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.
Modulated luminescence (LUM) technique was applied to analyze photophysical processes in the cortical layer of
human skull bones. The theoretical interpretation of the results was based on the optical excitation and decay rate
equations of the fluorophore and on the molecular interaction parameter with the photon field density in the matrix of the
bone. Using comparisons of the theory with the frequency response of dental LUM it was concluded that the optically
active molecular species (fluorophore) in the bones is hydroxyapatite. An effective relaxation lifetime of skull cortical
bone was derived theoretically and was found to depend on the intrinsic fluorophore decay lifetime, on the photon field
density, and on the thickness of the bone. The experimentally measured dependencies were in excellent agreement with
the theoretical model. The theory was able to yield measurements of the optical scattering coefficient, optical absorption
coefficient, and mean coupling coefficient. These results show that the quantitative LUM can be used as a sensitive
method to measure optical properties of the active fluorophore in cortical skull bones and the optical-field-induced
molecular interaction parameter. When calibrated vs. laser intensity, the modulated luminescence can also be used to
measure human skull thickness. These traits can be applied to monitor the bone mineral density (BMD) and, ultimately
can be used as potential markers of bone health or disease, such as osteoporosis or bone cancer.
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 mm×4 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.
Photothermal radiometry (PTR) and modulated luminescence (LUM) were applied to detect and monitor the
demineralization of root and enamel surfaces of human teeth to produce caries lesions and the subsequent
remineralization of the produced lesions. The experimental set-up consisted of a semiconductor laser (659 nm, 120 mW),
a mercury-cadmium-telluride IR detector for PTR, a photodiode for LUM, and two lock-in amplifiers. A lesion was
created on a 1-mm × 4-mm rectangular window, spanning root to enamel surface, using an artificial caries lesion gel to
demineralize the tooth surface and create small carious lesions. The samples were subsequently immersed in a
remineralization solution. Each sample was examined with PTR/LUM on root and enamel before and after treatment at
times from 1 to 10 days of demineralization and 2 to 10 days of remineralization. PTR/LUM signals showed gradual and
consistent changes with treatment time. At the completion of the experiments, transverse micro-radiography (TMR)
analysis was performed to correlate the PTR/LUM signals to depth of the carious lesions and mineral losses. In this
study, TMR showed good correlation with PTR/LUM. It was also found that treatment duration did not correlate well to
any technique, PTR/LUM, or TMR, which is indicative of significant variations in demineralization - remineralization
rates among different teeth.
Osteoporosis is a skeletal disorder characterized by a compromised bone strength predisposing a person to an increased
risk of fracture. The early detection of osteoporosis is important to a successful treatment. Current prominent bone
densitometry techniques include, among others, Dual Energy X-Ray Absorptiometry (DEXA) and Mechanical Response
Tissue Analysis (MRTA). However, DEXA uses ionizing radiation and MRTA results are often unreliable. Simultaneous
Photothermal Radiometry (PTR) and Modulated Luminescence (LUM) measurements can be a non-ionizing, noninvasive
and reliable alternative to the aforementioned diagnostics techniques. Controlled mineral loss was simulated
with sequential etching of a human skull bone. During the experiments, a low-power modulated laser illuminated the
sample surface. The absorbed incident optical energy was then
re-emitted either non-radiatively, in the form of thermal
waves (PTR), or radiatively as lumimescence light emission (LUM). The experimental setup consisted of a
semiconductor laser (635 nm, 20 mW), two lock-in amplifiers, a mercury-cadmium-telluride IR detector for PTR, a
photodiode for LUM and a computer. A one-dimensional, one-layer theoretical model for LUM and PTR was developed
to analyze the experimental data and extract optical and thermal properties of the sample.
A coupled diffuse-photon-density-wave and thermal-wave theoretical model was developed to describe the
biothermophotonic phenomena in a turbid medium under photothermal radiometry experimental conditions. The solution
of the radiative transport equation in the limit of the
diffuse-photon-density field was considered as a source term in the
thermal-wave field equation. The model was used to analyze laser induced photothermal phenomena in a demineralized
tooth sample as a function of depth. The analysis is based on a three-layer approach (demineralized enamel + healthy
enamel + dentin) and considering the influence of thermal and optical properties of each layer on the resulting optical
and thermal field.
Laser induced photothermal radiometry (PTR) was applied as a safe, non-destructive, and highly sensitive tool for the
detection of early tooth surface demineralization. In the experiments, teeth were treated sequentially with an artificial
demineralization gel to simulate controlled mineral loss on the enamel surface. 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 twice: before and after
treatment. The experiments showed very high sensitivity of the measured signal to incipient changes in the enamel
structure, emphasizing the clinical capabilities of the method. In order to analyze the biothermophotonic phenomena in a
sample during the photothermal excitation, a theoretical model featuring coupled diffuse-photon-density-wave and
thermal-wave fields was developed. The theoretical fits based on the three-layer approach (demineralizad enamel +
healthy enamel + dentin) allowed fitting thermal and optical properties of the demineralized layer. The theoretical
analysis showed that the dentin layer should be taken into account in the fittings.
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 (µ-CT), transverse microradiography (TMR), and scanning electron microscopy (SEM) analyses are performed. Although µ-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.
Recent trends in bioacoustophotonics and biothermophotonics of tissues are presented. The presentation is centered on the development of well-known frequency-domain photothermal and photoacoustic techniques to address issues associated with diffuse photon density waves during optical excitation of turbid media, both in hard tissues (teeth) and soft tissues. These methods have concrete advantages over the conventional pulsed-laser counterparts. In Part I we present biothermophotonic principles and applications to the detection of the carious state in human teeth as embodied
by laser photothermal radiometry supported by modulated luminescence. The emphasis is on the abilities of these techniques to approach important problems such as the diagnosis of occlusal pits and fissures and interproximal lesions between teeth which normally go undetected by x-ray radiographs. In Part II we present theoretical and experimental results in frequency-domain bioacoustophotonics of turbid media, such as soft tissues, and we describe the development of sensitive sub-surface imaging methodologies which hold the promise for sensitive diagnostics of cancerous lesions in e.g. a human breast. Results using tissue phantoms and ex-vivo specimens are discussed and the current level of subsurface lesion sensitivity compared to state-of-the-art pulsed photoacoustic techniques is examined. In summary, advances in coupled frequency-domain diffuse-photon-density-wave and thermal or thermoelastic responses of turbid media constitute new trends in bioacoustophotonics and biothermophotonics promising for their signal quality and high dynamic range.
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