Radiology Compare

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Hong Kong Den t J Vol 2 No 1 Jun e 2005 19

ORIGINAL ARTICLE Hong Kong Dental Journal 2005;2:19-23 HKDJJJJJ

Introduction

In the daily practice of dentistry, panoramic radiography (i.e. orthopantomogram) takes second place in importanceto intraoral radiography. However, panoramic imagingmay be more beneficial to patients because it offersboth excellent anatomical assessments and excellentevaluations of jaw fractures, tooth development, andmaxillary sinus disorders. It also has the additional

benefits of reduced radiation exposure compared withfull-mouth intraoral radiographs 1-3, and the availability of modifications to standard intraoral techniques for

pediatric and handicapped patients as well as those withgag reflex sensitivity.

In the last decade, many types of digital radiographicsystems, including those for panoramic radiography,have become available that are based on either storagephosphor plate (SPP) technology or charge-coupleddevices (CCDs). In contrast to conventional panoramicfilm radiographs, which have been evaluated in previous

studies4-7

, digital panoramic radiography allows thecorrection of errors in density and contrast becausethe digital image can be post-processed 8. Furthermore,due to their wider dynamic range, most digital receptorsare less sensitive to variations in exposure 8.

Charge-coupled device–based systems use a narrow2-dimensional detector in place of a moving cassette. Thedetector is composed of a CCD and a scintillator, similarto radiological intensifying screens that convert X-ray photons to the visible spectrum. A computer is required

* Private practice

Correspondence to:Dr. Bun-Ka YiuUnit 6D, 211 Johnston Road, Wanchai, Hong KongTel : (852) 2891 9759Fax : (852) 2891 6812e-mail : [email protected]

Digital dental panoramic radiography:evaluation of image quality in four

imaging systemsBun-Ka Yiu *, BDS, MGDSRCS (Edin)

Simon Chi-Man Ip *, BDS, PDipGDS, MGDSRCS (Edin)

Simon Chi-Yung Liu *, BDS, MGDSRCS (Edin), MSc, MBA

Philip Chi-Hong Ng *, BDS, FRACDS, PDipGDS, MFGDP, MGDSRCS (Edin)

Gerald Hin-Ho Siu *, BDS, PDipDS, MFGDP, MGDSRCS (Edin), MFDSRCS (Edin)

ABSTRACT Objectives.To evaluate and compare the quality of images obtained from digital panoramic diagnostic

imaging systems based on charge-coupled device or storage phosphor plate technologies. Methods. The four mostpopular digital panoramic imaging systems were identified, two charge-coupled device–based: DigiPan and Sidexis XG, and two storage phosphor plate–based: DenOptix and Digora PCT. Five landmark anatomical regions were chosenas representative of the clinical functionality of panoramic imaging: (1) the superior and inferior cortex of the mandibularcanal, (2) the superior and inferior margin of the mental foramen, (3) the lower and anterior border of the maxillary sinus, (4) the periodontal bone level, and (5) the trabecular bone pattern. A total of 160 digital panoramic image files(40 images from each system) were randomly selected and evaluated by five independent examiners using a 3-pointscale. Results. Images produced by the DenOptix system were rated significantly lower than those produced by theSidexis XG system in all regions (P<0.01), and significantly lower than those produced by the DigiPan system in allregions except the mandibular canal (P<0.005). Of the five anatomical regions, the mandibular canal was scored thelowest for all four systems. Conclusions. The Sidexis XG system produced the highest quality of digital panoramicimages among the four systems, whereas the DenOptix system was the most inferior in this study. In general, the quality of the images produced by the digital systems evaluated was acceptable to good, but no definitive conclusion could be

drawn as to whether charge-coupled device or storage phosphor plate is the superior modality.



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20 Ho ng Kong Den t J Vol 2 No 1 June 2005

for these systems because CCD sensors cannot storeinformation. Storage phosphor plate systems utilize aphosphor imaging plate that captures X-ray energy andis then placed in a scanner where it is stimulated by alaser beam to produce a digital image. Phosphor platesare often referred to as ‘multiple-use films’ because their

shape is similar to that of extraoral films and they canbe reused a number of times.

