A Conceptual Model of Dual-Mode Tomography Technique for Dental Diagnostics: Ultrasound and Light Propagation Analysis.

Many advanced imaging modalities, such as magnetic resonance imaging (MRI), X-ray Computed Tomography, Positron Emission Tomography (PET), Ultrasound, Single Photon Emission Computed Tomography (SPECT), and the most recent, Optical Coherence Tomography (OCT), have been developed for identifying dental tissues images and detecting changes in early carious lesions. Some modalities use high doses of radiation and energy to obtain more information, which may be harmful to patient’s health. Most early commercial OCTs had drawbacks such as its bulky size and limited image resolution. In order to overcome these concerns, this paper presents a dual-mode tomography technique that combines OCT and ultrasound method using the COMSOL Multiphysics. The application of an ultrasound device helps overcome the limitation of OCT in detecting the penetration depth of a caries lesion. Several simulations were performed to analyse the light and ultrasonic propagated waves with different diameters of carious lesions. In response to this goal, the combination data of OCT and ultrasound provide a 3D image which offers the best approach for displaying and examining changes in the oral cavity.


Introduction
According to the World Health Organization (WHO), oral disorders affect around 3.5 billion people worldwide, with three out of every four people affected living in middle-income nations [1].Many modern high-resolution diagnostic imaging technologies are being developed in clinical dentistry for detecting and treating anomalies caused by a variety of illnesses affecting the oral cavity from the early stage to the critical stage.
Due to a variety of disorders affecting the oral cavity, many high-resolution diagnostic imaging methods and technology in clinical dentistry [2] [3] are developed for detecting and treating abnormalities.Optical Coherence Tomography (OCT) has mostly been used due to its high-resolution image and low-coherence light to overcome the drawback of ionizing X-ray radiation with a small field of view (FOV) and penetration depth [4].Even though OCT can provide high-resolution diagnosis images, its acquisition rate and visual field of view (FOV) limitations made it difficult to acquire threedimensional (3-D) structures.In addition, due to their size, most commercial OCT devices only allow access to frontal views of the anterior teeth (incisors and canines) [2].
However, OCT's image acquisition and FOV limitation can be overcome by combining OCT with ultrasound technique.The previous study proved that combining both techniques can reduce multiple scattering as measured by the point spread function (PSF) by at least 20% [5] [6].
Therefore, this research will introduce a conceptual modelling of portable dental diagnostics based on wave (ultrasound) and light propagation analysis (OCT) via dual-mode tomography approach.These proposed techniques will help to overcome the issue of low-resolution image reconstruction.

Methodology
This simulation research focused on the incisors, the front teeth in the maxillary arch and carious lesion using COMSOL Multiphysics software.The simulation was observed in the plan view drawing of a tooth to investigate the light and wave propagation occur in the tooth.The diameters of the stimulated carious lesion are changed to 0.25mm, 0.35mm and 0.45mm.

Construction of simulation
The material used and 2D geometry of incisors were identified before generating a model in COMSOL Multiphysics.The structure of the incisor tooth was designed in a rectangular shape.The average width and depth of incisor tooth was defined as 9 mm width and 2mm depth.Enamel is the first or outer and hardest layer of a tooth.It is made up of hydroxyapatite (HA) and chemical formula is Ca5(PO4)3(OH) [7].Next, the caries lesion present on the incisor tooth was designed with a circle shape and the material used for carious lesion is acid buffer [8].

Light propagation (OCT).
A simulation of light propagation in Figure 1 involved determining object geometry and shapes to accurately represent incisor teeth size.A 2D geometry is defined, including the teeth dimensions and carious lesion to distinguish different scattering mediums.The rectangle shape was surrounded with Perfectly Matched Layers (PMLs) [9] to ensure accuracy and avoid reflections.The simulation uses Electromagnetic Waves and Frequency Domain module in COMSOL for computational physics.A parametric sweep at 1310nm is used for informed detection.

Wave propagation (Ultrasound).
A 2D space dimension was chosen in Figure 2 to simulate wave propagation in incisor teeth using direct transmission.Transmitting and receiving transducers were positioned to transmit and receive signals directly from the front surface of the enamel structure.COMSOL Multiphysics used modules like pressure acoustics, transient, solid mechanics, electrostatics, and electrical circuits for the simulation.A parametric step was introduced at 1us, with the highest frequency set at 25MHz [7].

