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Table of Contents   
ORIGINAL RESEARCH  
Year : 2013  |  Volume : 24  |  Issue : 1  |  Page : 98-103
Effects of image enhancement on reliability of landmark identification in digital cephalometry


1 Department of Orthodontics, Member of Orthodontic Research Center, Faculty of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran
2 Department of Oral and Maxillofacial Radiology, Member of Biomaterial Research Center, Faculty of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran

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Date of Submission25-Nov-2009
Date of Decision26-Aug-2012
Date of Acceptance22-Sep-2012
Date of Web Publication12-Jul-2013
 

   Abstract 

Introduction: Although digital cephalometric radiography is gaining popularity in orthodontic practice, the most important source of error in its tracing is uncertainty in landmark identification. Therefore, efforts to improve accuracy in landmark identification were directed primarily toward the improvement in image quality. One of the more useful techniques of this process involves digital image enhancement which can increase overall visual quality of image, but this does not necessarily mean a better identification of landmarks. The purpose of this study was to evaluate the effectiveness of digital image enhancements on reliability of landmark identification.
Materials and Methods: Fifteen common landmarks including 10 skeletal and 5 soft tissues were selected on the cephalograms of 20 randomly selected patients, prepared in Natural Head Position (NHP). Two observers (orthodontists) identified landmarks on the 20 original photostimulable phosphor (PSP) digital cephalogram images and 20 enhanced digital images twice with an intervening time interval of at least 4 weeks. The x and y coordinates were further analyzed to evaluate the pattern of recording differences in horizontal and vertical directions. Reliability of landmarks identification was analyzed by paired t test.
Results: There was a significant difference between original and enhanced digital images in terms of reliability of points Ar and N in vertical and horizontal dimensions, and enhanced images were significantly more reliable than original images. Identification of A point, Pogonion and Pronasal points, in vertical dimension of enhanced images was significantly more reliable than original ones. Reliability of Menton point identification in horizontal dimension was significantly more in enhanced images than original ones.
Conclusion: Direct digital image enhancement by altering brightness and contrast can increase reliability of some landmark identification and this may lead to more accurate cephalometric analysis.

Keywords: Digital cephalometry, image enhancement, landmark identification

How to cite this article:
Oshagh M, Shahidi S H, Danaei SM. Effects of image enhancement on reliability of landmark identification in digital cephalometry. Indian J Dent Res 2013;24:98-103

How to cite this URL:
Oshagh M, Shahidi S H, Danaei SM. Effects of image enhancement on reliability of landmark identification in digital cephalometry. Indian J Dent Res [serial online] 2013 [cited 2020 Oct 29];24:98-103. Available from: https://www.ijdr.in/text.asp?2013/24/1/98/114958
Cephalometry is an important tool in orthodontic diagnosis, treatment planning, evaluation of treatment results, and prediction of growth. [1],[2],[3] In conventional film radiography, the image quality is already determined during exposure and processing of the image, so that little can be done to improve the quality of the image. During the 1980s and early 1990s, research into the application of digital technologies to lateral cephalometric radiography emerged. Image processing is an intrinsic part of digital radiography which performed in a single data set to obtain image optimization of the final display version. The potential advantages of digital technology lies in the ability to manipulate the image, reduce patient dose, and improve storage and access of information. [4],[5],[6],[7]

It is important to know how accurately measurements on the films are being made and what are the sources of measurement errors. Two general classes of errors occur in the estimation of cranial dimensions from head films. The first may be termed "errors of projection" resulted from a two-dimensional shadow of a three-dimensional object. The second general class of errors may be termed "errors of identification," [1],[2],[3],[8],[9],[10],[11],[12] which are significant when transmitted to angular and linear measurements. Several studies found the main source of error in cephalometry to be the visual identification of the landmarks and thus one of the efforts to improve the precision in landmark identification should be directed toward improvement in the image quality. [8],[9],[11],[13],[14],[15],[16],[17],[18]

It seems logical to believe that cephalometric images with a high resolution and a high contrast should provide the best conditions for accurate landmark identification. Rossmann and Wiley showed that interpretation of radiographic images is dependent on radiological knowledge, pattern recognition, and physical image quality. According to their study, the physical image quality was the best important factor. Since the radiation dose should be limited to achieve, not the best physical image quality, but an image quality which is adequate for accurate diagnostic purposes, [15],[19] efforts to improve accuracy in landmark identification should be directed primarily toward the improvement in image quality. The computerized digital radiographic system permits a wide range of processing methods to be independently applied to each image. One of the most useful techniques of this process involves modifications of image contrast and brightness (edge enhancement), which can also be selectively applied to either darker or lighter areas or to the entire image. The amount of applied edge enhancement can also be varied. Although studies showed an improvement in image quality of digital cephalograms after digital enhancement, whether this degree of resolution translates into improved accuracy of outline tracing and landmark identification remains to be evaluated. [16],[20],[21] This study was performed to evaluate the effectiveness of digital image enhancements on reliability of landmark identification.


