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ESTIMATING DISTANCES ON DIRECT DIGITAL IMAGES AND CONVENTIONAL RADIOGRAPHS
-U= ESTIMATING DISTANCES ON DIIRECT DIGITAL IMAGES AND CONVENTIONAL RADIOGRAPHS KATRIEN H. VERSTEEG, D.D.S.; GERARD C. H. SANDERINK, D.D.S., PH.D.; FLORIS C. VAN GINKEL, M.A.; PAUL F. VAN DER STELT, D.D.S., PH.D.
Digital images produced by direct sensor systems are much larger than conventional radio-
graphic film images because of
monitor resolution and digital
image file sizes. This difference in size may introduce difficulties
when estimating distances (for
example, during endodontic treatment). The aim of this study was to compare observers' estimates of distances on digital im-
ages with those of conventional
radiographs.
)igital imaging has proven to be a great technical development, with great diagnostic potential in dental radiology.4 It will probably exist side by side with, or even replace, film-based imaging in the future. Several direct digital systems, based on Charge Coupled Device technology or storage phosphor plates, have been developed. A major benefit of direct sensor systems is the ability to produce instant images, which eliminates the time involved in processing conventional radiographic film, as well as the need for chemicals or a dark room. In addition, the radiation dose can be reduced as a result of the shorter exposure time and increased collimation permitted by the small sensor. Accordingly, the small sensor size makes these systems particularly appropriate for single-tooth imaging during endodontic treatment.
The dimensions of the digital images, when displayed on a monitor, are larger than those of conventional film images. The actual size of the digital images depends on the number of pixels in the x- and y-dimensions and the resolution of the monitor. The number of pixels per millimeter determines the spatial resolution in the digital images and, consequently, the magnification of the details in the image. Therefore, the size and magnification of digital images are much different from those of conventional film images, and may vary among image types produced by different sensor types or image acquisition devices. Magnification may introduce difficulties when practitioners assess root canal length (for example, during endodontic treatment). The aim of this study was to compare estimations of distances from the tip of an endodontic file to the radiographic apex of teeth on digital images and conventional radiographs in order to determine if observers get adjusted to larger images than they are accustomed to. We also included observer experience in our study to see if it affects the estimation of distances. MATERIALS AND MIVETHODS
One of us (K.V.) prepared a lower molar and premolar of a human cadaver jaw with soft-tissue-equivalent material for exposure with an endodontic file (Kerr no. 15). The file was positioned at various insertion lengths. This resulted in 14 arbitrary distances between the tip of the file and the radiographic apex; the minimum length was 0.9 mm and the maximum was 5.8 mm. We used a film holder (Rinn XCP, Rinn Corp.) with wax impressions to obtain reproducible images for the estimation of distance (Figure 1). JADA, Vol. 128, April 1997 439
_RESEARCHImage modalities. Exposures were made using Kodak Ektaspeed film (group A; Eastman Kodak), Vixa direct digital system (group B; Gendex Dental Systems) and the Sens-A-Ray direct digital system (group C; Regam Medical Systems AB). (Editor's note: Kodak Ektaspeed film has been replaced by Kodak Ektaspeed Plus film, and Ektaspeed film is no longer marketed by Kodak; however, the type of film used in this study has no effect on the results.) On the basis of test exposures, the exposure times used for the premolar and molar teeth were 0.22 and 0.24 seconds, respectively, for film; 0.08 and 0.08 seconds, respectively, for the Vixa system; and 0.10 and 0.12 seconds, respectively, for the Sens-ARay system. We measured the 14 distances three times on each imaging system. We used a digital ruler to measure the distances on film, and an electronic ruler (Emago Dental Software, SODS) to measure the distances on the digital images. The mean of the nine measurements for each distance was called the "measured distance," and was considered to be the best approximation of the true radiographic distance. This resulted in 14 measurements, with an accuracy of 0.1 mm (the accuracy of the rulers was 0.01 mm; measurements were rounded to the nearest 0.1 mm), which is consistent with the practical standards of clinical accuracy. Measured distances were as follows: premolar, 0.9, 2.2, 3.5, 4.4 and 5.8 mm; mesial root molar, 3.2, 3.4, 3.6 and 5.0 mm; and distal root molar, 1.3, 1.5, 2.2, 3.7 and 5.5 mm. The 14 images from sets A, B and C were mirrored to obtain a reduction of systematic errors (mirroring done with Paint Shop 440
JADA, Vol. 128, April 1997
Figure 1. Experimental set-up shows the premolar of a human jaw, prepared with an endodontic file, and the direct sensor in position.
