Optical coherence tomography ~OCT! is a noninva-sive and noncontact technique for obtaining high-resolution ~,20-mm! cross-sectional images ofbiologic structure.1 OCT was initially applied to to-mographic imaging of transparent tissue in the eyefor diagnosis of retinal macular diseases.2,3 Apply-ing OCT to other clinically relevant biologic struc-tures, however, has been complicated by the problemof optical scattering.4 The portion of backscatteredphotons from turbid tissue that passes the interfero-metric gate of the OCT system decays exponentiallywith depth. This rapid decay of signal from struc-tural interfaces translates to shallow penetrationdepths ~1–3 mm! in most biologic tissue. Applica-tion of OCT outside the retinal area has thereforebeen limited to relatively accessible regions of thebody, such as subsurface imaging of skin5 or opticalbiopsy of the vascular system with catheter-basedsystems.6 In this paper we focus on the use of OCTfor the imaging and the discrimination of hard- and
B. W. Colston, Jr., M. J. Everett, and L. B. Da Silva are with theLawrence Livermore National Laboratory, Health Care Initiative,7000 East Avenue L-399, Livermore, California. L. L. Otis is withthe University of Connecticut Health Center, 263 Farmington Av-enue, Farmington, Connecticut. P. Stroeve is with the Depart-ment of Chemical Engineering and Material Science, University ofCalifornia at Davis, 1 Shields Avenue, Davis, California 95616.H. Nathel is at P.O. Box 7607, Berkeley, California 94707-0607.
Received 11 July 1997; revised manuscript received 16 January1998.
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soft-tissue structures in the oral cavity, another siteon the body that has clinically relevant biologic tissuein proximity to the surface. Uses of imaging in theoral cavity include detecting demineralized enameland dental caries, identifying tooth fracture, andevaluating soft-tissue pathosis. Periodontal dis-eases, in particular, involve morphological changesthat are potentially detectable by an OCT system.
Periodontal diseases are plaque-induced disordersthat result in loss of connective tissue attachmentand resorption of alveolar bone. An important as-pect of periodontal disease assessment is determiningthe location of the soft-tissue attachment to the toothsurface.7 Currently, mechanical or pressure-sensitive probes are used to assess periodontal con-ditions.8 The periodontal probe is placed betweenthe soft tissue and the tooth. The depth of probepenetration ~probable pocket depth! is measured, andthe attachment level is estimated from a fixed refer-ence point on the tooth, the cemento–enamel junction~CEJ!. These probes can be painful for the patientand have several sources of error resulting from vari-ations in insertion force,9 inflammatory status of tis-sue,10 diameter of probe tips,11 and anatomical toothcontours.12 These errors limit the reproducibility ofthe measurements and reduce the accuracy to lessthan 60.25 mm, a significant error considering that adiseased pocket depth of greater than 3 mm is con-sidered clinically relevant.12 OCT may prove to be amore reproducible and reliable method for determin-ing attachment level. Moreover, directly imagingtooth and soft-tissue structures and contour in vivomay provide information that would allow diagnosisof periodontal diseases before attachment loss.
2. Experimental Methods
The OCT system is based on a single-mode fiber-opticMichelson white-light interferometer ~Fig. 1!. High-resolution cross-sectional imaging is achieved whenlight is focused from an optical low-coherence sourceon the biologic tissue and the intensities of the back-scattered reflections as a function of their axial andtransverse positions in the tissue are measured.The light is scanned axially through the tissue whenthe reference arm pathlength is varied with a longi-tudinal scanning mirror. Intensity modulation as-sociated with interference between light from thesample and the reference arm reflections ~heterodyn-ing! occurs only when the optical path lengths of thetwo arms are matched to within the coherence lengthof the source. The intensity of backscattered light isgiven by the amplitude of this heterodyned signal andis plotted as a function of axial position in the sample,generating an A-Scan ~Fig. 2!. Translating the sam-ple arm transversely generates a series of A-Scansthat are combined to create a B-Scan or a two-dimensional intensity plot ~Fig. 3!. The gray-scalevalues in the B-Scan correspond to the backscatteredintensity as a function of transverse and axial posi-tion in the tissue.