Statistical Analysis and 3D construction
The performance of the proposed conceptual modeling using light propagation and wave propagation were verified via statistical analysis.The statistical analysis of sensitivity maps was applied in light propagation while ultrasound was verified using time arrival using a Minitab software.Block diagram in Figure 3 shows the flow of 3D construction of incisor image.The data images of two difference propagations were merged to construct 3D image using 3D slicer that gave better image reconstruction in order to overcome the limitations of OCT in image penetration.

Results and Discussion
The simulation was performed to analyse the propagated waves (light propagation and wave propagation) of incisor teeth with different diameters of carious lesions.The diameter of carious lesions was changed from 0.25mm to 0.35mm and 0.45mm.Different methods were used to verify the propagation.Light propagation was verified using a sensitivity map while ultrasound was verified using time arrival.

Sensitivity map of light propagation
The penetration of the laser through the tooth formed a difference effect of scattered waves inside the non-homogeneous enamel structure with different diameters of the carious lesions.The value of the pixel was taken from the sensitivity map of the image for further analysis.The pixel values were brought information of carious lesions from light propagations and then analyzed using the normality test in Minitab software.Therefore, the normality test was used to determine the effect of the scattered waves in the light propagation of non-homogenous enamel with different diameters.A sensitivity map enables observing changes in light propagation when carious lesion size changes, providing insights into light interaction variations.The P-value's impact diminishes as the lesion radius increases, indicating larger lesions have a reduced effect on overall light propagation.Therefore, the techniques help to identify affected regions and measure carious lesion extent and dimensions.

Time arrival of wave propagation
The simulation of the arrival time was conducted to test and identify the ability of the transmission signal to receive at the receiver at a certain time since wave propagation cannot be evaluated in sensitivity maps and normality test.The arrival time was calculated from the second slope of signal where it determined the time when the signal received at the receiver.Table 2 shows the simulation results of time arrival in non-homogenous conditions with different diameters of carious lesions.Analysing time arrival allows for observing changes in wave propagation when adjusting carious lesion size.Larger lesions produced stronger signals, while defining and observing the depth of a carious lesion using appropriate techniques allows for visualization and determination of the lesion's depth below the surface.

Combination dual-mode tomography
A three-dimensional image of enamel can be produced from a two-dimensional non-homogenous enamel image [10].The result from the experiment then was plotted in the 3D scatterplot.Sensitivity maps from the light propagation were mapped on the x and y axes while time arrival from the wave propagation was mapped on the z axis.These two-difference propagations can generate better image reconstruction by combining the result of light propagation and wave propagation.Thus, it also can produce 3D images of teeth by combining 2D images of light propagation and wave propagation.This combination of dual-mode tomography makes it simple to detect changes when a tooth is in an abnormal state.As a result, the combination dual-mode tomography is a much better technique for tomography for tooth lesion examination.

Conclusion
In conclusion, this research provides conceptual modelling of portable dental diagnostics based on the dual-mode tomography method.This project successfully showed the difference of scattered waves between light and wave propagation from OCT and ultrasound methods.From the analysis, light propagation can identify the defect area of a carious lesion by using the sensitivity map while wave propagation can define the depth of a carious lesion by using time arrival.Other than that, 2D images from dual-mode tomography can construct 3D images of incisor tooth using data from sensitivity map and time arrival.This project may give many benefits to the field of dentistry by analysing the propagation of light and waves through an incisor tooth to treat carious lesions.

Figure 2 .
Figure 2. Construction of geometry in wave propagation on Man Machine Systems 2023 Journal of Physics: Conference Series 2641 (2023) 012008

Figure 4 .
Figure 4. 3D scatterplot of time arrival and sensitivity maps

Table 1 .
Table 1 shows the simulation results of the normality test in non-homogenous conditions with different diameters of carious lesions.Simulation results of non-homogenous enamel with difference diameter of carious lesion in light propagation 4 P-Value: 0.901 ii.Diameter of carious lesion (mm): 0.35 mm P-Value: 0.321 iii.Diameter of carious lesion (mm): 0.45 mm 5 P-Value: 0.251

Table 2 .
Simulation results of non-homogenous enamel with different diameter of carious lesion in wave propagation