Ex vivo human teeth imaging with various photoacoustic imaging systems

Dental caries cause pain and if not diagnosed, it may lead to the loss of teeth in extreme cases. Dental X-ray imaging is the gold standard for caries detection; however, it cannot detect hidden caries. In addition, the ionizing nature of X-ray radiation is another concern. Hence, other alternate imaging modalities like photoacoustic (PA) imaging are being explored for dental imaging. Here, we demonstrate the feasibility of acoustic resolution photoacoustic microscopy (ARPAM) to image a tooth with metal filling, circular photoacoustic computed tomography (cPACT) to acquire images of teeth with caries and pigmentation, and linear array-based photoacoustic imaging (lPACT) of teeth with caries and pigmentation. The cavity measured with lPACT imaging is compared with the X-ray computed tomography image. The metal filling and its boundaries are clearly seen in the ARPAM image. cPACT images at 1064 nm were a better representative of the tooth surface compared to the images acquired at 532 nm. It was possible to detect the cavities present in the dentine when lPACT imaging was used. The PA signal from the pigmented caries on the lateral surface (occlusion view) of the tooth was high when imaged using the lPACT system.


Introduction
Dental caries (also known as cavities) which affect the deciduous and permanent teeth, is a major concern in oral healthcare [1].Caries develop due to the dissolution and degradation of enamel layer by the lactic acid produced by the bacteria in the mouth [2].Caries start in the enamel, the surface of the tooth, and progress deeper into the tooth affecting the dentine region (rich in blood vessels and nerve endings) [3].Caries on an enamel can be treated using filling materials made of metal or cement.When the caries has impacted the dentine, either root canal treatment is done, or the tooth must be extracted.Root canal treatment needs multiple visits to the dentist and x-ray imaging during each visit to ensure the tooth cap is correctly placed.The gap created due to the extraction of the tooth leads to misalignment of the other teeth, which leads to further complications.Early detection of the caries is crucial to avoid the excruciating toothache and loss of tooth [4].One way to identify the caries is during visual examination by dentist or when detected on x-ray images.Often these caries are left undiagnosed because they are too small or hidden from the visual sight.Currently, x-ray imaging (dental radiograph) is widely used to identify caries.The resolution of the dental radiographs are poor [5][6][7], and exposure to ionizing radiation is another concern of dental radiographs [8].There is a risk of meningioma associated with the dental x-ray imaging [8].Therefore, there is a need for an alternative safe (non-ionizing) imaging for diagnosis of dental abnormalities.
Other than x-ray imaging, non-ionizing imaging methods such as magnetic resonance imaging (MRI) and ultrasound (US) imaging is also used for the diagnosis and treatment of dental diseases [9][10][11][12][13].The bulky and expensive MRI is not always accessible and therefore, is not efficient for screening patients.The US imaging is capable of only estimating the health of the soft tissue such as gingiva, as the sound waves cannot penetrate the dentine structure deep enough for caries detection.Other diagnosis done during the oral examinations is the probing of the gingiva tissue around the tooth to measure the clinical attachment loss [14].There is an attachment loss when the subject has periodontitis.Periodontitis is an inflammatory disease due to the growth of bacteria.The inflammation causes a gap between the tooth and the gingiva tissue (called pocket).The probing remains the gold standard to measure the depth of the pocket.There is ongoing research to substitute the probing with ultrasound imaging [11,[15][16][17][18][19][20].Another area where ultrasound imaging is used in dentistry is to assist in the dental implant [19].
Optical imaging techniques such as near-infrared hyperspectral imaging and optical coherence tomography (OCT) has been explored for dentine imaging [21][22][23][24].Pure optical imaging techniques provide only surface information because the optical photons cannot penetrate deep in the teeth.Since an image is acquired for each wavelength, the data acquired in hyperspectral imaging is large.One has to manually study the series of the images from all wavelengths to identify the caries.The time taken to acquire these images is longer than x-ray imaging.Another drawback of the hyperspectral imaging is that the subject is exposed to hyperspectral camera for a longer period which causes dryness in the oral cavity and discomfort to the subject.Recent OCT scanner developed by Shimada et al., is capable of providing 3D map of the teeth, which is quite comprehensive, but the drawback of this set-up is that performance of the system depends on the topology of the tooth [25].OCT imaging is only capable of identifying the presence of a cavity.The depth of the cavity is not available in OCT imaging because there is no contrast between the enamel and dentine.The treatment plan of filling, root canal treatment or tooth removal depends on the depth of the cavity.