Figure -. vxample of a lental film image (Eastman koaaKj) {AR) ( I X ' millimeters), a Vixa (Gendex) direct digital image (B) (75 x 95 mm) and a Sens-A-Ray (Regam) direct digital image (C) (100 x 145 mm).
Pro software, JASC Inc.). This resulted in a total of 28 images per set. Mirroring is the rotation of an image around a vertical axis. This means that the mirror image of a right molar, for instance, resembles the image of a left molar. Student and expert observers. We showed the three sets of images to six fourth-year dental students and six radiodiagnostic experts (from the departments of oral radiology, en-
dodontology and implantology, Academic Centre for Dentistry, Amsterdam, The Netherlands). For each group of observers, six possible sequences were used (for example, ABC, ACB and BAC), and the 28 images in each set were randomly grouped according to these combinations. All observations were performed under clinical viewing conditions. Conventional radiographs were viewed on a masked viewing box.
RBESEARCH11 TABLE
MEAN ERROR
Images Dental film images vixa imagest Sens-Ay i t Obserers St-udents Image Mirror
*
SD*
ERROR RANGE
(mnun
-0.88 -0.86
0.37 0.29 0.28
-1.82 to -0.41 -1.38 to -0.43 -1.04 to -0.18
0.36 0.37 0.37
-1.76 to -0.57 -1.83 to -0.58 -1.69 to -0.52
-1.05 to -0.14 -1.13 to -0.14 -1.04 to -0.14 -1.40 to -0.36
0.63t*
-0.99* -1.04 -0.93
-0.60§
E;xperts
a
(mm)
Image
-0.60
Mirror
-0.60
0.24 0.25 0.26
ean resu
-0.79
0.29
(mm)
SD: Standard deviation.
t Vixa digital images are manufactured by Gendex, Sens-A-Ray digital images by Regam. t Significant difference between the first two image modalities (dental film and Vixa) and Sens.-ARay images at P < .003. § Significant difference between the student and expert groups at P < .0001.
The digital images were displayed on a high-resolution SuperVGA monitor (NEC Multi Sync 3FG, 14 inches, 1,024 x 728 pixels, 256 colors, NEC Technologies) with the help of special application software (Emago Dental Software). This resulted in digital image dimensions of 75 x 95 mm (288 x 383 pixels) for the Vixa system and 100 x 145 (385 x 576 pixels) for the Sens-A-Ray system; the film dimensions were 31 x 41 mm (Figure 2). The observers were asked to estimate (without the help of rulers or other tools) the distance from the tip ofthe endodontic file to the radiographic apex, within an accuracy range of a quarter of a millimeter. We first showed each observer six digital images (three Vixa and three Sens-A-Ray) similar to those used in the study to mm
I
demonstrate the dimensions of digital images. RESULTS
To calculate the amount of overestimation or underestimation, we subtracted the measured distance from each estimate. Multivariate analysis of variance was performed, with these differences serving as the dependent variables. We entered the status of the rater (student or expert), the image modality (film, Vixa system- or Sens-A-Ray system) and orientation (image or mirror image) as within-subjects (in this case, images) factors. Interaction effects were also considered. The table shows the mean errors, standard deviations and error ranges. Main effects were significant for status of the rater (P < .0001) and image modality
(P < .003), but not for image orientation (P > .05). Differences between the film and Vixa images were not significant (P > .05). However, the amount of underestimation was significantly smaller for the Sens-A-Ray images than for the film and Vixa images (P < .003). Rater status and image orientation were involved in an interaction effect, which was significant (P < .045). Both students and experts underestimated the distances for all three image modalities, with students underestimating to a larger extent than the experts. Experts underestimated the distances on both of the image orientations to the same degree, whereas students underestimated distances more on one of the two orientations. None of the interaction effects found to be significant interferes with our ability to draw general conclusions about the status and image modality main effects. Figure 3 shows the correlation between measured distance and error of estimation (that is, estimated distance minus measured distance) using linear regression analysis. The association between error and measured distance was significant for all three image modalities (P < .05). The correlation coefficients were: rfilm = -0.83, rVixa = -0.66 and rS.AR = -0.80. Figure 4 shows the association between the ratio (that is, estimated distance minus measured distance/measured distance) and the measured distance (using linear regression analysis). The error was not a fixed fraction of the actual distance; the larger the distance, the smaller the absolute value of this fraction. The association between the ratio and the measured distance was significant for film and Vixa imJADA, Vol. 128, April 1997 441
DRESEARCH-
Figure 3. Linear regression lines indicate tne correlation between tne measurea estimation on film, Vixa (Gendex) and Sens-A-Ray (Regam) images.
