Even low-ratio compression changes what readers see

Changes in images introduced by compression algorithms at levels as low as 8:1 can be observed by readers when they are compared with uncompressed images, according to a research report presented at the SIIM conference. Researchers were unable to say what impact compression has on diagnosis, however, as the study did not address this question.

The study included nearly 15,000 images at three institutions. Readers were asked to flip between compressed and uncompressed images and to say whether they noticed any difference in the two groups. The readers found differences between 8:1 compressed and uncompressed images 78% of the time. The figures for 12:1 compression were 95% and for 16:1 compression, 99%.

The researchers concluded that even mild compression changes images in ways that are perceptible. But the perceived differences cannot yet be linked to diagnostic performance.

The study used a "flicker" method to compare compressed and uncompressed images. Readers can scroll between images shown in rapid succession, which creates a sense of motion when the two are different, said Elizabeth Krupinski, Ph.D., one of the principal researchers. That perception of motion may cue readers to the differences.

The test looked at thin-slice (0.625 to 1 mm) images from CT scans of the chest, abdomen, and pelvis. Interestingly, radiologists saw no difference between compressed and uncompressed images 38% of the time, said Krupinski, a professor of radiology at the University of Arizona.

That figure for Ph.D. readers was 28% and for residents, 29%.

"The trend is for radiologists to be more tolerant of differences or less sensitive," Krupinksi said.

Other researchers in the study were Dr. Bradley Erickson of the Mayo Clinic in Rochester, MN, and Katherine Andriole, Ph.D., of Brigham and Women's Hospital, Harvard Medical School.