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mean bit rate per compression level (bpp)

FIGURE 8 Mean subjective score vs mean bit rate for the MR study. The dotted, dashed, and dash-dot curves are splines fit to the data points for judges 1, 2, and 3, respectively. The solid curve is a spline fit to the data points for all judges pooled. Reprinted with permission, from Proceedings First International Conference on Image Processing, ICIP '94, 2:861-865, Austin, Texas, Nov. 1994.

range, clearly fail to fit a Gaussian model. We note that scores are assigned to the entire image rather than to any subsection of an image, such as each of the blood vessels in that image. More detailed information would be needed for a more thorough analysis since subjective score in the MR experiment is meant to reflect the quality of the image for vessel measurement, and this may differ for the different blood vessels. The Wilcoxon signed rank test showed that the subjective scores for the MR study at all of the six compression levels differ significantly from the subjective scores of the

FIGURE 7 Subjective ratings vs bit rate for the CT mediastinum FIGURE 9 Subjective score vs bit rate for the MR study. The x's indicate study.

data points for all images, pooled across judges and compression levels.

FIGURE 7 Subjective ratings vs bit rate for the CT mediastinum FIGURE 9 Subjective score vs bit rate for the MR study. The x's indicate study.

data points for all images, pooled across judges and compression levels.

originals at p<0.05 for a two-tailed test. The subjective scores at all the compression levels also differ significantly from each other. As discussed later, it appears that a radiologist's subjective perception of quality changes more rapidly and drastically with decreasing bit rate than does the actual measurement error.

5.1 Mammography Subjective Ratings

For the mammography study, Table 4 provides the means and standard deviations for the subjective scores for each radiologist separately and for the radiologists pooled. The distribution of these subjective scores is displayed in Figs 10-12. Level 1 refers to the original analog images, level 2 to the uncompressed digital, level 3 to those images where the breast section was compressed to 0.15 bpp and the label to 0.07 bpp, level 4 to those images where the breast section was compressed to 0.4 bpp and the label to 0.07 bpp, and level 5 to those images where the breast section was compressed to 1.75 bpp and the label to 0.07 bpp.

Figure 10 displays the frequency for each of the subjective scores obtained with the analog gold standard. Figure 11 displays the frequency for each of the subjective scores obtained

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