ages (P < .05), but not for SensA-Ray images (P > .05). The relationship was not very strong: r'film= 0.39, rVixa= 0.51, and r'S-A-R = 0.21. Using nonlinear regression analysis, we analyzed a quadratic relationship, which was stronger (r2film = 0.74, r'Vixa = 0.80, and r2S-A-R = 0.44) and significant for all three image modalities (P < .05). DISCUSSION
When performing endodontic treatment, practitioners usually determine the length of the root canal through measurement or estimation. The accuracy and reliability of this measurement will depend not only on the radiographic technique used, but also on the mathematical ability of the operator and the methods of measurement and calculation used. Direct measurement on radiographs (the distance between the tip of the endodontic file and the radiographic tooth apex) has proven to be very accurate.68 This direct method was advocated on the premise that distortion and calculation errors are small 442 JADA, Vol. 128, April 1997
enough to be of no practical consequence.9 When using the formula method (the ratio of the real length of the endodontic file to the projected length on film), a greater frequency of measurement and computation errors occurs.10 Electronic rulers may reduce the errors of measurement and computation. Emago software, for instance, includes an algorithm to calculate the ratio of real length to projected length quickly and accurately." Further investigation into the accuracy of the formula method using Emago's algorithm may be beneficial. In our study, we used estimation to determine the root canal length. The first aim of the study was to investigate the reliability of observers' interpretation of distance on magnified (digital) images. The second objective was to determine if experience facilitates accuracy. All images were displayed in the form in which they would be produced at chairside. The evaluators were asked to estimate the distance between the file tip and the radiographic apex, and not to
aistance ana tne error of
offer an opinion about the clinical importance of the distance measured. This eliminated the problem of defining the appropriate working length, as clinicians may differ in their preference for canal preparation and the distance of the canal from the radiographic apex. We realize the possibility of additional error being introduced by a foraminal exit on the buccal or lingual aspect of a root. However, this possible error was identically reproduced in all three image modalities; therefore, it had no effect on the results of this study. We made no comparisons with the actual file length in the specimen teeth, as this study was confined to a comparison of estimated distances using three image modalities that differed in size. When measuring a distance with a hand ruler, a maximum error of 0.5 mm is permitted.'2 The mean error of estimation in the group of student observers was 0.99 mm, and 0.60 mm in the group of expert observers. Although it seems that experi-
RESEARCH
theeurr o. estniar unre distane.corrneiner (utwert rnel meatureiainshpas fn to be rstro ne rigrotonmneas the error of estimation to measured distance. A nonlinear (quadratic) relationship was found to be stronger than the linear relationship for all three image modalities.
ence facilitates the estimation of distance, we advise practitioners to use a ruler for determining root canal length at all times. It is interesting that observers underestimated all 14 distances. A possible explanation is that observers were intentionally conservative in their estimates. In clinical situations, they may decide the insertion length approaches the working length. In this way, an apparently secure canal preparation is achieved, although a short preparation may be as insecure as a long preparation."3 We assumed that the larger the image, the worse the estimate would be, because of the unrealistic dimensions of structures on magnified images. However, the estimates of distance made on the largest image type (Sens-A-Ray) were better than the estimates made with the other two, smaller image types (dental film and Vixa). These results are difficult to explain, but may be addressed by knowledegable observers of tooth anatomy. Estimations of larger distances
did result in a relatively smaller error. If accurate determination of a small distance is clinically important, one should realize that a very small distance is difficult to estimate, and the relative error may be substantial. CONCLUSION
Estimations of distance on digital images were comparable with, or even better than, those of conventional radiographs; in addition, these estimates are facilitated by experience. Therefore, we can conclude that observers had no problem adjusting to larger images than they were accustomed to. This is an encouraging result since new diagnostic systems should be at least as efficient as the existing systems they are intended to replace. . Dr. Versteeg is a doctoral candidate, Department of Oral Radiology, Academic Centre for Dentistry Amsterdam, Louwesweg 1, 1066 EA Amsterdam, The Netherlands. Address reprint requests to Dr. Versteeg. Dr. Sanderlink is an associate professor, Department of Oral Radiology, Academic Centre for Dentistry Amsterdam, The Netherlands.