Photoacoustic (PA) imaging is also explored for dental imaging [26,27].Photoacoustic/optoacoustic imaging (PAI) is an hybrid imaging technique where the tissue produces ultrasound signals when illuminated with short pulsed light [28][29][30].The pulsed light is absorbed by the chromophores in the tissue such as melanin, hemoglobin etc. to convert light energy into localized temperature rise (in milli degree).The local temperature change gives rise to pressure wave that travels through the tissue as an acoustic wave (also known as PA waves).These PA waves are then acquired using ultrasound transducers (UST)/detectors on the tissue surface.The collected PA waves are then used to reconstruct/reveal the internal structure of the tissue.The contrast in PA images is coming from the light absorption, therefore, combining optical contrast with ultrasound resolution is what makes this hybrid imaging modality more potent than pure optical or pure ultrasound imaging alone.
Based on the optical illumination pattern and the orientation of the ultrasound transducer and signal detection geometry, PAI is mainly classified as photoacoustic computed tomography (PACT) [31][32][33] and photoacoustic microscopy (PAM) [34][35][36][37].In circular scan PACT (cPACT), the region of interest is illuminated, and the UST is positioned perpendicular to the direction of illumination.The UST is either a single element transducer that rotates around the sample in full circle (360 degree) or a curved array covering the full circle (or a full 3D spherical transducer array).In linear PACT (lPACT) the optical illumination and the UST (usually linear array transducer) is on the surface of the region of interest and the transducer usually has partial coverage [38].In cPACT/lPACT usually we obtain a 2D cross-sectional image and if a 3D volumetric data is needed one needs to perform several cross-sectional imaging and convert it into a 3D volume (in case of 3D spherical transducer array one can obtain full 3D volumetric imaging without any scanning).In PAM either the light illumination is focused [called optical resolution (ORPAM)] or the UST is focused [called acoustic resolution (ARPAM)] and the focused spot is raster scanned to obtain a high resolution image of the sample volume in 3D.PACT is widely used for imaging whole organs such as breast and feet [39,40] while PAM is used more for surficial imaging such as in dermatology (skin imaging) [41].
In the early application of PA in dentistry, PA signal from an extracted tooth with caries was studied [42].The amplitude of the PA signal from caries region was higher than that of the signal from healthy dental surface [43].PA signal was acquired with different concentrations of hemoglobin injected in the dentine [44,45].There was an increase in spectral intensities of the frequency components of the PA signal for higher concentrations of hemoglobin in dentine.Tasmara et al., acquired PA images using diode laser and condenser microphone [5].The advantage of this set-up is that it is non-contact, but the quality of the images is not acceptable for clinical decisions.Recent in vivo study by Ling et al., is where a hockey-stick shaped ultrasound transducer was integrated with the optical fiber based light delivery [46].The setup was used to study the gingiva tissue.The limitations of the setup include the bulkiness of the scanning head and the contrast agent (cuttlefish ink) used.Based on the anatomy of the subject some of the molars and gingiva tissue was not scanned due to the design of the scanning head.This imaging protocol cannot be used on everybody because some of the population is apprehensive towards animal products and a few others are allergic to them.
Even though there has been considerable research on PA imaging of extracted human teeth for caries and pigmentation, there are very few studies that validate the findings with x-ray computed tomography to enable the anatomical correlation [47][48][49].PA images are compared with x-ray images to convince clinical adaptation of PA imaging.In vivo trials for gingiva tissue imaging have also been demonstrated.Further research is needed to convince the clinicians that PA imaging can be used to measure the pocket as accurately as the probe.If the non-invasive PA imaging can provide the health of the tissue, the discomfort to the patients due to the manual probing can be reduced.Overall, PA imaging for dentistry is not yet clinically acclimatized.Feasibility of the PA imaging on the extracted tooth with filling is yet another area that needs active research.Dental implants which are metal based, needs imaging to check for cracks or degradation.This is not possible using x-ray or MRI due to the artifacts that metals introduce.PA imaging is expected to assist in this area as well.An understanding of how the current PA systems can be used for dental imaging is lacking.This needs a database of dental PA images from the PA systems along with the annotations from the dentists and the subject history.
It is important to understand, if PA imaging systems can image the caries or the fillings in teeth.It is meaningful to study the imaging performance of the well-established PA imaging set-ups to image the extracted tooth samples with caries or filling.The focus of this work is to demonstrate the feasibility of PAI, i.e., the ARPAM, cPACT, and lPACT on the extracted teeth samples.Three different PA imaging systems are used to image teeth samples (including caries, pigmentation, and metal filling).Two of the systems is in house built and the other one used is a commercially available system.We acquired the images from these systems and compared one of the samples with x-ray CT image.Due to the limitations of system availability and limited number of tooth samples, the results discussed here are confined to few representative extracted teeth having a particular abnormality.

