Contrast perception in digitized panoramic radiographs compared with their film-based origin

Contrast perception in digitized panoramic radiographs compared with their film-based origin Ralf Kurt Willy Schulze, Dr med dent,a Stephan Tobias Roman Rosing,b and Bernd d’Hoedt, Prof Dr med dent,c Mainz, Germany JOHANNES GUTENBERG-UNIVERSITY

Objectives. We sought to compare the visual image quality of film-based and digitized panoramic radiographs through use of a hole-containing test wedge. Study design. An aluminum wedge containing 100 cells, of which 90 were given shallow holes, was exposed in the film-based Orthophos CD panoramic unit. Two radiographs subjectively exhibiting optimum contrast were selected. Films were digitized with a charge-coupled device flatbed scanner at 300 dpi. Films and digitized images were rated cellwise by 2 similar groups of 50 observers each with respect to spot perception. Results. The mean sensitivity was 0.26 ⫾ 0.09 for film and 0.20 ⫾ 0.07 for digitized images (P ⫽ .000), with a pronounced decline in the latter in regions of high background density. The average specificity was 0.93 ⫾ 0.07 for film versus 0.92 ⫾ 0.08 for digitized images (P ⫽ 0.213). Conclusion. Film yielded a significantly higher sensitivity, but this absolute difference was actually small compared with that of the digitized images. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2002;94:388-94)

Emerging technologies such as teleradiology depend on digital data instead of film-based radiographs. Because the majority of radiographic units still operate with conventional film, digitization seems to be an ideal interim solution to create digital image files. Although laser film scanners are the de facto standard for highquality digitization, more and more charge-coupled device (CCD)– based scanners are available, which are commonly less expensive than laser scanners.1 Flatbed scanners typically operate by transillumination of the radiograph from a light source, then detection of the pixels line by line simultaneously with a linear array of CCD detectors.1 It is assumed that simple application and low costs make flatbed scanners the preferred digitization device for dental offices. Because the digitization process is based on the extraction of image information from the film radiograph, digitized images have to be evaluated with regard to their image content in comparison to their film-based origin. In other words, This article contains parts of the thesis of Stephan Rosing, titled “Comparative evaluation of direct digital and digitized panoramic radiography.” a Lecturer, Department of Oral Surgery, Dental School, Johannes Gutenberg-University, Mainz, Germany. b Scientific Assistant, Department of Oral Surgery, Dental School, Johannes Gutenberg-University, Mainz, Germany. c Head of Department, Department of Oral Surgery, Dental School, Johannes Gutenberg-University, Mainz, Germany. Received for publication Aug 1, 2001; returned for revision Dec 9, 2001; accepted for publication Apr 7, 2002. © 2002, Mosby, Inc. 1079-2104/2002/$35.00 ⫹ 0 7/16/126450 doi:10.1067/moe.2002.126450

388

the expected information loss has to be quantified to obtain reliable data for routine use. Despite this obvious need for reliable data, however, we are aware of only a few studies dealing with the quality of the digitization process of extraoral x-rays.2-4 Following the example of the well-known perceptibility curve test,5-8 we conceived of a test object equipped with details giving increasing or decreasing contrast relative to a homogeneous background adequate for this type of evaluation. A commercially available CCD-based flatbed scanner specifically designed for digitization of dental x-rays was applied for acquisition of digital data from the x-rays. Instead of comparing physical parameters such as contrast, resolution, and optical density (OD) separately for each mode (radiographic film/digitized images), our investigation intended to evaluate image properties that are caused by the complex interaction of those factors (ie, the perceptibility of small bur holes in panoramic radiographs). This study was based on the hypothesis that perception of these holes is equal in the film and digitized images. It was designed to detect possible loss of image information caused by the digitization process. MATERIAL AND METHODS Aluminum test object A stepless wedge of pure aluminum was manufactured (height: 10 cm, width, 7 cm; decreasing thickness, 2.0-0.5 cm). The surface (10 cm ⫻ 7 cm) opposite the oblique side of the wedge will be referred to as the front surface. On this front surface, lines parallel to the vertical edge were drawn at increments of 7 mm and,

