Near-infrared image-guided laser ablation of artificial caries lesions

Abstract

Laser removal of dental hard tissue can be combined with optical, spectral or acoustic feedback systems to selectively ablate dental caries and restorative materials. Near-infrared (NIR) imaging has considerable potential for the optical discrimination of sound and demineralized tissue. The objective of this study was to test the hypothesis that two–dimensional NIR images of demineralized tooth surfaces can be used to guide CO₂ laser ablation for the selective removal of artificial caries lesions. Highly patterned artificial lesions were produced by submerging 5 × 5 mm² bovine enamel samples in demineralized solution for a 9-day period while sound areas were protected with acid resistant varnish. NIR imaging and polarization sensitive optical coherence tomography (PS-OCT) were used to acquire depth-resolved images at a wavelength of 1310-nm. An imaging processing module was developed to analyze the NIR images and to generate optical maps. The optical maps were used to control a CO₂ laser for the selective removal of the lesions at a uniform depth. This experiment showed that the patterned artificial lesions were removed selectively using the optical maps with minimal damage to sound enamel areas. Post-ablation NIR and PS-OCT imaging confirmed that demineralized areas were removed while sound enamel was conserved. This study successfully demonstrated that near-IR imaging can be integrated with a CO₂ laser ablation system for the selective removal of dental caries.

2. INTRODUCTION

New optical caries imaging systems have been recently developed that are ideally suited to interface with lasers for the selective removal of dental caries. The most promising caries imaging systems employ fluorescence and near-IR imaging methods, including quantitative light fluorescence (QLF)¹, near-IR fluorescence², polarization sensitive-optical coherence tomography (PS-OCT)^3–5 and near-IR (NIR) imaging ^4,6,7. These imaging systems can be used for the acquisition of 2-D or even 3-D images of tooth demineralization on proximal and occlusal surfaces. Lasers have been used for many years for industrial marking and computer aided design/machining (CAD/CAM) and high-speed scanning systems are in routine use. It is feasible that similar high-speed computer controlled scanning systems that are integrated with a compact delivery system can be used to scan a laser over tooth occlusal surfaces to selectively remove either dental caries or composite restorative materials. In previous studies, we have already demonstrated that lasers can be scanned over tooth surfaces to remove dental composite and pigmented caries using acoustic and spectral feedback to differentiate these materials from sound enamel^8,9. However, these approaches proved to be unsuccessful for non-pigmented caries⁹. In this paper we have coupled NIR and PS-OCT imaging of simulated caries lesions with a high-speed scanning CO₂ laser ablation system to demonstrate that demineralized enamel can be imaged with high contrast and selectively removed by the laser.

Fluorescence based caries imaging and detection methods have been under development for more than 20-years and commercial systems such as QLF and the Diagnodent are readily available. Kavo^™ (Biberach, Germany) which sells the Diagnodent^™ and Er:YAG laser systems has recently introduced an Er:YAG laser hand-piece with an integrated fluorescence feedback system ¹⁰. The operator can monitor the fluorescence signal to determine when all the caries have been removed. Although this is a significant step forward, the fluorescence signal is non-localized and can arise from stains in the pits and fissures and does not correlate with the mineral loss. Therefore such an approach is not promising for integration with a scanning system.

At the wavelength of 1300-nm in the near-IR, healthy enamel appears transparent and does not strongly scatter light whereas demineralized enamel and carious lesions manifest reduced transmission and strong light scattering respectively¹¹. Therefore, demineralized enamel can be imaged with high contrast for discrimination from healthy enamel. NIR imaging can be used to acquire 2-D images of demineralized enamel on proximal and occlusal surfaces while PS-OCT is capable of acquiring depth-resolved images of demineralization so that 3-D images can be generated. The polarization sensitivity is necessary to remove the strong surface reflections from tooth surfaces for higher contrast images of demineralized areas³. These near-IR imaging methods can also be used to image demineralized enamel under composite for removal of secondary caries lesions⁵.

Image-guided procedures employing angiography, computed tomography (CT) or MRI are routinely used to acquire pre-operative or real-time medical images during treatment to minimize trauma and improve outcomes. A similar approach is feasible for dental surgery exploiting new optical imaging tools and lasers for selective and conservative removal of caries and restorative materials with an emphasis on the preservation of healthy tissue structure. The advantages of early detection and minimally invasive removal should improve the outcome of dental treatments. To implement a prototype of image-guided laser ablation system, we have divided the process into three phases: image acquisition, analysis, and treatment. Specifically, we will utilize NIR and PS-OCT images to generate optical maps of the caries lesions. In turn, the optical maps translated the NIR images into instructions to implement a computer-controlled CO₂ laser system to selectively remove lesions at a uniform depth.

