Results - Discussion - References.
Compression without loss of information yields an image that is identical to the original, and thus requires no comparison. The average size of the files was 112823 bytes (range 102943-119523) and the average compression was 1.57 (range 1.48-1.71), thus yielding a file that was 68% the size of the original file. The smallest compression was achieved with images 7 and 10; the greatest compression ratio was achieved with image 4 (Table 1).
For JPEG compression (with loss of information), the file sizes are presented in Table 1 and the compression ratios are presented in Table 2. The compression ratio for all the qualities and radiographic images reached a minimum mean value of 2.26 and a maximum of 19.2 with a range of 2.14 for image12 to 22.9 for image 4.
The mean and standard deviation values that resulted from the quantitative statistical analyses are presented in Tables 3 and 4.The results of the qualitative comparative assessment are presented in Table 5. A summary of the present conclusions is presented in Table 6.
Table 1. Size of the image files.Discussion.
Table 2. Compression ratios with and without loss of information.
Table 3. Mean grey values corresponding to the images that resulted from digital subtraction.
Table 4. SD values corresponding to the images that resulted from digital subtraction.
Table 5. Results of the subjective evaluation by the expert.
Table 6. Summary of results.
Handling large amounts of digital data requires fast, simple and efficient access, a stable communication system and a safe storing procedure within a Picture Archiving and Communications System (PACS). Accepted international standards such as TIFF for images, ASCII for text and DICOM in general must be used. File compression favours speedy access and data transmission and reduces the memory requirements for storage. JPEG compression involves irreversible loss of information, and could be severely detrimental to diagnostic interpretation.
The present study involved the comparative, qualitative and quantitative assessment of indirect digital images obtained from 14 conventional radiographs. They were submitted to a compression process with or without information loss, employing the WinZip 8.0 and Photoshop 5.0 software. The results presented in Table 1 reveal that compression with no information loss reduced the size of the file to 68% of the original, and is a valid alternative to storing original images. The range of compression achieved was below that for compression with information loss.
JPEG compression is a widely accepted standard and runs on multiple platforms. The different edition programs have different interfaces for interaction with the operator. Thus, each of these programs uses a different compression scale. Photoshop uses a compression quality scale of 0-10, whereas PhotoPaint uses a 0-100 scale. Within this context, the final result may be difficult to evaluate.
The analysis of Tables 1, 2 and 3 demonstrates an unexpected outcome, i.e. the JPEG compression qualities 4 and 5 seem to be inverted. The file size for quality 4 was larger than for quality 5. This implies a lower compression ratio for quality 4 that was confirmed, in turn, by a larger standard deviation for quality 5. This finding was confirmed by repeating the procedure to rule out possible errors, and might reveal a flaw in the identification of the JPEG compression algorithm by the Photoshop 5.0 program.
The images compressed to qualities10, 9, 8 and 7 and some of the images compressed to quality 6 did not show interpretation difficulties. Belowt his quality and up to quality level 2, visualization was overall acceptable, but the resulting images were unsatisfactory in terms of guaranteeing accurate diagnosis owing to loss of information on the edges and loss of fine details. Lower quality images were inadequate. The quantitative analysis performed from the viewpoint of Medical Physics employing digital subtraction allowed us to draw the following conclusions:
- The image that results from the subtraction between the original image and the compressed image is the one that causes visualization difficulties. This becomes more marked as the degree of compression increases.
- The standard deviation of the grey values of the image that results from the subtraction process indicates the quality of the image and its adequacy for use in diagnosis.
The comparative assessment of both techniques revealed that a JPEGlossy compression six times smaller than the original TIFF is compatible with diagnostic applications. A compression between six and 13 times was not considered suitable for diagnosis. However, the overall quality of the resulting images would be adequate for illustration purposes or when fine details are not diagnostically relevant. The standard deviation of the values corresponding to the image that results from digital subtraction was appropriate to assess the result of the compression process and guarantee adequate quality.
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