The source for our OCT system was a superlumi-nescent diode with a central wavelength of 1310 nm,a spectral bandwidth ~intensity at full width at half-maximum! of 47 nm, and a sample arm power of 70mW. The spot size of the beam was ;10 mm, and atransverse scanning resolution of 20 mm was used.The measured axial resolution of the system was ;17mm. A He–Ne laser at a 633-nm wavelength wasused for aligning the beam on the sample. The het-erodyne detection scheme produced a signal-to-noise
Fig. 1. Schematic of the dental OCT system.
ratio of 90 dB when a lock-in amplifier with a RC timeconstant of 30 ms at a Doppler shift frequency of 82kHz was used. We used polarization paddles to min-imize polarization mismatches between sample andreference arm reflections by adjusting the birefrin-gence in the sample arm fiber optic. Acquisitiontime for each B-Scan varied from 30 to 120 s, depend-ing on the image size.
Using our OCT system, we selected and imagedregions from the oral cavity of three 4–6-month-oldhealthy pigs within 2–4 hours after they were killed.Since the pigs were relatively young, the teethscanned were primarily deciduous premolars ~milkteeth!, which form within the first 2 months of life.13
We made histological confirmation of the OCT imagesby freezing the specimens and then slicing them ax-ially with a diamond-tipped rotating disk. The in-ternal tooth and soft-tissue microstructure was thusexposed and the critical tooth–gum interface main-tained. A digital camera mounted on a microscopewas used to obtain photomicrographs of the sectionedteeth.
3. Results and Discussion
Figure 3 ~right! is a photomicrograph showing thecross section of a typical porcine premolar tooth.The pulp, composed of loose connective tissue, formsthe core of the tooth. It is visible as a centrallylocated dark region in the photomicrograph. Sur-rounding the pulp is a dentin shell, made up of amixture of connective tissue and hydroxyapatitesalts. An enamel cap covers the dentin in the crown,whereas the dentin in the root has a thin cementumsurface. The enamel prisms show up as alternatingbands of dark and light radiating perpendicular tothe tooth surface. The enamel layer extends be-neath the oral mucosa in this specimen. The oralmucosa consists of a superficial epithelium layer sup-ported by connective tissue. Since no histologicalstains were used, the oral mucosa and the connectivetissue are seen in the photomicrograph as a light anda dark region overlying the lighter-colored bone.
The corresponding OCT image is shown in Figure 3~left!. No averaging or postprocessing has been usedin creating this image. The axial dimension shownon the OCT image is in terms of optical path length.
Fig. 2. A-Scan data from porcine periodontal tissue at lateral positions ~a! crown of tooth, ~b! near CEJ region, and ~c! below CEJ at oralmucosa. This A-Scan data was extracted from the B-Scan image ~Fig. 3, left! at marked locations along the vertical edges of the image.
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This scale needs to be divided by the refractive indexof the relevant tissue ~;1.3 for oral mucosa, ;1.6 forenamel, and ;1.5 for dentin! to obtain true physicaldimensions, resulting in an axial compression of theimage. The amount of coherently backscatteredlight in a highly turbid media such as biologic tissuedecays exponentially with depth. Since the back-scatter power is on a log scale ~i.e., decibel!, the imagebrightness should therefore decrease in an approxi-mately linear fashion from the front surface within agiven tissue type. The sudden transition in the OCTimage brightness at the anatomical crown corre-sponds to the enamel–dentin interface. Theamount of coherently backscattered light from theinternal structure of the dentin, taking into accountthe exponential decrease in signal with depth, isgreater than from the enamel microstructure @Fig.2~a!#. This is consistent with measurements of thelight scattering properties of enamel and dentin atnear-infrared wavelengths, where the scattering co-efficient of dentin was demonstrated to be over anorder of magnitude higher than enamel.14 Becausethe integrity of the dentin–enamel junction is an im-portant assessment in caries diagnosis, this tech-nique shows potential for using OCT images to detectincursion of caries below the outer enamel surface.15
Research is in progress to determine if current pen-
Fig. 3. Comparison of an OCT B-Scan image ~left! and a photomi-crograph of the corresponding region ~right! from the periodonitumof a young pig. Clinically relevant features are labeled. Lettersalong vertical edge of the B-Scan have corresponding A-Scan im-ages @Figs. 2~a!–2~c!#. The dark and the light bars in Figs. 3~a!and 3~b! represent 1 mm of optical distance.