   Materials and Methods Top


Twenty patients attending the private center of radiology were randomly enrolled. The gender, type of occlusion, and the skeletal growth pattern were not taken into consideration in the study design. Including criteria were as follows: The image was of sufficient quality to permit identification of the landmarks. This sufficient quality was confirmed by an expert radiologist. There were no unerupted or partially erupted teeth that could have hindered landmark identification.

A storage Phosphore Plate (Digora, Finland) was used as the basis. The cephalometric equipment included an X-ray unit (Planmeca-CC-Proline, Finland), a tube (Focal spot size 0.5 × 0.5 mm, 1.5 mm Al filter) together with a cephalostat. The radiographs were taken with the patients in the fixed head position in the cephalostat prepared in Natural Head Position (NHP). The focus-to-film distance was 160 cm.

The Phosphore Plates were processed (Digora-PSP processor) and saved in computer with JPEG format, with maximum quality setting by using Adobe Photoshop 8.0 software. The final image size was approximately 21 MB. The images were stored as PSD files each requiring about 21 MB of disk space. These images were subsequently opened using Adobe Photoshop 8.0 program. A 15-inch flat screen monitor with display resolution of 800 × 600 pixels was used for image viewing. The digital image resolution was 205 PPi. The images displayed consisted of a 1440 (width) × 1920 (height) pixels matrix.

Image enhancement, including brightness, contrast (without magnification) were done by an expert oral and maxillofacial radiologist to identify individual landmarks as precisely as possible.

Fifteen common landmarks including 10 skeletal and 5 soft tissues were selected on the cephalograms. Two observers (orthodontists) identified the 15 landmarks on the 20 original photostimulable phosphor (PSP) digital cephalogram images and 20 enhanced digital images, twice with an interval time of at least 4 weeks. [9] All original and enhanced images were coded and presented in a random order. The observers were allowed digital zoom function for image enlargement. Landmark identification was performed directly on the monitor-displayed image with a mouse-controlled cursor. After recording a landmark with the mouse, a dot on the monitor-displayed image indicated its position. The landmark position could be corrected until the operator was satisfied. For the images, landmark identification was performed in a dark room, the only illumination being from the PC-monitor.

In further attempt to simulate the range of variation which occurs in clinical practice, no conscious effort was made to increase interobserver reliability as to the manner in which landmarks were to be located except for obtaining consensual verbal agreement as to the definition for each landmark. [8]

The position of landmarks was recorded in the Cartesian coordinate system. The x-axis was defined as a line parallel to the upper margin of the cephalogram and y-axis was perpendicular to the x-axis.

The mean of intraobserver error was used as an indicator to determine the reliability of each cephalometric landmark identification. For reliability of each landmark, the absolute values of the differences between the coordinates of first and second identifications by two observers were assessed. The smaller the difference in millimeters, the higher was the reliability. In this study, differences less than 0.04 millimeter were assumed as zero. Therefore for each of the landmarks in each of two modalities (original and enhanced images), the mean y- and x-coordinates between two observers were calculated. The x-and y-co-ordinates were further analyzed to evaluate the pattern of recording differences in horizontal and vertical directions. The mean and standard deviation of these differences between two observers were calculated. Consequently, the reliability of cephalometric identification could be quantified and compared between original and enhanced images. Reliability of landmarks identification was analyzed by paired t test.

Landmarks are shown in [Figure 1]. [8],[22],[23],[24]
Figure 1: Landmarks definition

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   Results Top


There was a significant difference between original and enhanced digital images in terms of reliability of points Ar and N in vertical and horizontal dimensions and enhanced images were significantly more reliable than original images. Identification of A point and Pogonion points in vertical dimension of enhanced images was significantly more reliable than original ones. Reliability of Menton point identification in horizontal dimension was significantly more in enhanced images than original ones.

