目的 采用无损数字水印算法对医学图像进行篡改检测和恢复是一个重要的研究领域。针对现有方法在区域划分和块特征值选取上的不足，提出一种新的基于四叉树分解和线性加权插值技术的无损水印算法。方法 首先对原始的医学图像进行四叉树分解，得到非固定尺寸且具有高同质性的图像块；然后利用线性加权插值方法计算每个图像块的特征值作为水印信息，最后采用基于混沌的简单可逆整数变换进行水印嵌入。结果 在提取端当水印图像没有受到篡改时，原始的图像能被无损恢复；当受到篡改时，算法能精确定位篡改区域并能高质量恢复，采用本文算法恢复的图像质量较现有方法高出20 dB左右。另外，在水印图像遭到较大程度篡改时，本文算法的正检率和负检率均优于现有方法。结论 实验结果表明，本文算法相比现有方法具有更高的嵌入容量、篡改检测精确性、恢复图像质量。算法适用于医学图像的完整性认证和篡改检测中。

Objective With the rapid development of telemedicine and online diagnosis,more and more precious medical images have been transmitted through the open network,and the security problem of medical images has been taken much attention by many researchers.The integrity authentication of transmitted medical image is a key step in information protection.Watermarking technology is generally regarded as an effective method for integrity authentication.However,like other quality-sensitive images such as medical imagery,any slight distortions are not acceptable,because the distortion may cause wrong diagnosis.Therefore,using lossless watermarking method for medical image's tamper detection and recovery is a very important research field.Aiming at the existent algorithm's defects in region division and block's features chosen,a new lossless watermarking method based on quad-tree decomposition and linear weight interpolation is presented in this paper.Method First,the quad-tree decomposition is used to divide the original medical image into flexible size blocks with high homogeneity.Because this division method can reduce the length of feature value,much more information can be embedded.Then a new linear weight interpolation method is used to compute each block's features as watermark information in our method.Because in existent methods, the mean used of the block's pixels are used as recovery feature for each block,the tampered regions occurring in the low frequency region of an image(usually,the image's texture complex area),it is not the best choice for restoring all tampered pixels with mean values, because it will reduce the quality of the recovered image.Finally,in order to reduce the embedding and extraction algorithm's complexity and enhance watermarking method's security,these features are embedded by using simple invertible integer transformation based on chaos.The introduction of chaos system improves the presented method's security.Result In order to test the presented method's validity,we test some performance properties including reversibility,embedding capacity,watermarked and recovered image's quality,and the detection accuracy rate.The experimental results show that the original image can be recovered with any distortion if the watermarked image was not been tampered,otherwise the tampered region can be localized and recovered with high similarity in the extracting procedure.Compared with the existent method,the embedding capacity is obviously prior to the similar method and the recovered image' quality using our method is about 20 dB higher.In addition,when the watermarked image is tampered to a large degree,the right detection rate and wrong detection rate is obviously better than the existent method.Since all the algorithms are open to the public,we introduce the logistic chaotic mapping to select the reference pixel's position in watermark embedding procedure.Due to the initial sensitivity and randomness,an adversary cannot obtain the correct watermark embedding positions easily if the initial parameters of logistic mapping are secret.Conclusion Experimental results show that,compared with previous similar schemes,the presented method not only achieves high embedding capacity and good visual quality of restored image,but also has more accuracy for tampering detection.The presented method can be applied to realize medical image's integrity authentication and tampering detection.