From Volume 41, Issue 2, March 2014 | Pages 174-180
An update and overview of the use of optical coherence tomography (OCT) in dentistry is described. Specific aspects discussed include the evolution of the technology and the basic process of light beam interference used to obtain OCT images. In addition, aspects of the optical properties of dentine and enamel and the range of current diagnostic applications of OCT in dentistry are reviewed.
While ionizing radiation has generally been regarded as the dominant imaging modality able to provide useful imaging information within the field of dentistry, there is increasing interest in the use of optical coherence tomography (OCT) to provide unique imaging information relating to dental structures. Significant early advances in OCT technology for medical imaging were undertaken at the Massachusetts Institute of Technology and the Massachusetts General Hospital.1,2,3 The basic principle utilized of splitting, manipulating and recombining beams of light has been known in the world of physics since around 1881, with the invention of the Michelson interferometer.4 OCT has made greatest clinical impact in ophthalmology where it has become the method of choice for scanning of retinal structures. OCT technology was initially developed5 for internal inspection of precision fabricated optical structures and such industries continue to drive the core technologies of medical OCT applications.
OCT produces images by using optical radiation in some form of ‘echo sounding’ capacity where the image that is produced utilizes elements of light reflected or scattered from structures within the scan volume. A basic problem, however, associated with light in biological tissue is where photons tend to experience numerous scattering events before they are eventually absorbed, which causes the number of photons that can provide ‘echo’ information to decrease rapidly with penetration depth. This can limit image depth to 2–3 mm in hard dental structures, in contrast with the properties of x-ray photons, which are much less readily scattered.
In the classic ‘time domain’ OCT technique (Figure 1), light is split into a ‘sample arm’ for the sample being investigated and a ‘reference arm’ which is dynamically made to behave like a reflecting surface at varying depth. The returning signal from the ‘sample arm’ and the ‘reference arm’ are fed into a ‘co-incidence’ unit which registers an echo signal when they coincide. A classic way to implement the ‘reference arm’ is to include a mechanically driven mirror which oscillates at a high frequency. Such a system, however, suffers from limitations of driving a mechanical system and line scan rates are typically limited to around 10 kHz. Initially, OCT systems were developed using ultra short laser pulses. Fujimoto et al, for example, described3 a system where 65 femto second pulses from a specialist laser were passed into a sample in one optical line, and reflected pulses produced from within a sample detected against reference pulses which were made to undergo a delay by mirror displacement. The corresponding physical path length of light at this short value of pulse duration is around 20 microns.
A significant development within OCT has been the application of Fourier domain techniques, which allow essentially much higher scan speeds to be undertaken and with greater signal sensitivity. Within the Fourier domain approach, the methods of ‘spectral’ domain OCT and ‘swept source’ OCT are current. In the technique of spectral domain OCT (Figure 2), signals are diffracted into wavelength components and data read rapidly from a charge coupled detection array. In Figure 2, the combined signal of the reference beam from reference mirror and the signal beam from sample is split into spectral components by a spectrometer system with output read from a charge coupled detector array. A Fourier transform of the photodiode array signals reconstructs the scan information from within the sample.
In the swept source devices (Figure 3), the light within the reference laser system is dynamically swept through a range of wavelengths. In Figure 3, the swept source detector, a single photo detector, sums the signals from the direct mirror path and that of the sample reflections. This interference signal is processed by a fast Fourier transform to reveal the detail of reflections from interfaces within the sample. As this ‘A’ scan is repeated at different locations, a two-dimensional cross-section image is established. A three-dimensional image structure can be constructed by suitable translation of the sample beam.
The essential advantage of Fourier domain OCT over conventional time domain OCT is based on both speed of scanning and signal sensitivity. Fourier domain OCT systems essentially have no moving parts. Comparing the methods of spectral domain and swept source OCT, both methods have essentially comparable sensitivity gains over conventional time domain OCT, though swept source technology tends to have greater sensitivity in the infra-red wavelength range between 1 and 1.5 microns, which is also the region favoured for light penetration in hard dental tissues.
A key phase of development of OCT technology, however, has been to use much less specialist light sources and also alternative signal detection systems. Sections of a beam of light can be considered to have a given coherence length where wave oscillations over a section of a beam correspond to sequences of specific wavelengths which oscillate with similar phase. Normally, a high quality laser is associated with a long coherence length of many thousands of whole wavelengths. For optical coherence tomography, however, it is desirable that the light source should have a short coherence length – typically over a few dozen whole wavelengths, which in some ways is equivalent to the pulse length of an ultra short pulse laser. It is this physical length of coherence which largely determines the axial resolution of optical coherence tomography.
Light can be considered to have two separate polarizations at right angles to each other. Where a specific material presents a different velocity of light for each polarization direction, this phenomenon of bifringence presents as another optical property which can be exploited by OCT imaging techniques. Data from OCT images can be derived from separate polarization signals and polarization insensitive signals where both oscillations are combined and, in addition, the relative phase changes arising from bifringence within specific areas. Initial work by Baumgartner et al6 indicated that additional information was potentially available by determining the extent of polarization changes in OCT images using a Superluminescent Diode (SLD) at 830 nn with a bandwidth of 30nm. For the specific system used, however, the potential additional information was considered to have been degraded by the relatively poor penetration depth achieved.
Subsequently, work reported by Fried et al7 has indicated that techniques involving selective polarization have the ability to provide additional information at wavelengths providing increased penetration depth. Specific importance is attached to signal propagation in the ‘fast axis’ component of polarization where the effect of intense reflectance from the air-enamel boundary improves signal discrimination within the enamel surface. Elements of such polarization can be induced due to native bifringence of material and also the scattering from anisotropic structures. In general, further work remains to be undertaken to relate the changes in optical properties of tooth structures to optimize imaging techniques employing OCT.
