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 »  Home  »  Endodontic Articles 13  »  Indirect digital images: limit of image compression for diagnosis in endodontics
Indirect digital images: limit of image compression for diagnosis in endodontics
Introduction - Materials and methods.



M. Siragusa & D. J. McDonnell
Department of Endodontics, School of Dentistry, Universidad Nacional de Rosario, Santa Fe, Rosario, Argentina.

Introduction.
Several studies have compared the performance of direct digital intraoral radiographs (DDR) and traditional radiographs (Barbat & Messer 1998, Burger et al. 1999). Other studies have reported the results of the digital processing of images and their value as a clinical tool. These studies demonstrate the adequacy and feasibility of this technique and its advantages over conventional radiography (Mistak et al. 1998, Sullivan et al. 2000, Yousef & Saas 2000). The feasibility of digitizing conventional radiographs employing scanners for visualization and/or data transmission has been evaluated, and the minimum technical requirements for adequate digital processing have been established (Pluss et al. 1998).
Digital radiographs are byte maps where each byte represents the localization and intensity value of each element in the image (Gonzalez & Woods 1996). The quality of the image is directly proportional to the amount of descriptive elements. Most images have areas of slight variations that allow for the use of algorithms to reduce the size of the file. The aim of file compression is to minimize transmission time and memory requirements. The maximum degree of compression that is not detrimental to the quality of diagnosis in endodontics has not yet been established (Wenzel et al. 1996).
The aim of the present study was to analyse the effect of the information loss that results from the compression of a file containing a radiographic image.

Materials and methods.
Fourteen randomly selected intraoral radiographs with different degrees of diagnostic difficulty, including preoperative working length determination or postobturation images, were selected for study. They were digitized with an AGFAARCUS II scanner using the maximum optical resolution without interpolation, i.e. 300 ppi and a grey scale of 8 bytes. The evaluation scale was kept constant and independent of the natural histogram corresponding to each image. A digital image with no saturation or trimmings was obtained. The resulting image (176410-177114 bytes) was stored in Tagged Image File Format (TIFF).
Each of the resulting image files was compressed without loss of information employing WinZip 8.0. Compression with loss of information was performed with Photoshop 5.0 software (Adobe Systems Inc., San Jose, CA, USA) and the Joint Photograph Expert Group (JPEG) format. Each of the images was compressed from its original TIFF format to each of the available qualities (10- 0). The resulting 182 images were grouped as follows: 14 original images, 14 images compressed without loss of information and 154 images compressed with loss of information, in11groups of14 images each.
A qualitative evaluation by a single endodontic expert, to avoid the introduction of interobserver variables and to guarantee uniform assessment criteria, was performed. The aim was to compare the observations that resulted from each of the original TIFF images with those derived from each of the corresponding JPEG images. The observer had no knowledge of the degree of compression applied in each case and was allowed to adjust brightness and contrast freely for the original image. The same values were then applied to all the JPEG images. Observation was performed employing Photo-Paint 8 software (Corel Corporation, Ottawa, Ontario, Canada), the tool in which the observer was best trained. The observed images were scored as follows:
  • Adequate for diagnosis: when the observer could see the image details clearly on the monitor.
  • Adequate for illustration only: when the image details where blurred on the monitor.
  • Inadequate: when the image sharp details where lost.
For the quantitative analysis, each of the JPEG images was digitally subtracted from the corresponding TIFF original image, thus generating a new image in TIFF format as a result of the following operation performed using Adobe Photoshop 5.0. D(n,j).tif ј n.tif_n(quality j).jpg ю125 where n ј1. . .14, is the number of the image, j ј 0. . .10, is the quality applied by Photoshop 5.0 in JPEG format, 125 is a fixed half grey level in the 8 bits scale (125 ј256/2), D(n, j).tif is the digitally subtracted image in TIFF format, n.tif is the original image number n, in TIFF format and n(quality j).jpg is the image number n compressed in JPEG format, with quality j.
The histograms of the grey values corresponding to the 154 new images were statistically processed. The grey values of the pixels followed a normal distribution. The mean and standard deviation were calculated for each case.