Mr. van Ginkel is a statistician, Department of Orthodontics, Academic Centre for Dentistry Amsterdam, The Netherlands.
Dr. van der Stelt is a professor and head, Department of Oral Radiology, Academic Centre for Dentistry Amsterdam, The Netherlands. 1. Webber RL. Computers in dental radiography: a scenario for the future. JADA 1985;1 11(3):419-24. 2. van der Stelt PF. Improved diagnosis with digital radiography. Curr Opin Dent 1992;2(12):1-6. 3. Zubery Y. Computerized image analysis in dentistry: present status and future applications. Compend Contin Educ Dent 1992;13(11):964-73. 4. Sanderink GCH. Imaging: new versus traditional technological aids. Int Dent J 1993;43:335-42. 5. Vande Voorde HE, Bjorndahl AM. Estimating endodontic 'working length' with paralleling radiographs. Oral Surg Oral Med Oral Pathol 1969;27:106-10. 6. Grossman LI. Root canal therapy. 4th ed. London: Henry Kimpton; 1955:214. 7. Ingle JI. Endodontic instruments and instrumentation. Dent Clin North Am 1957;November:805-22. 8. Allred H, Grundy JR, Hatt SD. Precision in the reaming and filling of teeth. Dent Pract Dent Rec 1961;12:39. 9. Nicholls E. Endodontics. Bristol, England: Wright; 1967:110-6. 10. Lilley JD, Boreham NC. Operator errors in endometric endodontia. J Br Endo Soc 1977;10: 17-23. 11. van der Stelt PF. The Emago image processing environment for direct digital radiography in dentistry. In: Proceedings ofthe 15th Annual International Conference of the Institute of Electrical and Electronics Engineers/Engineering in Medicine and Biological Science. San Diego: IEEE; 1993:16234. 12. Bramante CM, Berbert A. A critical eva.luation of some methods of determining tooth length. Oral Surg Oral Med Oral Pathol 1974;37:463-73. 13. Grossman LI. Endodontic practice. 10th ed. Philadelphia: Lea & Febiger; 1981:212.
JADA, Vol. 128, April 1997 443
Digital images produced by direct sensor systems are much larger than conventional radio-
graphic film images because of
monitor resolution and digital
image file sizes. This difference in size may introduce difficulties
when estimating distances (for
example, during endodontic treatment). The aim of this study was to compare observers' estimates of distances on digital im-
ages with those of conventional
radiographs.
)igital imaging has proven to be a great technical development, with great diagnostic potential in dental radiology.4 It will probably exist side by side with, or even replace, film-based imaging in the future. Several direct digital systems, based on Charge Coupled Device technology or storage phosphor plates, have been developed. A major benefit of direct sensor systems is the ability to produce instant images, which eliminates the time involved in processing conventional radiographic film, as well as the need for chemicals or a dark room. In addition, the radiation dose can be reduced as a result of the shorter exposure time and increased collimation permitted by the small sensor. Accordingly, the small sensor size makes these systems particularly appropriate for single-tooth imaging during endodontic treatment.