In general, CCD-based systems are more advantageous,because their high sensitivity allows a greater reductionin radiation dosage compared with SPPs, and CCDimages (being directly digital) require no processingand can be acquired almost instantly. There are,however, several problems associated with bothof these technologies. For example, no scintillator is100% efficient: there is always a proportion of X-ray photons that flows through the scintillator without

being converted to visible photons. Not only do the‘lost’ or unconverted X-ray photons not contribute tothe image, they also impact the CCD at high energy,creating noise in the image and, in the long run,damaging the imager. Likewise, SPPs are not only sensitive to X-rays, but to visible light as well. Therefore,even a minor intrusion of ambient light can spoilthe image 9. Furthermore, if the plate is not properly erased after each use, the new image will be super-imposed on the remnants of the previous image, andthe overall image quality will be adversely affected.

Despite these drawbacks, digital radiography

provides more consistent image quality, and is moreefficient in terms of both time and space: digitalimaging eliminates the need for chemical processing,

which requires dedicated space and regular main-tenance and also creates unpleasant odors andchemical waste. In addition, digital radiography allows a tremendous reduction in radiation 10 andeases communication with patients. Because digitalimages are easily stored, retrieved, duplicated, andoptimized by digi tal processing, applicationsincluding teledentistry for consultation also becomepossible.

The aim of this study was to evaluate digital dentalpanoramic systems in order to compare the differences inimage quality of SPP- and CCD-based systems, and ratethe overall image quality of panoramic radiographsproduced by the digital systems studied.

Methods

We identified the four most popular and commercially available digital dental panoramic systems. Two were

CCD-based: Sidexis XG (Orthophos 3 DS, Sirona, Germany)and DigiPan (OrthoSlice 1000, Trophy Radiologie, France);two were SPP-based: DenOptix (Orthoralix 9000,Dentsply/Gendex, United States) and Digora PCT (CranexTome, Soredex, Finland).

For each radiographic system, 40 panoramic X-ray image files were submitted from a facility using therespective system. Images were obtained at the kV andmA recommended by the manufacturer. All the images

were acquired by a single operator in each facility. Noenhancement or optimization was made before the files

were saved 11. Altogether, 160 digital image files wererandomized and displayed on a 17-inch LCD screen(ViewSonic VE700; ViewSonic, United States) at aresolution of 1280 x 1024 pixels. Brightness and contrast

were fixed.

Five independent examiners (general dentalpractitioners), each with clinical experience ranging from15 to 20 years, rated the radiographs on a 3-point scale(1=unacceptable, 2=acceptable, 3=good). Image quality

was assessed by rating the visibility of five anatomicallandmarks commonly found on panoramic radiographs(Figure 1): (1) the superior and inferior cortex of themandibular canal, (2) the superior and inferior margin of the mental foramen, (3) the lower and anterior border of the maxillary sinus, (4) the periodontal bone level, and (5)

the trabecular bone pattern. For each image, the givenlandmark was rated as ‘good’ (image of excellent diagnosticquality), ‘acceptable’ (image of diagnostic quality butshould be improved), or ‘unacceptable’ (image not of diagnostic quality). Each image was then given an overallevaluation rating of ‘good’, ‘acceptable’, or ‘unacceptable’(Figure 2).

Prior to evaluation, examiners received verbal and written explanations of the nature of the study. They weretrained to interpret digital panoramic radiographs, and

Figure 1 Digital panoramic radiograph with the features tobe evaluated marked



Evaluating ima ge qu ality in digital dental imaging systems


were calibrated using 10 randomly selected samples fromthe systems. During the evaluation period, each examiner

was presented images randomly but in a constantsequence. Visualization of diagnostically significant

structures was compared and the results were analysedstatistically to identify any significant differences in imagequality among the four systems.

Statistical analysis

Overall differences among the digital systems anddifferences among examiners were analysed by theone-way analysis of variance, with the systems as thefactors. Paired comparisons of the systems were tested

with independent sample t tests using Statistical Packagefor the Social Sciences (Windows version 12.0; SPSS Inc.,

Chicago, United States). The image quality, as representedby each of the five landmark anatomical regions, and theoverall rating for each system were expressed as the meanscores of the five observers (P<0.05).

Results

Of 160 image files obtained, 154 were evaluated, whilesix acquired from the DenOptix system could not bedisplayed. Figures 3 and 4 illustrate each of thefour systems’ mean quality scores for each of the five

anatomical regions. The DenOptix system was rateddemonstrably lower than the other three systems forall regions except the mandibular canal. The Sidexis

XG system was scored significantly better than the

DenOptix system in all regions (P<0.01), while theDigiPan system was significantly better than theDenOptix system in all regions except the mandibularcanal (P<0.005). The DigiPan system was rated thelowest in the interpretation of the mandibular canal.

