Early Dental Caries and Demineralization Measurements

by Using Portable OCT Scanner

Serioh Okaneya

1,2 a

and Tatsuo Shiina

Phoenix-Dent Co., Ltd., 2649-3, Kameicho, Sano-shi, Tochigi, 327-0024, Japan

Graduate School of Science and Engineering, Chiba University, 1-33 Yayoi-cho,

Inage-ku, Chiba-shi, Chiba, 263-8522, Japan

Keywords: Optical Coherence Tomography, OCT, Time-Domain, Portable, Dental, Caries, Clinical Applications.

Abstract: There is an increasing need in dental research and clinical practice for accurate measurements of teeth.

Currently, methods such as x-rays and quantitative light-induced fluorescence (QLF) are used, but there are

problems such as the effects of radiation and limitations in resolution. Optical Coherence Tomography (OCT)

used in this study uses near-infrared light, which has no effect on the human body and provides very high

resolution in the range of tens of micrometers. OCT allows for non-invasive imaging, making it safer for

repeated use and ideal for monitoring caries progression over time. In this study, we developed a portable

OCT scanner specialized for dental use and measured and evaluated early caries and demineralization.

1 INTRODUCTION

In dental research and clinical practice, there is an

increasing need for precise measurements of the teeth,

such as the progression of early caries, caries between

adjacent teeth, and cracks on the tooth surface

(Colston, 1998; De Melo, 2005; Shimada, 2010; Imai,

2012; Leão Filho, 2013). Currently, methods such as

X-ray and quantitative visible light induced

fluorescence (QLF) are used, but there are problems

such as radiation effects and limited resolution

(Shiina, 2003; Shimada, 2020). In recent years, the

application of Optical Coherence Tomography

(OCT), already commercialized in the field of

ophthalmology, to dentistry has attracted attention,

and research on OCT for dentistry was initiated by

Sumi and colleagues around 2010 (Shimada, 2010;

Park, 2018). Non-destructive tomographic imaging of

bonded restorations (Makishi, 2011) and non-

destructive tomographic imaging using optical

coherence tomography (OCT) OCT was

commercialized as a medical device for dental use in

Japan in 2020. However, due to limitations in probe

size, product size and even price, it is currently used

more for research purposes than for actual clinical

practice.

https://orcid.org/0009-0009-2099-0906

https://orcid.org/0000-0001-9292-4523

OCT uses the low-coherence light interference of

an SLD as a light source. Cross-sectional images of

the target can be obtained in a non-erosive and non-

destructive manner, and the depth resolution, which

is determined by the coherence length of the light

source, is about a few tens of microns, which is higher

than that of X-ray or ultrasound measurements.

Currently, high-speed and high-dynamic-range

OCTs such as Fourier-domain OCT (FD-OCT) and

spectral-domain OCT (SD-OCT) are in practical use,

mainly in the medical field, but they are expensive,

large, and have complex systems (Colston Jr, 1998).

The TD-OCT (Time-Domain OCT) used in this study

can be designed independently in terms of the

wavelength of the light source used, measurement

range and resolution, and it can be configured at low

cost. TD-OCT is slower scanning speed than other

medical OCTs, but it allows a longer measurement

depth and does not require complex and extensive

computational processing, as the time information

can be directly converted into depth information.

In this study, a portable OCT scanner specially

designed with a tiny optical probe for dental uses and

its demonstration experiment has been performed to

scan, measure, and evaluate caries on extracted teeth.

In addition, the teeth were immersed in vinegar to

Okaneya, S. and Shiina, T.

Early Dental Caries and Demineralization Measurements by Using Portable OCT Scanner.

DOI: 10.5220/0013151700003902

In Proceedings of the 13th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2025), pages 95-101

ISBN: 978-989-758-736-8; ISSN: 2184-4364

reproduce the initial caries state and to measure and

evaluate the level of enamel demineralization on the

tooth surfaces (Tsai, 2019).

2 DEVICE CONFIGURATION

The optical configuration of the portable OCT

scanner developed in this study is shown in Fig.1. The

low-coherence light emitted from the SLD( 𝜆





1.3𝜇𝑚, Δ𝜆  53𝑛𝑚, 𝐴𝑛𝑟𝑖𝑡𝑠𝑢 𝐶𝑜. , 𝐿𝑡𝑑.) light source

is split into two beams by a fiber coupler and directed

towards the reference mirror and the measurement

target, respectively. The reference optical system of

this device utilizes a variable optical path mechanism.

By fixing the reference mirror and rotating the

reflector, the reflected light is returned to the fiber

coupler with a time delay. Meanwhile, the light

directed towards the measurement target passes

inside the target. The light is scattered (backscattered)

back in the original direction by each layer within the

measurement target. The light returning from these

two optical paths interferes and is received by the

Photo Diode (PD). In the receiving circuit, the

interference light is converted into an electrical signal

with the adequate filtering and magnification, and the

waveform can be observed on an oscilloscope.

