A low distorsion and reversible watermark: application to angiographic images of the retina.

Medical image security can be enhanced using watermarking, which allows embedding the protection information as a digital signature, by modifying the pixel gray levels of the image. In this paper we propose a reversible watermarking scheme which guarantees that once the embedded message is read, alterations introduced during the insertion process can be removed from the image. Thereafter, original pixel gray levels of the image are restored. The proposed approach relies on estimation of image signal that is invariant to the insertion process, and permits to introduce a very slight watermark within the image. In fact, the insertion process adds or subtracts at least one gray level to the pixels of the original image. Depending on the image to be watermarked, in our case angiographic images of the retina, it is expected that such image alteration will not have any impact on the diagnosis quality, and consequently that the watermark can be kept within the image while this one is interpreted.