http://jdr.sagepub.com/ Journal of Dental Research http://jdr.sagepub.com/content/68/2/107 The online version of this article can be found at: DOI: 10.1177/00220345890680020201 1989 68: 107 J DENT RES S.D. Peck, J.M. Rowe and G.A.D. Briggs Studies on Sound and Carious Enamel with the Quantitative Acoustic Microscope Published by: http://www.sagepublications.com On behalf of: International and American Associations for Dental Research can be found at: Journal of Dental Research Additional services and information for http://jdr.sagepub.com/cgi/alerts Email Alerts: http://jdr.sagepub.com/subscriptions Subscriptions: http://www.sagepub.com/journalsReprints.nav Reprints: http://www.sagepub.com/journalsPermissions.nav Permissions: http://jdr.sagepub.com/content/68/2/107.refs.html Citations: What is This? - Feb 1, 1989 Version of Record >> at Universitats-Landesbibliothek on December 28, 2013 For personal use only. No other uses without permission. jdr.sagepub.com Downloaded from at Universitats-Landesbibliothek on December 28, 2013 For personal use only. No other uses without permission. jdr.sagepub.com Downloaded from
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http://jdr.sagepub.com/Journal of Dental Research
http://jdr.sagepub.com/content/68/2/107The online version of this article can be found at:
at Universitats-Landesbibliothek on December 28, 2013 For personal use only. No other uses without permission.jdr.sagepub.comDownloaded from at Universitats-Landesbibliothek on December 28, 2013 For personal use only. No other uses without permission.jdr.sagepub.comDownloaded from
Studies on Sound and Carious Enamelwith the Quantitative Acoustic Microscope
S.D. PECK. J.M. ROWE and G.A.D. BRIGGS'
Department of Metallurgy and Science of Materials, University of Oxford, Oxford, England OX] 3PH
The scanning acoustic microscope gives strong contrast from smallcaries lesions in sections of human enamel. The uniqueness of theacoustic microscope lies in its ability to image elastic properties. Inaddition to revealing the extent and the shape of lesions, the micro-scope may also be used to measure the elastic properties point bypoint across an area of interest. Since enamel is anisotropic, mea-surements of the Rayleigh velocity and attenuation were made as afunction of direction on a section of sound enamel. The velocity wasgreatest parallel to the prism axis, and the attenuation was least inthis direction. Measurements ofV(z) across a section through a lesionarepresented. The variation ofattenuation can be interpreted in tennisof the development of demineralization, initially along prism boun-daries and then along cross-striations. The variation of velocity in-dicates a substantial reduction of elastic stiffness in the lesion.
J Dent Res 68(2):107-112, February, 1989
Introduction.
The uniqueness of the scanning acoustic microscope (s.a.m.)lies in its ability to image the elastic properties of a specimen.In a previous paper (Peck and Briggs, 1987), it was shownhow the acoustic microscope is able to image lesions in sec-tions of human dental enamel with a combination of spatialresolution and sensitivity to demineralization. In that paper, itwas explained how the contrast seen from a lesion dependedboth on the elastic properties of the enamel and on the defo-cusing conditions at which the picture was taken. The variationof the video signal, V, as a function of the distance, z, betweenthe focal plane of the lens and the surface of the specimen maybe plotted as a V(z) curve, and this was used to illustrate thebasis of the contrast. That paper ended with a figure that showedin a single picture [V(xz)] how the spacing and amplitude ofthe fringes in V(z) varied along a line through a lesion. Thispaper indicates how the information in such a measurementcan be extracted and interpreted.
In materials such as dental enamel that have relatively highacoustic velocities, it is possible to be rather more specificabout the dependence of the contrast on the elastic properties:For many purposes it may be said that the contrast is deter-mined by how Rayleigh waves propagate in the surface of thespecimen (Briggs, 1985). The velocity and attenuation of theRayleigh waves can be directly related to the periodicity anddecay of oscillations in the V(z) curve, so that these can bemeasured point by point across the surface of a specimen.
