MODERN RADIOGRAPHIC METHODS

IN THE DIAGNOSIS

OF PERIODONTAL DISEASE

P.F. VAN DER STELT

Department of Oral Radiology, Academic Center forDentistry Amsterdam (ACTA), Louwesweg 1,1066 EAAmsterdam, The Netherlands

Adv Dent Res 7(2):158-162, August, 1993

Abstract—For many years, radiographs have been a valuableaid in the diagnosis of periodontal disease and the evaluationof treatment effects. Computer-based image acquisition andprocessing techniques will now further increase the importanceof radiography in periodontal diagnosis.Temporal changes of lesions can be made easily visible bymeans of subtraction radiography based on digital images.This process requires a pair of images with identical gray-leveldistributions and projection geometry. The gray-leveldistribution and perspective projection of images can becorrected by means of digital image processing. A pair ofidentical images can thus be obtained without mechanicalalignment of patient, film, and x-ray source. Algorithms havebeen developed for automatical determination of the borders oflesions and can subsequently produce quantitative informationranging from simple distance measurements to advancedmultidimensional quantitation of image parameters. Accuratevolume measurements can be carried out by the utilization ofcalibration wedges in the image.

Image reconstruction procedures, such as tomosynthesis,provide information about the third dimension, which isnormally lost in conventional radiographic projections. Thebuccal and lingual sites of the alveolar crest can be inspectedseparately.

The progress of computer-aided procedures as discussed inthis paper appears to have great potential for the improvementof the radiographic diagnosis of periodontal lesions. Especially,the benefits of reproducibility and quantitative evaluation oftreatment effects will greatly improve the role of radiographyin periodontics.

Presented at the 12th International Conference on OralBiology (ICOB), "Modern Concepts in the Diagnosis ofOral Disease", held at Heriot-Watt University, Edinburgh,Scotland, July 6-7,1992, sponsored by the InternationalAssociation for Dental Research and supported by UnileverDental Research

We acknowledge the partial funding of this study byNATO Collaborative Research Grant No. CRG 910229.

T he diagnosis of periodontal disease is greatly facilitatedby the use of radiographs. Radiographs are helpful in theassessment of treatment effects and the prognosis ofdisease progress. Radiographs are considered a valuable

adjunct to the clinical examination, because essentialinformation is provided about the bony tissues covered by thegingiva that cannot be diagnosed by clinical inspection alone.

Radiographic image formation is based on the principle ofprojecting a three-dimensional object onto a two-dimensionalimage plane, and therefore this technique also has limitations.The radiographic image, in principle, lacks information aboutthe third dimension, at least in a way that is easily perceptibleby the human visual system. The radiographic image givesspatial information about the object in the x and y directionsonly, i.e., in the plane perpendicular to the direction of the x-ray beam. Consequently, a single radiograph is inappropriatefor obtaining information about this third dimension. Moreprojections are needed for this purpose.

Due to these characteristics of radiographic image formation,the orientation of the x-ray beam toward the object is a factorthat is very important for the resulting x-ray view. A differentorientation of the projection results in a different image, whichin its turn may affect the interpretation and diagnosis based onthat radiograph. It is for that reason that standardization andreproducibility are essential requirements for the reliability ofthe diagnosis.

It is obvious that radiographs are indispensable for thediagnosis of periodontal disease, but on the other hand there aresevere limitations in their abilities to provide informationabout the spatial relationship of anatomical and pathologicalentities in the direction parallel to the x-ray beam. The evolutionof digital radiography will reduce the drawbacks of radiographicimage formation as described in the previous paragraphs.Electronic intra-oral image receptors are now commerciallyavailable for dental applications, and the use of all-digitalradiographic procedures is possible on a routine base. Thispaper describes some procedures that are now available, andexplores some future trends in computer-based radiodiagnosisof periodontal disease.

DIAGNOSTIC REQUIREMENTSIn order to be useful for the purpose of diagnosis, radiographshave to satisfy the requirements of standardization andreproducibility. The ideal radiographic procedure should alsofacilitate the collection of quantitative data with regard to thecondition of (potential) lesion areas and should providesufficient information about the shape and the extension of thelesion in three dimensions. These requirements are even moreimportant in the radiographic diagnosis of periodontal disease.The progress of disease symptoms is relatively slow, so the

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VOL. 7(2) MODERN RAIOGRAPHIC METHODS IN PERIODONTOLOGY 159

Fig. 1—Example of subtraction radiography, (a) is theradiograph taken before treatment and (b) that taken aftertreatment. The difference between the two images is noteasily visible to the naked eye. Bone ingrowth is easilyvisible in (c) (arrow). The region of bone gain is isolated in(d) to permit area and volume measurements.

difference between subsequent radiographic views can beextremely small. The shape of a periodontal lesion in aradiograph is dependent, to a large degree, on the orientationof the radiographic projection. Comparison of radiographstaken with a time interval is possible only when the projectiongeometry is identical.

