Objectives: The aim of this study was to evaluate the effect of ambient light on the detection of contrast elements in digital dental radiographs. Materials and methods: A high-contrast standardized digital radiograph of an aluminum step wedge containing 32 boreholes of different depth was cut into 40 isometric images. Images were presented at random on a 17-in cathode ray tube (CRT) monitor at different ambient background illuminations of 0, 50, 200, and 500lx. Twenty observers stated twice their blinded decision whether or not they could perceive a dark spot on a five-point confidence scale. Areas (Az) under receiver operating characteristic curves were calculated and compared between the four different ambient illuminations using the Friedman test. Statistical significance was set at p < 0.05. Overall agreement was estimated determining the intraclass correlation coefficient. Results: The Az values (0.735 for 0lx, 0.728 for 50lx, 0.735 for 200lx, and 0.788 for 500lx) did not significantly differ (p = 0.796) between the four ambient lighting levels. Conclusions: The detection of small contrast features in digital dental radiographs on a CRT monitor seems to be comparable over a wide range of ambient background illumination. Clinical relevance: The ambient light in dental offices of <500lx may not negatively impact the diagnostic performance of digital dental radiograph

Dagassan-Berndt, Dorothea

Filippi, Andreas

Kühl, Sebastian

Mischak, Irene

Payer, Michael

Schriewer, Till

Schulze, Ralf

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ORIGINAL ARTICLEThe influence of ambient lighting on the detection of smallcontrast elements in digital dental radiographsTill Schriewer & Ralf Schulze & Andreas Filippi &Irene Mischak & Michael Payer &Dorothea Dagassan-Berndt & Sebastian KühlReceived: 12 July 2012 /Accepted: 1 October 2012 /Published online: 7 October 2012# Springer-Verlag Berlin Heidelberg 2012AbstractObjectives The aim of this study was to evaluate the effectof ambient light on the detection of contrast elements indigital dental radiographs.Materials and methods A high-contrast standardized digitalradiograph of an aluminum step wedge containing 32 bore-holes of different depth was cut into 40 isometric images.Images were presented at random on a 17-in cathode ray tube(CRT) monitor at different ambient background illuminationsof 0, 50, 200, and 500 lx. Twenty observers stated twice theirblinded decision whether or not they could perceive a darkspot on a five-point confidence scale. Areas (Az) under re-ceiver operating characteristic curves were calculated andcompared between the four different ambient illuminationsusing the Friedman test. Statistical significance was set at p<0.05. Overall agreement was estimated determining the intra-class correlation coefficient.Results The Az values (0.735 for 0 lx, 0.728 for 50 lx, 0.735for 200 lx, and 0.788 for 500 lx) did not significantly differ(p00.796) between the four ambient lighting levels.Conclusions The detection of small contrast features in dig-ital dental radiographs on a CRT monitor seems to be compa-rable over a wide range of ambient background illumination.Clinical relevance The ambient light in dental offices of<500 lx may not negatively impact the diagnostic performanceof digital dental radiographs.Keywords Radiography . Digital images . Imageperception . Visual perception . Light conditionsIntroductionDigital radiography has gained importance in daily clinicalpractice during the last decade [1]. When compared toconventionally film-based radiography, digital imagingreveals several advantages such as time economy, lowerradiation dose, and the possibility of image processing afteracquisition [2]. This, however, is associated with completelydifferent viewing conditions. Conventional radiographs aremainly viewed in darkened rooms to optimize or maintainthe perceived contrast of film images [3]. One known prob-lem when evaluating conventional radiographs in darkenedreading rooms is related to the extreme differences in illu-mination caused by the low ambient surrounding light andthe high luminance image [3]. These differences may lead toadvanced fatigue since the radiologist’s pupils contract anddilate as the visual focus intermittently shifts between thehigh luminance monitor and the darker background wall,thus negatively affecting the diagnostic performance [4, 5].Digital images in contrast are viewed on cathode ray tube(CRT) or liquid crystal display (LCD). One positive sideT. Schriewer :A. Filippi :D. Dagassan-Berndt : S. Kühl (*)Department of Oral Surgery, Oral Radiology and Oral Medicine,School of Dental Medicine, University of Basel,Hebelstrasse 3,4056 Basel, Switzerlande-mail: sebastian.kuehl@unibas.chR. SchulzeDepartment of Oral Surgery (and Oral Radiology),School of Dental Medicine, University Medical Centerof the Johannes Gutenberg University Mainz,Augustusplatz 2,55131 Mainz, GermanyI. Mischak :M. PayerDepartment of Oral Surgery and Radiology, School of Dentistry,Medical University of Graz, Austria,Auenbruggerplatz 12,8063 Graz, AustriaClin Oral Invest (2013) 17:1727–1731DOI 