The periodicity of the oscillations in the V(z) curve, Az,may be linked to the Rayleigh angle, OR, by the equation:
AdZ= vI2f(1 - cosOR)
where f is the operating frequency of the microscope and v1
the velocity of sound in the coupling liquid (Parmon and Ber-toni, 1979). The Rayleigh angle is related to the Rayleighvelocity, VR, by Snell's law:
sin(OR) = Vl/VR
Thus, by measuring the periodicity of the oscillations in V(z),one may determine the Rayleigh velocity. The attenuation ofthe Rayleigh waves is related to the rate of decrease in theamplitude of the oscillations as the specimen is moved towardthe lens (i.e., increasingly negative z).
For determination of the elastic anisotropy of a material suchas dental enamel, a non-imaging line-focus acoustic micro-scope may be used (Kushibiki et al., 1981). A cylindrical lenssurface brings the acoustic radiation to a line-focus on thespecimen, and thus Rayleigh waves are only excited in onedirection (Fig. 1). By variation in the orientation of the speci-men with respect to the line-focused acoustic beam, the atten-uation and velocity of Rayleigh waves in any particular directionmay be found (Kushibiki and Chubachi, 1985).
Elastic anisotropy in hard dental tissues was examined byLees and Rollins (1972). Using the critical angle reflectiontechnique described by Rollins (1968), they measured the lon-gitudinal and shear acoustic velocity components in the ex-posed surfaces of bovine incisors, both along the tooth axisand perpendicular to it. They concluded that the critical anglereflection technique was sensitive enough to detect variationsin velocity as a function of propagation direction, althoughthey were unable to attribute the variations to any particularstructural element of the bovine enamel.
In this paper we present the results from two sets of exper-iments. The first set of experiments investigated some aspectsof the anisotropy of the enamel structure, and the second thevariations in the elastic properties across a section of a smallcaries lesion. The results of the two studies are combined toexplain some of the features seen in s.a.m. images of lesions.
transducer
Fig. 1-Schematic diagram of the line-focus acoustic lens (Kushibikiet al., 1981).
107
Received for publication July 29, 1987Accepted for publication October 4, 1988This research was carried out under a collaborative SERC CASE
studentship sponsored by Beecham Products.ITo whom correspondence should be addressed
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Fig. 2(a)-Plots of Rayleigh velocity, VR, and attenuation, a, as functions of the direction of propagation across a longitudinal section of permanenthuman enamel. The inserts show the positions at which the propagation direction (marked by the arrowed line) was parallel to the probable direction ofthe long axes of the prisms in such a section. The error bars are ± one standard deviation.
Fig. 2(b)-Plots of Rayleigh velocity, VR, and attenuation, a, as functions of the direction of propagation across a polished buccal surface of permanenthuman enamel. The insert shows the position at which the propagation direction was approximately parallel to the long axis of the tooth. The error barsare ± one standard deviation.
Materials and methods.