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Table I. Distribution of bur hole depths (upper figures) over the aluminium phantom according to the grid pattern presented to the observers 1 2 3 4 5 6 7 8 9 10

A

B

C

D

E

F

G

H

I

J

0.21 (24.19)* 0.36 (24.04) 0.30 (23.86) 0.21 (24.10) 0.41 (23.84) 0.23 (24.13) 0.31 (23.82) 0.29 (23.85) 0.29 (24.05) 0.28 (23.93)

0.29 (21.55) 0.43 (21.44) 0.32 (21.41) 0.54 (21.23) 0.43 (21.39) 0.39 (21.48) 0.55 (21.30) 0.41 (21.58) 0.40 (21.44) 0.58 (21.19)

0.41 (19.50) 0.58 (19.33) 0.47 (19.58) 0.42 (19.54) 0.40 (19.66) 0.72 (19.16) 0.24 (19.62) 0.51 (19.26) –

0.20 (17.49) 0.68 (17.03) 0.37 (17.31) 0.19 (17.44) 0.44 (17.25) 0.24 (17.44) 0.21 (17.46) 0.30 (17.15) 0.17 (17.35) 0.18 (17.40)

0.18 (15.71) 0.24 (15.55) 0.25 (15.62) 0.25 (15.74) –

0.27 (14.03) –

0.29 (11.41) 0.34 (11.24) 0.25 (11.29) 0.31 (11.33) –

0.65 (9.31) –

0.19 (7.79) 0.13 (7.77) 0.24 (7.67) 0.72 (7.22) 0.28 (7.71) 0.43 (7.51) 0.34 (7.62) 0.33 (7.64) –

0.41 (5.86) 0.42 (5.94) 0.30 (6.18) –

0.22 (19.56)

– 0.23 (15.66) 0.14 (15.77) 0.54 (15.46) 0.33 (15.59)

0.19 (13.84) 0.18 (14.05) 0.49 (13.84) 0.16 (14.05) 0.29 (13.96) 0.26 (14.04) 0.19 (14.07) –

0.69 (10.97) 0.93 (10.66) 0.54 (11.18) 0.46 (11.16) 0.29 (11.28)

0.44 (9.71) – 0.26 (9.62) 0.29 (9.59) 0.15 (9.73) 0.24 (9.76) 0.46 (9.43) 0.22 (9.65)

0.21 (7.65)

0.34 (6.03) 0.47 (5.83) 0.18 (6.13) 0.29 (6.04) 0.30 (6.05) 0.57 (5.75)

*The lower figure in each cell (in parentheses) represents the calculated material thickness in beam direction behind the hole. All values are given in millimeters.

perpendicular to them, parallel lines to the horizontal edge at increments of 10 mm. This procedure resulted in a grid pattern (rows 1-10; columns A-J) containing 100 cells of 10 mm ⫻ 7 mm. Short pieces (approximately 3 mm) of wire (diameter, 0.3 mm) were glued to the surface with sticky wax at each endpoint of a line, providing clearly visible radiologic markers. On a drilling bench, shallow holes of cylindrical shape were drilled perpendicularly into the front surface of the wedge. Arbitrarily distributed over the grid pattern, 90 squares were prepared with 1 hole in a random location within each cell, with at least 2 mm to the cell boundary. Ten squares were left without a hole. The depths of the holes were subsequently assessed to the nearest 0.01 mm by means of a precise incremental lengthmeasuring instrument (Millitron; Mahr GmbH, Go¨ ttingen, Germany). The distribution of the holes over the grid pattern, their depth, and the resulting material thickness in beam direction is presented in Table I. Exposures A conventional film-based panoramic unit (Orthophos CD; Sirona Dental Systems GmbH, Bensheim, Germany) was used for the study. Radiographs were exposed on high-speed film (Kodak Lanex T-Mat G/RA Dental Film; Eastman Kodak Company, Rochester, NY) in combination with the corresponding intensifying screens (Kodak Lanex Medium Screens, Eastman Kodak Company). A positioning table was constructed to fit in the