The dimensions of the digital images, when displayed on a monitor, are larger than those of conventional film images. The actual size of the digital images depends on the number of pixels in the x- and y-dimensions and the resolution of the monitor. The number of pixels per millimeter determines the spatial resolution in the digital images and, consequently, the magnification of the details in the image. Therefore, the size and magnification of digital images are much different from those of conventional film images, and may vary among image types produced by different sensor types or image acquisition devices. Magnification may introduce difficulties when practitioners assess root canal length (for example, during endodontic treatment). The aim of this study was to compare estimations of distances from the tip of an endodontic file to the radiographic apex of teeth on digital images and conventional radiographs in order to determine if observers get adjusted to larger images than they are accustomed to. We also included observer experience in our study to see if it affects the estimation of distances. MATERIALS AND MIVETHODS
One of us (K.V.) prepared a lower molar and premolar of a human cadaver jaw with soft-tissue-equivalent material for exposure with an endodontic file (Kerr no. 15). The file was positioned at various insertion lengths. This resulted in 14 arbitrary distances between the tip of the file and the radiographic apex; the minimum length was 0.9 mm and the maximum was 5.8 mm. We used a film holder (Rinn XCP, Rinn Corp.) with wax impressions to obtain reproducible images for the estimation of distance (Figure 1). JADA, Vol. 128, April 1997 439
_RESEARCHImage modalities. Exposures were made using Kodak Ektaspeed film (group A; Eastman Kodak), Vixa direct digital system (group B; Gendex Dental Systems) and the Sens-A-Ray direct digital system (group C; Regam Medical Systems AB). (Editor's note: Kodak Ektaspeed film has been replaced by Kodak Ektaspeed Plus film, and Ektaspeed film is no longer marketed by Kodak; however, the type of film used in this study has no effect on the results.) On the basis of test exposures, the exposure times used for the premolar and molar teeth were 0.22 and 0.24 seconds, respectively, for film; 0.08 and 0.08 seconds, respectively, for the Vixa system; and 0.10 and 0.12 seconds, respectively, for the Sens-ARay system. We measured the 14 distances three times on each imaging system. We used a digital ruler to measure the distances on film, and an electronic ruler (Emago Dental Software, SODS) to measure the distances on the digital images. The mean of the nine measurements for each distance was called the "measured distance," and was considered to be the best approximation of the true radiographic distance. This resulted in 14 measurements, with an accuracy of 0.1 mm (the accuracy of the rulers was 0.01 mm; measurements were rounded to the nearest 0.1 mm), which is consistent with the practical standards of clinical accuracy. Measured distances were as follows: premolar, 0.9, 2.2, 3.5, 4.4 and 5.8 mm; mesial root molar, 3.2, 3.4, 3.6 and 5.0 mm; and distal root molar, 1.3, 1.5, 2.2, 3.7 and 5.5 mm. The 14 images from sets A, B and C were mirrored to obtain a reduction of systematic errors (mirroring done with Paint Shop 440
JADA, Vol. 128, April 1997
Figure 1. Experimental set-up shows the premolar of a human jaw, prepared with an endodontic file, and the direct sensor in position.
Figure -. vxample of a lental film image (Eastman koaaKj) {AR) ( I X ' millimeters), a Vixa (Gendex) direct digital image (B) (75 x 95 mm) and a Sens-A-Ray (Regam) direct digital image (C) (100 x 145 mm).
Pro software, JASC Inc.). This resulted in a total of 28 images per set. Mirroring is the rotation of an image around a vertical axis. This means that the mirror image of a right molar, for instance, resembles the image of a left molar. Student and expert observers. We showed the three sets of images to six fourth-year dental students and six radiodiagnostic experts (from the departments of oral radiology, en-
dodontology and implantology, Academic Centre for Dentistry, Amsterdam, The Netherlands). For each group of observers, six possible sequences were used (for example, ABC, ACB and BAC), and the 28 images in each set were randomly grouped according to these combinations. All observations were performed under clinical viewing conditions. Conventional radiographs were viewed on a masked viewing box.
RBESEARCH11 TABLE
MEAN ERROR
Images Dental film images vixa imagest Sens-Ay i t Obserers St-udents Image Mirror
*
SD*
ERROR RANGE
(mnun
-0.88 -0.86
0.37 0.29 0.28
-1.82 to -0.41 -1.38 to -0.43 -1.04 to -0.18
0.36 0.37 0.37
-1.76 to -0.57 -1.83 to -0.58 -1.69 to -0.52
-1.05 to -0.14 -1.13 to -0.14 -1.04 to -0.14 -1.40 to -0.36
0.63t*
-0.99* -1.04 -0.93
-0.60§
E;xperts
a
(mm)
Image
-0.60
Mirror
-0.60
0.24 0.25 0.26
ean resu
-0.79
0.29
(mm)
SD: Standard deviation.
t Vixa digital images are manufactured by Gendex, Sens-A-Ray digital images by Regam. t Significant difference between the first two image modalities (dental film and Vixa) and Sens.-ARay images at P < .003. § Significant difference between the student and expert groups at P < .0001.
