii for which accurate quantitative measurements of the thermal diffusivity of intracavity fluids can be made within the far simpler, but only approximate, one-dimensional approach conventionally adopted by users of thermal-wave cavities. It was shown that the major paramete...

Robust multiparameter method of evaluating the optical and thermal properties of a layered tissue structure using photothermal radiometry

The thermal and optical properties of multilayered dental tissue structure, the result of the surface-grown prismless layer on enamel, were evaluated simultaneously using multiparameter fits of photothermal radiometry frequency responses. The photothermal field generated in a tooth sample with near-infrared laser excitation was described using a coupled diffuse-photon-density and thermal wave model. The optical (absorption and scattering) coefficients and thermal parameters (spectrally averaged infrared emissivity, thermal diffusivity and conductivity) of each layer, as well as the thickness of the upper prismless enamel layer, were fitted using a multiparameter simplex downhill minimization algorithm. The results show that the proposed fitting approach can increase robustness of the multiparameter estimation of tissue properties in the case of ill-defined multiparameter fits, which are unavoidable in in vivo tissue evaluation. The described method can readily be used for noninvasive in vitro or in vivo characterization of a wide range of layered biological tissues.

Optothermophysical properties of demineralized human dental enamel determined using photothermally generated diffuse photon density and thermal-wave fields

Noninvasive dental diagnostics is a growing discipline since it has been established that early detection and quantification of tooth mineral loss can reverse caries lesions in their incipient state. A theoretical coupled diffuse photon density and thermal-wave model was developed and applied to photothermal radiometric frequency responses, fitted to experimental data using a multiparameter simplex downhill minimization algorithm for the extraction of optothermophysical properties from artificially demineralized human enamel. The aim of this study was to evaluate the reliability and robustness of the advanced fitting algorithm. The results showed a select group of optical and thermal transport parameters and thicknesses were reliably extracted from the computational fitting algorithm. Theoretically derived thicknesses were accurately predicted, within about 20% error, while the estimated error in the optical and thermal property evaluation was within the values determined from early studies using destructive analyses. The high fidelity of the theoretical model illustrates its efficacy, reliability, and applicability toward the nondestructive characterization of depthwise inhomogeneous sound enamel and complex enamel caries lesions.

Ultrahigh-resolution pyroelectric thermal-wave technique for the measurement of thermal diffusivity of low-concentration water-alcohol mixtures

Thermal diffusivities of water-methanol and water-ethanol mixtures were measured using a thermal-wave cavity with two techniques: conventional single-pulse photopyroelectric frequency scans and the common-mode-rejection demodulation dual-pulse scheme. The frequency-scan measurements showed maximum resolution of the photothermal signal in water at the level of 0.5% by volume in mixtures of methanol and ethanol. The common-mode-rejection demodulation method improved the resolution up to the level of 0.2% by volume, which is the highest thermophysical resolution of water-methanol and water-ethanol mixtures reported to date, to our best knowledge. The ultrahigh sensitivity of the method can be especially useful in environmental applications, specifically in real-time water pollution monitoring.

Quantitative analysis of incipient mineral loss in hard tissues

A coupled diffuse-photon-density-wave and thermal-wave theoretical model was developed to describe the biothermophotonic phenomena in multi-layered hard tissue structures. Photothermal Radiometry was applied as a safe, non-destructive, and highly sensitive tool for the detection of early tooth enamel demineralization to test the theory. Extracted human tooth was treated sequentially with an artificial demineralization gel to simulate controlled mineral loss in the enamel. The experimental setup included a semiconductor laser (659 nm, 120 mW) as the source of the photothermal signal. Modulated laser light generated infrared blackbody radiation from teeth upon absorption and nonradiative energy conversion. The infrared flux emitted by the treated region of the tooth surface and sub-surface was monitored with an infrared detector, both before and after treatment. Frequency scans with a laser beam size of 3 mm were performed in order to guarantee one-dimensionality of the photothermal field. TMR images showed clear differences between sound and demineralized enamel, however this technique is destructive. Dental radiographs did not indicate any changes. The photothermal signal showed clear change even after 1 min of gel treatment. As a result of the fittings, thermal and optical properties of sound and demineralized enamel were obtained, which allowed for quantitative differentiation of healthy and non-healthy regions. In conclusion, the developed model was shown to be a promising tool for non-invasive quantitative analysis of early demineralization of hard tissues.

Dental diagnostic clinical instrument ("Canary") development using photothermal radiometry and modulated luminescence

Since 1999, our group at the CADIFT, University of Toronto, has developed the application of Frequency Domain Photothermal Radiometry (PTR) and Luminescence (LUM) to dental caries detection. Various cases including artificial caries detection have been studied and some of the inherent advantages of the adaptation of this technique to dental diagnostics in conjunction with modulated luminescence as a dual-probe technique have been reported. Based on these studies, a portable, compact diagnostic instrument for dental clinic use has been designed, assembled and tested. A semiconductor laser, optical fibers, a thermoelectric cooled mid-IR detector, and a USB connected data acquisition card were used. Software lock-in amplifier techniques were developed to compute amplitude and phase of PTR and LUM signals. In order to achieve fast measurement and acceptable signal-to-noise ratio (SNR) for clinical application, swept sine waveforms were used. As a result sampling and stabilization time for each measurement point was reduced to a few seconds. A sophisticated software interface was designed to simultaneously record intra-oral camera images with PTR and LUM responses. Preliminary results using this instrument during clinical trials in a dental clinic showed this instrument could detect early caries both from PTR and LUM signals.