title: Regional difference in compression artifacts in low-dose chest CT images: effects of mathematical and perceptual factors.
abstract: OBJECTIVE
  The objective of our study was to investigate the difference of perceptible artifacts between the lungs and the chest wall and mediastinum in Joint Photographic Experts Group (JPEG) 2000-compressed low-dose chest CT images and to show that a perceptual image quality metric-the High-Dynamic Range Visual Difference Predictor (HDR-VDP)-can reproduce this regional difference.
  
  
  METHODS
  Twenty images were compressed reversibly and irreversibly to 6:1-30:1. To analyze the two regions separately (lungs; and chest wall and mediastinum), the compressed pixels outside each tested region were replaced with the original pixels. By comparing the compressed and original images, three radiologists independently rated the compression artifacts as grade 0, none, indistinguishable; 1, barely perceptible; 2, subtle; or 3, significant. At each compression level, the two regions were compared for the readers' responses, peak signal-to-noise ratio (PSNR), and HDR-VDP results. Wilcoxon's signed rank tests and exact tests for paired proportions were used with a p value threshold of 0.05.
  
  
  RESULTS
  Artifacts were rated as lower grades in the lungs than in the chest wall and mediastinum, showing statistical significances at 10:1-20:1 for reader 1, 8:1-15:1 for reader 2, and 8:1-20:1 for reader 3. Grade 0 was more frequent in the lungs, showing statistical significances at 10:1 for reader 1 and at 8:1-10:1 for readers 2 and 3. The results of PSNR indicated greater artifacts in the lungs (p < 0.001), whereas HDR-VDP results indicated fewer artifacts in the lungs (p < 0.001).
  
  
  CONCLUSIONS
  Although compression artifacts are mathematically greater in the lungs than in the chest wall and mediastinum, radiologists' artifact perceptions are the opposite, which can be successfully reproduced by HDR-VDP.
authors.affiliation: Department of Radiology, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Institute of Radiation Medicine, and Seoul National University Medical Research Center, Gyeonggi-do, Seoul 463-707, Korea.
authors.forename: Kil Joong
authors.initials: KJ
authors.lastname: Kim
authors.forename: Bohyoung
authors.initials: B
authors.lastname: Kim
authors.forename: Kyoung Ho
authors.initials: KH
authors.lastname: Lee
authors.forename: Tae Jung
authors.initials: TJ
authors.lastname: Kim
authors.forename: Rafal
authors.initials: R
authors.lastname: Mantiuk
authors.forename: Heung-Sik
authors.initials: HS
authors.lastname: Kang
authors.forename: Young Hoon
authors.initials: YH
authors.lastname: Kim
mesh_terms: D000328:Adult; D000368:Aged; D000465:Algorithms; D016477:Artifacts; D044962:Data Compression / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008482:Mediastinum / Q000000981:diagnostic imaging; D008875:Middle Aged; D011829:Radiation Dosage; D013902:Radiography, Thoracic; D015203:Reproducibility of Results; D003074:Solitary Pulmonary Nodule / Q000000981:diagnostic imaging*; D018709:Statistics, Nonparametric; D035441:Thoracic Wall / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed