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UTILITY OF OPTICAL COHERENCE TOMOGRAPHY- A NOVEL IMAGING TOOL IN PERIODONTICS
Dr. Sowmya Reddy Nandipati1, Dr. Devapriya Appukuttan2, Dr.P.S.G Prakash2, Dr. Sangeetha subramanian2
1Postgraduate, Department of Periodontics, SRM Dental College and hospital, Chennai.
2MDS, Reader, Department of Periodontics, SRM Dental College and hospital, Chennai.
3MDS, Professor, Department of Periodontics, SRM Dental College and Hospital, Chennai.
*Corresponding author: [email protected]
ABSTRACT
Biomedical engineering via technology has revolutionised the field of medicine to such great heights that the quality of health care has been enhanced. Imaging technologies have been a back bone in the field of medicine for diagnostic purposes and for evaluating the treatment outcomes. Multitude of diagnostic aids at hand serve the purpose, but are mostly complicated and are not patient friendly with more limitations. Hence, there is always an incessant quest for versatile advanced technologies to meet the need. In recent years, optical coherence technology (OCT) has been gaining its importance in the field of medicine and research, and has found its utility in the field of dentistry as well. It has been proven to be effective in evaluating enamel cracks, incipient caries, interproximal caries, bonding strengths of the materials, in prosthetic evaluation, in diagnosing the abrasive effects of tooth pastes and many more. Recently, in the field of periodontics it has been proven to be effective in evaluating the gingival conditions, evaluation of the periodontal pockets and evaluation of the peri implant conditions beforehand, making the treatment less complicated much before there is substantial damage to the tooth structures. OCT is expected to be a novel prospective next generation imaging diagnostic instrument and extensive research is being carried out to improvise its clinical use in dental medicine. This review article gives you an insight on the basics of OCT, principles and on the various studies demonstrating the utility of OCT in the field of periodontics.
Key Words: Imaging technologies, interferometry, optical coherence tomography, periodontal inflammation, periodontal disease
Introduction
Optical coherence tomography (OCT) is an advanced diagnostic optical imaging technique which non-invasively creates 2D and 3D images based on time delay and amplitude of back reflected/ scattered light from the tissues/structures. It produces micron scale images with excellent spatial resolution using a single beam low coherence broadband near infrared light source.1 The technique is comparable to an ultrasound but differs with respect to light source instead of sound, lesser tissue penetration and higher image resolution.
It has been recognized as a promising novel technology over other imaging techniques and is widely applicable in several branches of medicine such as cardiology, pulmonology, urology, dermatology, gastroenterology, neuroscience, oncology and more notably in Ophthalmology.
OCT can produce in situ and real time images with axial resolution of 1-15 micron without the need for direct contact with the tissues as it uses light as a medium which is an added advantage. Further, it can be incorporated with other diagnostic / therapeutic tools like
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laparoscopes, endoscopes, surgical probes because of the inbuilt fibre optic system and has excellent clinical feasibility as it is compact and portable. Initially, in the field of medicine it was used in ophthalmology for evaluating the transparent structures in the eye by David Huang in 1989 and the findings were reported in 19912. The first commercially available OCT named “OCT 1000” was marketed by Zeiss in 19963,4 .Slowly, OCT gained wide popularity and was swiftly adopted into clinical medicine due to their safety, reasonable cost, quality and high speed imaging, and therapeutic potential. Further, it is capable of capturing additional tissue characteristics such as blood flow, structural arrangement, birefringence etc via functional OCT extensions such as polarisation sensitive-OCT, dynamic contrast-OCT, doppler-OCT, optical coherence microscopy, optical micro-angiography. For the past few decades, it has also found its application in the field of dentistry as this technique is non- invasive, more sensitive, specific and clinically acceptable5.
In dentistry, it is used for examining the cracked tooth, dental caries, hard and soft tissues of the oral cavity6. This technique can measure accurately only to a very minimal depth of the exposed tissue, therefore to obtain a full image of the tissue structure multiple photographs are often required7. It has found its applicability in the field of periodontics to measure pocket depth, clinical attachment loss, gingival thickness, presence of calculus, biologic width and peri-implant tissues. The unique character of this technique is that it can differentiate minute changes in the structural anatomy in real time thereby helping the clinician to identify the disease in advance than other imaging diagnostics.