The objective of this pilot study is to develop a mapping system to align and calibrate two-dimensional NIR images to guide the CO₂ laser ablation system to selectively remove artificial demineralized lesions on tooth surfaces. With the development of the prototype, we aim to test the hypothesis that using the image-guided approach, highly patterned artificial lesions were removed selectively with minimal damage to sound enamel areas

3. MATERIALS AND METHODS

3.1 Sample Preparation

Ten blocks, 5 × 5 × 2 mm³, of bovine enamel were prepared from extracted bovine incisors acquired from a slaughterhouse. The samples had a layer of enamel approximately 1-mm thick over a layer of dentin. Surfaces were serially polished with 12, 9 and 3 μm embedded diamond polishing discs. A 3 × 3 grid was cut into the enamel surfaces of the samples to aid in the creation of patterned artificial lesions using a laser. The dimension of the each grid cell is 1.5 × 1.5 mm². Patterned artificial lesions were created by applying a thin layer of acid resistant varnish to sound enamel areas for protection before submersion in a demineralization solution. Artificial lesions were formed by exposure of the teeth for 9 days to a 40-mL aliquot of acetate buffer solution containing 2.0 mmol/L calcium, 2.0 mmol/L phosphate, and 0.075 mol/L acetate maintained at pH 4.9 and a temperature of 37°C. The demineralization solution produced lesions to a depth of approximately 100-μm on the exposed enamel smooth surfaces.

3.2 Near Infrared Imaging (near-IR)

Each sample was imaged using 1310-nm light using the system described in reference¹². Light from a single-mode fiber-pigtail coupled to a 1310-nm superluminescent diode (SLD) with an output power of 15-mW and a 35-nm bandwidth, Model SLED1300D20A (Optospeed, Zurich, Switzerland), was coupled to a 20-mm NIR fiber-collimator (μLS Micro Laser Systems, Garden Grove, CA). Samples were placed in the 20-mm collimated beam between the light source and imaging system with the enamel side facing the camera. We found that broadband SLDs were advantageous to avoid speckle. An InGaAs focal plane array (FPA) (318 × 252 pixels) the Alpha NIR^™ (Indigo Systems, Goleta, CA) with an Infinimite™ video lens (Infinity, Boulder, CO) was used to acquire all the images. The acquired 12-bit digital images were analyzed using IRVista^™ software (Indigo Systems, Goleta, CA) ¹³. Areas of demineralization appear opaque in the NIR images.

3.3 Polarization Sensitive Optical Coherence Tomography (PS-OCT) Systems

An all single-mode fiber autocorrelator-based Optical Coherence Domain Reflectometry (OCDR) system with polarization switching probe, high efficiency piezoelectric fiber-stretchers and an InGaAs receiver that was designed and fabricated by Optiphase, Inc., Van Nuys, CA was used for these measurements. A description of the scanning autocorrelator is described in reference ¹⁴. The OCDR was integrated with a broadband high power superluminescent diode (SLD) (Denselight, Jessup, MD) with an output power of 45-mW and a bandwidth of 35-nm and a high-speed XY-scanning system (ESP 300 controller & 850G-HS stages, National Instruments, Austin, TX) for in vitro optical tomography. The probe was designed to provide a spot diameter of 50-μm for each scan. Additional details of this system are described in references ^14,15.

3.4 Ablation apparatus

A transverse excited atmospheric pressure (TEA) CO₂ laser, Impact 2500 (GSI Lumonics, Rugby, United Kingdom) operated at λ=9.3 μm, was used to irradiate tooth samples with a fluence of 30 J/cm², and an energy of 14 mJ per pulse and a pulse duration of 16 μs. The laser energy was calibrated and measured using a laser energy/power meter, EPM 1000, Coherent-Molectron (Santa Clara, CA) with a ED-200 Joulemeter from Gentec (Quebec, Canada). The laser was focused with a planoconvex ZnSe lens of 100-mm focal length to a beam diameter of approximately 500-μm. The laser beam diameter (1/e²) at the position of irradiation was determined by scanning with a razor blade across the beam. Two and three dimensional images of the laser spatial profile was acquired using a Spirocon Pyrocam^™ I pyroelectric array (Logan, UT). Both laser beam profile and spatial profile showed that the laser was operated in a single spatial mode, Gaussian spatial beam. Incisions were produced by scanning the laser in spots spaced at 60-μm intervals, each spot receiving 10 pulses at a repetition rate of 300-Hz. A computer-controlled stage was used to create controlled movement of the samples for the incisions. A low volume/low pressure air-actuated fluid spray delivery system consisting of a 780S spray valve, a Valvemate 7040 controller, and a fluid reservoir from EFD, Inc. (East Providence, RI) was used to provide a uniform spray of fine water mist onto the enamel surfaces at 2 mL/min.