chinrest of the Orthophos CD panoramic unit. The table provided a horizontal platform on which the aluminum wedge was fixed in a reproducible position relative to the sharp image layer of the panoramic device. This was carried out in such a way that the front surface containing the bur holes was aligned to the center of the sharp image layer that had been determined experimentally according to the description presented in a previous article.9 In brief, 8 metallic spheres were positioned in a U-shape on the horizontal platform of the positioning table and then shifted and subsequently exposed until horizontal and vertical magnification of each sphere was identical. By connecting these positions the center of the image layer of sharp depiction was obtained. The front surface containing the drill holes was directed toward the radiographic film (Fig 1). For each bur hole, material thickness as measured along the central beam was calculated by means of the Pythagorean theorem. Panoramic radiographs were obtained at various exposure parameters covering the full exposure range of the unit. Films were processed immediately after exposure with fresh processing solution in an automated processing machine (Curix 242S; Agfa Gevaert N. V., Mortsel, Belgium). The contrast was assessed by two observers (R.S. and S.R.) who viewed the radiographs on a viewing box (Kaiser Polite 5000; Kaiser Fototechnik GmbH & Co KG, Buchen, Germany), and the 2 images showing the broadest range of shades of gray within the image of the phantom, according to the independent subjective decision of both

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ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY September 2002

spectrums are condensed. During the scanning process, both radiographs were placed in an identical position on the scanner surface and default settings were used. The ODs of the digitized images were assessed for each column according to the procedure described earlier, by means of commercial image-editing software (Adobe Photoshop 5.0; Adobe Software Inc, Mountain View, Calif). Image evaluation

Fig 1. Drawing of the positioning scheme of the wedge within the panoramic machine. To ensure optimum sharpness, the hole containing the front side of the wedge (dashed line) was aligned to the chord of the experimentally determined lateral part of the curve representing the center of the sharp image layer (dotted line).

observers, were selected for evaluation. The decision of both observers was identical. The ODs of the 2 radiographs were measured in triplicate for the right and left side of each column by means of a film densitometer (Digital Densitometer II Freiburg Physikalisch-Technische Werksta¨ tten Dr Pychlau GmbH, Freiburg, Germany) and averaged for both sides of each column. Film digitizing The 2 selected radiographic films were digitized at 300 ⫻ 300 dpi and an 8-bit depth of grayscale by means of a CCD-based, single-pass flatbed scanner (FRIACOM X-ray Digitizer; Friadent GmbH, Mannheim, Germany) specifically designed for digitization of radiographic images. Incorporating a transillumination unit, the scanner provides a maximum scan size of 210 mm ⫻ 270 mm and an optical resolution of 600 dpi ⫻ 1200 dpi. To reduce the file sizes of panoramic or lateral skull radiographs, however, the software provided by the manufacturer (FRIACOM DentalOffice, version 2.4.175) limits the resolution to 300 dpi. The manufacturer claims that the optical density is ⱖ3.3 OD. Before digitization the software requires determination of the density range of the source radiograph at 3 steps—from “overexposed,” via “ideal,” to “underexposed”. Because only the 2 radiographs exhibiting optimum contrast were used for our evaluation, “ideal” was selected for the digitization process. The underlying gamma curve has a sigmoid shape, with its steepest ascent in an interval of medium brightness. This results in a spreading of gray values located in the center of the total range, whereas values located in the dark and light