One of the added advantages is that using this technique we can avoid the risk of radiation exposure and relief from pain during periodontal probing while measuring the pocket depth8. Furthermore, in dental implantology it is sensitive in identifying the onset of periimplantitis at an early stage.
This review article gives insight on the basics of OCT and application of optical coherence tomography in the field of periodontics and implant dentistry.
History
1665 Robert Hooke – Observation of interference fringes. Colours in white light interference patterns are sensitive to the thicknesses between reflecting surfaces
9.
1879 Albert Abraham Michelson – Measured wavelength of light using international prototype meter and correlated that with white light interferometrics
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1887 Albert Abraham Michelson - Michelson interferometer used in Michelson–Morley experiment
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1971 M. A. Duguay- introduced use of light and optics in the field of medicine
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1972 P. A. Fluornoy- developed white light interferometric thickness gauge
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1978 Caulfield – Applied white light or multi colored interferometry for
calibrating gauge blocks
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1980 Adolf F Fercher and colleagues-started using white light interferometry in the field of medicine
12.
1987 Younquist- Developed calibration technique on gauge blocks using white light interferometry and use of white light interferometry with certain microscope systems
5,11
1990 ICO-15 SAT CONFERENCE- First two dimensional in vivo presentation of human eye fundus along a horizontal meridian based on white light interferometric depth scans
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1990- Naohiro Tanno—development of white light interferometry and named it as “Heterodyne Reflectance Tomography”
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1991 Huang et al- OCT was first developed in Prof. James Fujimoto’s laboratory at Massachusetts Institute of Technology
2
1994 CARL ZEISS MEDITEC—obtained patency for OCT
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1995 Groot and Deck—Practical implementation of use of white light interferometry with microscopes
1996- First generation OCT commercially available “OCT 1000”
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1998 Bill W. Colston and colleagues- First imaging of dental hard and soft tissues using OCT on porcine models
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1998-Warren et al provided details of the tooth structure along the vertical axis using OCT
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2000 Otis et al – suggested the use of OCT for dental applications
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2000- Commercially available second generation OCT“OCT 2000”
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2001 Wojtkowski et al- presented first in vivo SD OCT
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2002 –Approval of SD-OCT system for clinical use by US FOOD AND DRUG administration
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2004 Montgomery P- used white light interferometry for measurement of optical assemblie
14Types Of Scans
A SCAN/AXIAL SCAN/DEPTH SCAN: This scan is also known as the axial scan. This is obtained by focusing the light beam on to a point i.e surface of the sample to be tested and is recombined with the reflected light from the reference mirror. The information obtained will determine the depth of the tissue.4,10,15
B SCAN/LONGITUDINAL SCANS: These scans are generated by collecting multiple single axial scans both linearly and transversely across the tissues. They not only give the information on the depth of the tissue but also will give the lateral/angular details of the tissue. Multiple such scans can help in formation of a 3D structure.6,15
T SCAN: These scans are known as the enface scans. These scans are produced by transversally scanning the surface when the reference mirror is fixed resulting in producing lateral and angular details of the images. Multiple T scans will result in the formation of C SCAN which is also known as transverse slice scan. 15
Principle Of OCT
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The principle on which the OCT works is Michelson’s Interferometer. The light from the source when passed through the beam splitter divides into two equally energized beams where one beam hits the Mirror 1( reference arm which is fixed) and the other beam hits mirror 2( sample arm which is mobile and where we place the sample).These two beams after hitting the mirrors M1 and M2 gets reflected in the same path with slight deviation and they both combine at the coupler where it results in the formation of the interference pattern and this pattern will be transferred to the photoelectric device for image formation. Image formation depends on the type of the interference pattern that is produce i.e. either constructive or destructive interference. Constructive interference occurs when wavelengths of both the reflected beams are equal resulting in image formation, whereas when the wavelengths vary there would be a destructive interference pattern and no image will be formed.1
6
FIGURE 1: Principle of OCT (Michelson’s Interferometer) If L1=L2; Constructive Interference -Image formation takes place
If L1≠L2; Destructive Interference- there will be no formation of image
FIGURE 2: CONSTRUCTIVE INTERFERENCE
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FIGURE 3: DESTRUCTIVE INTERFERENCE
Types Of OCT:
1. TIME DOMAIN OCT (TD-OCT) 2. FOURIER DOMAIN OCT (FD-OCT)
Time Domain OCT:
In the TD-OCT, an image will be formed if the interference pattern is constructive and to be in constructive interference the wavelength of the split arms should be equal to the wavelength of source light. A scan of the sample occurs resulting in 2D images in this type of OCT. The Reference arm is not stationary in TD-OCT17. The images are obtained but are of greater noise making the images less clear which led to the invention of FD-OCT18.