3.5 Experimental procedures

Selective removal was accomplished in six distinct steps described in the flow chart below in Fig. 1.

Fig. 1.

Flowchart for selective removal.

Step 1: Generate patterned artificial lesions

Patterns of artificial lesions were generated on 10 bovine blocks. For details in sample preparation, see section 2.1.

Step 2: Capture pre-ablation images

Pre-ablation NIR and PS-OCT images were captured, Fig. 2. In NIR images, lesion areas appeared more opaque than sound tissue due to decreased transmission of light and increased light scattering at the lesion site. PS-OCT images were acquired at three positions across each sample (optical cross-sections) to include all data in the patterned lesions. In PS-OCT images, lesion areas manifested high reflectivity due to the higher scattering from the porous demineralized lesion areas.

Fig. 2.

Pre-ablation NIR and PS-OCT images

Step 3: Image analysis

An image processing module was developed using Matlab programming language to analyze the 12-bit NIR images. A gray-scale NIR image with the dimension of M × N pixels is represented as an M by N matrix containing values from 0 to 2¹²-1. The number zero represents black while the number 2¹²-1 represents white. Given the matrix representation of the gray-scale image, an NIR output was reconstructed as a 3D graph (Fig. 3) with the x and y coordinates identifying the position of the pixel and the z coordinate representing the gray-scale value of each pixel.

Fig. 3.

False color 3-D representation of the gray-scale NIR image and threshold determination based on the gray scale level.

An appropriate threshold was set to distinguish between the sound and lesion areas based on the original gray scale values.

Step 4: Generate optical maps

Based on the appropriate threshold, the image processing module traversed through each pixel of the image to identify the pixel as sound or demineralized. When the z value was less than or equal to the threshold, the tissue was classified as part of the lesion (Fig. 3) and was demarcated for selective ablation in the treatment phase. In contrast, when the z value was greater than the threshold, the pixel was marked as sound tissue and would be omitted from ablation. From the initial M × N matrix of the gray-scale image with value ranging from 0 to 2¹²-1, the image processing module translated the NIR image to a binary M × N matrix, namely the optical map containing either value of 0 or 1. The maps provided instructions to the laser ablation system to differentiate between sound and demineralized tissue and, the laser system subsequently utilized this information to achieve selectively removal of the caries tissues, Fig. 4.

Fig. 4.

Simplified optical map at low resolution.

Step 5: CO₂ laser programmed to selectively ablate the carious lesions

A program, written in the Labview^™ programming language (National Instruments, Austin TX) utilized the optical maps as the input parameters, and controlled the laser and scanning system to scan the samples across the laser beam. Depending on the value in the optical maps, the laser would either ablate or take no action at a given pixel. Each pixel position was separated by approximately 60 μm apart.

Step 6: Post-ablation NIR and PS-OCT images

Post-ablation NIR and PS-OCT images were captured to assess the selectivity of removal.

4. RESULTS

Post-ablation NIR and PS-OCT imaging confirmed that lesion areas were removed while sound enamel was conserved with minimal damage to sound tissue. Lesions areas in pre-ablation NIR images manifested more opacity than sound tissue (high-contrast) whereas the ablated area in the post-ablation NIR images exhibited similar or more translucency compared to the sound tissue (low-contrast), Fig. 6. PS-OCT images were acquired at three positions across the patterns to include all data in the patterned lesions. In pre-ablation PS-OCT images, carious tissue displayed strong surface reflections due to its light scattering property. The post-ablation PS-OCT images illustrated the removal of the lesions with uniform depth, Fig. 7. Optimal removal of lesions with conservation of sound tissue was achieved by increasing the resolution of the optical map, Fig. 8. The sound tissue is a circular area that was best approximated with a higher resolution optical map.

Fig. 6.

Near-Infrared (NIR) pre-ablation and post-ablation images for the artificial lesion patterns of Fig. 5.

Fig. 7.

PS-OCT pre-ablation and post-ablation images.

Fig. 8.

Improved accuracy by increasing resolution.

5. DISCUSSION

Post-ablation NIR and PS-OCT images confirmed that demineralized enamel was selectively removed with minimal damage to sound tissue. This pilot study successfully demonstrated that near-IR imaging can be integrated with a CO2 laser ablation system for the selective removal of dental caries. The next step is to demonstrate that various depth lesions can be removed with an iterative process of the uniform depth removal, and to verify that this approach can be applied to natural occlusal caries in the pits and fissures. Further studies will also focus on the development of an integrated system for the real-time control of ablation depth and measurement of remaining caries depth.

Fig. 5.

Artificial lesion patterns.