System description
For this study, we used the ARPAM system that was operated at 1064 nm [50].cPACT was operated at 532 nm and 1064 nm.lPACT scans was done using the commercial linear array-based system [with light emitting diode (LED) light illumination] which was operated at 850 nm [51][52][53].Further details of each of the system is described below.

Acoustic resolution photoacoustic microscopy (ARPAM)
The ARPAM system [Fig.1(a)] [50] used a diode-pumped solid-state Nd:YAG laser (INNOLAS, Edgewave, Wurselen, Germany) producing laser pulses of 1064 nm.The laser beam was passed through a series of lenses and filters to form a doughnut shaped beam on the sample.The laser beam was collimated by passing through lenses L1 (LJ1328L2-B, Thorlabs, Newton, New Jersey) and L2 (LA1257-C, Thorlabs).Then, the residual 532 nm beam was removed by passing the beam through a filter (FGS900-A, Thorlabs).The beam intensity was adjusted using a variable neutral density filter (NDC-50C-4 M, Thorlabs).The beam was then focused by lens L3 (LA1257-C, Thorlabs) into a multi-mode fiber (MHP910L02, Thorlabs) which was mounted on a linear translation stage (XYT1, Thorlabs).The fiber is connected, via a fiber-tip holder, to a 60 mm cage on a translational stage (CT1, Thorlabs).This created the scanning head [also shown in Fig. 1(a)].The fiber had a damage threshold of ∼50 W at 980 nm.Inside the scanning head the laser beam was routed through a focusing lens, L4 (LA1951, Thorlabs) and a conical lens of apex angle 130°, CL (1-APX-2-B254, Altechna, Vilnius, Lithuania) in order to transform the beam into a doughnut-shape [54].The doughnut-shaped beam was passed through a custom acrylic optical condenser that has cone angles of 75°and 105°which contained a 30 MHz ultrasound transducer (V214-BB-RM, Olympus-NDT, Waltham, Massachusetts).Attached to the transducer via UV curing optical adhesive (NOA61, Thorlabs), is a 6 mm diameter diverging lens (LC1975-C, Thorlabs) with a 12.4 mm radius of curvature.This configuration created an acoustic focal diameter of ∼80 µm.The 1064-ARPAM had a working distance of 15 mm.The optical focus was larger than the acoustic focal diameter at ∼4 mm in diameter in an optically clear medium.The maximum energy per pulse was ∼1.8 mJ which illuminated an area of ∼1.75 cm 2 .Therefore the energy density on the sample surface is ∼1.02 mJ/cm 2 , which is within the safety limits prescribed by American National Standards Institute (ANSI) [55].Depth resolved PA signals (A-lines) from the sensors had the Hilbert transform applied and then were further processed using MATLAB.This was done to form maximum amplitude projection (MAP) images.

Circular scan photoacoustic computed tomography
The circular scan photoacoustic tomography system [Fig.1(b)], cPACT, used a Q-switched Nd:YAG laser having laser pulses of 5 ns at 532 nm wavelength with 10 Hz pulse repetition rate.The laser energy density on the sample surface was ∼3.18 mJ /cm 2 (less than the ANSI safety limit of 20 mJ/cm 2 at 532 nm) [55].Unfocused US transducer (Olympus NDT, V306-SU) was used to obtain the PA signal.These were 13 mm in diameter active area and 2.25 MHz central frequency with ∼70% nominal bandwidth.This system configuration allowed the sample data to be acquired continuously at a rotational speed of 0.75°/second.This correlates to a 360°a cquisition time of 480 seconds.The PA signals were sampled at a rate of 25 MS/s.Once the PA signals were acquired, they were regrouped into 800 A-lines.The signals were recorded on a desktop data acquisition card (GaGe, Compuscope 4227) after being amplified and filtered via a pulse amplifier (Olympus-NDT, 5072PR).The reconstruction of the acquired data is done using a simple delay-and-sum reconstruction algorithm [56].