FIGURE 4: IMAGING USING OCT
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Fourier Domain OCT:
Consists of two types:
1. Spectral Domain OCT (SD-OCT) 2. Swept source (SS-OCT)
Spectral Domain OCT (SD-OCT)
This is also similar to TD-OCT but the major difference is that the reference arm is fixed, thereby providing high resolution images with less noise. The depth of the scan is also greater than TD-OCT. The type of scan is B scan and the images obtained are 3D images10.
Swept Source OCT(SS-OCT)
It is the latest advancement which is an extension of TD-OCT and FD-OCT. This is also known as Optical Fourier Domain OCT19. The major difference from the other two is that, it uses a broad band light source which provides a facility to produce an output with time varying wavelength instead of producing the whole spectrum at the same time. It uses a single detector so that the interference signals can be captured continuously19
Application of OCT in The Field of Dentistry
In the field of dentistry, OCT has become a reliable tool, more idealistic as it is an easy chair side diagnosing tool for the dentists because of its handy usage. It is used for in-depth visualization of the tooth structures and to detect and resolve the problem in the initial stages itself. The unique feature of OCT is that it can image through saliva, water and even plaque.
It also has the ability to image minute structural changes of the oral mucosa, hard and soft tissues of the oral cavity in the field of conservative dentistry it is highly accurate in evaluating the early dental caries which are not visualized using the radiographs. 11 It can help in evaluating the extent of carious lesions and also accurate in differentiating stains, enamel dysplasia, presence of active decay, early enamel caries, sensing of tooth micro leakage, evaluation of micro gaps, examining white spot lesions and identifying enamel cracks. It is helpful for evaluating the remaining dentin thickness in crown preparations. In endodontic therapy, OCT is highly effective in detection of the root canals and makes root canal treatment less complicative. The OCT imaging is also useful in evaluating the hard and soft tissue changes of the oral cavity.
Shimada et al in 201520, evaluated the effectiveness of SS-OCT in detecting proximal caries and it proved to be a more reliable tool not only for the detection of the proximal caries but also the depth of the carious lesion compared to bitewing radiographs. Maia et al in 201621, assessed the enamel caries using OCT and compared it with the quantitative light induced fluorescence and found out that OCT is effective in detecting early enamel caries. Cassimiro
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Silva et al in 201622, demonstrated that OCT can be useful in predicting the severity of enamel erosion that is induced by different commercially available toothpastes.
de Oliviera Mota et al in 201723, evaluated the ability of SD-OCT and SS-OCT in diagnosing dentinal apical root cracks when compared with micro-CT and suggested that they can be used as a useful tool for diagnosing apical microcracks. SS-OCT proved to be effective in identifying pulp exposure that can arise during access preparation using Er:YAG laser24
Carla Andree Madaras et al25 suggested that OCT technology can be an early diagnostic tool for detecting faults in the structure of the ceramic crowns before being luted over the crown preparations. Further, it can be utilised to evaluate the defects / gaps at the implant-abutment interface thereby increasing the success of dental implant therapy26
OCT in the Field of Periodontics OCT imaging for soft tissues:
OCT has been gaining its importance in all the fields of dentistry, particularly in the field of periodontics it has been effective in examining the changes in the gingival structures, for estimating probing pocket depths, identifying plaque and calculus, to differentiate calculus from the tooth structures and also in detecting early tissues changes in peri-implantitis conditions.
Sven Prestin and colleagues27, evaluated the thickness of oral mucosa at 7 different sites (anterior floor of the mouth, lateral floor of the mouth, anterior palatal arch, palatine uvula, lateral tongue, hard palate, buccal mucosa) using NIRIS type of OCT system. Their findings demonstrated that OCT can be an efficient, non-invasive and convenient alternative to painful excisional biopsies for diagnosis of malignancies.