10.1007/s00784-012-0858-2effect of these different viewing conditions represents thelower diffuse reflection coefficient which allows a higherflexibility in adjusting the ambient light when compared toconventional viewing [4]. With regard to this, the back-ground illumination may be elevated without negative in-fluence on diagnostic performance. An elevated backgroundillumination could positively influence the fatigue processduring image evaluation. Modern dental offices are increas-ingly provided with monitors located in the direct surround-ing of the dentist’s chair. The background illumination inthis surrounding is mainly elevated since light is needed fortreatment purposes and additionally to create a friendlyatmosphere. It could be shown that high levels of ambientlight may decrease diagnostic accuracy by as much as 11 %[6]. In contrast to this, others found a slight backgroundillumination to improve the diagnostic performance with apositive effect on the sensitivity for digital images whencompared to darkened conditions [3].Nevertheless, there is a lack of information on the influenceof ambient illumination on the diagnostic performance fordigital dental radiographs. The aim of the present study wasto evaluate the influence of four different ambient light con-ditions on the depiction ability of small contrast elements indigital dental images. By means of an exactly known testobject, a large number of observers, and a receiver operatingcharacteristics (ROC) analysis, the influence of ambient lighton observer performance should be investigated.Materials and methodsTest objectAn aluminum step wedge (40×30 cm) with eight steps ofincreasing thickness (3–10 mm) containing 32 boreholes (di-ameter, 1 mm; depth, 0.05, 0.10, 0.30, and 0.50mm) was usedas described in a previous study [7]. Each step was dividedinto five equal segments (length 6 mm; width, 5 mm), four ofwhich were provided with one borehole (Fig. 1). The locali-zation of each hole was randomized avoiding a close relationto the border zones of each segment. Based on CNC deviceand a plain drilling machine, the holes were burred with aprecision of 0.01 mm in length and width (Chiron-WerkeGmbH & Co. KG, Tuttlingen, Germany). Thus an equalthickness of aluminum inside the boreholes could be realized.Image acquisitionA direct digital cephalometric unit (Orthophos DS Ceph,Sirona Dental System GmbH, Bensheim, Germany) equippedwith a CCD sensor with an effective detector array of 138.6×5.9 mm2 was used for image acquisition, providing a spatialresolution of 282×282 dpi. The wedge phantom waspositioned in such a way that its back (and front) side wasaligned parallel to the receptor at a distance of 26 mm asdescribed in a previous study [8]. Radiographs were producedover the entire exposure range and the image exposing themaximum contrast as assessed from the extreme grayscale atthe thick and the thin end of the wedge was selected (Photo-paint 11.0, Corel Corporation, Ottawa, Ontario, Canada) forfurther evaluation. The mean gray value for each segment wasadditionally determined using the identical software.Image processingThe original radiograph was exported as uncompressed 8-bittiff file and subsequently separated into its single cells imagesby the image editing software specified above, resulting in 40single images with a spatial resolution of 282 dpi2 each.Thirty-two images comprised a small contrast feature in termsof a borehole; eight images were without. In order to obtain a1:1 numeric distribution between images with and withoutborehole, each of the eight images without borehole wasrepresented four times. This resulted in a total of 64 images(32 with borehole, 32 without). For display on monitor, eachsingle image was presented as uncompressed tiff file using ablack background.Image evaluationImages were presented on a 17-in CRT monitor (XTA 3813MT, Iiyama Electric Co. Ltd., Nagano, Japan; resolution:1,024×768 pixels, true color, blacktest positive, 170 cd/m2).The monitor represented the standard at the time of study beginand was quality tested according to German regulations fordental radiographic viewing monitors both on a daily andmonthly basis. The eye-to-screen distance was between 60and 80 cm and the angle for viewing around 35°. Differentbackground illuminations of 0 lx (completely dark), 50 lx(corresponding to 50 candles at 1 ft distance), 200 lx(corresponding to the illumination of floors), and 500 lx (brightillumination in offices/receptions) were created by dimming anindirect halogen light (floor uplighter, positioned 2 m in frontFig. 