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Indirect digital images: limit of image compression for diagnosis in endodontics
1Department of Endodontics, School of Dentistry, Universidad Nacional de Rosario, Santa Fe, Rosario, Argentina. msiragus@arnet.com.ar
Aim:
To analyse the impact of the loss of information that results from the compression of a file containing a radiographic image.
Methodology:
Fourteen intraoral radiographs were digitized employing an AGFA ARCUS II scanner at an optical resolution of 300 ppi and with a grey scale of 8 bytes. The images were stored in Tagged Image File Format (TIFF). The files were compressed with no information loss with the WinZip 8.0 program. Compression with information loss was performed using the Photoshop 5.0 program (Adobe Systems Inc., San José, CA. USA) and the Joint Photograph Expert (Group (JPEG) format. Each of the images was compressed to one of the 11 qualities available (10-0). An expert performed the qualitative analysis. The quantitative analysis involved digital subtraction with each of the JPEG images to yield a new image using Photoshop 5.). The histograms of grey values were submitted to statistical analysis. The mean and standard deviation were calculated for each image.
Results:
The data revealed that a JPEG lossy compression, six times smaller than the original TIFF, is compatible with diagnostic applications.
Conclusions:
The compression ratio calculated as the quotient between the file sizes and the standard deviation of the values corresponding to the image that resulted from digital subtraction may be employed to assess the outcome of the compression process and guarantee adequate quality.