Wenger N A, Tewson D H T K, McDonald F
Department of Oral Health, West Block, Norfolk and Norwich University Hospital, Colney Lane, Norwich, UK.
Med Eng Phys. 2006 Jul;28(6):560-7. doi: 10.1016/j.medengphy.2005.09.010. Epub 2005 Nov 11.
This study used an aluminium test object to assess the effect of the Joint Photographics Expert Group (JPEG) compression algorithm, on direct digital cephalometric image quality.
The aluminium block of 15 steps, with 20 holes in each step was radiographed in a Planmeca Proline 2002 digital cephalometric machine with Dimaxis2 software. Six different JPEG compression ratios were used to capture the cephalometric images. These ratios were 60%, 70%, 80%, 90%, TOP QUALITY JPEG (TQJPEG 98%) and TIFF (uncompressed). The images were taken at 68 kV and 12 mA with a 7 s exposure. Six experienced observers viewed the monitor displayed images, which were presented randomly. This was repeated one month later. The number of holes detected by each observer was plotted against each compression ratio. Intra-observer and inter-observer reproducibility was calculated using the Mann-Whitney U-test. Differences between the compression ratios were assessed using a Kruskal-Wallis one-way analysis of variance.
When comparing intra-observer reproducibility, it was found that there were only four of 36 comparisons that showed a statistically significant difference (Observer 1: 60% (P=0.004), TQJPEG (P=0.019); Observer 2: TIFF (P=0.005); Observer 3: 90% (P=0.007)). Statistically, there was no significant difference with inter-observer reproducibility. There was no statistically significant difference between the image quality obtained from each compression ratio.
The results showed that JPEG compression does not have any effect on the perceptibility of landmarks in the aluminium test object used in this study.
本研究使用铝质测试物体评估联合图像专家组（JPEG）压缩算法对直接数字化头影测量图像质量的影响。
在配备Dimaxis2软件的Planmeca Proline 2002数字化头影测量机中，对具有15个阶梯且每个阶梯有20个孔的铝块进行X线摄影。使用六种不同的JPEG压缩比来采集头影测量图像。这些压缩比分别为60%、70%、80%、90%、顶级质量JPEG（TQJPEG 98%）和TIFF（未压缩）。图像在68 kV和12 mA下以7秒曝光时间拍摄。六名经验丰富的观察者观看在显示器上随机呈现的图像。一个月后重复此操作。将每位观察者检测到的孔数与每个压缩比进行绘图。使用曼-惠特尼U检验计算观察者内和观察者间的可重复性。使用克鲁斯卡尔-沃利斯单向方差分析评估压缩比之间的差异。
在比较观察者内可重复性时，发现36次比较中只有4次显示出统计学上的显著差异（观察者1：60%（P = 0.004），TQJPEG（P = 0.019）；观察者2：TIFF（P = 0.005）；观察者3：90%（P = 0.007））。在统计学上，观察者间可重复性没有显著差异。从每个压缩比获得的图像质量之间没有统计学上的显著差异。
结果表明，JPEG压缩对本研究中使用的铝质测试物体中标志点的可感知性没有任何影响。