The digital images were displayed on a high-resolution SuperVGA monitor (NEC Multi Sync 3FG, 14 inches, 1,024 x 728 pixels, 256 colors, NEC Technologies) with the help of special application software (Emago Dental Software). This resulted in digital image dimensions of 75 x 95 mm (288 x 383 pixels) for the Vixa system and 100 x 145 (385 x 576 pixels) for the Sens-A-Ray system; the film dimensions were 31 x 41 mm (Figure 2). The observers were asked to estimate (without the help of rulers or other tools) the distance from the tip ofthe endodontic file to the radiographic apex, within an accuracy range of a quarter of a millimeter. We first showed each observer six digital images (three Vixa and three Sens-A-Ray) similar to those used in the study to mm
I
demonstrate the dimensions of digital images. RESULTS
To calculate the amount of overestimation or underestimation, we subtracted the measured distance from each estimate. Multivariate analysis of variance was performed, with these differences serving as the dependent variables. We entered the status of the rater (student or expert), the image modality (film, Vixa system- or Sens-A-Ray system) and orientation (image or mirror image) as within-subjects (in this case, images) factors. Interaction effects were also considered. The table shows the mean errors, standard deviations and error ranges. Main effects were significant for status of the rater (P < .0001) and image modality
(P < .003), but not for image orientation (P > .05). Differences between the film and Vixa images were not significant (P > .05). However, the amount of underestimation was significantly smaller for the Sens-A-Ray images than for the film and Vixa images (P < .003). Rater status and image orientation were involved in an interaction effect, which was significant (P < .045). Both students and experts underestimated the distances for all three image modalities, with students underestimating to a larger extent than the experts. Experts underestimated the distances on both of the image orientations to the same degree, whereas students underestimated distances more on one of the two orientations. None of the interaction effects found to be significant interferes with our ability to draw general conclusions about the status and image modality main effects. Figure 3 shows the correlation between measured distance and error of estimation (that is, estimated distance minus measured distance) using linear regression analysis. The association between error and measured distance was significant for all three image modalities (P < .05). The correlation coefficients were: rfilm = -0.83, rVixa = -0.66 and rS.AR = -0.80. Figure 4 shows the association between the ratio (that is, estimated distance minus measured distance/measured distance) and the measured distance (using linear regression analysis). The error was not a fixed fraction of the actual distance; the larger the distance, the smaller the absolute value of this fraction. The association between the ratio and the measured distance was significant for film and Vixa imJADA, Vol. 128, April 1997 441
DRESEARCH-
Figure 3. Linear regression lines indicate tne correlation between tne measurea estimation on film, Vixa (Gendex) and Sens-A-Ray (Regam) images.
ages (P < .05), but not for SensA-Ray images (P > .05). The relationship was not very strong: r'film= 0.39, rVixa= 0.51, and r'S-A-R = 0.21. Using nonlinear regression analysis, we analyzed a quadratic relationship, which was stronger (r2film = 0.74, r'Vixa = 0.80, and r2S-A-R = 0.44) and significant for all three image modalities (P < .05). DISCUSSION
When performing endodontic treatment, practitioners usually determine the length of the root canal through measurement or estimation. The accuracy and reliability of this measurement will depend not only on the radiographic technique used, but also on the mathematical ability of the operator and the methods of measurement and calculation used. Direct measurement on radiographs (the distance between the tip of the endodontic file and the radiographic tooth apex) has proven to be very accurate.68 This direct method was advocated on the premise that distortion and calculation errors are small 442 JADA, Vol. 128, April 1997
enough to be of no practical consequence.9 When using the formula method (the ratio of the real length of the endodontic file to the projected length on film), a greater frequency of measurement and computation errors occurs.10 Electronic rulers may reduce the errors of measurement and computation. Emago software, for instance, includes an algorithm to calculate the ratio of real length to projected length quickly and accurately." Further investigation into the accuracy of the formula method using Emago's algorithm may be beneficial. In our study, we used estimation to determine the root canal length. The first aim of the study was to investigate the reliability of observers' interpretation of distance on magnified (digital) images. The second objective was to determine if experience facilitates accuracy. All images were displayed in the form in which they would be produced at chairside. The evaluators were asked to estimate the distance between the file tip and the radiographic apex, and not to
aistance ana tne error of
offer an opinion about the clinical importance of the distance measured. This eliminated the problem of defining the appropriate working length, as clinicians may differ in their preference for canal preparation and the distance of the canal from the radiographic apex. We realize the possibility of additional error being introduced by a foraminal exit on the buccal or lingual aspect of a root. However, this possible error was identically reproduced in all three image modalities; therefore, it had no effect on the results of this study. We made no comparisons with the actual file length in the specimen teeth, as this study was confined to a comparison of estimated distances using three image modalities that differed in size. When measuring a distance with a hand ruler, a maximum error of 0.5 mm is permitted.'2 The mean error of estimation in the group of student observers was 0.99 mm, and 0.60 mm in the group of expert observers. Although it seems that experi-
RESEARCH
theeurr o. estniar unre distane.corrneiner (utwert rnel meatureiainshpas fn to be rstro ne rigrotonmneas the error of estimation to measured distance. A nonlinear (quadratic) relationship was found to be stronger than the linear relationship for all three image modalities.