Standardization of projection geometry is required toproduce radiographic images that can be compared with'normals'. The clinician is not able to make a reliablediagnosis unless he can compare the radiograph of a particularpatient with the normal and abnormal reference images hehas in his mind. Therefore, the use of film-positioningdevices and the paralleling technique is strongly recommendedfor high-quality images to be obtained.

It must be emphasized that film developing is an importantfactor in standardization. A careless developing procedure caninterfere with the detection of small or ill-defined lesions.

Reproducibility is required for reliable comparisons ofradiographs taken at sequential time intervals. In most cases,this is done by connecting the x-ray machine, the patient, andthe film mechanically and sometimes by using film holdersprovided with individual bite blocks in order to establish theexact position of the film for each examination (Duckworthet ai, 1983). It is clear that even subtle changes in theocclusal surfaces of teeth—when, for instance, new restorationshave been made—will reduce the quality of this procedure.Jeffcoat et al. (1987) used cephalostats to overcome thisproblem. They report an angular disparity betweenrepositionings of the patients within the cephalostat of as lowas 0.33 ±0.1°.

The human visual system is very skillful in recognizingglobal structures, but not in detecting small details. It isimpossible for the clinician to make a very accurate estimationof the absolute amount of bone loss. Other means are needed

Fig. 2—Computer-aided recognition of angular periodontalbone resorption. Two lesions are visible in the interdentalbone. Lesion size is corrected for the width of theperiodontal ligament space, based on measurements of theaverage width of the periodontal ligament space in theapical part of the region of interest. The amount of boneresorption in the bucco-lingual direction is indicated by thegray value of the pixels within the lesion area (black, lessbone loss; white, more bone loss). Note that this method isbased on the assumption that density differences areindicative of changes in lesion volume.

for the quantification of bone loss or the assessment of treatmenteffects when successive radiographs are compared (Hildeboltet ai, 1991). The extraction of quantitative information fromthe radiographs will facilitate a reliable comparison.

In addition to this, methods are needed that will provideinformation for the clinician about the missing third dimension(parallel to the x-ray beam) as well. These methods will giveeither sectional views of the object or real three-dimensionalrepresentations.

SUBTRACTION RADIOGRAPHYSubtraction radiography can be used to visualize smalldifferences of bone density and bone volume over time(Grondahl and Grondahl, 1983; Okano et al, 1990). Fig. 1demonstrates this technique. The development of subtractionradiography has been greatly facilitated by the availability ofdigital imaging techniques (Grondahl et al., 1983).

For subtraction radiography, the requirements ofstandardization and reproducibility are even stronger than inconventional visual interpretation of radiographic images.The projection geometries of the two radiographs that formthe pair of images used for the subtraction process in thecomputer need to be identical. Otherwise, the results willdisplay the difference in registration, and this will hide thechanges due to the disease process. Digital subtraction

160 VAN DER STELT Am DENT RES AUGUST 1993

radiography requires reproducibility of the projection geometrywithin an angle of 2 or 3 degrees (Grondahl et al., 1984).Mechanical devices have been used to maintain correctrepositioning of films over time, but these methods are rathercumbersome and not useful for routine applications.

Methods have been developed aiming at the reconstructionof images that have an arbitrary projection geometry into theprojection geometry of a reference image (Ruttimann et al.,1986; Van der Stelt era/., 1989; Dunn and Van der Stelt, 1992).The result of the reconstruction is a pair of images with theidentical projection geometries, as required for subtractionradiography. These methods eliminate the need for mechanicalconnection of x-ray source, patient, and film (Yen et al., 1990).Dunn and Van der Stelt (1992) report that reliable measurementscan be made in dental radiographs with up to 16 mm translationerrors and angulation errors of up to 32°. They consider this apromising approach for creating useful image pairs forsubtraction radiography.

Even when the selection of exposure time and thedevelopment of the film are performed very carefully, smallfluctuations of the optical densities of sets of films are sometimesunavoidable over a longer period of time. Small differences ofthe gray-level distribution in a pair of images can be correctedby digital image processing. Ruttimann and Webber (1986)and Ohki et al. (1988) describe software-based methods tocorrect the gray-level distribution of one image according tothe distribution of another image. This so-called "digital gammacorrection" is a convenient method for correcting the problemof different exposure conditions in apairof subtraction images.Application of this procedure, in many cases, is a prerequisitefor doing reliable measurements on subtraction images.