Changes in the optical properties of tooth enamel and dentine occur as a result of tooth demineralization associated with caries progression. In addition, the presence of bacteria associated with dental plaque gives rise to porphyrins which produce fluorescence upon excitation with red light and has been the principle of systems to detect such areas on tooth surfaces.8 The intrinsic fluorescence properties of enamel and dentine vary also with the extent of demineralization associated with caries progression. The so-called QLF (quantitative laser fluorescence) method has also been investigated as a means of in vitro quantitative measurement of caries progression,9 though in general OCT is anticipated to be able to provide more useful diagnostic information relating to the nature and extent of such lesions. Various in vitro studies, for example, have correlated the reflectivity of signal with the depth of lesion detected.10
It is the optical properties of enamel and dentine and the associated disease states of these materials which are of key relevance for selecting the optimum wavelength for OCT in dentistry. The transmission characteristics of both enamel and dentine are dominated by the scattering characteristics rather than the absorption characteristics. In OCT images, the enamel tends to provide for reduced scattering, with dentine presenting the appearance of increased signal due to higher scattering. Photons of light can much more likely be removed from the ‘prompt’ outgoing beam and corresponding returning beam as a result of scattering than absorption. Also, the effective transmission characteristics have also to be related to the spot size of the beam used for such measurements. While smaller spot sizes lead to improved lateral resolution, beam signal within a narrower beam decreases generally more rapidly than a wider beam. For enamel, the scattering coefficient is proportional to the inverse cube of the wavelength, while dentine demonstrates no significant wavelength dependence. Observations by Otis et al11 of separate system operating at 850 nm and 1310 nm indicated that images obtained at 850 nm tended to be poor due to insufficient penetration of signal into teeth. The specific refractive indices of enamel and dentine12 are usually identified as 1.63 and 1.45.
Currently, work is in progress at King's College London Dental Institute to evaluate the use of OCT as a means of management of dental implants at risk of failure. Using the VivoSight multi-beam OCT scanner system of Michelson Diagnostics (UK) Ltd (Orpington, Kent), it is anticipated that the ability of OCT to take an image of soft tissue infrastructure, including blood vessels, when combined with conventional x-ray and cone-beam computerized tomography, will significantly improve the management of ‘at risk implants’.
One of the main application areas for OCT dentistry has been variously described as that of caries detection where as a ‘non-contact’ method it has obvious advantages, especially with children. A study, undertaken by Darling et al,13 measured the optical characteristics of a range of samples of natural and artificially demineralized dental enamel at 1310nm and compared these with values of mineral density derived using an ultra high resolution digital microradiology system. This indicated that the optical scattering coefficient increased exponentially with increasing mineral loss. Within the range of demineralization investigated from both natural and artificially demineralized dental enamel, a difference in scattering coefficients of two orders of magnitude was observed.
Confirmation of the use of high-speed swept source OCT technology for the detection of carious lesions has been confirmed by Shimada et al14 where comparisons in excised teeth of visual inspection without magnification, confocal laser scanning microscope of thin sections and swept source OCT were undertaken. Using the microscopy finding as a reference, the swept source OCT technique was shown to be superior to that of visual inspection alone without magnification.
In a further development of caries diagnosis, Choo-Smith et al15 also describes the potential of polarized Raman spectroscopy (PRS) to confirm potential caries lesions identified by OCT, where the PRS method detects lower values of phosphate peak of hypoxyapatite from sound tooth material. The technique of PRS uses fluorescence-based techniques to detect composition of dental materials.
In the USA, Lantis Laser has been developing an OCT system designed for routine dental applications, and with a key focus on developing the role of OCT as a management system to minimize the development of carious lesions through the process of active tooth remineralization. The non-contact approach of identification of carious lesions is contrasted with the conventional approach of ‘explorer’ contact, which may not be able to detect the smallest carious lesions able to be detected by OCT and also may contribute to more rapid deterioration of sites due to mechanical damage.
Most current examples of OCT images within dentistry are derived from Fourier domain type systems, though the availability of images to demonstrate the potential of the technology readily is currently somewhat limited to research-based applications rather than routine dental practice use. Figure 4 indicates a basic identification of the enamel-dentine interface in a normal tooth. Figure 5 indicates tissue differentiation within a ground tooth. The OCT systems of Michelson Diagnostics used to capture these images utilizing a four component sample beam, which provides a high resolution zone 1 mm in length within the sample.
Products for OCT in dentistry are likely to develop from more general OCT systems designed for tissue characterization of skin or via endoscopic examination. A typical product is the Niris® Imaging System developed by the Imalux Corporation (Cleveland, OH, USA) (Figure 6). The probe tip of 2.7 mm diameter (Figure 7) incorporates a mechanical scan element which sweeps the scanning beam laterally by 2 mm. While typically used in dentistry for soft tissue characterization,16 images of hard tissues can also be obtained.
The majority of clinical studies within OCT in dentistry have been undertaken using essentially research-based systems rather than equipment systems that are commonly found within dental practices. Such studies have generally outlined key aspects of potential improvement for routine practice management. The emergence of OCT systems into the dental marketplace is awaited in order to evaluate aspects of OCT utilization within routine dentistry. This is likely to take place when the cost of OCT systems falls as more broadly based tissue characterization systems become established in clinical practice within a range of specialties. With an ever widening array of proprietary products for tooth management based on remineralization, OCT has the potential to provide a sound technique both for remineralization product development and for tracking mineralization properties of individual patients.
