Basu A, Kamal A D, Illahi W, Khan M, Stavrou P, Ryder R E J
Department of Diabetes, Endocrinology and Metabolism, City Hospital, Birmingham, UK.
Diabet Med. 2003 Sep;20(9):766-71. doi: 10.1046/j.1464-5491.2003.01022.x.
The National Screening Committee (NSC), whilst recommending the use of digital mydriatic retinal photography for diabetic retinopathy screening, has not yet accepted the use of digitally compressed images for grading. By greatly reducing the file size, however, compression of images is invaluable for storage and for its rapid transmission across computer networks. We undertook a study to compare the different levels of JPEG compression with the original bit-mapped image to determine whether there was any loss of clinical detail following compression.
Three hundred and thirty images were analysed in this study. These images had been captured from 66 eyes consecutively photographed in a diabetic retinopathy screening programme, using a Sony DXC-950 P 3CCD colour video camera mounted on a Canon CR6-45NMf fundus camera. Single 45 degrees macula-centred images were taken from each eye. The images were compressed using the JPEG algorithm within Adobe Photoshop (version 4.0) and then displayed with a Sony Trinitron colour monitor. Four different levels of compression were used, JPEG-1, JPEG-2, JPEG-3, JPEG-4, and an objective analysis was undertaken using 'lesion counts'. The compressed images were assessed separately and blindly and the results compared with their original BMP images.
Eight BMP images could not be evaluated (five right eye and three left eye). A total of 290 images were therefore used in the final evaluation. All the JPEG-1 images with file sizes between 16 and 24 kb were found to be 'pixelated', while the JPEG-4 images (66-107 kb) appeared similar to the original BMP (1.3 Mb) images. Both JPEG-2 and JPEG-3 images had significantly lower counted lesions than the BMP images.
From our findings we can conclude that only some degree of image compression (compression ratios of 1 : 20 to 1 : 12) with file sizes of 66-107 kb is permissible using JPEG format, whereas the images obtained after higher compression ratios may not be suitable for diabetic retinopathy screening.
国家筛查委员会（NSC）虽推荐使用数字化散瞳视网膜摄影术进行糖尿病视网膜病变筛查，但尚未接受使用数字压缩图像进行分级。然而，通过大幅减小文件大小，图像压缩对于存储及其在计算机网络上的快速传输非常重要。我们开展了一项研究，比较不同程度的JPEG压缩与原始位图图像，以确定压缩后临床细节是否有任何损失。
本研究分析了330张图像。这些图像是在糖尿病视网膜病变筛查项目中，使用安装在佳能CR6 - 45NMf眼底相机上的索尼DXC - 950 P 3CCD彩色摄像机，从66只眼睛连续拍摄获取的。每只眼睛拍摄一张以黄斑为中心的45度单幅图像。使用Adobe Photoshop（版本4.0）中的JPEG算法对图像进行压缩，然后用索尼特丽珑彩色显示器显示。使用了四种不同程度的压缩，即JPEG - 1、JPEG - 2、JPEG - 3、JPEG - 4，并使用“病变计数”进行客观分析。对压缩图像分别进行盲法评估，并将结果与其原始BMP图像进行比较。
8张BMP图像无法评估（右眼5张，左眼3张）。因此，最终评估共使用了290张图像。发现所有文件大小在16至24 kb之间的JPEG - 1图像都出现了“像素化”，而JPEG - 4图像（66 - 107 kb）看起来与原始BMP（1.3 Mb）图像相似。JPEG - 2和JPEG - 3图像的计数病变均明显低于BMP图像。
根据我们的研究结果可以得出结论，使用JPEG格式时，仅允许一定程度的图像压缩（压缩比为1 : 20至1 : 12），文件大小为66 - 107 kb，而更高压缩比后获得的图像可能不适用于糖尿病视网膜病变筛查。
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