OCT technology has been widely successful in

ophthalmology for detecting and diagnosing eye

conditions. The non-invasive nature of OCT,

combined with its high resolution, makes it a

promising tool for dental applications. In comparison

to X-rays, OCT eliminates the risk of radiation

exposure, making it safer for repeated use. To

uniformly measure the enamel, which is a random

medium, a mechanism for moving the probe and a

algorithm for processing the oscilloscope signals on a

PC are integrated. This allows continuous observation

the interference waveform and obtaining average

optical properties.

The appearance of the device is shown in Fig.2. It

is small, with dimensions of 180mm in width, 240mm

in length, and 105mm in height, and weighs

approximately 3kg. It operates with a DC power

supply, with a rotation reflector for varying the

optical path length and an SLD light source output of

2mW[max], making it low in power consumption and

be portable. The OCT probe is shown in Fig.3. The

numerical aperture (N.A) of the lens used is 0.11,

resulting in a lateral resolution of 9.87 μm. The

measurement of the distance from the probe and the

signal strength indicated that the focal length is

0.8mm.

The probe, intended for intraoral use, is enough

small with a diameter of 3.6mm and a length of 12mm,

consisting of a single hemispherical lens. This allows

for easy handling within the oral cavity, from the

anterior teeth to the molars, and both buccal and

lingual sides.

Figure 1: System configuration of TD-OCT.

Figure 2: Device appearance of a portable OCT.

(a)

(b)

Figure 3: Probe for dental OCT/ (a) Schematic diagram (b)

Photograph.

PHOTOPTICS 2025 - 13th International Conference on Photonics, Optics and Laser Technology

3 EXPERIMENTAL

PROCEDURES

The setup for the tooth measurements in this study is

shown in Fig.4. First, the extracted tooth is mounted

in a hole on the stage so that the emitted light,

indicated by the green arrow, is perpendicular to the

tooth surface. Next, the probe is scanned horizontally

along the tooth in the direction of the red arrow. The

measurement interval is set to 1 scan (1 line in depth

direction) per 0.20 mm. To capture the fine surface

irregularities when measuring the tooth shape,

scanning is performed at 1 scan per 0.10mm.

However, in order to measure the optical properties

within the enamel quickly and uniformly, a wider

interval was used. Since the width of the teeth used in

the experiment is about 6 to 8 mm, a single

measurement can collect about 30 to 40 lines, which

is sufficient for the study.

Figure 5 shows photographs of some of the human

teeth used for OCT measurements for caries checks

and demineralization processing. Extracted human

teeth with no visible damage or caries were selected,

including a total of 17 teeth from both anterior and

posterior regions. The black lines on the surface of the

teeth were used as reference marks for measurement.

The probe was positioned perpendicular to the tooth

surface at these marked positions and the distance

between the probe and the tooth surface was adjusted

for focus. Measurements were taken only on the

enamel side surfaces because the occlusal surfaces

were unsuitable due to their irregularities.

Figure 6 illustrates the workflow of the

demineralization experiment. First, healthy teeth

were measured with an OCT scanner. After they were

immersed in a demineralization solution, they were

measured again with OCT, and this process was

repeated until the demineralization time reached 36

hours. After each demineralization treatment, the

teeth were thoroughly rinsed with water and the

surfaces were dried to ensure that no water droplets

were present during the measurement. That care was

taken not to rub the surface during drying. Fresh

demineralization solution was used each time after

the teeth were removed. The same positions were

measured using reference marks as shown in Fig.5.

4 EVALUATION METHOD

In the field of dentistry, there have been reports that

suggest demineralization potentially infiltrate

internally using SS-OCT (Damodaran, 2016). During

Figure: 4 Optical probe scanning process.

Figure 5: Some of the measurement samples of human teeth.

Figure 6: Demineralization Experiment Workflow.

the analysis, it is helpful to consider binarizing the

signals using a threshold. In healthy teeth, the OCT

signal is initially visible only at the surface, but

gradually, it expands under the demineralization. This

phenomenon, which is known as superficial

demineralization, refers to the early stages of caries

progression, where demineralization can be observed

spreading in the enamel's surface layer

Early Dental Caries and Demineralization Measurements by Using Portable OCT Scanner

(approximately 0.1 to 0.5 mm). It has been suggested

that dental OCT offer a means of detecting early

caries that are difficult to confirm visually. However,

it should be noted that these results have not yet been

subjected to a quantitative evaluation, as they are

based solely on comparisons of OCT images.

In this study, we enhance the quality of the raw

data by performing a series of processing steps,

including focal position correction, distance-squared

correction, normalization at the peak position, and

natural logarithm processing. We then applied

binarization using a threshold to the processed data.

