Occlusal caries detection by using thermal imaging
Introduction
Dental caries is a dynamic disease that is still amongst the most prevalent diseases in the world. It is characterised by tooth demineralisation that initially leads to increased surface enamel porosity. Leaving these lesions untreated can result in the formation of cavities that can reach the dentin and pulp and eventually cause tooth loss. Occlusal and aproximal surfaces are amongst the most susceptible sites of demineralisation due to acid attack from bacterial by-products in the biofilm.1
The use of preventive agents to inhibit or reverse the demineralisation process is advised on the detection of lesions at an early stage.2, 3, 4, 5 It is therefore desirable to identify and manage incipient caries on time in order to avoid invasive clinical intervention. However, detecting early lesions is a difficult diagnostic task and quantitative techniques assisting in the decision-making are required. Monitoring of therapeutic success after interventions is also challenging requiring relatively large changes in lesions before they can be reliably detected using most of the currently available diagnostic aids.
The initial stage of the disease is characterised by dissolution of hydroxyapatite and this leads to a non-uniform mineral loss and an increase in enamel porosity—this is commonly known as a white spot lesion.5 Therefore, estimating the amount of enamel surface porosity or mineral loss can assist in the detection and quantification of early tooth demineralisation non-invasively.
Radiographical methods are not ideal due to the patient exposure to ionising radiation and their lack of sensitivity at very early stages of the disease.6, 7, 8 Electrical caries’ monitoring has demonstrated good performance for single point measurements9; however, imaging methods have the advantage of being more illustrative and allow performing relative measurements with respect to other sites within the same tooth. Current imaging methods are based on the observation of changes in the light transport within the tooth, namely absorption, scattering10, 11, 12 and fluorescence.10, 11, 12 In general, porous media scatters away more light than uniform media and stain within the tooth tends to absorb light; stain is therefore a strong confounding factor in caries detection with visible imaging techniques. This is of particular interest when looking at occlusal surfaces where uptake of stain may be an indicative of lesion arrest.
Continuous evaporation of water accumulated inside the pores produces a thermodynamic transient on the tooth surface that will last until a new thermal equilibrium is reached when the tooth dries. The temporal profile of the temperature will depend on the amount of water stored inside the lesion as well as the shape of the lesion and can therefore contain information related to its degree of porosity and severity. Thermal imaging for caries’ detection has been little explored in the past. Kaneko et al.13 reported a method to quantify tooth decay on flat tooth surfaces using an infrared camera; the method was tested with a set of 18 extracted incisors with artificial lesions. Their method relies on sensing the temperature time-decay due to water evaporation from the tooth surface as it is dried by an air-jet. A similar study was reported a year later using thermal imaging to look at the temperature time-decay immediately after a pulse of heat delivered onto flat tooth surfaces using a xenon flash light; 19 incisors with artificial lesions were used in these experiments.14, 15 Such artificial lesions may exhibit greater porosity than natural lesions where the dynamic process of de- and re-mineralisation has occurred.
Recently, another technique was proposed to identify dental carious tissues using frequency-domain laser infrared photothermal radiometry (PTR).16, 17, 18 PTR uses the optical to thermal energy conversion resulting from tooth laser absorption and studies the changes observed in the thermal wave diffusion inside teeth with various degrees of demineralisation. The technique appears promising for the detection of pit and fissure caries up to 5 mm below the surface; however, according to the authors, it is less appropriate for imaging.
Although thermography has shown to be useful in the evaluation of smooth surface caries, no evidence is found of such evaluation at occlusal surfaces. The purpose of this study is to assess the performance of thermal imaging for quantifying natural demineralisation, both early and advanced, on occlusal surfaces.
Section snippets
Teeth
A set of 25 extracted teeth (premolars and molars) with different natural lesion severities were used for this study. In order to have independent measures at a tooth level, a uniform distribution of lesion severities across the 25 samples used in this experiment was selected based on their ICDAS code; having 5 teeth per codes 1–5. The teeth were collected from the Oral Health Centre of the University of Indiana, USA. Soft tissues were removed from the collected teeth and the teeth were
Results
Infrared video sequences were obtained from a total of 25 teeth. In addition, a total of 72 histological slices with different lesion severities were selected and lesions were identified as either from the surface or the fissure pattern. Note that lesions reaching the dentin were only in the fissures. Two random groups of 48 samples were then formed, both balanced for severity and location of the lesion by sorting the data from the tooth slices ascending in histological score and divided per
Discussion
Every object above zero Kelvin will radiate infrared (IR) electromagnetic energy as described by the black-body radiation theory.19 This energy can be detected with an IR camera and temperature measurements can be performed. Models describing the thermodynamics of the drying process of porous media have been reported in the past.20, 21
Tooth temperature will be influenced by a number of factors such as ambient and body temperature, breath and moisture. The amount of water stored inside the tooth
Conclusions
Our investigations demonstrate that thermal imaging has the ability to discriminate between (a) areas that are either sound or with an early lesion on the outer half of the enamel and (b) areas with lesions extending to the middle of the enamel or deeper. This could potentially be useful for in vitro measurements where other methods such as radiography have low sensitivity to early tooth decay but high sensitivity to decay at E2 level and above. However, the viability of taking in vivo
Acknowledgements
The authors would like to acknowledge access to the infrared camera used for the experiments to Dr. Mark Dickinson and Dr. David Binks from the Photon Science Institute at the University of Manchester. We thank Prof. Kim Ekstrand and Prof. David Ricketts for scoring the histological slices. We also thank Prof. Helen Worthington for useful discussions on statistical analysis. Dr. Iain Pretty is funded by a NIHR Clinician Scientist Award.
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