The overall image quality of each of the four systemsby pooled analysis of all five regions and each of thefive examiners is shown in Figure 5. All five examinersassessed the DenOptix system as that with the lowestimage quality, and the Sidexis XG system as that withthe highest image quality. Of the five anatomical regions,

the mandibular canal was scored the lowest for all foursystems. The maxillary sinus and trabecular bone pattern were scored the highest in three of the four systems.

Discussion

Subjective image quality, as measured in this study, canbe used to compare different systems, especially whenthe examiners are experienced in the common range of diagnostic tasks the systems typically perform in generaldental practice.

(a)

(c)

(b)

(d)

Figure 2 Digital panoramic radiographs with (a) Sidexis XG, (b) DigiPan, (c) DenOptix, and (d) Digora PCT



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22 Ho ng Kong Den t J Vol 2 No 1 June 2005

The images in the present study were exportedfrom their own software to a neutral software programthat provided identical facilities for image enhance-

ment. Use of a neutral software program for imageenhancement was chosen for two reasons: to blind theimages in order to avoid preference bias, and to performthe assessments in the same way the images wouldbe used in clinics in the case of joint consultation. Inclinics and institutions where many different digitalsystems are in use, it is common practice for all imagesto be exported to a universal software program thatcan handle (enhance, displace, and save) images fromdifferent systems and that can communicate withpatient-file programs. However, owing to the study design, the possibility cannot be excluded that some

of the systems might have obtained different quality scores if they had been evaluated with their ownsoftware.

The various available panoramic systems differ inthe means by which rotational movement is modified inorder to image the dental arch. Other factors influencingimage quality are errors in exposure, the projectionangle, the accuracy of patient positioning, and thelocation of the tongue. However, because the imagesin this study were acquired with default exposure

MC MF MS PBL TBP

1.00

1.50

2.00

2.50

3.00

Landmark

DigiPan

Sidexis XGDenOptix

Digora PCT

Meanscore

Figure 3 Mean image quality scores for each of theanatomical regions evaluated by four digital panoramicimaging systems; MC=mandibular canal (superior andinferior cortex), MF=mental foramen (superior and inferiormargin), MS=maxillary sinus (lower and anterior border),PBL=periodontal bone level, and TBP=trabecular bonepattern

Figure 4 Mean image quality scores of four digitalpanoramic imaging systems by landmark anatomical regionevaluated; MS=maxillary sinus (lower and anterior border),PBL=periodontal bone level, TBP=trabecular bone pattern,MC=mandibular canal (superior and inferior cortex), andMF=mental foramen (superior and inferior margin)

3.00

2.50

2.00

1.50

1.00

DigiPan Sidexis XG DenOptix Digora PCT

Dental panoramic system

Meanscore

MSPBLTBPMCMFOverall

DigiPan Sidexis XG DenOptix Digora PCT

Dental panoramic system

Meanscore

1.00

1.50

2.00

2.50

3.00 ○ ○ ○ ○

○ ○ ○ ○

Observer 1

Observer 2

Observer 3

Observer 4

Observer 5

Figure 5 Mean image quality scores reported by each ofthe five examiners by digital panoramic imaging systemevaluated (pooled scores of all landmarks and overallratings)



Evaluating ima ge qu ality in digital dental imaging systems


settings and were selected randomly, effects associated with these factors were minimized.

This study was performed on images obtained frompatients, rather than from dry skulls or phantom heads,because image quality is strongly affected by anatomical

variation and the presence of soft tissues. Subjectiveassessment of image quality by experienced dentalpractitioners was adopted in this study to ensure that thefindings would be applicable to real-world dental practice.

Of the five anatomical landmarks evaluated, the visibility of the mandibular canal obtained the lowestscore in all four systems. Whether this is also true for film-based panoramic systems should be investigated. In anearlier study, CCD images were judged to be inferiorto film radiographs 12, while two other studies foundno significant differences in image quality 13-15.

The overall highest quality scores in this study were obtained with the Sidexis XG system, whereasimages from the DenOptix were ranked significantly lower. In general, the image quality of digitizedpanoramic radiographs was acceptable to good, withthose generated by the Sidexis XG being the best.Because of the variation within each of the two systemcategories, no conclusion as to the superiority of eitherSPP or CCD systems can be drawn.

Acknowledgements

The authors would like to thank Prof. Edward CM Lo andDr. Thomas KL Li for their contribution as advisors to thisstudy.

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