Among soft tissue landmarks, identification of Pronasal in vertical dimension of enhanced images was significantly more reliable than original ones.

Identification of other points (S, Or, ANS, A', Ls, B, B', Pog') was not significantly different between two modalities in terms of reliability [Table 1] and [Figure 2].
Table 1: Mean and standard deviation of distance (in millimeter) of two identifications by two observers in horizontal and vertical dimensions in digital and enhanced cephalograms


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Figure 2: Mean distance ( in millimeter ) of two identifications by two observes in landmarks with significant differences in two modalities

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   Discussion Top


The most important source of tracing error is uncertainty in landmark identification. [14],[25] When we take advantage of direct digital cephalometry and image enhancements, it is important to question whether the enhanced digital image yields better level of performance in terms of landmark identification than original digital image. The current study showed that by enhancement, some landmarks were more identifiable vertically such as Pronasal, A Point, and Pogonion. Whereas some points such as Menton was identified more reliably horizontally. This pattern of reliability was approximately similar to that of error distribution for landmark identification, which is associated with complex radiographic images and the topographic orientation of the delineated anatomic boundary. [8],[26] The uncertainty in location of Pronasal, A, Pogonion, and Menton points may be caused by the difficulty in delineating landmarks on the curved anatomical boundary. In this study, landmarks were identified without the aid of construction lines, which may be helpful during the identification of defined landmarks. However, others reported that tracing of constructed lines did not increase the reliability in landmark identification and it seems that the more direct the identification, the higher the landmark reliability. [1],[26]

Also Baumrind explained that the observer is most often asked to estimate the position of a point on an edge. The precision of this operation is a function of how sharply the edge in the region of the point being estimated. Where the edge folds very sharply, the estimates would be very good indeed. However, where the edge is a gradual curve such as point A or Gonion, the task is rendered more difficult and the errors tend to be proportionately larger and to be distributed along the edge itself. This condition also appears to hold true in the cases of Pogonion and points, both of which proved more variable than has been expected. [8] Chen's study also stated that significant differences on the horizontal axis were seen at the Me, Gn, ANS, PNS, and LIA points between traditional and computer-aided digital cephalometry. [1]

Some landmarks with great superimposition of adjacent structures, such as Or and Po, have no direct physical vision on image, and the observer is asked to estimate its position as a conceptual operation based upon his general knowledge. We do not contend that conceptual judgments are invalid, but one should not be surprised to find them more variable than judgments for perceived points. Studies also showed that identification of Porion and Orbitale was inconsistent and unreliable. [1],[27]

In this study, identification of Or, ANS, A, Ls, B, B, and Pog was not significantly different between enhanced and original digital images. But Chen described that using the digital enhancement for landmark digitization allowed for a less obscured view of Porion or Orbitale. A further problem appears to be rigor of definition. For example, Orbitale had been defined in terms of the "more prominent" orbit. Some of the errors in location of this point involved differences in opinion as to which orbit was the more prominent. [8] Difficulties involved in locating different landmarks varied depending on the nature of the landmark examined and radiographic complexities. This is in agreement with Bjork's study. Also Richardson and Geelen described that some landmarks were more reproducible vertically than horizontally and vice versa. [1],[4],[13],[28] Baumrind and Frantz found that magnitude of errors in identification varied from landmark to landmark and the distribution of the errors for each landmark was usually noncircular and characteristic of that landmark. The factors influencing accurate identification were quoted as distinctness of structural detail, noise from adjacent structures due to superimposition of conflicting anatomical details, and conceptual judgment, a factor which is largely based on the past experience and radiological knowledge of the observer. [8],[15]

In this study, two observers identified landmarks of each image twice, with an interval of 1 month. It is clear that average reproducibility of two observers is more accurate than one observer. Although McWilliam described that interobservers differences may often be greater than the effects of physical image quality. [13],[15],[29],[30],[31] It is important that more than one observer are included when a new cephalometric technique is evaluated. Although Baumrind reported that reliability is not sufficient if one landmark in cephalometric image is identified twice, but Midtgard found that an interval of 1 month between two registrations did not significantly affect the reproducibility of the landmark examined. [8],[9]