The digital image data were exported as uncompressed tagged-image file format files and subsequently implemented in Power Point software (MS Power Point 97; Microsoft Corporation, Redmond, Wash). Because the study originally included corresponding radiographs obtained from a direct digital panoramic machine that were later omitted from evaluation because of shortcomings in the experimental design, the digitized images had to be enlarged by a factor of 1.8 (screen size: width ⫽ 175 mm, height ⫽ 286 mm) to compare with the direct digital radiographs. The enlargement procedure was done by means of the implemented feature in MS Power Point 97 software for enlargement of embedded tagged-image file format images. The grid containing 10 columns (A-J) and 10 rows (1-10) was transferred onto the digitized images by connecting the wire images with white lines (Fig 2). Clear transparencies were mounted on top of each radiograph, on which the grid was drawn with a thin white marker. Evaluation of the digitized images was carried out on a 17-inch cathode-ray monitor (Vision Master 17“; Iyama Corporation Ltd, Nagano City, Japan), with a display of 1280 ⫻ 1024 monitor pixels, whereas the x-ray films were viewed on a viewing box for the radiographs specified earlier. Both evaluations were performed in the same darkened and quiet room. Because of the change in the original study design and the resulting delay, 2 similar groups (groups D and F) containing 50 observers each were recruited mainly from the students of the Dental School in Mainz. The inclusion criterion for students was completion of their primary radiologic course at least a half year before image evaluation. Detailed data on the composition of the 2 observer groups are given in the ”Results‘ section. No observer with obviously poor eyesight was included. Before image evaluation, all observers were briefed on the procedure. Observers had to rate whether they perceived a spot representing a hole in each cell of the grid by a simple yes/no decision, transferring this decision onto a sheet with grid patterns identical to those on the images. Contrast adjustment was not allowed for the digitized images, nor was magnification applied for the x-ray films. Although no time limits

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Schulze, Rosing, and d’Hoedt 391

Fig 2. Radiographic digitized image provided with the grid pattern as displayed to the observers. Note that, in contrast to the film and the monitor display, only very few spots are perceivable.

were set, the approximate duration of an evaluation period for either mode was 10 to 15 minutes. Statistical evaluation Both exposures per mode were combined for the statistical evaluation. True positive, false-positive, true negative, and false-negative observations were counted for each mode and observer. The sensitivity (SNT) and specificity (SPF) were calculated from these results for each observer (⫾SD), averaged for each image modality, and compared by means of the paired t test. This test was also applied for the comparison of the characteristics of the 2 observer groups. Regression analysis was performed with SNT and SPF as dependent variables to calculate cofactors influencing observations, such as the depth of the drill hole, the thickness of the aluminum wedge at the respective site, and the radiologic experience of the observers. Results were considered significant when P ⬍ .05. RESULTS Mean radiologic experience did not differ significantly between the 2 observer groups (mean digitized, 3.1 years; mean film, 2.8 years; P ⫽ .374). Sex composition was almost equal, with group D incorporating 27 female and 23 male observers, while group F consisted of 26 female and 24 male observers. Only 7 observers (14%) were identical in both groups. Film OD values ranged from 0.33 to 3.11, whereas the corresponding digital grayscale values ranged from 217 to 14 (Table II). The depth of the bur holes ranged

between 0.13 mm and 0.93 mm (mean, 0.35 mm; Table I). The average SNT was low, with a mean of 0.26 ⫾ 0.09 for film- based radiographs and 0.20 ⫾ 0.07 for digitized radiographs. This difference was significant (P ⫽ .000). The average SPF (film, 0.93 ⫾ 0.07; digitized images, 0.92 ⫾ 0.08) did not differ between the image modalities (P ⫽ .213). Boxplots of the average SNT and SPF per observer are shown in Fig 3. The SNT was highest in column F (mean gray value, 58) for digitized images, whereas maximum values for film images were reached in column G (mean OD, 2.04). An abrupt decline to very low SNT values (ⱕ0.11) from column G on is evident for the former (Table II). The influences of cofactors on sensitivity and specificity were calculated by regression analysis (Table III). No significant interaction was calculated between either SNT or SPF and the radiologic experience of the observers. However, the depth of the bur holes did have a significant influence on the SNT in both modalities (film, P ⫽ .000; digitized images, P ⫽ .024). The material thickness in beam direction had a significant influence on the SNT and the SPF only for the digitized radiographs. DISCUSSION Because digitization of conventional radiographic images is commonly reported to reduce storage space and to offer numerous advantages in terms of digital data management, our study aimed to answer the ques-