Light emitting diode-based photoacoustic computed tomography
The lPACT system used to acquire the linear array based images was a commercially available PA and US imaging system (AcousticX, CYBERDYNE INC, Tsukuba, Japan).The system used LED light source for illuminating the sample [Fig.1(c)].Two LED arrays each with 4 rows of 850 nm LED elements were attached to the linear array ultrasound probe at an angle.The probe and the LED arrays were connected to a DAQ (data acquisition) component which was connected to a computer.The LED arrays were producing 70 ns light pulses with ∼0.4 mJ per pulse.The receiving ultrasound transducer was a 7 MHz linear array probe with 128 channels and 80% bandwidth.The pulse repetition frequency (PRF) of the LED arrays was 4 KHz.The system acquired signals at a sampling rate of 40 MS/s for PA and 20 MS/s for US.Data can be simultaneously obtained from the 128 channels.The system was used at a display frame rate of 10 Hz.The AcousticX system used an offline DAS based reconstruction algorithm with a coherence factor for 2D images.The reconstruction was done on a frame averaged over a large number of samples to compensate for the lower energy of the LEDs and improve the signal-to-noise ratio (SNR).

Computed X-ray tomography
The x-ray tube used here was FXE-225.45 from YXLON.Products.It is a 225 kV µ-focus Comet tube, operated at 1 mA.The maximum tube power was 225 W and the target power was 100 W. At the minimum focus-to-object distance of ∼6.75 mm, the focal spot was around ∼6 µm.The tooth was secured on a plastic tube of diameter ∼2 cm using clay.Dexela 2923NDT was the detector, whose pixel size was 74.8 µm.The detector had 3888 by 3072 pixels.An in-house algorithm was used to reconstruct the CT images.

Dental sample
The human dental samples used for this study were acquired from the dentist.All these samples were extracted due to the caries or demineralization.Informed consent was obtained from the subjects before samples were used for the experiments.The dentist also provided the annotations of the caries and cavities for the teeth samples.The samples used in the experiments are selected such that each of them represents a particular abnormality.The tooth sample used for the ARPAM imaging was filled with metal to treat caries on the enamel, which was extracted after a few years due to the untreatable cavity that had developed over time.Teeth used for cPACT were extracted due to demineralized caries and pigmented caries.Samples with demineralized caries and pigmented calculus for studying the imaging capability of lPACT.

Results
A dental sample with a metal filling (Fig. 2(a)) was imaged using ARPAM system at 1064 nm illumination wavelength.Lateral resolution of the system was ∼130 µm and axial resolution was ∼57 µm.The metal filling is clearly seen in the MAP image shown in Fig. 2(b).Metals have high optical absorption, hence gives strong PA signal.Therefore, clearly visible in the PA image.There are areas where the intensity is lower around the boundaries.This can be attributed to the less dense metal filling that is percolated into the tooth cavity.The cPACT system was used to image a tooth with a cavity due to demineralization (Fig. 3(a)) and a tooth with pigmentation caries (Fig. 3(d)).Both 1064 nm and 532 nm wavelengths were used for imaging.The spatial resolution of this imaging system was ∼165 µm.There are bright spots seen in the images.These spots represent the location of demineralization (green arrows in Figs.3(b) and 3(c)) and pigmentation (blue arrows in Figs.3(e) and 3(f)).On observation of the images from 532 nm (Figs.3(b) and (e)) and 1064 nm (Figs.3(c) and (f)), it is evident that the 1064 nm provides a better representation of the tooth surface compared to that of 532 nm.It is interesting to see the curved patterns visible in the tooth are also seen in the PA images.The ripple pattern in the images could be due to the configuration of the illumination and the transducer.The illumination is on the surface of the sample while the transducer is perpendicular to the plane of illumination.The noise is observed more prominently for tooth samples due to the hardness of the sample.Linear array-based PACT systems are very similar to the US imaging systems that is widely used in the clinics.Lateral, axial and elevational resolution of the cyberdyne system used here is 0.3 mm, 0.12 mm and 2.1 mm, respectively.A dental sample with demineralization cavity (affected till the dentine) was imaged (Fig. 4(a)).PA image is interlaid on the US image here.The PA signal shown in green circle (Fig. 4(b)) is from the dentine layer of the tooth.The US/PA imaging is promising for clinical translation because the US image provides the structural information while the PA image gives the information of depth of caries.With better illumination and more averaging, one can even measure the width of the dentine, which is crucial for the root canal treatment planning.Another tooth was imaged using the lPACT had pigmentation along the lingual view (Fig. 4(c)).The PA signal from the pigmentation (Fig. 4(d)) can be seen in the combined US/PA image.This is very promising because this implies that lPACT can be used for identification of hidden caries which are not obviously visible in visual inspection or any other imaging modality easily.
The capability of the US/PA system to image the cavity was compared with CT slice.The cavity was measured from the PA image was 2.8 mm (Fig. 5(a)), while the cavity (red line) measured from the x-ray slice was 2.5 mm (Fig. 5(b)).The slight mismatch in the accuracy of the PA is because of the resolution of the system.This study implies that reliable measurements can be obtained from PA imaging.