As demineralization progresses in the enamel surface

layer, it would seem that the signal expands internally

within the enamel. This will etch the surface and

cause demineralisation but will not cause weakening

of the outer enamel and subsequent sub-surface

mineral loss. It is helpful to define this range as the

demineralization range, and to calculate the average

depth. Following normalization, a threshold is set,

and areas with values exceeding this threshold are

displayed in white. Figure 7 illustrates the alterations

in OCT images prior to and following binarization.

The threshold setting was informed by the published

paper (Damodaran, 2016). The average

demineralization depth is calculated by dividing the

area of the white region by the horizontal length

(corresponding to the vertical axis in Fig.7).

Figure 8 illustrates a typical B-scan result from the

demineralization experiment. It seems that the

demineralization depth will expand due to the

treatment, which could potentially increase the range

where interference signals appear. This illustrates

how acid gradually erodes the surface, gradually

progressing deeper into the enamel. It would seem

that the eroded enamel generates a great many fine

cavities, which become increasingly porous and thus

increase backscattering. For the sake of clarity, we

would like to present the demineralization range

shown in Fig.8 in a different format. This is done by

using a threshold to create a binarized image, which

we will show in Fig.9. It is proposed that the areas

with color changes in the binarized data be considered

as the demineralization depth.

5 MEASUREMENTS OF EARLY

CARIES

Figure 10(a) and (b) illustrate the anterior tooth

samples utilized for OCT measurement. That the

dotted lines in the figures indicate the B-scan

measurement position, with measurements taken

(a) OCT Image immediately after logarithmic

processing (b) Binarized OCT image

Figure 7: Changes in OCT images due to binarization.

(a) Before demineralization

(b) After demineralization

Figure 8: Changes in B-scan due to demineralization.

(a) Before demineralization

(b) After demineralization

Figure 9: Binarized B-scan image using threshold.

(a)

(b)

PHOTOPTICS 2025 - 13th International Conference on Photonics, Optics and Laser Technology

from right to left in the photograph. The

measurements were taken in a horizontal direction on

the surface. It will be observed that within the dotted

circles in Fig.10(a) and (b), these areas appear to be

more opaque than other parts. This is an indication of

early caries (CO). The results of the measurements

across these areas affected by caries are presented in

Fig.10(c). In regions where no caries is observed at

either end, no signals are detected below the surface.

In contrast, in the areas with caries, signals are

detectable up to approximately 0.39 mm below the

surface. Measuring the thickness of the white opaque

area within the dotted circle in Fig.10(b) gives a value

of 0.54 mm. Considering the displacement between

the section location shown in Fig.10(c) and the

measurement location, the results are satisfactory.

Figure 11(a) and 12(a) illustrate the molar

samples utilized for OCT measurements. With regard

to molar sample B, it was deemed appropriate to take

measurements from the occlusal surface. For

convenience, the dotted line in each figure indicates

the measurement positions, with B-scan

measurements taken from right to left across the page.

Figure 11(b) is the measurement results for the

molar sample B. It would seem that early caries (C1)

will be present in the region indicated by the dotted

circle, and staining is visible in the region indicated

by the solid circle. In the carious area, it seems that

signals from demineralized regions within the tooth is

detected in addition to the surface signals. It is helpful

to consider the yellow dotted line as a representation

of the predicted surface shape in a healthy state. It

would appear that the progression of enamel

demineralization will result in a collapse of the

surface shape.

For molar sample C, measurements were taken at

intervals of 0.2mm in the direction indicated by the

arrows in each figure, with the aim of observing

changes in signals at different measurement positions.

Fig.12(b) presents the measurement results, with the

dotted line results on the left and the results from a

0.2mm shift in the arrow direction on the right.

It is observed that the dotted and solid circle

regions in Fig.12(a) appear to correspond to the

carious area and the staining, respectively. Figure

12(b) presents the measurement results for sample C,

which appear to indicate the presence of early caries

(C1) in the dashed circle region. It can be seen from

the OCT cross-sectional image that there is a clear

progression of internal demineralization, which

allows for a quantitative assessment of the depth of

progression.

(a) Surface lateral measurement position (b) Cross-

sectional view of measurement position (c)

Measurement results

Figure 10: Anterior tooth sample A.

(a) Photograph (b) Measurement results

Figure 11: Molar sample B.

(a) Photograph (b) Measurement results

Figure 12: Molar sample C.