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ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY September 2002

Table II. Mean sensitivity and mean specificity in correlation to the optical density ranges in columns A B C D E F G H I J F: 0.33-0.39 F: 0.41-0.51 F: 0.60-0.73 F: 0.81-1.00 F: 1.15-1.34 F: 1.55-1.79 F: 1.92-2.16 F: 2.30-2.56 F: 2.66-2.85 F: 2.98-3.11 D: 217-207 D: 206-188 D: 185-157 D: 154-116 D: 109-76 D: 71-45 D: 43-31 D: 25-18 D: 18-14 D: 4-14 SNT film (mean) SNT digitized (mean) SPF film (mean) SPF digitized (mean)

0.06

0.26

0.41

0.40

0.28

0.45

0.63

0.18

0.02

0.02

0.04

0.13

0.36

0.47

0.40

0.51

0.11

0.00

0.01

0.01

na

na

0.91

na

0.90

0.96

0.79

0.99

0.99

1.00

na

na

0.92

na

0.79

0.85

0.99

1.00

0.99

1.00

F, Film analog density range; D, digital grayscale range; SNT, mean sensitivity; SPF, mean specificity; na, not applicable.

Fig 3. Boxplots of observers’ mean senstitivity (SNT) and specificity (SPF) for both image modalities. Each box represents the interquartile distance covering 50% of the values, with the median indicated as a horizontal line within each box. Values lying no more than 1.5 interquartile distance from the respective edges of the boxes are marked by the whiskers, whereas all values exceeding these limits (extreme values) are displayed as crosses (⫻). As per the definition, the SPF cannot exceed 1.0, resulting in the truncated appearance of the SPF plots at the upper end.

tion of whether digitization is able to sufficiently translate radiographic film characteristics into digital image files. Images were digitized at a resolution of 300 dpi, which is considered to be a clinically acceptable compromise between storage size and image information.10 All other parameters were set according to the default settings provided by the manufacturer. Inconsistencies among scans as reported by Chen and Hollender2 were prevented by digitization of both radiographs in an identical location and orientation on the scanner surface.

Radiographic image characteristics are commonly described in terms of contrast, resolution, and OD11, whereas radiographic diagnosis is based on an intuitive evaluation of the complex interaction among those parameters. To accommodate the objectives of our study, a test model yielding information on all 3 parameters and on the underlying physical properties had to be designed. By means of an aluminum wedge with shallow drill holes, slight contrast differences relative to a homogeneous background were created. The presence or absence of a spot representing a bur hole in defined cells had to be rated by a simple yes (presence) or no (absence) decision. Although we used an uneven distribution of 90 holes and only 10 “nonholes,” our approach allowed false-positive findings, thus facilitating the determination of SNT and SPF. It should be noted that SPF was calculated from only 10 instances of true absence of a hole. This imbalance certainly affected the statistical outcome; thus the results on SPF indicate a trend and should be investigated further. Test SNT refers to the relative proportion of instances of the presence of a hole that produced positive findings (“yes”), whereas test SPF was defined as the relative proportion of times when the absence of a hole (“no”) is correctly verified.12 The simplicity of the radiologic decision requested from the observers did not require substantial experience; thus, we were able to obtain data from many observers (50), mainly recruited from among our students. One major weakness in our study is that x-ray films and their digitized counterparts were evaluated by 2 separate sets of observers, with an overlap of only 7 observers. This was caused by the interval between the evaluation of the digitized images, which was approximately 1.5 years before that of the radiographic films. Considering the high number of observers, in combination with the similarity of the groups, however, we believe that their readings are

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Table III. Influence of cofactors on SNT and SPF as calculated from regression analysis

Material thickness Depth of bur hole Experience of observer

SNT film (mean)

SNT digitized (mean)

SPF film (mean)

SPF digitized (mean)

