Compression artifacts appear significantly higher in thin-section CT images

Compression artifacts on thin sections are greater than on thick sections in JPEG 2000 compressed chest CT images, according to a new study from Korea.Image compression ratios of 5:1 to 10:1 are acceptable thresholds in compressing lung CT studies. What has not been clear until now, however, is whether these thresholds are optimal for images of various section thickness (AJR 2008;191:W38-W43)."In our study, the lung shows significantly more JPEG 2000 compression artifacts on thin-section CT images at a lung window setting than on thick-section images," said Dr. Kil Joong Kim of the radiology department at Seoul National University Bundang Hospital. Compared with thick sections, thin sections showed more mathematical artifacts (smaller peak signal-to-noise), higher grade artifacts, and more frequent perceptible artifacts, Kim said. Section thickness should therefore be taken into consideration when adjusting the compression level for lung CT images, he said."Using a lower compression level would be prudent for thin sections," Kim said.Likewise, thick sections might be compressed to a higher level than the 5:1 and 10:1 thresholds recently reported in studies that limited their analysis to thin (1- to 2-mm) sections.Irreversible image compression is advocated to effectively manage image data generated by CT scanners. Among the many factors affecting compression artifacts -- including compression algorithm, image content, and image acquisition parameters -- section thickness is of particular interest because thin and thick sections are both used for chest CT in clinical practice. "The ability to change section thickness after image acquisition is one of the most advantageous features of modern CT scanners," Kim said. "Many radiologists routinely reconstruct two complementary data sets of thin and thick sections from a single acquisition to emphasize spatial resolution along the z-axis and low contrast resolution, respectively."

The study relied on readers' subjective evaluation in grading compression artifacts in 35 thin-section (1 mm) and 35 corresponding thick-section (5 mm) images. In each compressed image, pixels outside the lung were replaced with those from the original image. By comparing the compressed and original images, three radiologists independently rated compression artifacts using grades:

• 0: none, the two images are indistinguishable
• 1: barely perceptible image difference
• 2: image differences subtle
• 3: significant image differences

Not only did thin sections have smaller peak signal-to-noise, they had higher grades of artifacts, showing significant differences at compression levels 10:1 and 15:1, Kim said.