QUANTITATIVE ANDCOMPUTER-AIDED INTERPRETATION

Changes of bone density can be measured by means of computer-aided procedures. By use of a calibration wedge with knownradiation attenuation properties, the measurements of densitydifferences can be converted into estimations of volume

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changes. Aluminum or hydroxyapatite is often used because ofits similarity to bone in terms of radiation attenuationcharacteristics. In vitro studies have shown agreement within10% between digital volume calculations and determination ofthe lesion size by weight (Ruttimann et al., 1985). Bragger etal. (1988) report a sensitivity of 82%, a specificity of 88%, anda diagnostic accuracy of 87% for a system measuring surgically-induced bone loss in humans. The results of this techniquewere significantly better than those obtained through visualexaminations of the radiographs by a group of experiencedperiodontists.

When the features of anatomical details and abnormalitiesas they are depicted on radiographic images can be expressedmathematically, then computer algorithms can be developed tointerpret radiographs automatically (Ruttimann et al., 1985;Van der Stelt et al., 1985; Benn, 1991). A computerizedprocedure has been reported to describe the shapes and sizes ofangular periodontal lesions (Fig. 2). When applied to series ofradiographs taken over a certain period of time, this methodcan be used to follow the effects of clinical intervention, e.g.,in patients treated for juvenile periodontitis. These types ofcomputer-aided procedures can be a great adjuvant to clinicaldecision-making. They add objective information to the clinicalassessment of the patient (Van der Stelt and Geraets, 1991).

THREE-DIMENSIONAL IMAGINGThe local contrast in a radiographic image is the result ofspatial attenuation differences of the x-rays passing throughthe object. Differences are caused by different materials (softtissue, bone, etc.) and by different path lengths (e.g., thick bonelayers vs. thin bone layers). The effect of both causes is thesame; in many cases, it is not possible to distinguish, merely bythe radiographic density, between different materials ordifferentdimensions of structures in the object. When thin layers of anobject are visualized, this problem is much less important,because the effective path length through the object is reduced,and local contrast is more likely the result of different materials,corresponding to different anatomical structures.

Tomosynthesis is a software-based

^

3 reconstruction method to produce thin layersor slices of dental structures (Groenhuis et al.,1983; Ruttimann et al., 1983). The method isnot yet useful under clinical circumstances,because the basis projections need to be takenaccording to a very strict imaging geometry(Groenhuis et al., 1984). It is beyond doubt,however, that new hardware and softwaresolutions will be available in the near future toestablish more conveniently the registration ofthe basis projections and make this methodfeasible for clinical use (Van der Stelt et al.,1989) (Fig. 3).

Fig. 3—Example of two tomosynthetic reconstructions. The leftreconstruction shows a cross-section of the lingual cortical plate; note thealveolar bone level (arrows) which is easily recognizable without disturbingthe superimposition of other anatomical structures. The right image shows acentral section with the periodontal ligament space clearly visible.

THE FUTUREThe progress of computer technology is veryfast. It seems therefore precarious to predictthe role of digital image processing in dentalradiology in the distant future. Some trends,

VOL. 7(2) MODERN RAIOGRAPHIC METHODS IN PERIODONTOLOGY 161

however, may be indicative of the way dental radiography willbe practiced in the near future. These trends are:

• automated interpretation of radiographic images to improvethe objectivity of the diagnosis;

• development of quantitative procedures to support thediagnosis of lesion progress over time;

• advanced procedures for the reconstruction of radiographicimages to obtain standardized and three-dimensional views.

The infrastructure for high-speed transmission of large datasets, like images, will be available in many geographic areas,thus enabling dentists to exchange images, to consult expertsimmediately, and to use knowledge bases as an aid to diagnosticproblem-solving (Stheeman et al., 1992).

It is evident that the diagnosis of periodontal disease willbenefit from these developments. The difficulties of visualizingthe buccal and lingual plates of the alveolar process withoutsuperimposition of tooth structures will be overcome. Theshapes and sizes of angular bone lesions can be judged fromtheir three-dimensional reconstruction. Subtraction radiographyfor longitudinal evaluation of treatment effects can be performedon a routine basis, without the need of meticulous fixation ofthe patient, the x-ray source, and the image plane during theexposure.

However, notwithstanding the development of all thesenew and promising technologies, new procedures mustdemonstrate their effectiveness and reasonable cost/benefitcharacteristics in extensive clinical trials (Goin and Hermann,1991). Deas et al. (1991) compared probing attachment losswith digital radiographic density measurements. They foundmore bone loss with the digital method; in their conclusions,however, they point out that the digital method may producemore positives due to the increased sensitivity as comparedwith conventional (non-digital) measuring methods that haveas-yet-undetermined clinical significance. Specificity,sensitivity, and other parameters of diagnostic performancemust be determined and carefully evaluated before newtechnologies can be fully introduced in radiodiagnosis andeventually replace existing diagnostic tests.

REFERENCESBenn DK (1991). Automatic analysis of radiographic images:

I. Theoretical considerations. Dentomaxillofac Radiol19:187-192.

Bragger U, Pasquali L, Kornman KS (1988). Remodeling ofinterdental alveolar bone after periodontal flap proceduresassessed by means of computer-assisted densitometric imageanalysis (CADIA). / Clin Periodontol 15:558-564.