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etration depths are adequate for visualizing caries atinterproximal contact points where they are mostprevalent.
The enamel layer becomes thinner and eventuallyends below the oral mucosa at the CEJ @Fig. 2~b!#.The location of the CEJ, where the outer layer of thetooth transitions from enamel to cementum and theroot of the tooth begins, is an important referencepoint for determining the attachment level. Somestructure is also evident in the oral mucosa layer,although the lack of stained histological sectionsmakes it difficult to characterize @Fig. 2~c!#. Soft-tissue inflammation, a key indicator of periodontaldiseases, can lead to changes in the character or thecomposition of the oral mucosa microstructure.These changes should be quantifiable by OCT imag-ing and therefore useful as indicators of disease pro-gression. The coherence spikes in this A-Scan~approximately 45 dB down from the peak air–tissuereflection! are visible but do not degrade the overallimage quality since they are outside the penetrationdepth of the system. An instrument with deeperimaging capabilities would ideally have a source inwhich the coherent spikes lie outside the axial scan.These echoes can alternatively be removed by digitalprocessing of the data, since their location and am-plitude relative to specular reflections are known.
Although the periodontal tissues of the porcinespecimens used in this study were healthy, the im-ages demonstrate the ability of OCT to locate thetooth surface and contour beneath the oral mucosa.This is important in determining the feasibility ofusing this technique for diagnosis of periodontal dis-eases. An issue that requires further attention isthe attenuation characteristics of biologic fluid in thegap between the oral mucosa and the tooth. Pa-tients who have periodontal diseases often have amixture of blood, plaque, and tissue fluid in thispocket. The presence of this fluid could complicatelocation of the tooth surface, although it is expectedthat the fluid–tooth interface should still have a largeenough index of refraction change to make it visible.The penetration depth of the current OCT system isalso of some concern, as the imaging capabilities ofthis instrument is limited to approximately the firstmillimeter of tissue in the oral cavity. Visualizationof the tooth–mucosa interface, potentially necessaryfor disease detection, would therefore be difficult inthe distal portion of the oral cavity where the oralmucosa is thicker. The penetration depth can beimproved, however, by improving the dynamic rangeof the OCT system. The largest gains in dynamicrange can be accomplished by increasing the amountof incident radiation on the tissue.16 The sourcepower used in these initial studies falls far below theAmerican National Standards Institute ~ANSI! stan-dard for tissue optical damage. The ANSI damagethreshold for skin tissue is 96.2 mW, assuming nomore than 8 h of continuous irradiation with a1.3-mm wavelength source. The power incident onthe tissue in the current setup is over a thousandtimes less than the ANSI damage criterion, with
much shorter exposure times. If the source powerused was at the 96.2-mW level, the dynamic range ofthe system would be increased by approximately 30dB, and the penetration depth would be approxi-mately doubled.
We have demonstrated in this paper the potentialof OCT for noninvasively imaging periodontal tissuesin the porcine model. This technique creates cross-sectional images of the periodontal region with inter-nal structural information on the gingiva, the tooth,and the interface between the two. It has potentialclinically as a painless, more accurate substitute forcurrent mechanical probing methods, and it has po-tential in research efforts aimed at discovering thecomplex and the currently poorly understood etiologyof periodontal diseases. Further efforts will beaimed at analyzing animal models of diseased tissueand developing a prototype OCT device with fasterimage acquisition times and better penetrationdepths for in vivo human studies.
This research was performed under the auspices ofthe U.S. Department of Energy by Lawrence Liver-more National Laboratory under contract W-7405-Eng-48.
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