The present investigation has shown that enhancement of digital cephalometric images improves reliability of landmark identification and it is not in agreement with some other studies which showed that physical image quality is the least important factor which causes errors in landmark identification. [15],[19] In contrast, Geelen et al. reported that the precision of landmark recording in 11 of 21 landmarks was lower for enhanced monitor-displayed images than for both films and digitally enhanced hardcopies, but it was most probably of little clinical significance. This may be attributed to differences in study method. In their study, six observers recorded landmarks, reproducibility was defined as an observer's deviation from the mean between all observers, and digital images were produced by scanning in an image reader. But in this study, we only evaluated reliability and not accuracy of landmark identification. [4]

In this study, reliability of Nasion and Articulare identification in enhanced images was significantly more than original digital images in vertical and horizontal axis. It is not in agreement with Wenzels et al. study which reported that digital processing did not improve the landmark reliability in radiographs with lower quality. [32] It may be attributed to the fact that radiographs in that study had low quality and special pixel size. [17] The cephalometric radiographs used in this study were randomly selected and represented the quality of daily routine work. In Chen's study, the reliability of Ar, Po, ANS, UM in digital images was inferior to traditional cephalometry. The contributing factors to these differences included the nature of cephalometric landmarks, quality of original radiographs, resolution of digital images, and the training level or experience of observers. The most important factor seemed to be the nature of landmark itself. [1]

In this study, there was no significant differences in Sella identifying between enhanced and original images. There was a significant difference between original and enhanced digital images in terms of reliability of points Ar and N in vertical and horizontal dimensions and enhanced images were significantly more reliable than original images. Better reliability of Ar identification in enhanced image could be attributed to better visual quality of this area, which has a great superimposition of skull base. Nasion is an especially important point, since a very large number of clinically employed angular relationships are based on the line SN. In Baumrind's study, the estimates for this point were quite good but there was a disquieting number of gross errors in their study. In their study, Sella presents a unique problem because it involves visual estimation of the center of a structure. [8] We should not be surprised to find that performance of such a task is quite good in this study, since visual estimation of midpoints is a kind of mental averaging process yielding means of reduced dispersion.

In this study, 50% of hard tissue landmarks and only 20% of soft tissue landmarks were more identifiable in enhanced images. This is not in agreement with Eppley's study which concluded that in enhanced images hard tissue landmark identification was similar to conventional cephalometric radiography and soft tissue landmarks were more identifiable. However in their study, only 4 hard tissue and 5 soft tissue measurements were recorded and they compared conventional cephalometry with digital enhanced ones. [16]

It is doubtful whether the higher variation in some landmark recording has a clinically significant influence on the outcome of the cephalometric analysis. [4] In Chen's study, differences between original and digital images were significant but clinically acceptable. [14] Bruntz et al. also described that small horizontal and vertical discrepancies between original, digital, and hard copy of images were clinically insignificant. [27] This was not evaluated in the present study. Although it is sometimes contended that, while great precision may be required for research procedures, estimating with lower accuracy is sufficient for "routine clinical judgments," but it seems that the opposite is the case because findings of a research is valuable only if it is applicable in clinical use. [8]

Increased familiarity with the medium in our digital cephalometric system may reduce the error and improve the reliability in landmark identification. [1] Also it should be explained that the mouse cursor used in this study had comparable gray scale with background image in some areas and became indistinct. This may explain the decreased reliability in some areas. This source of error can be eliminated by altering the software.

Also it must be stated that statistically significant differences of some landmarks between two identifications were less than a tenth of a millimeter and this may be clinically insignificant. Finally, according to this study, image enhancement with modification of brightness and contrast is recommended for the identification of hard tissue landmarks in digital cephalograms. It should be emphasized that, however, these enhancements increase overall visual quality of image but this does not necessarily cause a better identification of a specific landmark and this is due to anatomic specifications of that landmark.


   Conclusion Top


It is concluded that in direct digital cephalograms, image enhancement by altering brightness and contrast can increase reliability of some landmark identification and this may lead to more accurate cephalometric analysis.

 
   References Top

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Correspondence Address:
S H Shahidi
Department of Oral and Maxillofacial Radiology, Member of Biomaterial Research Center, Faculty of Dentistry, Shiraz University of Medical Sciences, Shiraz
Iran
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Source of Support: The authors thank the vice-chancellery of Shiraz University of Medical Sciences, for supporting the research. Also the authors thank Dr. M.Vossoughi from the Dental Research Development Center, for the statistical analysis, and Dr SH.Hamedani for improving the use of English in the manuscript., Conflict of Interest: None


DOI: 10.4103/0970-9290.114958

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