1 Aluminum step wedge with boreholes1728 Clin Oral Invest (2013) 17:1727–1731of the monitor, illuminating the ceiling with approximately45°) manually. Lux represent a standardized unit to expressthe perception of brightness for human and is equal to 1 lumenper m2. The background illumination was assessed with a luxmeter (Luxmeter LM-1010, ELVOS GmbH, Ludwigsburg,Germany) in the middle of the monitor which was turned offduring evaluation. The time for adapting the eye to the differentbackground illumination was at least 10 min for each observer.Twenty observer (students of the Dental School in theirfinal year) stated their blinded decision whether or not theycould perceive a dark spot representing a hole using thefollowing five-point confidence scale: 1 0 “hole definitelyvisible”, 2 0 “hole probably visible”, 3 0 “uncertain, whetheror not a hole is visible”, 4 0 “hole probably not visible,” and5 0 “hole definitely not visible.” The order of imagesshown was randomized with respect to the particular back-ground illumination. All observers were briefed and calibratedon the procedure prior to participation by showing them onesample image with and without a borehole. There were notime limits for evaluation. All images were viewed subse-quently at different background illumination in one sessionof an appropriate duration of 20 min.Windowing and levelingwas not allowed for the observers. The ratings were recordedby one author (T. S.) and transferred into spreadsheet software(Microsoft Excel 2003, Microsoft Corporation, Redmond,USA) for statistical evaluation. The evaluation was repeatedunder randomization after a time interval of at least 30 days,thus resulting in total of 10,240 readings (64 images × 4background illuminations × 20 observer × 2 observations).Statistical data evaluationFor each observer and background illumination, the respectiveROC was calculated, with the threshold rating assigned to thenegative proportion of observations. The area beneath eachcurve (Az) was estimated nonparametrically. Accordingly,sensitivity (SN) and specificity (SP) were obtained. Az valueswere compared between the different background illumina-tions using the nonparametric Friedman test to determinestatistically significant differences between the four backgroundilluminations. The level of significance was set at p<0.05.The data were averaged over all observers and both obser-vations and the intra-rater reliability was estimated using theintraclass correlation coefficient (ICC). Additionally, the SNand SP were determined for the main background grayscaleand differences between the four background illuminationswere evaluated separately.ResultsSN averaged over all observers and both observations wasthe highest for 0 lx and the lowest for 200 lx backgroundillumination (Table 1). SP was the highest for 200 lx and thelowest for 0 lx background illumination (Table 1). Thedifferences between the four background illuminations weremarginal for SN and SP (Table 1). Accordingly, Az valuesshowed only slight variance between the four viewing con-ditions (Table 1) without significant difference (p00.796).The intra-rater reliability, i.e., concordance between firstand second observation, was highest for the viewing con-ditions at 500 lx and the lowest for 0 lx (Table 2). Thehighest numbers of positive (correct) results (evaluations)Table 1 Mean sensitivity (SN), specificity (SP), and areas beneath thecurves (Az) for the different background illuminations (in lux)Ambient light SN SP Az0 0.58 0.7 0.73550 0.53 0.72 0.728200 0.5 0.74 0.735500 0.53 0.73 0.788Table 2 Intraclasscorrelation coefficient(ICC) and 95 % confi-dence interval showingthe readers reliability atdifferent backgroundilluminations (in lux)Ambient light ICC0 0.677 (0.758)50 0.706 (0.781)200 0.683 (0.762)500 0.737 (0.806)Fig. 2 Boxplots showing correct evaluations (red) and incorrect evalua-tions (blue) according to the mean gray value of the images and back-ground illumination. At 200 and 500 lx, fewer incorrect evaluations wereobserved in low gray value images than at 0 and 50 lxClin Oral Invest (2013) 17:1727–1731 1729were obtained at a background illumination of 200 and500 lx, at brighter mean grayscale of the images (Fig. 2).The differences were not statistically significant.DiscussionThe digitization of dental radiology in dental offices isassociated with completely different viewing conditionswhen compared to conventional radiography. There is onlyscarce knowledge on the effect of ambient light on thediagnostic performance in digital dental radiology. Howev-er, knowledge on this is important for the daily practicesince ambient light generally is known to be very variablebetween different dental offices.The present study showed that within a range of 0–500 lx, the diagnostic performance in terms of sensitivityand specificity was not significantly influenced by the dif-ferent background illuminations. Pollard et al. evaluated theinfluence of ambient light of 50 lx when compared to 0 lx inchest radiology for the detection of suspicious nodule, andin agreement to our findings, they found no statisticallysignificant differences in sensitivity and specificity forslightly elevated background illuminations [5]. It could ad-ditionally be shown that a moderate background illumina-tion of <50 lx may even improve the observers performancewhen LCD monitors are used [3]. Chawla and Samei con-cluded a background illumination of 75–150 lx range to bean ideal setup for typical LCD’s [4]. The good diagnosticperformance at slightly elevated background illuminationmay be explained by the low diffuse reflection coefficientsand high luminance ratio of LCD monitors [4]. The