(i) Anisotropy measurements. -A longitudinal section and abuccal surface from two mature caries-free human permanentmolars were prepared (Peck and Briggs, 1987). The sectionedsurfaces were ground with successively finer grades of siliconcarbide papers, ending with 1200-grade. They were thenmounted in a precision polishing jig and lapped by use of asolder-faced lapping wheel impregnated with oil-soluble paste
containing 1-xm-diam diamonds. The flatness of the surfaceproduced by this procedure was checked by optical interferencemicroscopy with green light and a IOx objective. The straight-ness of the fringes indicated a flatness better than 300 nm overthe field of view. This is crucialtfor reliable results in acousticmicroscopy. Care was taken to ensure that the specimens didnot dry out during the preparation procedure.The specimens were measured in a line-focus beam acoustic
microscope, and V(z)s were taken at 10° intervals. In both
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range of z ir hcs as 9 m, and te maximum defouswasu miTjTh couuliingwaoe ept at 25 -4- 2C
throughout the, experimeni. The 17z)data were Fourier-ana-lzed togiv the~Rayleig veocity, from the equaltions gien
in the "Intrduction, as well as thRayleihwv a nation and the~lnituin alwa velocity
() The lnepfcus acou ~c microscope ind~ -Fig. 2a shows~,the Rayl igh veloci,vl and attenuation, a for ways propagating in the longitudinal Section, p otted as functions of prop-agation diretin Fig. 2b~ shows similar plots for te bcasurface (note the different vertical scales in a and b). Tangles at which the propagation of the Rayleigh waves wasapproximately parallelwto the probable mean dietino thelong axes of the prisms (Fig 2a), and to the long axis of the"amietothaFg 2b), ar 'indicated by sketches w rossgi3 00 inAhdrcion in each case. The~maximIum and minimumre're"le~ree how in te TaiThe errors
esesiae tbe I ss than 0.2 o he value oftheity a hu5 th v lu sof theattenuation.
0-60 0) Pozi Pffoc V )sacrs c/ esion. Fig. 3a ~ hows thcs I.a~ micrograph of the lesion taken at a defocus o-15~~xm,, whle, in Fig. 3b the Rayleigh velocities ~calculated~
- fr m the Vz s are plotted against their positions across teleson ef the1n ma kd on Fig. 3a). Four rcpresenta vVz) curveae shown in Fig measure atpints()()
and (m i Fi.3.Bcause of he anistopy, ths hudaj ~~~~~~~~~~~~~~beregarded as mean values of Rayleigh velocity (Somekh et
al,1~) h attenuation dedcdfo teurves isa8)he t iAdcdf~h41lso~h wn,;but aga e ueo te arisotropy these values o~should be intcrpreted with caut0ion The acuayof the 'Ray-
leigh vlocrty wka about I o in sound enamel, but was reducedrn the core of the lesion b as' f the severe attenuto Theerror nthe atteruatior was about O in son 1eae but
Fg3( c imrg hot acare esloin 00mahyiron50 i tehso Te tnutoFignamotaiTop raphgfeqc fpartsoope e uI isde0 borp i IA dctigwtu h aeilvelocity*(leftordinateand attenuation (righ hnordoraThIuttcdatorV)pottedsauctionf pos ion aong 'iI rc howI ( incrases wihde asing speimn dhenst)Thlaraiosi
ithe~velocity and attention ploscttedringF tig.3b corespondtoandtheTALdhevri os nth pr diiy rdd ayofte sclltinMAXIMUMANMINIMUMVLUES OF R\YLFIGH EOCII7Y intheVls) cuweadtin ofg 4 i nryit h a
a a~~~~~~~~~~ aol r ivelciy,(eftorint msand esadRlii 1 eotdtath datg ft~iLongitudiialsection3124 294 0 8t 0.08 ~~icritical angle efleto ec niqeoert puseehomthods forplottedasafiiii~ ~ ~ ~ tht i dd ot eqir tw srfaesa inie isanc aar
Hwvrth c iticangernAtnato iefdcior tehiqe altoughpn incasestelensascouled o h spec ~r wit a drole of pincipl ab nto edetetaosi anstoycnbusdolfreshlydstilledwter.The mpe atureof he d olet was in rlttvel largaurveas,suchg assraeso oin nml
c Iareulymoire byaMIIMM themocupeSTFRYLIH aELcsic r-TutepolmWsoitdwt h rprto fsrc
wassetionedlongitdinallthrogh thelesionand prpared ithtesan olinefcs acousti microcopethe choictae ofspeciasbeforeforscanni~~~~~~~~~~~~~~ngaosi irsoyTelso a e slted byte lergt h of te line-fcso the specimeLnimagdwithSethens 31.,adV2 er hn ae witha~.8 point- surfiace. h len used ninth6resen tstd ha a ie ouso
Bulasrt teanaoia ufc costeeinhe14A fisth()eumela(the p olihe aruawa approximy(atel 2.amndwastaknataistanceof10 k from te anatoical suface. oaeter)bufothelori gitdalsnfeiel ectinhnlyueregion where
Threfloperatin requency of thermicoscouple.Tew asou370MzTe Thes tename blwasthickrthan ithisdimensocrprtoul be cosid-
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ered. The lens was positioned over high lateral enamel, avoid-ing the cusp.The results of the acoustic investigation show both the lon-
gitudinal section and the buccal surface to be elastically ani-sotropic. In the longitudinal section, it would appear, from theestimated positions at which the line-focus was either normalor parallel to the long axes of the prisms, that the anisotropywas closely related to the prismatic orientation. The Rayleighvelocity was greatest and the attenuation lowest parallel to theestimated mean direction of the long axes of the prisms, andthe Rayleigh velocity was smallest and the attenuation highestin a direction estimated to be normal to the long axes of theprisms. The results showed a 5.8% difference in the velocitiesand a 77% difference in the attenuation recorded for the twopropagation directions.