Discussion
Various types of photoacoustic imaging systems were used to image different extracted teeth samples.ARPAM imaging was able to image the metal filling with very high contrast.Imaging metal filling with x-ray is challenging due to the artifacts generated by the metal in the x-ray radiographs.Another ARPAM work by Lee et al., imaged a dental implant in an excised porcine jawbone embedded 10 mm under chicken tissue [57].The amplitude of the PA signal was different for the two parts of the implant (fixture and the abutment) due to the different grades of the titanium used in the manufacturing process.PA amplitude was significantly higher for fixture for wavelengths ranging from 680 nm to 1064 nm.These studies show that ARPAM imaging can be used in clinics for studying the dental implants and metal fillings.
Circular scan photoacoustic computed tomography was done for samples which had caries and pigmentation.The images were acquired at 532 nm and 1064 nm.1064 nm images were relatively more representative of the dental structure compared to the 532 nm images.Even though the cPACT has been extensively studied for extracted tooth samples [58], the information about caries obtained in this imaging setup is limited.The imaging system has to be developed further to correlate the anatomy of the teeth with the images acquired.The difficulty to obtain meaningful images from cPACT could be due to the orientation of the transducer.The acoustic signal generated in the center of the tooth (dentine) has to travel the hard enamel to reach the transducer.The circular geometry of the scanning setup makes cPACT inconvenient for in vivo imaging of tooth.Nonetheless this imaging system is ideal for studying either caries or dental fillings in the extracted tooth samples.Cheng et al., acquired PA images of the human tooth using a similar PACT system operated at 532 nm [58].Along with the optical absorption map (the traditional PACT reconstructed image) the authors also reconstructed the spectral maps of the tooth.The spectral maps represent the microstructure and the mechanical properties of the tooth.Teeth being hard tissue high frequency components of the ultrasound signal are attenuated [58].Hence, USTs of lower frequencies are preferrable.The drawback of the cPACT is the low SNR in the images.
Linear array-based PACT which has the ability to acquire US and PA images, does seem promising for dentistry.This imaging setup can identify the caries in dentine.Pigmentation on the lingual view of the tooth can be seen in the PA image.This implies that the hidden caries in the lingual surface can be imaged using linear array probe.Even though, there was commercially available hockey-stick based transducer used for imaging gingiva [46], this is the first time, the traditional linear-array based PA probe with LED has been demonstrated for dental imaging.With further improvements in the LED in terms of the illumination pattern and the wavelength used, clinical imaging of the tooth through the cheek should be possible.
The limitation of this work is that all the imaging systems are explored for very few samples.It would have been ideal to compare all the teeth samples discussed here with x-ray CT images.Due to the limited resources this was not feasible.An interesting comparison would have been to image the teeth before and after the metal filling with the ARPAM to quantitatively measure the boundary between the teeth and metal.cPACT images shows presence of bright spots.Further investigation is required to understand the source of these spots.lPACT being the most flexible in terms of handheld scanning, positioning the probe at any location etc, however, for teeth imaging the current probe size needs to be optimised.The illumination pattern used here is also larger than the size of the tooth.Further experiments are needed with various illumination patterns to improve the image quality.The quantitative measurement of the cavity was done only for one tooth, it would be interesting to compare more PA images and x-ray images of more samples.
The wavelength used for our experiments are 1064 nm, 532 nm and 850 nm.The choice of these wavelengths are due to repeated dental studies done using these wavelengths [20] and easy availability of these wavelengths compared to other wavelengths.The laser energy used to illuminate the sample here is significantly less than the ANSI safety limit.This is confined by the maximum available energy from laser.The SNR of the images is expected to increase with increased laser energy.In the future, we will do statistical analysis on the depth of cavity that each of the imaging systems can detect for the same set of samples.This will require large number of dental samples to be extracted.Once the database is generated for PA images with annotation, one can use machine learning algorithms to improve the quality of the images.External contrast agents based on indirect immunofluorescence staining, using a high-titred polyclonal antiserum can be studied to stain the Streptococcus mutans that cause the caries [59].
Future work of PA in dentistry is crucial for clinical translation.One needs to perform more experiments with ARPAM system to image dental samples with filling.If ARPAM provides significant details of the filling and its boundaries, this imaging setup can be miniaturized and used in clinic.Research has been done to miniaturize ARPAM probes for dermatological imaging [60].Clinical ultrasound probes can be integrated with LEDs of multiple wavelengths to enable multispectral imaging.This would enable us to identify the hidden caries which is demonstrated in Fig. 4. Spectral unmixing of the PA signals from multi-wavelengths will help us identify the health of the gingiva tissue.Estimating the health of the gingiva tissue using multispectral PA would eliminate the need for contrast agent.It is important to understand if PA imaging can be done through cheek tissue.If the pigmentation seen in Fig. 4(d) can be seen even through the cheek tissue, then that would be a potential application of PA in dentistry.
It is important to note that not all caries are dark in color.Since the PA signal strength is dependent on the absorption of light by the pigmentation of the caries, therefore, not all caries will produce strong PA signal.Thus, PA imaging may not be able to detect lightly pigmented caries or cavities due to demineralization.However, with the use of external contrast agent, PA signal strength can be enhanced.If the linear array is replaced with a three-dimensional spherical array and suitable illumination distribution, volumetric analysis of the caries would be feasible.It is expected that with the constant development in the PA systems, the resolution of the systems will improve, and the caries can be detected better.Deconvolution with speckle structure illumination and localization of flowing absorbing particles are some of the ways to improve the axial resolution by 80 times and 30 times in ARPAM.The same techniques improve axial resolution of cPACT systems by 160 times and 78 times [61].Spectral analysis of the caries and gingiva tissue is essential to understand the most efficient wavelength for dental imaging.Based on the observations from this work, there is scope for PA imaging in dentistry.