ence facilitates the estimation of distance, we advise practitioners to use a ruler for determining root canal length at all times. It is interesting that observers underestimated all 14 distances. A possible explanation is that observers were intentionally conservative in their estimates. In clinical situations, they may decide the insertion length approaches the working length. In this way, an apparently secure canal preparation is achieved, although a short preparation may be as insecure as a long preparation."3 We assumed that the larger the image, the worse the estimate would be, because of the unrealistic dimensions of structures on magnified images. However, the estimates of distance made on the largest image type (Sens-A-Ray) were better than the estimates made with the other two, smaller image types (dental film and Vixa). These results are difficult to explain, but may be addressed by knowledegable observers of tooth anatomy. Estimations of larger distances
did result in a relatively smaller error. If accurate determination of a small distance is clinically important, one should realize that a very small distance is difficult to estimate, and the relative error may be substantial. CONCLUSION
Estimations of distance on digital images were comparable with, or even better than, those of conventional radiographs; in addition, these estimates are facilitated by experience. Therefore, we can conclude that observers had no problem adjusting to larger images than they were accustomed to. This is an encouraging result since new diagnostic systems should be at least as efficient as the existing systems they are intended to replace. . Dr. Versteeg is a doctoral candidate, Department of Oral Radiology, Academic Centre for Dentistry Amsterdam, Louwesweg 1, 1066 EA Amsterdam, The Netherlands. Address reprint requests to Dr. Versteeg. Dr. Sanderlink is an associate professor, Department of Oral Radiology, Academic Centre for Dentistry Amsterdam, The Netherlands.
Mr. van Ginkel is a statistician, Department of Orthodontics, Academic Centre for Dentistry Amsterdam, The Netherlands.
Dr. van der Stelt is a professor and head, Department of Oral Radiology, Academic Centre for Dentistry Amsterdam, The Netherlands. 1. Webber RL. Computers in dental radiography: a scenario for the future. JADA 1985;1 11(3):419-24. 2. van der Stelt PF. Improved diagnosis with digital radiography. Curr Opin Dent 1992;2(12):1-6. 3. Zubery Y. Computerized image analysis in dentistry: present status and future applications. Compend Contin Educ Dent 1992;13(11):964-73. 4. Sanderink GCH. Imaging: new versus traditional technological aids. Int Dent J 1993;43:335-42. 5. Vande Voorde HE, Bjorndahl AM. Estimating endodontic 'working length' with paralleling radiographs. Oral Surg Oral Med Oral Pathol 1969;27:106-10. 6. Grossman LI. Root canal therapy. 4th ed. London: Henry Kimpton; 1955:214. 7. Ingle JI. Endodontic instruments and instrumentation. Dent Clin North Am 1957;November:805-22. 8. Allred H, Grundy JR, Hatt SD. Precision in the reaming and filling of teeth. Dent Pract Dent Rec 1961;12:39. 9. Nicholls E. Endodontics. Bristol, England: Wright; 1967:110-6. 10. Lilley JD, Boreham NC. Operator errors in endometric endodontia. J Br Endo Soc 1977;10: 17-23. 11. van der Stelt PF. The Emago image processing environment for direct digital radiography in dentistry. In: Proceedings ofthe 15th Annual International Conference of the Institute of Electrical and Electronics Engineers/Engineering in Medicine and Biological Science. San Diego: IEEE; 1993:16234. 12. Bramante CM, Berbert A. A critical eva.luation of some methods of determining tooth length. Oral Surg Oral Med Oral Pathol 1974;37:463-73. 13. Grossman LI. Endodontic practice. 10th ed. Philadelphia: Lea & Febiger; 1981:212.
JADA, Vol. 128, April 1997 443