0.210* 0.000*† 0.733‡

0.008*† 0.024*† 0.230‡

0.137a na* 0.451‡

0.005*† na* 0.385‡

*Calculation based on mean SNT/SPF per cell. †Significant result. ‡Calculation based on mean SNT/SPF per observer.

comparable. Furthermore, if we follow the common process of drawing conclusions from the observations of 1 group of observers regarding generality, we implicitly assume that the observations of two similarly composed observer groups are also similar. Our data reveal very low overall sensitivity for the entire study, combined with relatively high SDs. The latter translates to a high magnitude of interobserver discrepancies between identical radiographic decisions (ie, low reliability). Although low reliability of radiologic decision processes is a well-known fact and has already been reported in the 1970s by Goldman et al,13,14 we had expected a somewhat higher reliability because of the simple yes/no decision. However, 2 factors account for the very low overall SNT and the high SD: First, the design of the testing device consisted of a relatively massive aluminum wedge containing shallow drill holes, with their depths distributed randomly over the phantom. This design produced very slight contrast differences relative to the surrounding background, making perception a difficult task for the observers. Secondly, motion blur typical for panoramic radiographs15 made perception of the holes even more difficult. It should also be mentioned that, despite our efforts to have the phantom aligned to the center of the sharp image layer, this central line is curved according to the shape of the dental arch. This results in different sharpnesses of the hole images in different columns. However, because this effect pertains to both digitized and film images and our approach is purely comparative, it will not directly affect the results of the study except for an overall negative impact on the SNT. Of course, the viewing conditions and the respective eyesight of an individual observer have a fundamental influence on his perception performance. However, the second factor should not play a major role in this study because normal eyesight should be one of the basic requirements for a dental student. Nevertheless, the influence of different viewing conditions for digitized and film images cannot be ignored. It is assumed that the average observer is more familiar with viewing radiographs on a viewing box than on a computer monitor. This factor might have partially influenced our finding that, on an overall low basis, the SNT was

significantly higher for the radiographic films than for their digitized counterparts (P ⫽ .000). On the other hand, the enlarged display of our digitized images presumably enhances the visibility of the spots. As expected, the depth of a hole did have a significant influence on the SNT in both modalities. Obviously, the SNT was highest in the middle of the wedge (ie, in areas with medium background brightness) and declined on either side of the phantom. Particularly in the columns of high OD, the SNT declined rapidly for the digitized radiographs; this finding was less pronounced for films. These results may also explain the evident influence of material thickness in beam direction on the SNT only for digitized images. Because the OD is directly correlated with material thickness and the SNT is much more evenly distributed over the density range in our radiographic films, the expected influence of this parameter may be lost in the digitization process because of the spreading of medium density values caused by the sigmoid-shaped gamma curve. Obviously, small contrast differences within a relatively dark background are not sufficiently translated by the digitizing unit when operated with the settings specified earlier. It would be interesting to evaluate whether a different setting of the gamma curve or a completely different curve would have altered these results. Because of the selection of radiographs with the best possible contrast, however, our images exhibited a wide range of ODs from 0.33 to 3.11. This translates into a variety of shades of gray, which are assumed to be representative of a typical clinical radiograph with sufficient contrast (ie, an “ideal” image). On the basis of these considerations, even a possible superior performance under different scanner settings would not be clinically relevant as long as the instructions for the user are not changed accordingly. Our findings are in concordance with those of Hangiandreou et al,1 who describe generally increasing noise and decreasing signal and OD exceeding 3.0 for 4 evaluated CCD scanners. Shortcomings in the correct translation of radiographic images containing high ODs, therefore, seem to be a typical technical problem of CCD-based flatbed scanners, irrespective of the specific device. Consequently, in a clinical situation, the detection of contrast differ-