Conclusion
Photoacoustic imaging of extracted teeth samples was acquired using three different systems.The images were acquired from ARPAM at 1064 nm, cPACT system at 532 nm/1064 nm, and lPACT system at 850 nm.There were dental structures seen in all the PA imaging modalities at these wavelengths.ARPAM can be used for the further research on dental filling.The clinical application is not feasible with the current cPACT set-ups in dentistry due to its scanning geometry and poor image quality.Multispectral imaging of the tooth using linear array PA probe could assist clinicians in identifying the health of gingiva.Statistical analysis must be done for quantitative information from PA images.Further work has to be done to generate a database for developing testing and training data for machine learning and deep learning algorithms.Such algorithms are expected to improve the quality of the dental PA images.When PA is combined with US imaging, one can identify the depth of caries and determine if it has impacted the dentine.This information is essential to decide on the treatment options.

Fig. 1 .
Fig. 1.Schematic of the PA systems used to image the teeth samples.(a) Acoustic resolution photoacoustic microscopy system operated at 1064 nm.(b) Circular photoacoustic tomography system operated at 532 nm and 1064 nm.(c) Linear array-based PAT system operated at 850 nm.

Fig. 2 .
Fig. 2. ARPAM for a dental sample with metal filling.(a) Photograph of the tooth.(b) MAP from ARPAM.

Fig. 3 .
Fig. 3. cPACT images of dental samples with demineralization (indicated with green arrows) and pigmented caries (indicated with blue arrows).(a and d) are the photographs of tooth samples.(b and c) are the cPACT images of demineralized tooth acquired at 532 nm and 1064 nm respectively.(e and f) are the images of pigmented caries acquired at 532 nm and 1064 nm respectively.

Fig. 4 .
Fig. 4. lPACT images of dental samples with a cavity due to demineralization and pigmented caries.(a and c) are the photographs of tooth samples.(b) is the US/PA image of (a).The green circle indicates the PA signal acquired from the dentine of the tooth.(d) is the US/PA image where the PA signal corresponds to the pigmentation.Coloured photoacoustic image is interlaid on gray scale ultrasound images.

Fig. 5 .
Fig. 5. (a) US/PA image of a tooth with pigmented cavity.(b) x-ray CT-slice along the cavity.Cavity (red line) was 2.8 mm in US/PA image and 2.5 mm in x-ray CT image.