Deas DA, Pasquali LE, Yuan CH, Kornman KS (1991). Therelationship between probing attachment loss andcomputerized radiographic analysis in monitoringprogression of periodontitis. / Periodontol 62:135-141.

Duckworth JE, Judy PF, Goodson JM, Socransky SS (1983).A method for geometric and densitometric standardizationof intraoral radiographs. / Periodontol 54:435-440.

Dunn SM, Van der Stelt PF (1992). Recognizing invariantgeometric structure in dental radiographs. Dentomaxillofac

Radiol 21:142-147.Goin JE, Hermann GA (1991). The clinical efficacy of

diagnostic imaging evaluation studies; problems, paradigms,and prescriptions. Invest Radiol 26:507-511.

Groenhuis RAJ, Webber RL, Ruttimann UE (1983).Computerized tomosynthesis of dental tissues. Oral SurgOral Med Oral Pathol 56:206-214.

Groenhuis RAJ, Ruttimann UE, Webber RL (1984). A prototypedigital tomographic x-ray system for dental applications.Proceedings, IEEE International Symposium on MedicalImages and Icons. New York: IEEE, 218-221.

Grondahl H-G, Grondahl K (1983). Subtraction radiographyfor the diagnosis of periodontal bone lesions. Oral SurgOral Med Oral Pathol 55:208-213.

Grondahl H-G, Grondahl K, Webber RL (1983). A digitalsubtraction technique for dental radiography. Oral SurgOral Med Oral Pathol 55:96-102.

Grondahl K, Grondahl H-G, Webber RL (1984). Influence ofvariations in projection geometry on the detectability ofperiodontal bone lesions; a comparison between subtractionradiography and conventional radiographic technique. /Clin Periodont 11:411-420.

Hildebolt F, Vannier MW, Shrout MK, Pilgrim TK (1991).ROC-analy sis of observer-response subjective rating data—application to periodontal radiograph assessment. Am JPhys Anthropol 84:351-361.

Jeffcoat MK, Reddy MS, Webber RL, Williams RC, RuttimannUE (1987). Extraoral control of geometry for digitalsubtraction radiography. / Periodont Res 22:396-402.

Ohki M, Okano T, Yamada N (1988). A contrast-correctionmethod for digital subtraction radiography. / Periodont Res23:277-280.

Okano T, Mera T, Ohki M, Ishikawa I, Yamada N (1990).Digital subtraction of radiograph in evaluating alveolarbone changes after initial periodontal therapy. Oral SurgOral Med Oral Pathol 69:258-262.

Ruttimann UE, Webber RL (1986). A robust digital method forfilm contrast correction in subtraction radiography. /Periodont Res 21:486-495.

Ruttimann UE, Groenhuis RAJ, Webber RL (1983). Computertomosynthesis: A versatile three-dimensional imagingtechnique. Proceedings, 7th Conference of the Society ofComputer Application in Medical Care (SCAMC), 783-786.

Ruttimann UE, Webber RL, Groenhuis AJ, Troullos E, RethmanM (1985). Automated estimation of lesion size. Proc SPIE,Vol. 135. Bellingham (WA): SPIE, 325-330.

Ruttimann UE, Van der Stelt PF, Webber RL (1986). Use ofimage similarity for the selection of projections forsubtraction radiography. SPIE Med XIV/PACS IV, Vol.626. Bellingham (WA): SPIE, 301-307.

Stheeman SE, Van der Stelt PF, Mileman PA (1992). Expertsystems in dentistry; past performance—future prospects. /Dent 20:68-73.

Van der Stelt PF, Geraets WGM (1991). Computer-aidedinterpretation and quantification of angular periodontalbone defects on dental radiographs. IEEE Trans BiomedEng 18:334-338.

162 VAN DER STELT ADV DENT RES AUGUST 1993

Van derSteltPF, Van der Linden LWJ, Geraets WGM, Alons Van der Stelt PF, Ruttimann UE, Webber RL (1989).CL (1985). Digitized image processing and pattern Determination of projection for subtraction radiographyrecognition in dental radiographs with emphasis on the based on image similarity measurements. Dentomaxillofacinterdental bone. / Clin Periodontol 12:815-821. Radiol 18:113-117.

Van der Stelt PF, Webber RL, Ruttimann UE, Groenhuis RAJ Yen L, Dunn SM, Van der Stelt PF (1990). Finding invariant(1986). A procedure for reconstruction and enhancementof anatomical relationship in dental radiographs. Proc. IEEE/tomosynthetic images. Dentomaxillofac Radiol 15:11-18. EMBS, Vol. 12. New York: IEEE, 2076-2077.