Claudia Mota and colleagues carried out animal and human studies in three stages to demonstrate the utility of OCT as a diagnostic and monitoring tool for evaluating periodontal tissues in health and disease and to evaluate the disease activity following periodontal therapy respectively. Mota et al,201528 in an ex-vivo study on porcine jaws compared the effectiveness of two different types of FD-OCT systems operating at different wavelengths (1325 nm SS- OCT, 930 nm SD-OCT) and demonstrated their ability to identify free gingiva, attached gingiva, calculus, gingival thickness and gingival sulcus depth. The gingival thickness and sulcus depth could be measured from 0.8mm up to 4 mm. These authors suggested that OCT with greater wavelength i.e. 1350 nm has better tissue penetration and diagnostic abilities.
The researchers further demonstrated that SS-OCT (1325nm) was more accurate in measuring the gingival sulcus depth when compared with traditional probes namely North Carolina manual probe and Florida automated probe. The average sulcus depth measured on the healthy buccal sites of anterior teeth using SS-OCT by two calibrated examiners was 0.85⩲ 0.27mm and 0.87⩲ 0.27mm respectively. Further a significant finding was that there was no discomfort or pain associated with OCT imaging for sulcus depth estimation. They suggested that OCT has the potential to be a reliable tool for in vivo periodontal tissue evaluation and for reproducible sulcus depth measurements.29 OCT as an auxiliary tool for detecting changes in the periodontium following periodontal therapy was demonstrated based on their findings in patients with periodontitis.30Comprehensive findings from their extensive research suggested that OCT was able to precisely measure the periodontal pockets depth,
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capture the calculus and/or biofilm deposition, and determine the thickness of inflamed soft tissues.
Diagnostic accuracy of OCT was further evaluated by Jin young et al in 201731, wherein they compared the ex vivo images of gingival sulcus depth measured using Dental OCT, micro CT system and histological sections. Their findings suggested that Dental OCT can provide high resolution images up to a depth of 1.2mm-1.5mm from the surface, however blurred or improper images of structures were captured at deeper levels. They concluded that there is a need for further development of the OCT device with respect to the image scanning range.
Meng Tsan Tsai et al in 201732, developed a handheld OCT for in vivo visualizations of the microstructural and microvascular features of oral mucosa. Scanning probe was developed that consisted of a probe body fabricated by a 3D printer, miniaturized two-axis galvanometer, relay lenses, and reflective prism.Their results showed that the epithelium and lamina propria layers, fungiform papilla and salivary gland were differentiated. Moreover, various microcirculation features at different mucosal sites were identified that are potentially effective indicators for the diagnosis of premalignant lesions. Consequently, the results indicated that the developed hand-held OCT system is a promising tool for non-invasive imaging of oral mucosa.
Kakizaki et al33 evaluated the effectiveness of OCT in examining the periodontal tissue profiles and observed that the penetration and reflection were limited to a certain depth of 1.5mm. They noticed that some images were not clear even at the depth of 1.5mm. However, OCT seems to be a promising tool for non-invasive observation of the periodontal tissue profile and measurement of internal periodontal structures including biologic width in the anterior region. Gingival tissue inflammation was evaluated in in vitro conditions based on pixel densities by Petra Surlina et al34 in gingival tissue samples from patients with periodontal disease; patients with periodontal disease and a systemic comorbidity;
periodontal and systemic healthy patients. 2D images of the samples were obtained using OCT and the images were assessed using a software that allowed the quantification of pixels on a given segment in the epithelium. It was observed that the gingival tissues with periodontal conditions showed less pixel density compared to healthy tissues.
OCT for detection of plaque and calculus
The primary objective in periodontal therapy is complete removal of plaque and calculus, however most often thorough debridement is never accomplished. Examination of calculus remnants and inadvertent cementum removal via tactile perception of the subgingival area is occasionally unreliable. A novel non-invasive method for measuring plaque and calculus against the classic probing technique was attempted by Yao Sheng Hsieh et al in 201135 wherein they used SS-OCT imaging. It was effective in detecting calculus and could differentiate between caries and calculus. Later, Meng Chun et al36 demonstrated the usage of a miniature endoscopic optical coherence tomography (0.9mm) which is a scanning fibre probe used with SS-OCT to examine healthy enamel and dental calculus. It was found to be effective and the size of the probe was an added advantage for insertion into the gingival sulcus. SS-OCT has been found to be effective in detection of calculus and root cementum only up to a certain depth based on evaluation both in invivo and invitro conditions by Masaki Tsubokawa etal in 201837. Later in 2019, an in vitro evaluation of calculus imaging on the root surface using SD-OCT was performed on extracted periodontally affected teeth by Felis Krause38. The teeth were sectioned and images were taken under wet (blood, saline) and dry
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conditions. The images obtained were clear and irrespective of the medium used for imaging calculus could be differentiated.