(c)

(b)

0.54m

(a)

Early Dental Caries and Demineralization Measurements by Using Portable OCT Scanner

6 MEASUREMENT OF TOOTH

DEMINERALIZATION

Figure 13 offers insight into the impact of utilizing

undiluted demineralization solution and a 10-fold

diluted solution on the depth of demineralization. The

horizontal axis represents the immersion time in the

demineralization solution, and the vertical axis

represents the demineralization depth. With regard to

the results obtained with the undiluted solution (pH

2.7), it seems that the demineralization depth will

increase in a linear fashion until approximately 12

hours, after which it appears to plateau at a depth of

approximately 0.06 to 0.08 mm. This is in line with

what the B-scan images appear to show, which seems

to indicate that the demineralization process reaches

a certain point where it plateaus in the depth direction.

With regard to the 10-fold diluted solution, it seems

that the increase continues until around 20 hours, after

which it appears to plateau at approximately 0.08mm.

It seems reasonable to suggest that the longer time

taken to reach the plateau is due to the decreased

acidity of the diluted solution.

Similarly, it appears that the surface signal

intensity will increase gradually until around 12 hours,

after which it also seems to reach a plateau. This leads

us to the possibility that demineralization is occurring

not only internally but also on the surface, with the

surface structure potentially changing over time.

Furthermore, this observation is of great importance

as it highlights the necessity of monitoring both

internal and surface changes during demineralization

processes to gain a comprehensive understanding of

the extent and progression of caries.

Figure 14 presents a graph with the

demineralization depth on the horizontal axis and the

surface signal intensity on the vertical axis. It

illustrates the results obtained when using the 10-fold

diluted solution. The change is logarithmic rather

than linear. This result suggests the possibility that

there are differences in the pace of erosion within the

tooth and the degree of surface erosion. It is the case

that the slower increase in surface signal intensity

compared to internal erosion is due to differing acid

resistance between the internal and surface enamel.

Given the observed difference in acid resistance

between the enamel surface and internal, these

findings are to be expected. It seems plausible to

suggest that the enamel surface will be more acid-

resistant than the internal enamel, which could

explain these results. Additionally, figure 14 suggests

that the surface signal intensity will saturate earlier,

indicating that the demineralization solution will first

infiltrate the surface before progressing inward. It

seems that the initial saturation suggests that surface

erosion occurs relatively quickly, followed by a more

gradual internal demineralization process. This could

be an indication of the layered nature of enamel

degradation.

(a) When using undiluted solution (pH2.7)

(b) When using 10-fold diluted solution

Figure 13: Relationship between demineralization time and

demineralization depth.

(When Using 10-Fold Diluted Solution)

Figure 14: Relationship between demineralization depth

and surface signal intensity.

Depth of demineralization(mm)

0 0.02 0.04 0.06 0.08

(

)

(

)

PHOTOPTICS 2025 - 13th International Conference on Photonics, Optics and Laser Technology

100

7 CONCLUSIONS

In this study, we have developed a cost-effective and

portable TD-OCT scanner that could potentially offer

a more efficient alternative to existing dental OCTs.

For the measurement of early caries, we used

human teeth after tooth extraction and evaluated the

measured signal in the early caries area. In the

measurement of anterior teeth, the depth of the

clouded area was measured to be approximately 0.39

mm by OCT, and generally good results were

confirmed in comparison with the actual cut surface

measurement of 0.53 mm. Although numerical

comparisons were not made for the molar area due to

the severe surface irregularities, clear differences in

measurement signals were observed between the

caries-affected and non-caries-affected areas.

We also aimed to gain a deeper understanding of

the effects of demineralization on teeth. To assess the

effects of demineralization, we immersed extracted

human teeth in commercially available vinegar and

evaluated changes in demineralization depth, surface

signal intensity, and attenuation coefficient as

demineralization progressed. We observed that the

demineralization depth, surface signal intensity, and

attenuation coefficient increased at rates of 2.6 μm/h,

2.7/h, and 0.18 mm⁻¹/h, respectively. After

approximately 36 hours, these values appeared to

stabilize. The analysis methods were based on

previously published methods, and the obtained values

were similar to those measured using microscopes.

In order to gain a better understanding of the effects

of drying, we conducted an analysis of the OCT signals

for both non-demineralized and demineralized teeth

after drying. In order to dry the samples, silica gel was

used, and the samples were placed in a container with

less than 10% humidity for 30 minutes. While non-

demineralized teeth showed minimal effects from

drying, demineralized teeth exhibited increased

demineralization depth (approximately 1.20 times),

increased surface signal intensity (approximately 1.33

times), and decreased attenuation coefficient

(approximately 0.63 times). It seems that these changes

in signals will be attributed to the formation of fine

voids in the HA crystals, which are the main

component of enamel, due to demineralization. This

could allow moisture to enter and exit.

From these results, we were able to gain insight

into the changes in OCT signals within teeth caused

by early dental caries, demineralization and drying

using TD-OCT. Further work could include

developing devices for use in clinical settings and

devising methods to measure and evaluate in real time

within the oral cavity.

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