394 Schulze, Rosing, and d’Hoedt

ences in dark areas (eg, interproximal crestal bone) might be hindered because of the digitization with such scanners. Hangiandreou et al1 concluded from their findings that CCD digitizers are not able to produce a reliable digital translation of plain radiographs because of their limitations in density range.1 Considering the complexity of clinical decision making based on radiographic evaluation, we feel that the conclusion of Hangiandreou et al1 and our findings should be substantiated by additional clinically oriented experiments. It should also be kept in mind that the actual difference calculated for SNT was relatively small (⬃6%). Conclusions regarding the clinical relevance of such differences would be highly speculative. In conclusion, we aimed to clarify whether image information is lost by the CCD-based digitization of conventional panoramic radiographs as compared with their film-based origin. Our results demonstrate a significant loss of SNT (ie, loss of information, particularly in dark zones of high OD). Despite being statistically significant, the actual difference between both modalities was relatively small. Because of the experimental design of the study, a transfer to a clinical situation is beset with difficulties. We conclude from our findings that whenever a x-ray film is digitized with a flatbed scanner, possible information loss should be kept in mind and diagnostic conclusions should be considered carefully in this context. REFERENCES 1. Hangiandreou NJ, O’Connor TJ, Felmlee JP. An evaluation of the signal and noise characteristics of four CCD-based film digitizers. Med Phys 1998;25:2020-6. 2. Chen SK, Hollender L. Digitizing of radiographs with a flatbed scanner. J Dent 1995;23:205-8. 3. Chen SK, Chiang TC. Digitizing of radiographs with a rollertype CCD scanner. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997;83:719-24.

ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY September 2002 4. Davidson HC, Johnston DJ, Christian ME, Harnsberger HR. Comparison of radiographic image quality from four digitization devices as viewed on computer monitors. J Digit Imaging 2001; 14:24-9. 5. Hayakawa Y, Kitagawa H, Wakoh M, Kuroyanagi K, Welander U. Assessing the image quality of a CCD-based digital intraoral radiography system: application of perceptibility curve test. Bull Tokyo Dent Coll 2000;41:9-14. 6. Yoshiura K, Stamatakis H, Shi X-Q, Welander U, McDavid WD, Kristoffersen J, et al. The perceptibility curve test applied to direct digital dental radiography. Dentomaxillofac Radiol 1998; 27:131-5. 7. Yoshiura K, Kawazu T, Chikui T, Tatsumi M, Tokumori K, Tanaka T, et al. Assessment of image quality in dental radiography, part 1: phantom validity. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999;87:115-22. 8. Yoshiura K, Kawazu T, Chikui T, Tatsumi M, Tokumori K, Tanaka T, et al. Assessment of image quality in dental radiography, part 2: Optimum exposure conditions for detection of small mass changes in 6 intraoral radiography systems. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999;87:123-9. 9. Schulze R, Schalldach F, d’Hoedt B. Auswirkung von Positionierungsfehlern auf Vergro¨ sserungsfaktoren im Unterkiefer bei der digitalen Panoramaschichtaufnahme. Mund Kiefer Gesichts Chir 2000;4:164-70. 10. Attaelmanan A, Borg E, Gro¨ ndahl H-G. Digitisation and display of intra-oral films. Dentomaxillofac Radiol 2001;29:97-102. 11. Teslow TN. The laser film digitizer: density, contrast, and resolution. J Digit Imaging 1997;10:128-32. 12. Imrey PB. Considerations in the statistical analysis of clinical trials in periodontitis. J Clin Periodontol 1986;13:517-28. 13. Goldman M, Pearson AH, Darzenta N. Endodontic success— who’s reading the radiograph? Oral Surg Oral Med Oral Pathol 1972;33:432-7. 14. Goldman M, Pearson AH, Darzenta N. Reliability of radiographic interpretations. Oral Surg Oral Med Oral Pathol 1974; 38:287-93. 15. McDavid WD, Welander U, Kanerva H, Tronje G, Morris CR. Definitions of unsharpness and layer thickness in rotational panoramic radiography. Oral Surg Oral Med Oral Pathol 1984;57: 96-101. Reprint requests: Ralf K. W. Schulze, Dr. med. dent. Poliklinik fu¨ r Zahna¨ rztliche Chirurgie Augustusplatz 2 55131 Mainz Germany [email protected]

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