Kim Kil Joong, Kim Bohyoung, Lee Kyoung Ho, Kim Tae Jung, Mantiuk Rafal, Kang Heung-Sik, Kim Young Hoon
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.
AJR Am J Roentgenol. 2008 Aug;191(2):W30-7. doi: 10.2214/AJR.07.3462.
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.
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.
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).
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.
本研究的目的是调查在联合图像专家组（JPEG）2000压缩的低剂量胸部CT图像中，肺与胸壁及纵隔之间可察觉伪影的差异，并表明一种感知图像质量指标——高动态范围视觉差异预测器（HDR-VDP）能够再现这种区域差异。
将20幅图像分别进行可逆和不可逆压缩，压缩比例为6:1至30:1。为了分别分析两个区域（肺；胸壁及纵隔），将每个测试区域之外的压缩像素替换为原始像素。通过比较压缩图像和原始图像，三名放射科医生独立将压缩伪影评为0级，无，无法区分；1级，勉强可察觉；2级，轻微；或3级，明显。在每个压缩级别，比较两个区域的读者反应、峰值信噪比（PSNR）和HDR-VDP结果。采用Wilcoxon符号秩检验和配对比例的精确检验，p值阈值为0.05。
肺内伪影的评级低于胸壁及纵隔，读者1在10:1至20:1时、读者2在8:1至15:1时、读者3在8:1至20:1时显示出统计学意义。0级在肺内更常见，读者1在10:1时、读者2和读者3在8:1至10:1时显示出统计学意义。PSNR结果表明肺内伪影更多（p < 0.001），而HDR-VDP结果表明肺内伪影更少（p < 0.001）。
尽管从数学角度看，肺内的压缩伪影比胸壁及纵隔更多，但放射科医生对伪影的感知却相反，而HDR-VDP能够成功再现这种情况。