While the majority of the apatite crystallites within the prismheads [the pit-floor phase crystallites (Boyde, 1987)] are alignedwith their c-axes parallel to the long axes of the prisms, theorientation of the embryological interpit crystallites is in strongcontrast. The porosity of the prism boundaries also appears tobe higher. These two factors would result in higher acousticattenuation in these regions.
If the prisms and the crystallites were perfectly aligned, thenthe propagation of an acoustic wave along the c-axis of a prismwould not be significantly disrupted by the enamel at the prismboundaries. However, if (as expected) there are distortions inthe alignment of the prisms, then the Rayleigh velocity will
be reduced and the attenuation increased, because the acousticwave will be forced to traverse the more attenuating interpitregions where the orientation of the crystallites is quite differ-ent from that in the bulk of the prisms. Thus, in the longitu-dinal direction the measured values v.. and Amin whichcorrespond to Rayleigh wave propagation parallel to the longaxes of the prisms are likely to be smaller and greater, re-spectively, than if the prisms had been perfectly aligned.Ile relationship between structure and the elastic anisotropy
detected on the buccal enamel is less straightforward. Therewas a 2.1% difference in the velocities recorded parallel andperpendicular to the long axis of the tooth (maximum velocitybeing parallel to the long axis of the tooth), but there was noclear relationship between the propagation direction of the ul-trasound and its attenuation.The prism structure in cross-section is often described as a
keyhole pattern. Transmission electron microscope studies onsuch sections (Meckel et al., 1965) have shown that the crys-tallites at the tails of the keyholes have a preferred orientationwith their c-axes at approximately 20° to the plane of the sec-tion. The tails of the keyholes point cervically. This preferredorientation of crystallites in the tails of the prisms and theorientation of the tails might account for the increased Rayleighvelocity in a direction parallel to the long axis of the tooth.
For prisms sectioned transversely, there is comparativelylittle difference in the amount of prism boundary material inany given direction across the section. If, as for the longitu-
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Fig. 5-A schematic representation of the spread of demineralizationassociated with the enamel caries lesion as suggested by scanning acousticmicroscopy. The lines represent pathways of demineralization; the heavierthe line, the more demineralized the pathway. Only the solid lines arediscernible in acoustic micrographs. The diagram is not to scale.
dinal prism sections, it is the change in the orientation of thecrystallites at the prism boundaries that is assumed to be themost important factor in determining the attenuation of theRayleigh wave, then little difference in attenuation will berecorded between different propagation directions on the trans-verse sections. This is borne out by the results. The relativelylow values of attenuation can be accounted for by the higherdensity of surface enamel compared with interior enamel(Weatherell et al., 1966).
Turning now to the measurements that were made with theimaging microscope on lesions, the interpretation of the atten-uation may be guided by the line-focus results obtained fromsound enamel. The large difference in attenuation for differentpropagation directions was accounted for by the variations inthe amounts of acoustically attenuating prism boundaries en-countered by the Rayleigh waves. Scanning acoustic micro-graphs of lesions presented in the previous paper suggestedthat the changes in elastic properties associated with demin-eralization proceed initially along the prism boundaries andthen, as demineralization becomes more advanced, across thecross-striations (this is shown schematically in Fig. 5). There-fore, we should expect that for regions where there is only arelatively small loss of mineral, the attenuation of an acousticwave propagating parallel to the long axes of the prisms willremain almost unchanged, while a wave propagating acrossthe prism boundaries will be more attenuated because of theincreased amount of interprismatic material it must cross. Inthe lesion core, where the demineralization is greatest and min-eral has been lost at the cross-striations as well as along theprism boundaries, the Rayleigh wave will be attenuated notonly as it propagates normal to the long axes of the prisms(across the prism boundaries), but also as it propagates alongthe prisms themselves, because at each cross-striation it willencounter a plane of demineralization. The spherical lens inthe imaging microscope is sensitive to Rayleigh wave propa-gation in all directions on the surface of the specimen. Inanisotropic materials, the signal will be dominated by Rayleigh
propagation in the low-attenuation direction; for sound andslightly demineralized enamel, this direction is parallel to thelong axes of the prisms. If the enamel is extensively deminer-alized and mineral loss has occurred both along the prismboundaries and at cross-striations, as acoustic micrographs ofthe lesion core suggest, then the Rayleigh wave would be se-verely attenuated in all directions. The analysis of the V(z)staken over sound, slightly demineralized, and extensively de-mineralized enamel [Fig. 3 points (m), (j), and (f)] is consistentwith this model.
In the measurement of elastic properties by acoustic mi-croscopy, it is important that the area illuminated by the lensshould be reasonably homogeneous. As the negative defocusof the lens increases, so the area sampled increases. In thepresent study, the maximum defocus was -67 WLm. At thisdefocus, the Rayleigh wave contribution to V(z) is derivedfrom an area of enamel approximately 80 pum in diameter (es-timated geometrically assuming a Rayleigh angle of 30°).However, in the V(z) curves in Fig. 4, the signals becomerather small after the first three oscillations or so, so that mostof the contribution to the analysis comes from a defocus lessthan 40 pLm, corresponding to an area sampled about 45 pumacross.
In relating the measurements presented here to more con-ventional measurements of mineral content, one must remem-ber that each technique measures a different property.Microradiography measures primarily the density of mineralcontent. Microhardness and acoustic microscopy both measuremechanical properties, but microhardness involves plastic flowbeyond yield, whereas in acoustic microscopy the strains arepurely elastic. It was explained in Peck and Briggs (1987) thatsince the density in a lesion is in general less than in soundenamel, and the velocity of an acoustic wave in a mediumvaries inversely with the square root of the density, then thechange in density by itself would lead to an increase in veloc-ity. The reduction in density in the core of a lesion may be atleast 25% (Darling, 1958), and so if this were the sole effectof demineralization, an increase in velocity of 15% would beexpected. But the results presented in Fig. 3 in fact indicatenot an increase in the Rayleigh velocity, but a decrease of 15%in the core of the lesion. Since acoustic velocity varies inproportion to the square root of the elastic modulus, the mea-surements using quantitative acoustic microscopy show that ina lesion there is a decrease in the elastic stiffness of abouttwice the magnitude of the decrease in density that is deducedfrom other techniques.
Acknowledgments.We wish to express our thanks to colleagues who have de-
veloped the experimental and analytical facilities that have beenused in this study, especially Drs. J. Kushibiki, J.M.R. Weaver,and D.S. Spencer. The development of acoustic microscopyat Oxford is in collaboration with Harwell Laboratory. Weparticularly wish to thank Dr. W.B. Davis and his colleaguesat Beecham Products for their support. We also thank Dr. T.Bromage and Professor A. Boyde for detailed comments onthe paper.
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