back-ground illumination in the present study included elevatedambient light levels of 200 and 500 lx. These were usedaccording to recommendations of the DIN-norm 6868–57.In these a background, illumination between 300 and 500 lxis recommended for diagnostic of radiographs exposed onmonitors [9]. Interestingly when evaluating the diagnosticperformance according to the mean gray value of theimages, the study showed for elevated background illumi-nations of 200 and 500 lx less diagnostic errors in lowergray images than for 0 and 50 lx. This seems to be incontrast to studies which showed that values above 450 lxreduce perception of micronodules and pulmonary lines inchest image on cathode ray tubes [10, 11]. A recent studycould show that the detection of caries lesions at a back-ground illumination of 1,000 lx is significantly inferior tothe detection at 50 lx background illumination on a LCDmonitor [12]. It is difficult to compare the results since it isunknown to what extend the different types of monitors,e.g., TFT, LCD or CRT, as used in our study may beinfluenced by the ambient background illumination withregard to the resulting diagnostic performance. However,several studies proved elevated diagnostic performance inthe detection of lung nodules for LCD when compared toCRT as used in the present study [13]. This is mainly relatedto the lower luminance ratio of CRT [13]. Nevertheless, itseems that CRT is superior in low-attenuation areas ofthorax radiographs, whereas LCD may be advantageous inhigh-attenuation areas [14].Contrary to the clinical approach of all cited studies, webased our evaluation on experimental radiographs. Theseincluded small contrast features which had to be detected bythe viewer. The use of phantoms allows clear determination ofthe sensitivity and specificity of a diagnostic system, andresults are not depending on individual interpretation [15].The rather low sensitivity values indicate that the diagnostictask, i.e., perception of small round-shaped dark spots atvarying locations within an image of gray was not as simpleas suggested. Finally and within the limits of a phantom-basedstudy, the data indicate that the impact of background illumi-nation seems to be moderate within the range of 0–500 lx fordetecting small contrast features in digital dental images dis-played on CRT monitors. With regard to the different monitordevices and potential impact on diagnostic performance, wesuggest a clinical study design to confirm the findings of thepresent study. A comparison between CRT and LCD for thediagnostic performance of digital dental radiographs couldadditionally reveal important information on optimum moni-tor display of digital dental radiographs.Conflict of interest The authors declare that they have no conflict ofinterests.References1. Geissler O, Rother U (2007) The film-based printout using tetenalprinting solution II. Int J Comput Dent 10:285–2912. Wenzel A (1999) Matters to consider when implementing directdigital radiography in the dental office. Int J Comput Dent 2:269–2903. Pollard B, Chawla AS, Delong DM, Hashimoto N, Samei E (2008)Object detectability at increased ambient lighting conditions. MedPhys 35:2204–22134. Chawla AS, Samei E (2007) Ambient illumination revisited: a newadaptation-based approach for optimizing medical imaging readingenvironments. Med Phys 34:81–905. Pollard BJ, Samei E, Chawla AS, Beam C, Heyneman LE,Hurwitz-Koweek LM et al (2012) The effect of ambient lightingin chest radiology reading rooms. 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Scharitzer M, Prokop M, Weber M, Fuchsjager M, Oschatz E,Schaefer-Prokop E (2005) Detectability of catheters on bedside chestradiographs: comparison between liquid crystal display and high-resolution cathode-ray tube monitors. Radiology 234:611–61611. Goo JM, Choi JY, Im JG, Lee HJ, Chung MJ, Han D et al (2004)Effect of monitor luminance and ambient light on observer perfor-mance in soft-copy reading of digital chest radiographs. Radiology232:762–76612. Hellén-Halme K, Lith A (2012) Effect of ambient light level at themonitor surface on digital radiographic evaluation of approximalcarious lesions: an in vitro study. Dentomaxillofac Radiol 41:192–19613. Buls N, Shabana W, Verbeek P, Pevenage P, De Mey J (2007)Influence of display quality on radiologist’ performance in thedetection of lung nodules on radiographs. Br J Radiol 80:738–74314. Balassy C, Prokop M, Weber M, Sailer J, Herold C, Schaefer-Prokop C (2005) Flat-panel display (LCD) versus high-resolutiongrays-scale display (CRT) for chest radiography: an observer pref-erence study. Am J Roentgenol 184:752–75615. Otis L, Sherman RG (2005) Assessing the accuracy of cariesdiagnosis via radiograph. Film versus print. J Am Dent Assoc136:323–330Clin Oral Invest (2013) 17:1727–1731 1731

The influence of ambient lighting on the detection of small contrast elements in digital dental radiographs

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The influence of ambient lighting on the detection of small contrast elements in digital dental radiographs

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