OCT imaging for dental implants
Ionita and Reisen in 200939, imaged the Osseointegrated dental implant interface using FD- OCT with swept source in dog mandible in vitro. Based on their findings they suggested that the images obtained with 1325nm laser source SS-OCT showed significant contrast and spatial resolution to evaluate either bone adherence or resorption around the dental implants indicating its utility invitro for peri-implant tissue evaluation. Likewise, in 2016, Bordin and colleagues40 evaluated the ability of OCT images to identify early signs of peri-implant mucositis in minipig models. They observed that OCT images correlated well with the histologic sections following experimental ligature induced peri-mucositis and could serve as a valuable tool in early interception of implant failures. Sanda et al in 201641, studied the effectiveness of OCT images in identifying residual luting cement around dental implants and also its ability to visualise implants embedded in jaw bone under mucosa of varying thickness. Nonetheless, OCT showed limited ability in identifying implants under mucosal thickness of more than 1 mm and in visualising excess cement under mucosal thickness of more than 3 mm, it could be a potential effective instrument for diagnosis of peri-implant pathologies. Sulhee Kim et al in 201842, using porcine mandibles with intentionally created dehiscence defects followed by implant placement demonstrated that OCT images can be used to identify peri-implant bone level and loss. Furthermore, the OCT measurements closely correlated with that of a digital calliper indicating its utility as a potential diagnostic non -invasive tool with high resolution and accuracy.
Discussion
OCT has been an effective tool in accurate visualization of both hard and soft tissues thus providing a clear view on physiological and pathological structures of the tooth. However, the drawback is that it has a very low visual accuracy range when compared with other diagnostic technologies. It has the ability to record even minute alterations in the structures than any other diagnostic instruments but it is able to image only a few mm each time making it difficult to use for larger abnormalities, as more images are required for getting a clear view of the entire defective dental structure. The invention of OCT probes for oral application has made it more advantageous for imaging intraoral structures and more convenient for patients. It is highly effective in detecting minute changes in the peri implant tissues thereby playing a crucial role in early diagnosis and preventing implant failure. It is not a complicated or technique sensitive procedure and can be used easily by any clinician on the chair side. The amount of coherently backscattered light in highly turbid media decays exponentially with depth, resulting in axial compression of the image. For this reason, the OCT axial depth scale should be calculated by dividing the refractive index of the relevant tissue regions, such as 1.3 for oral mucosa, 1.6 for enamel and gum, 1.5 for dentin and 1.4 for gingival sulcus, to obtain true physical dimensions.
OCT is one of the most important imaging modalities in bio photonics. It has been widely used in Ophthalmology as it provides optical biopsies with micrometre spatial resolution.
OCT being very effective in several biomedical fields has been used drastically for evaluating its use in the field of dentistry. It has been effective in visualization of the cracks, early enamel caries, interproximal caries and evaluating the bonding effectiveness of the brackets, evaluating the prosthetics etc. In the field of periodontics, periodontal probing and evaluation
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of the periodontal status manually usually is not a very accurate process as it is prone to errors and also a discomfortable examining procedure. This leads to failure in the identification of disease at the initial stage itself. So there is always a need for alternative techniques which could be ideal for examining and detection of the disease in the early stage.
Although there are some technical limitations to OCT, such as light penetration depth and scan window lower than the size of the pockets, it is non-invasiveness and can be used on the chair side. Therefore, looking at the advantages if the technology can be further improvised eliminating the drawbacks OCT, it would be a game changer in the field of periodontics and in the near future can replace the current diagnostics and maybe can evolve into a 6th generation periodontal probe.
Acknowledgement – Nil Conflict of interest: Nil
The manuscript has not been published and is not under consideration for publication in any other journal .
All authors approved the manuscript and its submission to the journal.
The manuscript has been submitted solely to this journal and is not published, in press, or submitted elsewhere. All the research meets the ethical guidelines, including adherence to the legal requirements of the study country.
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